Optically Functional Metallic Micro/nanostructure Fabrication via a Highly Photosensitive Direct Laser Reduction Method A thesis submitted for the degree of Doctor of Philosophy by Sahar Tabrizi Principle supervisor: Assoc. Prof. Baohua Jia Co-supervisor: Dr Han Lin External supervisors: Prof. Min Gu and Dr. Ben P Cumming Centre for Micro-Photonics Faculty of Engineering and Industrial Sciences Swinburne University of Technology Melbourne, Australia 2017
Transcript
Optically Functional Metallic Micro/nanostructure
Fabrication via a Highly Photosensitive Direct Laser
Reduction Method
A thesis submitted for the degree of
Doctor of Philosophy
by
Sahar Tabrizi
Principle supervisor: Assoc. Prof. Baohua Jia Co-supervisor: Dr Han Lin
External supervisors: Prof. Min Gu and Dr. Ben P Cumming
Centre for Micro-Photonics
Faculty of Engineering and Industrial Sciences
Swinburne University of Technology
Melbourne, Australia
2017
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A person who never made a mistake never tried anything new.
Albert Einstein
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Abstract
The constantly growing demand for fabrication of metallic structures with micro/nanometre
scale and feature size requires the two-photon photoreduction (2PR) based three-dimensional
(3D) scanning direct laser writing (DLW) system. Owing to the creation of surface plasmon
polariton on the surface of the metallic structures, that can control the electromagnetic (EM)
field and confine it in a nanoscale, it has attracted a remarkable attention from various
research fields such as metamaterials, plasmonics, surface-enhanced Raman spectroscopy
(SERS). In addition to the high feature resolution and 3D structure, having a high electrical
conductivity is of a great value and it is considered as the main requirement for a functional
metallic micro/nanostructures.
Development in nanofabrication techniques in order to meet all the required
characteristics for functional metallic micro/nanostructures has exceedingly increased,
recently. Due to the nonlinear multiphoton absorption confinement in the focal spot and the
material threshold response, DLW based on 2PR process has a high spatial resolution beyond
the diffraction limit with the potential of 3D structure fabrication, leading this approach as a
pioneer among all other introduced techniques as a single step approach with high flexibility
and efficiency. Therefore, 2PR can directly fabricate multidimensional micro/nanostructures
from a solution with a great simplicity; though, realizing functional plasmonic
micro/nanostructures providing resonances in the optical region is still challenging because of
the large linewidth of the structure, rough surface, resulting in a low electrical conductivity. It
seems that all those problems are associated with the material and its potential in interaction
with the incident light. Therefore, cited challenges correlated to the photoreduction material
are tackled in this thesis by improving the material regarding increasing the electrical
conductivity, surface smoothness and also the feature resolution of the structures. To exploit
the new material and its response to the fabrication of functional plasmonics structure and
also its flexibility to fabricate various designs via the 2PR technique, the research conducted
in this thesis concentrates on the following key areas.
As the photoreduction material properties -in interaction with the incident light- are the
main objectives to study the 2PR outcome, we concentrate on the required parameters of the
material for a plasmonic structure. The possibility of a large area thin film fabrication is
examined using both a UV lamp and a UV laser source. The characteristics of the thin film
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and the contained NPs have been studied both electrically and morphologically to find out the
optimized fabrication conditions. In addition, the potential of the material for the fabrication
of large area devices such as the microfluidic chip, have been discussed.
Single-step DLW of a functional silver nanoresonator array is verified by means of a
highly sensitive two-photon laser reduction procedure. The realized conductive periodic
silver nanoresonators with reduced surface defects reveal strong optical resonances in the
near to mid-infrared regions; it is proven that the fabricated nanoresonators response to the
incident polarization and it is also confirmed that the resonant frequency is tuneable across a
wide range by modifying the fabrication parameters. This simple photoreduction-based DLW
technique qualifies a scalable and cost-effective procedure for fabricating functional photonic
devices based on metallic nanostructure building blocks.
In order to demonstrate the flexibility of the technique for the realization of arbitrary
designs and also the possibility to fabricate metallic structures beyond two-dimensional (2D),
we explored different fabrication processes and eventually, we are able to confirm the
potential of the material and the fabrication technique in this regard. In addition, we also
explored the potential of achieving parallel processing through spatial light modulator (SLM)
assisted direct laser printing (DLP) technology.
The uniqueness of the research presented in this thesis is to propose a new technique for
fabrication of functional metallic micro/nanostructures with an especial version of
photoreduction solution, that could solve the poor electrical conductivity of the structure, low
surface smoothness, and the low feature resolution; hence, we could demonstrate that key
challenges in the fabrication of functional metallic micro/nanostructures are solvable and also
we provide our advice and suggestions for future development in this field. In this case, the
application range for this fabrication can be extended broadly to metamaterials, plasmonics,
SERS, lab-on-a-chip (LoC).
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Acknowledgment
I wish to express my gratitude to a number of people who became involved with this thesis,
one way or another. Firstly, I would like to express my sincere gratitude to my supervisor
Assoc. Professor Baohua Jia at the Swinburne University of Technology for her constant
support and for providing me with the opportunity to carry out this research and publish my
findings in journals and conference papers. I am also indebted to my co-supervisor Professor
Min Gu for his vital scientific advice and invaluable support.
I wish to give my best thanks to my co-supervisors Dr. Benjamin. P. Cumming for his
helpful discussions on the experimental performance, the software maintenance, and his
technical help, and Dr. Han Lin for his scientific discussions and his useful technical support
in setting up spatial light modulator (SLM) system successfully. I wish to thank Dr. Yaoyu
Cao for his contribution to this research by preparing the photoreduction material and his
scientific discussions and supports.
My thanks are extended to Ms. Pierrette Michaux, Dr. James Wang, Dr. Xijun Li and Dr.
Xiaohan Yang for their valuable assistance during my laboratory works. Also, I need to thank
Mrs. Barbara Gillespie, Mrs. Amable Lou and Mr. Riaan Lourens and other staff for their
help and assistance for all meeting arrangements and paperwork during my Ph.D. research.
I believe I owe deepest thanks to all people in my entire family who have supported me
since I was born. I wish to appreciate my parents, Mehrangiz and Firouz, without their
continuous support and encouragement I never would have been able to achieve my goals.
And I wish to thank my incredible sister Maryam and her husband Alireza Foroughi, for their
real support from the first day of staying in Australia and during my study. Their real love
made my student life easier and finally, my warm thanks must be to my amazing sisters
Nooshin and Rashin and my lovely brother Aziz for their encouragement and support in all
steps of my life and especially in my study.
Lastly, special thanks to my brilliant fiancée, Dr. Alireza Mohammadinia, for his
continued and unfailing love, support. I appreciate the time we spent for many scientific chats
regarding my project and thanks to him for helping me with technical support in writing,
drawing figures and understanding underpins my persistence in the graduate career and
makes the completion of this thesis possible.
Sahar Tabrizi
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Declaration
I, Sahar Tabrizi, declare that this thesis entitled:
“Optically Functional Metallic Micro/nanostructure Fabrication via a Highly
Photosensitive Direct Laser Reduction Method”
is my own work and has not been submitted previously, in whole or in part, in respect of any
other academic award.
Sahar Tabrizi
Centre for Micro-Photonics Faculty of engineering and Industrial Sciences Swinburne University of Technology Australia
Table of Content Abstract ................................................................................................................................ v
Acknowledgment ................................................................................................................ vii
Declaration......................................................................................................................... viii
List of Abbreviations............................................................................................................ ix
Table of Content .................................................................................................................. xi
List of Figures ..................................................................................................................... 14
List of Tables ...................................................................................................................... 20
Figure 5-11. (a) SEM image of the fabricated structure with the incident power of 2 mW,
green coloured c-shape structure on the top right side of the image makes the structure clear.
(b) enlarged image of a single out-of-plane c-shape structure. ........................................... 125
Figure 5-12. SEM images of fabricated out-of-plane c-shape structure with the different
incident powers of (a) 2 mW, (b) 3 mW and (c) 4 mW. Green coloured c-shape structures on
the top right side of the images make the structure clear and more recognizable. ............... 127
Figure 5-13. SEM images of fabricated structure on the different surface of (a) z=149 nm and
(b) z=150 nm. Green coloured c-shape structures on the top right side of the images make the
structure clear and more recognizable. ............................................................................... 128
Figure 6-1. Experimental setup for the DLP nanofabrication system. The arrow indicates the
polarization direction of the laser beam. Inset: the displayed phase modulation.[196] ........... 132
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List of Tables
Table 2.1. Classification of past studies ............................................................................... 36
Table 3.1. Material properties of thin films made by a UV lamp .......................................... 69
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Introduction
Problem statement
Metallic nanostructures are one of the significant components that strongly support a broad
range of micro/nanodevices, such as nanoelectronic integrated circuits, metamaterials,
micro/nanofluidics, and micro-electromechanical systems (MEMS) and
nanoelectricalmechanical system (NEMS) [1-5]. Succeeding in the fabrication of functional
optical structures that can manipulate light at the optical frequencies depends on the creation
of conductive metallic structures at the nanoscale with low structural defects [6, 7].
Currently, most optical nanostructures are fabricated either by the sophisticated electron
beam lithography (EBL) system, like the one in Figure 1-1 (a), or the focused-ion beam (FIB)
milling systems, which are too expensive, complex and time-consuming. Most important of
all, fabricating three-dimensional (3D) structures that are required to achieve comprehensive
functionalities is challenging. In comparison, the direct laser writing (DLW) technique has
proved its high spatial resolution and cost-effectiveness in the 3D fabrication of
micro/nanostructures [8-12].
Figure 1-1. (a) EBL system at Centre for micro-photonics, Swinburne University of Technology, (b) Two-beam optical beam lithography (OBL) has the advantage of fabricating 3D arbitrary geometry with nanometer feature size and resolution comparable to EBL by the photoinhibition strategy. The image in the inset shows the focal spot of the writing beam and the inhibition beam [13].
(a) (b)
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Furthermore, the recent breakthroughs in superresolution [14] nanofabrication as
shown in Figure 1-1 (b) and parallel writing [15-17] have improved the fabrication resolution
and throughput of the DLW process and made it one of the most promising future
nanofabrication technologies.
Research method
There have been different approaches for using the DLW method to achieve optical
micro/nanostructures, such as the coating process [18], the inversion process [19] and the
photoreduction [20] method.
According to the experiment that has been conducted by Radke et al. [18] in 2011 for the
fabrication of 3D bichiral plasmonic crystal, firstly, they fabricated a 3D bichiral crystal by
two-photon femtosecond DLW in a negative-tone photoresist coated on the substrate via spin
coating technique. Then, the fabricated dielectric template was coated with a metal film
through electroless silver platting, through which all exposed surfaces were coated without
any external current. Because during the electroless deposition process the substrate was also
coated with silver, the fabricated structure was detached from the coated substrate with a
glass capillary and deposited on a coverslip for transmission spectroscopy characterization.
Different research groups have used this method for metallization of templates of the
structure such as a 3D dielectric photonic crystal [21] generated by two-photon
polymerization technique inside a negative photoresist and coting with a metallic thin layer
such as silver via electroless platting method.
The idea of inversion process is to infill a polymer template by electrochemical
deposition of a metal such as gold or silver [19]. According to the work of Gansel et al. [19]
at 2009 helix-shape air holes were created in a positive-tone resist by 3D DLW system. A
glass substrate covered with 25-nm thin optically transparent film of indium-tin oxide (ITO)
as the cathode before the photoresist is spun on for the electrochemical deposition process.
After the development process, the structure was located in an electrochemical cell. A voltage
was applied between the ITO film and the anode, which was immersed in the electrolyte.
After gold infiltration by the current density and the growth time, the polymer was removed
by an etching process by exposing the composite structure to the air plasma and eventually,
an array of standing 3D gold helices achieved.
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Both the coating [18] and inversion [19] methods rely on a two-step process, starting
with the DLW of polymer templates followed by either a solution based metal layer coating
process or a chemical vapor deposition or electrochemical deposition inversion process. Even
if the polymer template has adequate quality, it is still challenging to succeed a uniform
coating. Additionally, such coatings and inversions do not preserve the geometry of the
original template resulting from the complex process. Compared to the two-step templating
methods, the single-step photoreduction method [22] can directly fabricate multi-dimensional
metallic micro/nanostructures from solution easily. Yet, it is still a challenge to achieve
functional nanostructures with response in the optical regime because of the large linewidth
of the fabricated metallic structures, rough surfaces, and the consequent poor electrical
conductivity.
We investigated in this thesis the fabrication of metallic multidimensional
nanostructure arrays via single & two-photon laser reduction process with the aim to achieve
smooth surfaces, high conductivity and high fabrication resolution. The fabricated metallic
nanostructure arrays show pronounced optical resonances in the near-infrared region (NIR)
with polarization sensitivity. Through tuning the parameters of the metallic
micro/nanostructure arrays, the optical frequency can be tuned across a large dynamic range
offering a viable cost-effective fabrication solution for optical micro/nano devices.
Thesis objective
The objective of this thesis is to develop a metallic aqueous solution for the fabrication of
metallic multidimensional optical micro/nanostructures arrays through DLW method based
on both the single-photon photoreduction and 2PR technique. The generated
multidimensional patterns can be realized to introduce a fast and cost-effective method with
high-quality, high design flexibility, large-area, and high-resolution fabrication capability.
In the direction of this goal, the studies have been conducted in three aspects.
Due to the close relation between material and two-photon photoreduction (2PR)
technique, investigating the light-matter interaction properties to achieve possible effective
ways with proper material selection have been extensively studied. Therefore, comprehensive
electrical and optical characterizations have been done on our material which is metal-ion
aqueous solution consists of diammine silver ions (DSI) as the silver resource and a nitrogen-
atom containing alkyl carboxylate Figure 1-2 (n-decanoyl sarcosine sodium, NDSS) as the
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O O
CH3
surfactant (which is able to control the resolution of the fabricated structure in photoreduction
process) and 2-hydroxy-4’-(2-hydroxyethoxy)-2-methylpropiophenone (HHMP) as
photosensitizer. The resolution of features in different structural and material conditions has
been investigated through theoretical and numerical calculations. As a final step in this part,
metallic microstructures have been fabricated with the single-photon process through UV
light, and the structures have been characterized morphologically, electrically and optically.
Also, the potential of the structures has been investigated to make a device such as a micro
heater for micro-fluidic applications.
𝐶𝐻3 − (𝐶𝐻2)8 − 𝐶 − 𝑁 − 𝐶𝐻2 − 𝐶 − 𝑂−𝑁𝑎+ (𝑁𝐷𝑆𝑆)
Figure 1-2. Chemical formulation of NDSS [20].
The second objective of the Ph.D. is to fabricate two dimensional (2D) metallic
micro/nanostructures with the 2PR process through the scanning technique and characterizing
the optical properties of the fabricated metallic structures experimentally. Towards this aim,
we fabricated silver c-shaped array for plasmonics applications; and to figure out the
functionality of the fabricated silver structures, Fourier transform infrared spectroscopy
(FTIR) has been used and plasmonic resonance of c-shaped array has been confirmed.
Therefore, we could investigate the possible functionality of the fabricated nano device such
as nano resonators.
Finally, the potential of generating the c-shaped arrays with the 2PR process via a
single exposure of the incident light has been explored. This fabrication is via the
superposition of two concentric circularly polarized beams. Our experimental demonstration
with a spatial light modulator (SLM) confirms that the fabrication of a metallic out-of-plane
c-shape array is possible. Our single laser exposure process enhances the nanofabrication
efficiency. By using a phase-modulated circularly polarized vortex beam and spatially
shifting the positions of the phase vortices, it is possible to create out-of-plane c-shape
patterns without the need for laser scanning. The fabrication material and method is used in
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this thesis deepens the research into the fabrication of a real 3D helical structure and also the
fast fabrication of large area chiral optical metamaterials in the infrared regime.
Thesis outline
This thesis has been divided into 6 chapters. A brief description of each chapter is described
in the next paragraphs:
Chapter 1: Introduction
Chapter 1 is composed of an introduction into the present research study in general and
highlights the problem statement, objectives, research method and outlines of the thesis.
Chapter 2: Literature review
Chapter 2 reviews the chemical scheme of 2PR fabrication process based on DLW method
along with the explanation of the proper related optical setup for this fabrication, and
introduces the current investigations on material and optical improvements to achieve a
functional structure in the optical region. Fabrication procedure, sample preparation method
and setup configuration followed in this research are also explained extensively for the
experimental part. Then the feasibility, simplicity, and flexibility of the 2PR method is
illustrated for the fabrication of metallic micro/nanostructures for variety fields of
applications, which provides the basis for this thesis. Then, an overview of the influence of
different chemical and optical parameters on the fabricated structure are presented. To set the
benchmark, a general and brief introduction to plasmonics and to the fabrication of out of the
plane structure is presented.
Chapter 3: Single photon fabrication via UV sources
Based on achieving a large area film fabrication of the material to simplify the material
characterization, making thin films via single photon reduction process are under
investigation in this Chapter. Thin films fabricated via a UV lamp and a UV laser in order to
achieve low and high-resolution structures, respectively. Both kinds of films have been
studied regarding their surface roughness and electrically conductivity depend on different
incident power and scanning speed. Considering different optical fabrication conditions to
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achieve an appropriate control on the properties of the final fabricated films, an extensive
discussion on the morphology of the nanoparticles (NPs) forming films is also presented.
Chapter 4: Functional optical plasmonic resonators fabricated via 2PR
Chapter 4 describes the investigation on the successful fabrication of 2D c-shape array with a
very smooth surface, high electrical conductivity, and high feature resolution. The
morphology and the optical properties of the structure are analyzed in order to its optical
functionality. Then, it is demonstrated that the structure has an adequate quality to introduce
surface plasmonic resonances in the optical regime. It is also investigated that fabricated c-
shape structure is sensitive to external light polarizations and readily tuneable in a broad
dynamic range by only changing the laser parameters.
Chapter 5: Fabrication of metallic micro/nanostructures beyond 2De
In Chapter 5, the potential of 3D metallic structure fabrication is explored towards achieving
chirality function using the 2PR through two techniques of scanning the focal spot inside the
material and via single incident light exposure. It is also described how to achieve multifocal
fabrication with our material based on the 2PR method. In addition, related fabrication
challenges in material aspects have been discussed. Moreover, the fabrication results have
been studied with respect to the potential of the material and its mechanical stability for
future 3D fabrication.
Chapter 6: Conclusion and outlook
Chapter 6 is to conclude the research presented in this thesis. Merits and limitations of using
this method for fabrication of metallic multidimensional micro/nanostructures considering
proper photosensitive material and laser parameters are discussed here thoroughly with
emphasis on future challenges and perspectives. Future outlook has been put forward
according to the recent advances in DLW technology and material engineering.
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Literature review
Introduction to two-photon reduction
Metallic micro/nanostructures have free electrons flowing on the surface that create surface
plasmon polaritons (SPP) through which electromagnetic (EM) field can be controlled and
localized in a nano-scale. This property makes metallic nanostructures competent candidates
for broad research fields such as metallic photonics crystals, metamaterials, plasmonic
structures, microfluidic chips and also surface-enhanced Raman scattering (SERS) because
metallic nanostructures are capable of controlling the optical energy in a sub-wavelength
region [18, 23-30].
To achieve functionalities for different applications, metallic micro/nanostructures need
to meet several key requirements. Firstly, micro/nanostructures need to possess a high
conductivity since high free electron mobility could increase surface plasmons formation;
Secondly, the structures are required to be in the sub-wavelength range, since the working
wavelengths of the devices are proportional to the structure feature size. For instance, to
achieve plasmonic devices in the optical regime, structure feature down to a few hundreds of
nanometer are required. Finally, to realize the maximum spatial confinement, three-
dimensional (3D) micro/nanostructures are required. However, simultaneously satisfying
these conditions, which means fabrication of 3D highly conductive metallic
micro/nanostructures with a small-feature-size down to the nanometer scale, is challenging.
But it holds the key for diverse applications in electronics and nanophotonics [31, 32].
Advances in fabrication accuracy and resolution require improvement in fabrication
methods. Great progress in decreasing the metallic feature size to nano-scale has been
achieved via nanofabrication techniques such as excimer laser lithography, nanoimprinting
lithography, focused ion beam (FIB) milling and electron beam lithography (EBL) [22, 33-
36]. However, these nanofabrication processes involve multiple steps including patterning of
photoresist, metal deposition and removal of the photoresist, which are not only complex and
time-consuming but also somehow degrade the quality of the fabricated structures. More
importantly, it becomes extremely challenging for the fabrication of 3D metallic structures
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using these methods because neither the method is capable of 3D direct fabrication. On the
other hand, using alignment in 3D is far more too complicated.
Recently, direct laser writing (DLW) has achieved enormous successes due to its
capability of one-step fabrication of arbitrary 3D micro/nanostructures with high throughput
and high resolution in a broad range of materials, including dielectrics and metals [8-10, 17,
27, 37-40]. DLW method tightly focuses an ultrafast laser beam into a tiny spot, in which the
chemical reactions of materials are controlled via the multiphoton absorption process that
enables the fabrication of micro/nanostructures with complex geometries [9, 39-46]. Due to
the 3D confinement of the nonlinear multiphoton absorption only in the focal region and the
material threshold response, high spatial resolution surpassing the diffraction limit can be
achieved [47]. In addition, the detrimental thermal effect that degrades the spatial resolution
can be minimized by controlling the pulse width and the repetition rate of the ultrafast laser
[48].
To realize functional metallic micro/nanostructures different DLW approaches have
been demonstrated, such as the coating method [18], the inversion method [19] and the
photoreduction method [19, 20, 49-51]. Both the coating and inversion methods depend on a
multistep procedure and start with the DLW of polymer templates followed by either a
solution based metal layer coating process [49, 52] or a chemical vapor deposition or
electrochemical deposition inversion process. Eventually, the polymer templates are removed
by the etching technique, as shown in Figure 2-1 (b) [19].
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Figure 2-1. (a) Fabrication scheme of the coating method. A glass cover slip serves as the substrate on which a double layer of negative-tone photoresist is deposited via spin coating. The thinner bottom layer is pre-crosslinked by UV flood exposure and serves as an adhesion layer. The top layer is exposed via 3D direct laser writing. Post-baking and developing converts the latent image formed during the exposure into a free-standing photoresist template of a 3D bichiral crystal resting on four small posts, one at each corner. All surfaces of this template as well as the adhesion layer are coated with a conformal silver film via electroless plating. To facilitate transmission spectroscopy, the bichiral crystal is detached from the silvered substrate with a thin glass capillary and deposited on a clean glass cover slip [18, 21]. (b) A positive-tone photoresist (blue) is spun onto a glass substrate covered with a 25-nm thin film of conductive indium-tin oxide (ITO) shown in green. After 3D DLW and development, an array of air helices in a block of polymer results. After plating with gold in an electrolyte, the polymer is removed by plasma etching, leading to a square array of freestanding 3D gold helices [19].
With these methods, even the polymer templates have adequate quality, it remains
challenging to achieve a uniform coating or complete infiltration due to the complicated 3D
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micro/nanoenvironment of the fabricated structures. Furthermore, such coatings and
inversions do not normally maintain the exact geometry of the original templates due to the
harsh procedures involved, which tends to attack the templates. Compared to the two-step
templating methods, the single-step photoreduction method can directly fabricate 3D metallic
micro/nanostructures from solution with a great simplicity using the multiphoton reduction
mechanism [20-24, 26-29, 53] However, due to the structure formation mechanism, which is
based on the nucleation of small nanoparticles (NPs), the current challenges in this method lie
in the realization of high-resolution metallic structures with small surface roughness and high
electrical conductivity towards functional plasmonic nanostructures with resonances in the
optical regime.
Recently, highly sensitive two-photon reduction (2PR) technique [54-58] has been
developed to fabricate conductive 3D micro/nanostructures in one step with high flexibility
and efficiency. It has been demonstrated that the quality of the metallic structures fabricated
using the 2PR method is excellent compared with other methods [59] providing a new and
enabling platform for functional nanophotonic device fabrication. In this Chapter, we provide
a comprehensive review of the development of the 2PR method for the fabrication of metallic
structures. We introduce the 2PR method as a cost-effective nanofabrication approach for
realizing functional metallic micro/nanostructures in the optical regime in Section 2.2. We
discuss the material properties and the fabrication threshold in Section 2.3. The fabrication
parameters and characterization regarding conductivity and smoothness of the fabricated
structures are presented in Sections 2.4. The potential and relevant applications, remaining
challenges and future prospects of the 2PR method are given in Section 2.5. A brief
introduction to the plasmonics and chirality of the metallic micro/nanostructures are
presented in Section 2.6 and 2.7, respectively.
Two-photon photoreduction process
2PR process is realized when two photons are absorbed, simultaneously –they can be
identical or different in terms of frequency- to excite a molecule to a higher energy level as it
is shown in Figure 2-2. The amount of energy for both photons should be equal to the energy
difference between two energy states of the molecule. Two-photon absorption is known as a
third order nonlinear procedure depends on the square of the light intensity [60], which is
much weaker than the linear absorption that happens at low light intensity. So, through the
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multiphoton photoreduction process, a metal ion absorbs two or more photons simultaneously
to be reduced into metal NPs.
Figure 2-2. Schematic of energy states of two-photon excitation process [61].
According to the literature, the basic mechanism for photoreduction of metal ions is the
absorption of two photons by the metal ions either through the molecules of a dye material
such as 2-hydroxy-4’-(2-hydroxyethoxy)-2-methylpropiophenone (HHMP) [59]- which will
act as a photosensitizer in the process- or directly from a laser beam [20]. In the case of using
dye material in the 2PR solution, transferring of an electron from the excited dye to the metal
ion leads to the formation of a metal atom, which can either (a) react with other metal atoms
to nucleate, or (b) add onto an existing particle, or (c) undergo a charge combination [59, 62].
Thus the formation and growth of metal particles are limited by the competition between
the rate of growth or nucleation and charge recombination, as well as local depletion of metal
ions. Since the growth rate is generally much greater than the nucleation rate and it depends
on the number of nucleation centres, it is reasonable to state that the introduction of NP seeds
into the composite could significantly enhance the formation efficiency of a continuous metal
phase. The photoreduction of the metal nanostructures can be divided into three stages:
nucleation, growth, and aggregation of metal NPs to form micro/nanostructures.
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The single-photon absorption rate is measured using the Beer’s law [63] as shown in
equation (2.1). For 2PR process, this law is still valid but it needs to be modified to equation
(2.2). The light intensity is a function of the path length (x), the concentration c and the initial
light intensity (I0). α and β are the absorption coefficients of one and 2PR processes,
consequently [64].
𝐼(𝑥) = 𝐼0 𝑒−𝛼𝑐𝑥 (2.1)
𝐼(𝑥) = 𝐼0
1+𝛽𝑐𝑥𝐼0 (2.2)
To excite the two-photon absorption (TPA) in the 2PR process, tightly focused ultrafast
laser pulses have to be employed. Ultrafast laser has the ability to transfer a massive amount
of photon energy (with a peak power on the order of ~1014 W/cm2) in a small fraction of the
time [65]. In other words, the photon energy could be deposited in the lattice much faster
while the material is exposed to the femtosecond pulses compared to the energy
transformation with electrons through phonon emission. This is especially beneficial in 2PR
in which thermal diffusion effect can impose a negative impact on the fabrication resolution
[47, 66, 67].
A typical setup for the 2PR fabrication of metallic structures is shown in Figure 2-3 (a).
Femtosecond laser pulses first pass a neutral density (ND) filter, where the beam intensity
could be properly adjusted. A beam expansion system allows the uniform illumination over
the back aperture of a high numerical aperture (NA) objective lens. The laser beam could be
tightly focused into the photoreduction solution in a nanometric volume on the order of λ3/4,
(λ is the wavelength of the incident light) by the high NA objective lens as shown in Figure
2-3 (b). The beam is focused into a sample to initiate the photoreduction. The sample, the
photoreduction solution, is often sandwiched between two coverslips as shown in Figure 2-3
(c), which are separated with a sticky tape with a thickness in ~100-200 micrometer range. A
3D piezoelectric scanning stage, which supports the sample, provides the 3D precise
movement on the subnanometer scale [20, 24, 42, 43, 68]. The motion of the scanning stage
is pre-programmed and controlled by a computer to form desired 3D micro/nanostructures.
The fabrication process is monitored in a real time by a charge-coupled device (CCD). The
requirements for photoreduction materials are discussed in Section 2.3. After the fabrication,
the sample is developed through a simple post-processing procedure to achieve the stand-
Figure 2-3. (a) Schematic apparatus of laser 2PR system used in our experiment; M: mirror, L: lens. (b) the detailed configuration of the sample for the bottom-up fabrication of 3D structures. (c) Schematic of material preparation for photoreduction fabrication.
There are two types of photoreduction processes, one is the photosensitized method
developed by means of dye molecules in 2PR solution and the other is the non-
photosensitized process. In the former route, the 2PR process starts from the absorption of
two photons simultaneously by the molecules of the photosensitisers, which can initiate the
reduction of the metal ions into metal NPs as shown in Figure 2-4 (a). A metal atom cannot
gain electrons, the metals can only lose electrons. A metal atom with the standard number of
electrons is not an ion, a metal atom that has lost electrons has is a metal ion. So metal ions
are cations with a positive charge. Then electrons transfer from the lowest unoccupied
molecule orbital level of the excited photosensitizer to the metal ions and create silver atoms.
The generated metal atoms could either link with other metal atoms and nucleate or undergo
34
charge recombination. As the growth rate of metal NPs is much faster than that of nucleation,
metal poly-crystal grows continuously inside the laser spot through three main sequential
reactions by tunning the focal spot in the material [62]. Figure 2-4 (b) shows a schematic of
the typical reduction process of metal ions. The presence of photosensitizer in the
photoreduction solution enhances the electron transferring process to the metal ions leading
to an increase in the formation of metal atoms.
Consequently, nucleation process speeds up due to the improved interaction between
metal atoms [53]. This is a photochemical synthesis process through which the structures can
be fabricated in different environments like the surfactant micelles in aqueous solution.
During the creation of NPs, they could act as a stabilizing polymer to control the size and
shape of the NPs [20, 69]. The main benefits of this technique are the high spatial resolution,
fast process, efficient formation of NPs and also the ability to control the incident light
wavelength since it depends on the sensitizer rather than the metal source [22, 69].
In the latter case, the nucleation process initiates with the absorption of photons by metal
ions. Then silver nuclei grow up to create silver NPs and ultimately, aggregation of NPs with
different non-uniform shapes and sizes form the final pattern [20]. Hence, the 2PR
mechanism could be defined in three different steps: first, the creation of seeds due to the
reduction of silver ions in a laser focal spot; second, the growth of silver seeds to NPs and
last, the aggregation of NPs, which form the silver pattern eventually as shown in Figure 2-4
(b).
This process contains fundamental route of chemical reactions, which allows the laser
light to interact directly with atoms or molecules. Through changing the material or tunning
the conditions of the incident light, it is possible to make different modifications inside the
material. In particular, due to the threshold behavior of the 2PR process, resolution beyond
the diffraction limit can be realized by controlling the laser pulse energy and the number of
applied pulses. Therefore, to achieve the best fabrication outcomes, the laser parameters,
including the wavelength, pulse width, repetition rate, power level and scanning speed need
to be optimized.
35
Figure 2-4. (a) The basic scheme of photoinduced transfer from the dye molecules to metal ions for the reduction of metal ions. Symbols denote Ag+ , silver ion; Ag, silver atom; ET, the electron transfer; CR, charge recombination; HOMO, the highest occupied molecular orbital; LUMO, the lowest unoccupied molecular orbital. (b) schematic of photoreduction process of metal ions and forming a metallic pattern in three steps; first, nucleation; second growing; final step, aggregation [20].
As shown in Table 2.1 most of the 2PR experiments employed near-infrared (NIR)
femtosecond laser pulses readily from a Ti: Sapphire laser. However, it has been found that
the laser illumination wavelength has a profound impact on the fabrication resolutions in two
2013/Cao/APL[22] Silver dots Nanophotonic 0.022 um Increasing electron donor 800 nm, 140 fs, 80MHz, NA1.4
NA
2015/Wang/Sci.Technol.Adv.Mater[80]
Silver NPs Lines Chip/Microfluidic chip
Wire:0.19 um Silver NPs: tens of nm
Controllable assembly of AGNPs/ roughness:11nm
800 nm,120 fs, NA=1.45
NA
2015/Kang/Nanotechnology[32]
Silver/ Gelatin matrix
3D(15) THz metamaterial devices
Sub100 nm 3D Ag in a stable dielectric matrix
795 nm,50 fs,11 MHz, NA
2015/Gu/Adv.opt.matt [59]
Silver aqueous solution
2D c-shape array
nanoresonators resonant at 100 & 75 THz
Sub 200 nm Surfactant & photosensitiser assisted 2D nanostructure
532 nm, 250 fs, 50 MHz 1.55 Ω/
38
First of all, the Abbe’s diffraction limit [81] is proportional to the illumination
wavelength, the shorter the wavelength, the higher the achievable resolution given the two-
photon absorption condition is still satisfied. Due to this reason, some groups introduced
optical parametric oscillator (OPO) to convert the NIR femtosecond pulses to the visible
ones, which has improved the fabrication resolution significantly [40, 66, 82-85]. Secondly,
tuning the wavelength of the femtosecond laser can increase the photosensitivity of the
photoreduction solution therefore significantly reducing the required threshold power leading
to a much smaller fabricated feature size. It has been demonstrated by tuning the laser
wavelength from 800 nm to 580 nm, the weight of the reduced silver dot fabricated with the
laser energy in the threshold region increased faster with increasing the laser energy, which
suggested that the shorter wavelength rendered a higher photosensitivity [22]. As a result, the
fabricated feature size of the silver dot decreased from 220 nm to 130 nm. Through
combining with the threshold fabrication mechanism and a highly photosensitive
photoreduction medium the fabrication of the silver dot as small as 22 nm, which is
equivalent to λ/26, far beyond the diffraction limit, has been achieved [22, 47]. This result is
by far the highest resolution achieved for silver dot fabrication. For line fabrication, the
minimum line width of 120 nm was reported by Atsushi Ishikawa et al. [62] as summarized
in Table 2.1.
At a given laser wavelength, the laser power and scanning speed are normally used to
control the fabrication conditions and provide the essential energy to achieve different sized
fabrication results. At an energy level higher than the threshold, the fabrication feature size
normally has a monotonic dependence on the laser power or scanning speed when one of the
parameters is fixed. The feature size increases with elevated power and reduced speed and
reaches saturation at a certain level. According to the results presented by Maruo et al. [50] in
2008, the linewidth can be controlled precisely from 0.2 to 1.7 μm when the power is
changed from 1 mW to 5 mW. On the other hand, increasing the scanning speed from 1 to
100 μm/s at a constant fabrication power of 4 mW, resulting in the reduction in the linewidth
from 1.2 to 0.3 μm. In this experiment, a Ti: sapphire laser of 752 nm wavelength with a
pulse width of 200 fs and a repetition rate of 76 MHz was used. The objective had an NA of
1.25. Depending on the repetition rate of the fabrication laser, too high scanning speed can
lead to discontinued lines due to the long time interval between two consecutive pulses.
Therefore, to ensure high quality and comparatively fast fabrication, the scanning speed is
39
normally maintained at a medium level while varying the laser power to adjust the fabrication
feature size.
Figure 2-5. Fabrication line width dependence of laser power and scanning speed with a laser of 752 nm wavelength, a pulse width of 200 fs, a repetition rate of 76 MHz, and an objective of 1.25 NA [50].
Laser pulse width and repetition rate are the two parameters that affect the thermal effect
in the DLW fabrication and have been less discussed in 2PR. The laser pulse width can
impact the fabrication feature size in two ways. On one hand, decreasing the pulse width
increases the peak intensity at a constant power. Therefore, the photon density in the focal
region is higher leading to a lower threshold, therefore, improved fabrication resolution [65].
On the other hand, the pulse width determines the temporal distribution of the energy. Since
the spatial distribution of the focal spot is proportional to the temporal distribution owing to
the inclusion of more spectral components [86, 87], a shorter pulse can lead to a higher
resolution focal spot, which can eventually reduce the fabrication feature size. The laser
repletion rate is relevant to the 2PR in terms of the thermal diffusion. Normally a lower
repetition rate results in less thermal diffusion effect, which in turn benefits the 2PR
fabrication resolution [65].
40
Material for 2PR fabrication
Since materials properties have a profound impact on the 2PR fabrication outcomes, it is
important to know the material characteristics to optimize the fabrication product.
Formulations may vary based on different application requirements. Here we focus on the
metallic materials that are useful for plasmonic relevant applications. The conductivity,
feature size and surface roughness of the fabricated structures have been used as the basic
criteria. In this section, we present a review of different materials that have been used so far
in 2PR fabrication. The fabricated structures are discussed considering the resolution,
conductivity, surface roughness and also functionality.
Fabrication materials in 2PR can be categorized into two types: one is a metal-ion
aqueous solution containing metal salt with dye as the main ingredients; the other one is a
metal-ion doped polymeric film. Among all metals, silver is the most popular material to be
used in the 2PR fabrication process -either as a metal salt to make the aqueous solution or as
metal-ions to be doped in polymer films- due to different reasons such as its high chemical
stability, controllability, good plasmonic properties and low-cost compared to gold.
Commercially available silver salts such as AgNO3, AgClO4 and AgBF4 could be mixed with
de-ionized water and eventually with either dye or polymer for preparing aqueous solutions
and polymer films, respectively; while for gold structures, HAuCl4 is often used as the metal
salt.
The beginning of the study on photoreduction of silver ions dates back to 1976 [88]. A
lot of research efforts have been devoted to developing 2PR fabrication technique in the last
decade [51, 85], which resulted in a rapid progress in this field recently.
Many research groups have used metal-ion doped polymeric films to fabricate metallic
structures by 2PR because using polymer can reduce the surface roughness much lower than
structures made from the aqueous solution. This is because metal ions distribute more
uniformly in polymer matrix and polymer prevents detrimental metal ion diffusion.
According to the state-of-the-art, multidimensional continuous structures have been
fabricated in polymeric matrix [50, 85, 89-91]. These structures show a proper
interconnection between NPs which in turn helps to develop the electrical conductivity of the
structure; Continuous 2D/3D structures with a smooth, continues surface and resistivity of
3.4810-7 Ωm has been reported by Maruo et al. [50] as the only conductive structure
reported among all groups. In their experiment, they used a Ti: Sapphire laser of 752 nm
41
wavelength with a pulse width of 200 fs and a repetition rate of 76 MHz with a 1.25 NA
objective, as shown in Figure 2-6. According to their report, only the appropriate amount of
the photon density in the focal region and a proper density of silver ions in the polymer
matrix could make such continuous lines of 200 nm width. With the same mixture of material
but not enough photon and silver density, the final structure would not be smooth and well
conductive [91].
In this method, a polymer material, such as polyvinylpyrrolidone (PVP), is used as a
polymer matrix and it is dissolved in ethanol. In the second step, silver nitrate is dissolved in
deionized water. Polymer solution and silver solution are mixed in the next step and stirred
for a while. Eventually, a cover slip is coated with the final solution to make the thin film.
The process contains prebake at a high temperature above 100 C after coating, through
which silver ions are likely to aggregate and prevents achieving a high spatial resolution [50].
Figure 2-6. Experimental procedure for sample preparation and 3D laser drawing of metallic microstructures using metal-ion doped polymeric films with a laser of 752 nm wavelength, a pulse width of 200 fs, a repetition rate of 76 MHz, and 1.25 NA objective [50].
Tsutsumi et al. [74] introduced a polymer based 2PR fabrication method but they
performed the fabrication at a laser wavelength of 508 nm, which was different from the
usual 780 to 800 nm sensitizer excitation wavelengths, to improve the lateral spatial
resolution. In this experiment, a femtosecond laser of 800 nm wavelength, a repetition rate of
1 kHz, and a pulse width of 100 fs and an oil-immersed objective lens with NA of 1.30 was
used and the wavelength was converted to 508 nm wavelength using an optical parametric
amplifier (OPA). The light with the wavelength of 508 nm could directly excite the carbonyl
group of PVP via the 2PR process and reduce the silver ions. The scanning speed of laser
light was 2 µms-1 and the laser energy was considered in the range of 6 µW-50 µW. One pair
of electrons in PVP-stabilized silver ions and decreased the molecular weight of PVP
preventing silver clusters growing larger. As a result, narrow silver clusters of linewidth
PDMS
casting
42
ranging from 300 nm to 400 nm were fabricated in the PVP matrix, which was an
advancement towards structure resolution improvement.
Although excellent progress in optical properties of metallic microstructures using this
method has been achieved, there are still challenges to further advance this technique. For
instance, being a multi-step process as shown in Figure 2-6, it makes this method a time-
consuming approach, in addition to the spatial resolution limit of the conductive structures.
This might be due to the prebake process.
Metal-ion aqueous solution is introduced to fabricate metallic micro/nanostructures via
the 2PR technique by Tanaka et al. [51]. Tanaka proposed the fabrication of a self-standing
3D microstructure with high electrical conductivity via the 2PR method using metal-ion
aqueous solution. They prepared the aqueous solution of AgNO3 and HAuCl4 for the
fabrication of silver and gold structures, respectively. The coverslip was coated with 3-
aminopropyltrimethoxysilane to attach the metal pattern firmly on the substrate surface.
According to their report, the average resistivity of 5.3010-8 Ωm was achieved. However,
the surface roughness was still high and limited the functionality of the structures.
Developing the photoreduction material to gain higher electrical conductivity and feature
resolution was continued by Ishikawa et al. [71]. They used a two-photon sensitive dye
(coumarin 440) in the reduction aqueous solution containing AgNO3 and investigated that the
presence of dye could reduce the diameter of the fabricated dots from 3.39 μm to 0.55 μm,
while the required fabrication power has changed in the case of the absence and presence of
dye from 14.97 to 1.88 mW, respectively. Improvement in resolution for wires fabricated in
this work was 400 nm and 3D structure of free-standing rod was successfully fabricated. In
2008 by modifying photoreduction material Cao et al. [20] could fabricate silver strips with
the linewidth of 120 nm and 3D structure with the size of 180 nm, which is the best
resolution reported to date based on the 2PR technique. They succeeded in achieving such a
result by adding NDSS as the surfactant to the reduction solution to prevent the growth of
unwanted silver particles. This way, they provided a simple method to precisely control the
position of NPs, in addition to the efficient creation and accumulation of the NPs.
As mentioned previously, to enable functionalities in nanophotonics, microelectronics,
and micro-electromechanical systems (MEMS) technology, metallic microstructures need to
be well controlled on the size of the aggregated NPs and the surface smoothness.
43
Cao et al. [92] proposed the method to fabricate the structures with respect to the
different carbon chain lengths (Cn: n= 4,5,7,9) of fatty salt in the photoreduction solution as
shown in Figure 2-7 (a-h). The fatty salts used in the sample solutions include sodium
valerate with four carbons in its chain (C4), sodium hexanoate (C5), sodium caprylate (C7)
and sodium decanoate (C9). According to the results increasing the chain length from C4 to
C9 results in increasing the resolution from about 600 to 450 nm. Also, it can improve the
surface smoothness, which has a key role in making the structure efficient for low-loss
photonics applications. The reason for this is a larger hydrophobic force existing in the longer
carbon chain chemical which prevents the growth of NPs efficiently.
Figure 2-7. (a-d) SEM images of the silver dots fabricated in sample solutions with different fatty salts, (a) for C4, (b) for C5, (c) for C7, and (d) for C9, which are under the laser power of 4.6 mW and the laser duration of 800 ms; (e-h) SEM images of the silver dots fabricated in sample solutions with different fatty salts, (e) for C4, (f) for C5, (g) for C7 and (h) for C9, under the laser power of 1.3 mW and the laser duration of 800 ms; scale bars: (a-h) 500 nm [20].
Ishikawa et al. [62] reported the first fabrication of 2D functional metallic nanostructure
for plasmonics metamaterials in 2012. The silver rod-pair array was fabricated using the same
2PR technique and the same mixture of the aqueous solution as mentioned above by this
group. Magnetic resonances with negative permeability at the far-infrared frequency were
demonstrated by these silver rod pair array. This was the starting point on the functionality
track of metallic micro/nanostructures towards micro/nanodevice fabrication using the 2PR
technique, which has been improved further since then. In 2013 Lu et al. [79] fabricated gold
nanostructures using a new stabilizer, amino-terminated ionic liquid with AuCl-4 ions. NPs
with sizes less than 5 nm were achieved, which eventually lead to the high smoothness of the
fabricated structure. The fabricated u-shape planar metamaterials of 228 nm line width
44
showed a spectral response in the terahertz region, which achieved another milestone towards
the fabrication of microphotonics/electronic devices. However, the structure was not
responding to the polarization of incident light, which in turn limits the potential applications,
although the calculated electrical resistivity of 16.510-8 Ωm was achieved, which is in the
same order of magnitude as that of bulk gold.
Although promising and exciting progress have been reported so far from various
research groups in this field, there are still key challenges to achieving an excellent functional
metallic nanostructure. From the functionality point of view, there is always a discrepancy in
conductivity between bulk silver and fabricated silver nanostructures because of oxidization,
sulfurization and also the surface roughness which degrade the functionality of the structure.
The same challenges exist for fabricating gold nanostructures.
In fact, the fabricated structures are the pile of the NPs and they are not like the bulk
metal with high crystalline structure. Therefore, the conductivity is quite low [79]. The
inclusion of organic or other residual chemicals such as surfactants [20, 59, 79] and
photoinitiator [93] also reduces the conductivity, although their presence in the solution
decreases the size of the produced NPs. On the other hand, the pile of the particles increases
the surface roughness, which introduces losses for electrons [51, 75]. These problems can be
potentially solved by reducing the NPs size to nanometer range less than 10 nm [22, 79, 92,
94] and allow them to densely packed together by tunning the carbon chain length [79, 92]
thermal annealing or even with tighter focusing [13, 59], which can also remove the organic
chemicals if there is any remaining.
Recently, functional silver plasmonic resonators that are responsive to light polarization
have been demonstrated [59]. The fabricated silver c-shaped array has a low root-mean-
square surface roughness of 22 nm, a small sheet resistance of 1.55 /square and high feature
resolution of less than 200 nm, leading to measured optical resonances in the near to mid-
infrared region. The aqueous solution was prepared by mixing three components: a
photosensitizer, a growth inhibitor and a source of silver. A water soluble dye HHMP was
used as the photosensitizer and a surfactant NDSS was used as the growth inhibitor, which
improved the fabrication resolution in the 2PR process. The conductivity of the structure can
potentially be tuned by changing the concentration of the growth inhibitor to prevent the
formation of the insulating layer on the metal NPs [22]. Therefore, the achievement of the
functional plasmonic structures via the 2PR process has manifested the importance of
controlling NPs size on the improvement of the feature resolution, conductivity and surface
45
smoothness. It is obvious, the smaller the NP size is, the higher the achievable resolution,
surface smoothness, and conductivity are if no insulating layer is involved on the surface of
the NPs. Hence, to achieve a functional metallic nanostructure via the 2PR process a great
attention should be dedicated to controlling the NP size. Using metal-ion doped polymer
films for 2PR makes it quite challenging the sizes of NPs to be reached less than 50 nm.
While fabrication in aqueous solution under optimized conditions enables the formation of
NPs on a scale less than 50 nm [22, 79], establishing an excellent platform for a high-
resolution metallic structure with a great smoothness and a high conductivity. NP size can be
adjusted by controlling the level of light confinement in the focal region. The stronger light
confines the faster reduction process it will lead to, which results in the decrease of the NP
size.
State-of-the-art of 2PR structures
It is important to characterize the fabricated structures regarding their physical properties
such as electrical conductivity, surface roughness, fabrication resolution and optical
properties in order to evaluate their functionalities. Hence, a brief review has been provided
in this section due to the significance of characterization as the final stage of the experiment,
which can guide researchers to optimize the fabrication products.
2.4.1 Conductivity
In the experiment, there are two main methods for characterizing the electrical conductivity
of the fabricated structures and their composites. Either a digital voltmeter or a four-point
probe can be used in this case. To measure the electrical conductivity through a digital
voltmeter, usually, two metallic electrode pads are fabricated of silver, gold or cupper on a
glass substrate and then metallic lines can be fabricated via the 2PR process between them to
connect the electrodes as shown in Figure 2-8. Eventually, the resistivity can be calculated
using the measured resistance from the gradient of the current versus the applied voltage and
the geometrical parameters of lines such as length, width, and height [51, 79].
46
Figure 2-8. (a) SEM image of a gold metallic nano line between two Au electrodes. (b) AFM image of the gold nano line [79].
The latter method with a four-point probe is usually used to characterize a large area or a
thin film [59]. Since the distance between the two end-probes in a four-point probe tool is 4
mm, the structure should be at least in a 4 mm area according to the Figure 2-9.
Figure 2-9. Schematic of a four-point probe for sheet resistivity measurement [95].
Measurement of the sheet resistivity with a four-point probe is through the following
equation:
𝜌𝑠𝑞𝑢𝑎𝑟𝑒(
𝛺
𝑠𝑞𝑢𝑎𝑟𝑒)
=𝜋
ln (2)
𝑉
𝐼 (2.3)
where: 𝜋
ln (2)= 4.53;
47
For the bulk resistivity measurement, we need to consider the bulk thickness as well, so
the equation has a very small variation as below:
𝜌 =𝜋
ln (2)𝑡 (
𝑉
𝐼) = 4.523𝑡 (
𝑉
𝐼) (2.4)
According to the state of the art, the acceptable and the best range of reported resistivity
obtained via these methods is on the order of 10-8 Ωm -10-6 Ωm (bulk silver and gold
resistivity order), which supports the functionality of the structures for the different level of
applications.
2.4.2 Resolution
The resolution and morphology of the fabricated structures could be characterized by various
microscopes, including a SEM or a transmission electron microscopy (TEM). According to
Table 2.1, the best-achieved resolution of the fabricated silver dots via 2PR is 22 nm [22],
which could be potentially used to produce nanophotonics/electronic devices for sensing and
SERS applications. The best-reported resolution for line fabrication is around 120 nm [20, 62,
72] for applications such as integration of microcircuits in MEMS, LoC, microfluidic devices
and plasmonics and metamaterials resonator at far-field in the THz region [59, 96]. As is
discussed earlier in this Chapter, controlling the growth of NPs during the 2PR process is of
great value to improve the resolution of linewidth in the structure. So in addition to the
material properties, laser parameters also need to be optimized to target different types of
applications [77, 79, 80].
2.4.3 Surface roughness
The surface roughness of the fabricated structure is characterized by atomic force microscopy
(AFM). Surface roughness level is determined by the average roughness over a certain area
[59]. As an example, the topography and cross-sections of silver lines fabricated via 2PR are
shown in Figure 2-10 (d-i) [20].
48
Figure 2-10. The left column contains SEM images obtained from samples with [NDSS]= (a) 0.013 M, (b) 0.033 M, and (c) 0.099 M, respectively; (d–f) show corresponding topography and (g–i) cross-sectional images taken by AFM. Scale bars are 100nm in (a), (b), and (c) [20].
AFM characterization provides the measurement of the surface roughness of the
structure. Different level of smoothness is necessary for the different range of applications.
For instance, plasmonic structures and metamaterials need a highly smooth surface of below
100 nm roughness to be responsive effectively to incident light with a low loss [59].
Applications
As has been discussed extensively in this Chapter, 2PR can be a versatile technique to
fabricate metallic micro/nanostructures, which have a great potential to be applied in many
applications such as plasmonics, metamaterials, nanowires, microfluidics chips, lab-on-a-chip
(LoC) devices and also as SERS substrates. Different research groups have reported the
fabrication of plasmonic structures- such as grating-like [97], disconnected and connected
nanostructures [32, 40, 78, 98], metamaterials [79, 99-112], which can bend, scatter, transmit
or otherwise shape EM radiation in ways that no natural material can. Here we provide a
49
summary of the applications of multidimensional micro/nanostructures fabricated through the
2PR method.
2.5.1 SERS substrates
Since the SERS was used for single molecule detection due to its strong enhancement,
different methods have been introduced to fabricate SERS substrates for a larger field
enhancement [113, 114]. Among all the approaches the 2PR process is known as the most
repeatable and reliable method to prepare SERS substrates in a cost-effective manner. With a
femtosecond laser at the optimized fabrication conditions, regularly arranged grating-like
nanostructures were directly fabricated and silver NPs could form on the nanostructure
simultaneously with high enhancement factor (EF) of 109 via a one-step fabrication technique
[96, 115-117]. This new method for fabrication of multifunctional integrated microchips and
SERS substrate has a great potential application for LoC technology [82]. Xu et al. [118]
successfully fabricated localized flexible integration of SERS monitors. The patterned silver
substrate at the bottom of a microchannel was fabricated through 2PR with a high
enhancement factor of108. The 2PR process allows the design of flexible structures that
could form unique SERS substrates into various patterns and could be placed at any desired
point of the microchannels. This group also fabricated on-chip silver microflower arrays
within the microfluidic channel as a catalytic microreactor to allow in situ SERS monitoring
via 2PR [119]. The fabricated structure shows a high catalytic activity and SERS
enhancement of 108.
2.5.2 Flexible electronics
Direct fabrication of 3D metallic micro/nanostructure is the most significant advantage of the
2PR technique comparing to other photolithography methods.
It has been investigated that complex geometries could be fabricated by 2PR into
photoreduction solution on the planar substrate. Moreover, 2PR affords to fabricate the
design on either nonplanar substrate or on the flexible substrate; which broadens the
application spectra and makes this fabrication technique extremely powerful. Xu et al. [72]
succeeded to demonstrate flexible metal nanowire on a nonplanar substrate as shown in
Figure 2-11(a). Also, they provided the fabrication of microheater inside the microchannels
for microfluidic applications as shown in Figure 2-11 (b). The patterned silver nanowires
50
maintained a low resistivity of about 1.610-7 Ωm, that makes it a great candidate for
circuitry and electronic interconnection applications that required a high conductivity, for
instance as a heating circuit.
Figure 2-11. (a) SEM image of the silver microwinding on a hemisphere, (b) SEM image of silver microheater inside a microchannel ( 80 μm in width and 20 μm in depth) [72].
Ng et al reported on the fabrication of a helical silver track written onto a cylindrical
polyimide substrate as shown in Figure 2-12 (a) [73]. This process has introduced a low-cost
manufacturing method for 2.5D structures such as spiral inductors used in components as
capacitors or inter-digital electrodes in Figure 2-12 (b). Although this process needs several
steps to be completed, the potential of 2PR has been confirmed for fabricating
micro/nanostructures on a flexible and nonplanar substrate.
Figure 2-12. (a) An electroless Cu plated micro-coil fabricated by DLW. (b) Long silver helix track fabricated by a laser on glass pipette coated with a polyimide film. The inset shows the magnified image of the track with a linewidth of 15 μm [73].
51
2.5.3 Microfluidic devices
Microfluidic chips or LoC systems are widely used in synthesis, analysis, detection, sensing
and therapy in various field such as biology, chemistry, physics, material science,
pharmaceuticals because of its integrated multifunctional system [96, 120].
When SERS is combined with LoC, a sensitive system can be created for optofluidic
detection. Integration of solid-state SERS substrate with microfluidic devices (Figure 2-13 (a-
d)) could be realized through two different methods. One way is the chemical route and the
other is the laser micro/nanofabrication technique. With the help of DLW, 3D complex
structure could be fabricated in a broad range of materials. Xu et al. [119] presented
nanoflower array fabricated via 2PR inside the microfluidic channel for a catalytic reaction.
Figure 2-13. (a,b) Schematic illustration of the fabrication of a silver microheater inside a microfluidic channel. (c) Heating test of a microheater fabricated inside a microchannel. Optical micrographs of the heating process. (d) The intensity ratio of monomer to excimer of PS-Na used to quantitatively calculate the local temperature and dependence of temperature on heating time [120].
According to the literature, one of the major challenges in the fabrication of microfluidic
devices and integration with solid-state SERS substrate is the high cost and sophisticated
52
fabrication method, which can be solved with the TPR technique. In addition, 2PR technique
could aid miniaturization of SERS-enabled LoC systems to achieve a portable detection
system. Moreover, the powerful 2PR technique could also offer the design and integration of
more functional units dramatically increasing the functionality and complexity. Considering
the advances in 2PR fabrication in the near future, it is promising to expand application
ranges for microfluidic chips.
2.5.4 Plasmonic and metamaterials
Using the 2PR method, the first functional planar metamaterials structure was made by Lu et
al. [79] in 2013. The fabricated u-shape gold resonance rings with a laser of 780 nm
wavelength- demonstrates a quite smooth surface as it is shown in Figure 2-14 (a) A low
resistivity of 16.510-8 Ωm was achieved. Optical characterization of the structure with an
FTIR confirms an electric resonance around 63 THz for the x-polarized wave. However, no y-
polarization resonance was observed according to the Figure 2-14 (b).
Figure 2-14. (a) SEM image of the U-shape gold resonance rings on a glass substrate, which was fabricated under the laser power of 1.57 mW and the scanning speed of 2 μm/s using the sample solution with L=1.9 μ, H=150 nm, W=640 nm, P=3 μm. (b) Measured transmission and reflection spectra for the metamaterials with x-polarized illumination [79].
Introduction to plasmonics
SPP are EM excitations propagating at the interface between a dielectric and a conductor
[121]. Free electrons in the metal surface support fluctuation of charge density during the
light-metal interaction. This phenomenon is called a surface plasmon wave. Light can be
53
coupled to the metal surface through surface plasmons. As a result, the light-matter
interaction could be enhanced significantly based on the application requirements. This
strong light-matter interaction results in two important consequences, light can be confined in
an area beyond the diffraction limit, and the local EM field intensity can be enhanced
considerably. Since plasmonics enhances light-matter interactions by many orders of
magnitude, minute changes in the local environment can be amplified significantly to enable
ultrasensitive detection. Applications include photonic data storage, light generation, and bio-
photonics [1, 122-124]. Because the propagation constant () is bigger than the wave vector
(k), which results in decaying wave amplitude at both sides of the interface.
In addition to the excitation of SPPs using charged particles, there are various optical
techniques for phase-matching such as using a prism or a grating to couple the light in as well
as using the tightly focused light beam. Wave vectors in excess of k can also be achieved
using illumination in the near-field, making use of the evanescent waves in the immediate
vicinity of a sub-wavelength aperture. In this Section, we explain how we benefit from
excitation of charged particles on the surface of metallic micro/nanostructures to create SPPs
for characterization optical properties of the fabricated nanostructures and confirming their
functionality.
The electric field of a propagating EM wave is stated by:
𝐸 = 𝐸0𝑒𝑥𝑝[𝑖(𝑘𝑥𝑥 + 𝑘𝑧𝑧 − 𝜔𝑡)] (2.5)
where k and 𝜔 are the wave number and the wave frequency, respectively. By solving
Maxwell’s equations for the EM wave at an interface of two materials with relative dielectric
function 𝜀1, 𝜀2with the appropriate continuity relation the boundary conditions are [125, 126]
𝑘𝑧1
𝜀1
+𝑘𝑧2
𝜀2
= 0
and
𝑘𝑥2 + 𝑘𝑧𝑖
2 = 𝜀𝑖 (𝜔
𝑐)
2
; 𝑖 = 1,2
where c is the speed of light in vacuum, and kx is the same for both media at the interface
for a surface wave. Solving these two equations, the dispersion relation for a wave
where 𝜓 is the polarization angle and 𝜃 is the angle from the z-axis in the xz+-plane.
There are two significant results of these equations. Firstly, if ψ = 0 (s-polarization), |W|2 =
0 and the scattered light dI
dΩI°= 0. Secondly, the scattered light has a measurable profile,
which is readily associated to the roughness [127].
From these analyses, one can see that in order to minimize the scattering loss, it is
important to reduce the surface roughness. In this thesis, we are exploring how to degrade the
surface roughness with tunning different fabrication conditions to optimise the fabricated
structure with a lower scattering loss to enhance the functionality.
2.6.2 Mie theory
When an EM wave encounters an obstacle or nonhomogeneity such as NPs, some part of it
may redirect from the original path, which is known as the scattering phenomenon. During
the interaction of incident light and discrete particles, the electron orbits inside the particles
are affected and oscillated periodically with the same frequency of the incident beam. This
periodic oscillation creates induced dipole moment within the particles, which are normally
radiation sources for EM and lead to light scattering process as shown in Figure 2-17.
Figure 2-17 Light scattering by an induced dipole moment due to an incident EM wave [128].
There are two classifications for the light scattering process; one is based on the
Rayleigh theory [128-130] that discusses scattering from small, dielectric and spherical
particles and the other one is based on the Mie theory [131-133] that has no limitation on the
particles regarding their absorption and size. Therefore, this theory is typically practical for
spherical particle scattering systems [134]. Regarding the polarizability , which is defined
58
through 𝑃 = 𝜀0𝜀𝑚𝛼𝐸0, is related to a small sphere of sub-wavelength diameter in the
electrostatic approximation and can be expressed as below:
𝛼 = 4𝜋𝑎3 𝜀−𝜀𝑚
𝜀+2𝜀𝑚 (2.14)
Therefore, NPs behave like electric dipoles and either absorb or scatter incident beam
resonantly.
In summary, we discussed here how an incident EM wave interacts with NPs
(molecular/atomic structure), which introduces light scattering. Which is a complex process
[63, 126, 135] and normally act as a loss mechanism in the SPP structure. In this thesis, we
provide a solution on how to minimize the scattering loss in the DLW of metallic
micro/nanostructures to realize functionalities.
Chirality from metallic micro/nanostructures
Obtaining strong chirality is of great interest for researchers in the field of plasmonic and
metamaterial [136-138]. Chirality is intrinsically a 3D phenomenon. Chirality happens in
helices; for example DNA (Figure 2-18), cholesteric LCs, screws, and circular metal helices
[139].
Figure 2-18 The helix structure of a DNA [140].
Chiral optical materials combine electrical and magnetic responses to manipulate the
excitation of magnetic dipoles by the electric part of the light field. Obviously, the reverse
direction of this process (i.e. manipulation of the electric component of light with excitation
59
due to magnetic dipoles) can have significant application. To achieve pure chirality, the
locally induced magnetic (electric) dipoles should be parallel to the local exciting electric
(magnetic) field. In this case, the eigen polarizations correspond to the circular polarization of
light, whereas they are elliptic in the more general nonparallel (i.e., bianisotropic) case [141,
142]. To define the cross-coupling between the electric field and magnetic field going
through a chiral medium a dimensionless chirality parameter is used to describe this cross-
coupling effect. The refractive indices of right-handed circularly polarized (RCP) and left-
handed circularly polarized (LCP) waves become different due to the existence of . The
differences will be explained in detail in the following sections. Some of the examples of the
application of chiral metallic metamaterial structures are giant gyrotropy, circular dichroism,
and negative phase velocities at microwave and far-infrared frequencies.
Chiral media have been studied for a long time. The optical rotation in quartz crystals
and some liquids and gasses was discovered by Biot and others researchers [143]. Biot
suggested that the phenomenon originates from the molecules. Pasteur (1840) confirmed the
handedness nature of the molecules in optically active materials [144]. The discoveries were
implemented for analytical chemistry and pharmaceuticals application. The word ‘chirality’
was first used by Lord Kelvin in 1873 to describe the handedness [145]. Lindeman
introduced the optical activity phenomenon in visible light to radio waves. He used a
collection of helical coils serving as artificial chiral ‘molecules’ [144]. The studies of chiral
media in the microwave region have found applications in many areas such as antennas,
polarizers, and waveguides [146].
Tretyakov et al. (2003) discussed the possibility of realizing negative refraction by chiral
nihility [147]. The authors’ first proposal was to fabricate a metamaterial which is composed
of helical wires functioning as chiral particles. To achieve negative refraction for one of the
circular polarizations, needs to be larger than√εμ. Natural materials such as quartz and
sugar solutions have a considerably smaller than 1, while √εμ is generally larger than 1.
Thus natural chiral materials would not experience negative refraction. However, with chiral
metamaterials, the macroscopic parameters can be designed. The idea of chiral nihility is to
make the negative refractive index for one circular polarization when and of a chiral
medium are small and very close to zero even though is small.
There are a number of goals for a good design. Not only are high values of optical
chirality needed, but large, continuous “hot“ regions with the enhanced optical chirality that
60
are easily accessible for the chiral species are also most desirable. Easier fabrication is yet
another goal. We have investigated different designs and compared their usefulness for
practical chiral applications. We’ve discovered a number of basic design rules. For example,
nanoscale elements with a strong twist but without sharp corners lead to the best results, and
3D chiral elements generate notably stronger optical chirality than 2D ones. A superstructure
that is composed of both handed species of a chiral metallic element should be used for
simultaneous sensing of both species of a chiral molecule.
Recently, a lot of experimental work of chiral metamaterials fabricated by planar
technologies has been published [148, 149]. The interest is due to the strong optical activity,
circular dichroism, and to the predictions [136, 147, 150] that chiral metamaterials can be
used to achieve negative refraction. Theoretical studies also show properties such as focusing
of circularly polarized waves with a chiral medium slab [151], and negative reflection in a
strong chiral medium. The strong optical activity and circular dichroism in planar chiral
metamaterials have been studied experimentally by several groups since 2003. Fabricated
planar chiral metamaterials [152] that give negative refractive index was published in 2009.
Similar to metamaterials designed for linear polarized waves, chiral metamaterials are also
periodic arrangements of artificial structures. These artificial structures in chiral
metamaterials are chiral so that cross-coupling between the magnetic and electric fields
happens at the resonance. The cross-coupling is described by the chirality parameter so that
the constitutive relations of a chiral medium is given by
𝐃 = 𝜀0𝜀𝑟𝐄 + 𝑖𝜅√𝜇0 𝜀0𝐇 (2.15)
𝐁 = 𝜇0𝝁𝒓𝐇 − 𝑖𝜅√𝜇0𝜀0𝐄 (2.16)
where εr and μr are the relative permittivity and permeability of the chiral medium, and
ε0 and μ0 are the permittivity and permeability of vacuum, respectively. The eigensolutions
of the EM waves in chiral media are the RCP (+) wave and the LCP (-) wave. Due to the
rotational asymmetry, the polarization plane of a linearly polarized wave will rotate when it
passed through a chiral medium, introducing the optical activity.
Also, the RCP and LCP waves interact with the chiral particles differently and are
absorbed to a different extent, causing circular dichroism. Moreover, the presence of 𝜿 causes
the difference of refractive index of RCP (n+) and LCP (n−) waves. n± = n ± κ, where n =
√εrμr. When |κ| is large enough, either n+ or n− becomes negative. While in conventional
metamaterials, both negative εr and μr are required to achieve negative n, neither ε nor
61
μ needs to be negative in CMs. This makes negative refraction easier to achieve and offers
simpler designs of metamaterials [153].
Various research groups have been studying 2D Periodic chiral metallic nanostructures
[154] as a usual split ring resonator (SRR) and a ring with a slit and 3D periodic chiral
metallic nanostructures as a helix due to their ability to control and manipulate circular
dichroism, gyrotropy and negative phase velocities [19].
Research on planar chiral metamaterials has been reported by many groups, both in
theory and experiments [148, 149, 155]. It is more convenient to fabricate planar structures
than bulk media. Moreover, there are potential applications and interesting behaviours such
as strong optical activity and circular dichroism in a thin film in planar structures. Zheludev
and co-workers at the University of Southampton first reported the optical activity of a planar
chiral structure with experiments done in the optical regime [148].
The helical structure was further studied [150] in the innovative work of Tretyakov et al.
[147]. A theoretical model of helical inclusions as building blocks of bulk chiral metamaterial
was designed. A quasi-planar version of helices was proposed in 2007 by Marques et al [156]
which can be easily fabricated on printed circuit boards (PCB). Based on the work of Baena
et al. [157] and Marques at al. [156], Jelinek et al. suggested the development of 3D, isotropic
chiral metamaterials [158]. Recently, a non-planar metamaterial slab made of chiral SRRs has
been fabricated and studied by Wang et al. [153]. Strong optical activity, circular dichroism,
as well as negative refraction, have been demonstrated by both numerical calculations and
experimental measurements.
Nanofabrication development is essential for plasmonics and metamaterial advancement.
Conventional methods like EBL, FIB are too costly with a low throughput. The DLW
method has thrived recently, due to its advantages such as being as a maskless approach and
feasibility for 3D structures with a broad range of materials. Based on the multiphoton
absorption process, DLW has been demonstrated to be able to fabricate microstructures with
complicated geometries [20]. More importantly, using this method, metallic nanostructures
can be directly developed with a high level of design flexibility and structure quality [8, 10,
159], providing a good way to fabricate functional plasmonics devices.
Based on the multiphoton absorption process, DLW has been demonstrated to be able to
fabricate microstructures with complicated geometries [20]. Two step inversion [19] and
coating [49] have been used so far to fabricate SRR regarding chiral structures. These
62
methods are more complicated compared to DLW and different processes may affect the
quality of the structure.
A one-step DLW method suggested by Cao et al. [20] is a simple and low-cost solution
to the problem. DLW was used by Cao et al. to fabricate silver nanodots and in this method
no Pre/post treatment was employed. But no control on surface smoothness was employed.
The surface loss is one of the substantial barriers to the production of effective metallic
plasmonic devices and fabricating smooth surface is challenging.
In this study, we work on the generation of the multifocal c-shaped arrays through the
superposition of two concentric circularly polarized beams using one step DLW fabrication
method via 2PR without using any mask or evaporation step. We also predict the creation of
a multifocal array of patterns in the shape of SR and Helix (Figure 2-19). In the next step, we
work on the fabrication of 3D patterns of helices. Our experimental demonstration with a
spatial light modulator (SLM) confirms that an array of SR microstructures can be fabricated
in the metal. Our single laser exposure process enhances the nanofabrication efficiency.
Figure 2-19 Different paths toward a chiral metamaterial, which can rotate the polarization of incident light. (a) A planar array of metallic split-ring resonators is chiral when light strikes the surface of the plane at an oblique angle. (b) An elongated split-ring resonator is chiral even at normal incidence. (c) The bilayered metallic chiral structure has a negative index of refraction at microwave frequencies [152].
63
Conclusion and outlook
In this thesis, we had a first attempt to fabricate the 2D array of metallic micro/nanostructures
with the DLP method as detailed in Chapter 4. Considering all the successes that have been
achieved in this field so far, it is still very challenging to fabricate a functional metallic
micro/nanostructure to be responsive to the external light polarizations. We are also
investigating the fabrication of 2D metallic micro/nanostructure that are responsive to the
polarization of light in the optical region with investigation on electromagnetic resonances of
SPPs on the surface of our design, in Chapter 4 of this thesis. Moreover, fabricating a
structure with tunable property in a large dynamic optical range by changing laser fabrication
parameters would be of a great value. In addition, there is also a very big challenge to
fabricate metallic micro/nanostructure beyond 2De via 2PR technique; we are exploring this
challenge in Chapter 5 of this thesis with the fabrication of out of the plane silver
micro/nanostructure.
64
Single photon fabrication via a UV light source
Introduction to the fabrication by a UV light source
Since in single-photon reduction process material response is intrinsic, it is important to know
the material properties in order to fabricate plasmonic structures with the required quality.
Here we focus on our silver solution based material that is useful for many applications. To
develop any functional structures valuable in the plasmonics field, it is important to
investigate the level of conductivity and surface smoothness this method can achieve with the
specific material. In this Chapter, we present our material characterization on different
aspects using two ultraviolet (UV) light sources, first by means of UV lamp, and a continuous
UV laser.
On one hand, using a UV lamp helps us to fabricate a large area thin film of 4×4 mm2 in less
than 10 minute, which makes the initial sample characterization much easier and it is a
reasonable and simple way for sample characterization from both electrical and
morphological aspects. On the other hand, using a UV laser source lets us putting one step
forward to make the thin film in conditions closer to the final stage; fabrication of lines
through scanning the UV beam on the sample and increasing the fabrication resolution to a
couple of micrometres in addition to achieving a large area sample for characterization, are
valuable advantages of using a UV laser here.
In both cases, we are able to produce a large area of the films, which have been analysed
electrically and morphologically in order to achieve optimized fabrication parameters to
make a proper structure with a high level of conductivity and surface smoothness for
plasmonic applications.
In the last part of this Chapter, after confirming the potential of the material for the
fabrication of functional structures, a circuitry structure has been presented which is made by
means of a UV laser source and the flexibility of this method for fabrication of arbitrary
structures has been confirmed.
In summary, this Chapter intends to answer the following questions:
65
What are the requirements for the materials to introduce the most effective interaction
with light; what is the method to fabricate large-area metallic thin films with UV light
sources; what is best surface smoothness and electrical conductivity we can achieve with
different source and how that is comparable to bulk metal; what is the optimized laser
parameters, including the scanning speed and power in order to fabricate a proper plasmonics
structure; how the nanoparticles (NPs) behave, distribute and size or shape change in respect
with the scanning speed and laser power; how the silver weight in the fabricated structures
changes with the laser parameters; what is the best silver ratio to harness plasmonic
functionalities;
Characterization of silver solution via a UV lamp
For electrical characterization of the sample, a four-point probe is a preferable option. But
there is a size limitation of the sample for using a four-point probe tool and the sample needs
to be at least in a 4×4 mm2 area. To this end, using UV lamp to form a large-area thin film is
possible without scanning process in less than 10-minute time and enables us to easily
characterize the sample electrically.
In the single-step UV laser photoreduction fabrication process, silver ions are reduced
from the solution into silver seeds in the focal region of the laser. These silver seeds grow in
size to NPs and the accumulated NPs eventually form silver patterns following the movement
of a scanning stage. After fabrication, the samples are rinsed with deionized water and dried
in the air [23].
The key to achieving metallic nanostructures with plasmonic resonance in the optical
region relies on improving the fabrication resolution, material conductivity, and structure
surface smoothness. We tackled these challenges by two strategies. First, we developed a
photoreduction solution that contains three engineered functional components including a
photosensitizer, a growth inhibitor and a source of silver as shown in Figure 3-1 (a). A water
soluble dye 2-hydroxy-4’-(2-hydroxyethoxy)-2-methylpropiophenone (HHMP) is used as the
photosensitizer and a surfactant n-decanoyl sarcosine sodium (NDSS) is used as the growth
inhibitor, which can improve the fabrication resolution in photoreduction process [23].
To reinforce the conductivity of the silver structures as fabricated, the concentration of
the growth inhibitor is tuned to avoid the formation of an insulating layer on the silver
particles. In the meantime, the local density of silver ions is adjusted to be sufficient to
66
prevent the discontinuity of the silver structure that is usually controlled by the silver ions
diffusion during the fabrication. Optimizing all these factors, we prepare the solution by
mixing silver nitrate, ammonia, NDSS, HHMP with the concentrations of 1.2, 2.4, 0.016 and
0.027 M, respectively, in deionized water.
Figure 3-1. (a) Photo-reduction solution materials (b) the optical absorption spectrum of the photoreduction solution. The arrows indicate the laser wavelengths for a UV laser at 445 nm and a femtosecond laser at 532 nm.
532nm 445nm
67
Thin films fabrication by the UV lamp setup is shown in Figure 3-2, which contains
different NDSS ratios as characterized in this Section. The OLYMPUS UV lamp of the U-
ULS100HG model with the specification of 19V and 100W is used here, while a lens of 10
cm focal length is used to create a proper focus to achieve a reduced area of 4×4 mm2 (Figure
3-2).
Figure 3-2. (a) Silver ion reduction setup using a UV lamp of the U-ULS100HG model, 19V, and 100W. (b) The power generator of UV lamp of OLYMPUS U-RFL-T-200 with the specification of 220-240V~1.8A 50/ 60Hz.
The losses of a plasmonic resonator are determined by not only the ohmic losses but also
the scattering losses from the surface imperfections and from the grain boundaries. [30] It is,
therefore, important to control the level of the surfactant NDSS that largely affects both the
grain size of the particles and the ability to conduct charges between neighboring particles.
To account for this, we have performed electrical conductivity and reflection measurements
of the prepared samples with different NDSS concentrations after the morphological
characterization of the films. Here, a three-dimensional (3D) optical profiler microscope is
used for morphological analyzing of the thin films. As it is shown in Figure 3-3 (a) the thin
film has a very smooth surface with a surface roughness of tens of nanometer and a low level
of scattering. This smooth surface guarantees to achieve a proper electrical conductivity.
In the experiment, the electrical conductivity of the photo-reduced silver is measured
through a four-point probe (MWP-8, Jandel Engineering), which indicates the carrier
mobility, and thus the intrinsic plasmonic resonance of the material. Inset of Figure 3-3 (b-i)
show a 5×5 mm2 silver thin film prepared by the UV light-induced photoreduction of the
68
sample used in the experiments. By decreasing the concentration of NDSS, we found that the
sheet resistance is decreased from 2.108 Ω/square to 1.55 Ω/square, while the reflection of
the silver thin film is increased from 53% to 89%, indicating ultrahigh surface smoothness
has been achieved (Table 3.1).
Figure 3-3. (a) 3D optical profiler measured surface roughness of a UV reduced thin film. (b) Sheet resistance measurement of the UV reduced samples with the different ratio of NDSS. Inset (i) Mirror-like thin film made with UV reduction compared to (ii) a commercial mirror.
69
The improved surface smoothness can be attributed to the increased silver content and
the reduced nanoparticle sizes. When the concentration of NDSS is decreased to 0.016 M, the
reflection of the silver film at 632.8 nm becomes comparable to a commercial silver mirror
(inset of Figure 3-3 (b-i, ii)) with only 6% less in reflection, indicating minimized light
scattering loss. In the meantime, the conductivity is tuned close to that of bulk silver
indicating improved structure continuity as shown in Figure 3-3 (b). In addition to the
conductivity and reflection measurements, the film roughness is also characterized by an
AFM.
Table 3.1. Material properties of thin films made by a UV lamp
As shown in Figure 3-4 (a-b), it is remarkable to find that a low root-mean-square
surface roughness of 22 nm has been achieved for the film with the highest conductivity. The
high conductivity and low surface roughness provide the prerequisites for the fabrication of
plasmonic resonators.
Figure 3-4. (a) AFM measurement of a UV reduced Ag thin film. (b) Height plot along the white cross section line in (a). The route mean square surface roughness of the film is 22 nm.
Sample NDSS (M) R (Ω/) Reflection Efficiency
Commercial silver mirror 95%
Reduced thin film 0.016 1.55 89%
Reduced thin film 0.032 1.67 68%
Reduced thin film 0.048 2.06 54%
Reduced thin film 0.064 2.108 53%
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To compare the surface roughness of different samples, AFM analysis of each sample
with introducing average surface roughness is presented in Figure 3-5 (a-d). As discussed
previously the thin film made with less NDSS ratio has less sheet resistance and more
reflection efficiency, which in turn indicates less surface roughness.
Since it is shown in Figure 3-5, the average roughness is increased from 54.74 nm for the
thin film made with the NDSS ratio of 0.016 M in Figure 3-5 (a) to about 500 nm for the thin
film made with NDSS ratio of 0.064 M in Figure 3-5 (d); the smoothness of the surface can
be calculated through the colour bar in each image of Figure 3-5 (a-d). The trend of changing
smoothness of the surface with respect to different NDSS ratio is shown in Figure 3-5 (e). It
can be concluded that increasing NDSS ratio in the solution results in creating the bigger size
of NPs and then makes the structure rougher; so with controlling NDSS ratio in a specific
amount we would be able to control the thin film properties such as specific surface
roughness, conductivity and eventually functionality of the final structure.
The characterization of the thin film made by the UV lamp confirmed the optimized
material condition with the NDSS ratio of 0.016 M, which resulted in a high-quality thin film
with the surface smoothness of 54.74 nm, the reflectivity of 89% and the sheet resistance of
1.55 Ω/square. Since, these parameters are promising for the fabrication of various functional
applications introduced earlier in Chapter 2, we are going to explore further fabrication
conditions such as scanning speed and the incident light power on the quality of the formed
thin film. To this end, it is useful to make a thin film by a UV laser, that enables us to make
the thin film through a single-photon photoreduction process. This experiment is discussed
extensively in the following Section.
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Figure 3-5. (a), (b), (c) and (d) AFM images of UV reduced thin films with NDSS ratios of 0.016 M, 0.032 M, 0.048 M and 0.064 M, respectively. (e) Surface roughness plot of all samples made of silver solution using UV source depends on the NDSS ratio.
72
Characterization of silver solution via a UV laser source
Appling UV laser source with a scanning stage that is able to move up to tens of centimeters
in all directions as shown in Figure 3-6 (UV source set up), provides us with the opportunity
of fabricating a large area film for conductivity measurement purpose. The aim of this
measurement is to find the trend of conductivity change with respect to the scanning speed
and laser power. Considering this potential, a 4×4 mm2 square of lines was fabricated with
different scanning speed for the first set of experiments.
Each sample contains twice fabrication of the square with 90 degrees of rotation to make
sure lines are connected to each other and the current can flow properly through the structure.
The effect of scanning speed on the conductivity of the material could be found through the
fabrication. It is clear that with a lower speed the incident light shines longer on the sample;
so more light applies at the same position, the more reduction happens.
Figure 3-6. (a) UV source direct laser fabrication setup. PM: power meter, PG: power generator, SS: scanning stage, L: UV laser, S: sensor. (b) Schematic of the UV laser fabrication setup.
The Sheet resistance of the samples has been measured using the four-point probe
(MWP-8, Jandel Engineering), which indicates the average resistance of a sheet by passing
current through the outside two points of the probe and measuring the voltage across the
inside two points. The measured sheet resistance indicates a very high conductive thin film in
a range of 2 Ω/square to about 50 Ω/square with the scanning speed of 2 mm/min to 80
mm/min, respectively as it is shown in Figure 3-7. Performing this 3D laser printing
fabrication with a UV laser source gives us the opportunity to fabricate a large area thin film.
73
It is clear that fabrication a large area in a very short time is possible using a UV laser. For
instance, fabrication of one square of 4×4 mm2 takes only 12 min with the scanning speed of
80 mm/min and the sheet resistance is still very low (50 ohms/square).
The sheet resistance of 12 samples fabricated by the UV laser with different scanning
speeds in the range of 2 to 300 mm/min sits in the ohm per square range and in the case of
250 mm/min and above that increases to kilo-ohm per square range. But with the scanning
speed of more than 350 mm/min, four-point probe gives the contact limit error, which might
be resulted from the narrow width of the fabricated lines not overlapping with each other. At
a low scanning speed, the sheet resistance obtained so far is comparable to that coated by
sputtering.
Figure 3-5 shows that the thin films made by both the UV lamp and a UV laser can lead
to sheet resistance comparable to the bulk silver, which demonstrates the material itself is
promising to make any further functional structures.
It is worth exploring why the fabricated silver thin film using UV light source is highly
conductive. Since it is mentioned in UV lamp film fabrication part, the smoothness of the
surface plays a significant role in creating a descent conductive thin film for various
plasmonic applications. So one of the important reasons is the surface morphology of thin
film. The smoother is the surface, the less electron scatter loss and the higher conductivity
can be achieved in the thin film.
Considering the 2PR process with UV laser source of 445 nm wavelength, it is predicted
to achieve a smooth surface similar to the result of thermal evaporation method. According to
the AFM measurement of 1 μm2 areas of the surface of a silver thin film fabricated with a 2
mm/min of scanning speed, a highly smooth thin film is obtained as it is shown in Figure 3-8
(a). The averaged roughness from the AFM measurement of the fabricated thin film is 54.43
nm (Figure 3-8) (b). It is interesting that the surface roughness of silver thin film made by UV
lamp reduction is 54.78 nm according to Figure 3-5 (a), which is comparable with the result
achieved via the UV laser fabrication. Therefore, the UV laser setup is promising for
fabricating functional nanostructures.
74
Figure 3-7. Sheet resistance and length of fabrication for samples made by a UV laser with different scanning speeds.
As it is known, fabrication with a UV laser source provides us with the opportunity to
achieve a higher resolution and being able to make any arbitrary designs in a large area for
different applications and this is an absolutely significant advantage for direct laser
fabrication system over the other technique.
Because in this case, free charges can flow through the surface (connected lines here)
and a proper electricity current flow on a surface with lowest defects can confirm a higher
conductivity. Therefore, the smoothness of the surface is one of the key factors contributing
to a high conductivity of the fabricated silver thin film.
75
Figure 3-8. (a) AFM images of the fabricated thin film with the speed of 2 mm/min and the laser power of 120 mW. (b) Histogram plot of the fabricated thin film with the average surface roughness of Sa = 54.43 nm. (c) Surface plot of the fabricated thin film with the speed of 3 mm/min and the power of 120 mW from the 3D point of view.
To calculate the conductivity of the sheet resistance of the fabricated thin film with a
four-point probe device, we need to measure the thickness of the thin film as well. For this
aim we selected the sample made with a 2 mm/min scanning speed and 120 mW power and
made a step with tweezers on top of the thin film as it is shown in Figure 3-9 (a), using
atomic force microscopy (AFM) we could be able to measure the cross section and find out
the thickness of the film, which is measured to be 164.618 nm by the AFM in Figure 3-9 (b).
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Figure 3-9. (a) AFM cross-section measurement of the step created on the thin film with the speed of 2 mm/min and a laser power of 120 mW with the average surface roughness, Sa of 54.43 nm, (c) the plot of the thickness of the step created on the thin film is 164.618 nm.
The 3D surface profile of the cross section of the step created on the film in Figure 3-9
(b) confirms that the peak-to-peak film thickness is around 160 nm, but plotting the raw data
obtained from AFM imaging, gives an accurate thickness of 164.618 nm. Giving the film
thickness about 160 nm we can confirm how thin the film we could make using the UV light
source. However, this thickness is corresponding to the structure of two fabrications on top of
77
each other to ensure the good overlap between the adjacent lines. As a result, it would be able
to make the structure thinner with the optimized parameters for fabrication in future.
Measurement of the sheet resistivity with four point probes is through Equation (2.3).
And for the bulk resistivity measurement, we need to consider the bulk thickness as well. So
the Equation (2.3) has a small variation as equation (2.4):
𝜌 =𝜋
ln (2)𝑡 (
𝑉
𝐼) = 4.523𝑡 (
𝑉
𝐼) (3.1)
Hence giving the bulk resistivity of 2.5 Ω/square and the thickness of 164.618 nm, we
Comparing to the bulk silver conductivity as 6.3107 [160, 161], it can be seen that our
fabricated silver thin film here using the UV laser is highly conductive and it is only one
order of magnitude less than that of bulk silver. This excellent result indicates that the laser
fabricated silver structures can be directly used for functional micro/nanostructure formation,
which can be employed for a broad range of applications.
3.3.1 Scanning speed dependency
To accurately control the fabricated structures with the required morphology and conductivity
to achieve the desired functionality, it is critical to control the experiment conditions. To this
end, we need to understand the structural morphology as a function of the fabrication
conditions. Analyzing the difference of NPs aggregation depends on the scanning speed
provides us more chance to control the experiment production. In this Section, the
morphology characterizations of the samples as a function of the scanning speed are
presented.
For measuring the conductivity of the thin film, a continuous large area is needed. To
this end, we fabricated two layers of lines on top of each other with a small rotation of 10
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degrees to fill the gap of between lines as it is shown in Figure 3-10. It is clear that
overlapped lines create a proper area with the maximum filling ratio, which makes the
conductivity measurement much easier.
Figure 3-10. Schematic of overlapped lines fabricated in two layers on top of each other to make a proper and continuous large area thin film.
The scanning electron microscopy (SEM) images of one fabricated overlapped lines with
the scanning speed of 20 mm/min is shown in Figure 3-11. According to Figure 3-11 (a),
lines are fabricated in two different directions are overlapping appropriately and a proper
connection forms between them to render a decent current flow. With a high magnification
image in Figure 3-11 (b-c), it is clear that the thin film is reasonably smooth and uniform,
which is necessary to make any functional structure in future. Uniform size of NPs is also
shown in Figure 3-11 (d), which also shows how the NPs are connecting well with each other
due to a low scanning speed and sufficient reduction.
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Figure 3-11. SEM images of the fabricated area (overlapped lines) in different magnifications.
From the throughput point of view, a higher scanning speed is preferred. Therefore,
Figure 3-12, the SEM images of thin film fabrication at a different speed from 2 mm/min in
Figure 3-12 (a) to 450 mm/min in Figure 3-12 (g) are presented. Obvious changes in the size
and shape of the silver NPs can be clearly seen when the scanning speed is increased.
When a lower speed is applied, bigger NPs are created. The particles tend to have more
aggregation at low speed. According to Figure 3-12 (a-f), it can be seen that the size of NPs
depends on the scanning speed of fabrication and size reduces as the speed increased, which
is resulted from a reduction in exposure time to the light source. More interestingly, the
packing density of the nanoparticles also changed significantly at different speeds. When the
scanning speed increased from 2 mm/min to 300 mm/min in Figure 3-12 (a-d), the created
NPs are packed and connected to each other tightly so there is not noticeable space between
them. But in the cases of 350 mm/min onwards Figure 3-12 (e-f), although the size of NPs is
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not changed much, the space between smaller NPs is increased, which limits the current flow
among NPs. Consequently, the conductivity of the fabricated thin film is decreased.
Figure 3-12. SEM images of the fabricated thin films with different scanning speed of (a) 10, (b) 60, (c) 80, (d) 100, (e) 180, (f) 450 mm/min, respectively.
It can be investigated that with a cube like NPs the filling ratio is higher than round
shape NPs. Moreover, bigger size of NPs in cases of lower speeds helps to increase the filling
ratio and eventually make the created silver thin film more conductive. Analyzing the SEM
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images confirm this conclusion as it is shown in Figure 3-12 (f) that big spaces between NPs
leave obvious voids and make the surface of cover slip visible.
Evaluating the UV laser reduction mechanism leads us to consider two different steps
after silver ions absorb the photon energy and are reduced. In the first step, aggregation of
silver atoms occurs to create NPs and in the second step, if the exposure time increases,
bigger NPs are created and cube shaped NPs are formed. In addition, the localized heating
anneals the NPs to form more aggregation.
Moreover, since in UV photons have more energy and also it has a continuous wave, the
light moderately heats the sample leads to more uniform thin film in all cases of speeds.
As is discussed extensively in the SEM images shown in Figure 3-12, NP sizes are
changing with tuning the scanning speed. Here we continue our study about another aspect of
NPs, which is NPs distribution. We have investigated that how the distribution of different
sizes of NPs is changing depending on different scanning speed in Figure 3-13.
Analyzing the NP distribution shown in Figure 3-13 indicates that the number of NPs in
a fixed area is higher for lower scanning speeds. The number of NPs is much higher in the
case of Figure 3-13 (a) with big diameters of above 250 nm. So the distance between NPs is
reasonably small here and compact NPs introduce a high filling ratio. According to the films
conductivity measurement, this trend continues until the speed is less than 100 mm/min in
Figure 3-13 (a-c). According to Figure 3-13 (a-c) the amount of NPs are still high with
increased speed from 10 mm/min (Figure 3-13 (a)) to 60 mm/min (Figure 3-13 (b)) and 80
mm/min (Figure 3-13 (c)). Also the number of small sized NPs with the diameter of less than
50 nm increases. For low scanning speed at 10 mm/min, it is challenging for the image
analyzing software to differentiate the connected NPs. To avoid this problem and ensure the
accurate the measurement of the particle size for the first case (Figure 3-13(a)), we present
the profile of NPs as an inset of each figure. A rapid reduction in the number of NPs is found
when the scanning speed is increased from 100 mm/min to 450 mm/min in Figure 3-13 (d-f).
The filling ratio has a substantial reduction in these cases. Small filling ratio makes the
structure rougher and preventing the formation of a conductive film, which is not our interest
in this project.
As the quantity of the NPs is decreasing from lower speeds to higher ones, NPs profile
has been changing as well. As it is shown in the inset of Figure 3-13 (a) the profile of NPs are
quite wide and reaches to more than 100 nm for 10 mm/min speed, however, it continues
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with this range with a little bit reduction until the speed of 100 mm/min in Figure 3-13 (d).
NPs profile is becoming narrower in respect with increasing the speed to 180 mm/min in
Figure 3-13 (e) and 450 mm/min in Figure 3-13 (f), which show the profile of about 50 nm
for each case.
Figure 3-13. Distribution and profile plot of NPs in each sample with scanning speeds of (a) 10 mm/min, (b) 60 mm/min, (c) 80 mm/min, (d) 100 mm/min, (e) 180 mm/min and (f) 450 mm/min.
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This phenomenon can be explained referring to the laser reduction mechanism. The
silver ions are reduced to silver atoms absorbing the incident light. If being exposed
continuously longer after creating NPs, a strong heat distribution in the focal region helps
NPs to aggregate with other particles - however, the scale of this contribution is not clear in
detail because it is difficult to get access to the sample temperature during the fabrication-
and create bigger particles, which happens in the fabrication with low speeds. Eventually,
bigger particles will have smaller distance and surface roughness of the film will decrease
significantly. In turn, it makes a well conductive structure approaching the evaporated thin
films. It is clear that high scanning speed of 450 mm/min forms particles with sharp and
narrow profiles, which prevent them to properly connect with each other. Hence, the number
of NPs in a same size of the area will change with tuning the scanning speed and this method
offers a neat way in controlling the surface roughness of the structure with optimized
parameters.
To have a more accurate idea of the size of NPs in each case, using SEM we measured
the diameters of the NPs in each sample and made an average size to plot as a final size of
NPs for each scanning speed, which is shown in Figure 3-14.
Figure 3-14. Nanoparticle size dependence on the scanning speed.
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The created NPs at high scanning speeds have the sizes of 70-80 nm and their sizes
increase with increased exposure time and eventually reach several hundreds of nanometer at
low speeds. According to Figure 3-14, NPs fabricated at low speeds like 2 mm/min have
sizes of about 350 nm and their sizes decrease with decreased exposure time at high speeds of
450 mm/min to about 70 nm. This broad range of size and shape of NPs make our technique
very useful in controlling NPs morphology to control the surface roughness and conductivity
of the film.
The conductivity of the fabricated film is also highly relevant to the metal content.
Performing elemental analysis and evaluate thin films based on the materials they contain,
provide us the amount of silver in each sample. Also, it is useful to figure out the dependence
of the silver ratio at different scanning speed and to find out whether NPs sizes are relevant to
the silver ratio.
To this end, we introduced energy- dispersive X-ray spectroscopy (EDX) analysis of
different areas and samples. According to the results shown in Figure 3-15 (a-b), the thin film
fabricated with the scanning speed of 2 mm/min shows the silver weight of 57.10% and the
thin film fabricated with the scanning speed of 40 mm/min contains silver of 31.74%. These
results confirm that the amount of silver inclusion in the UV laser fabricated thin films are
high.
As it is shown in Figure 3-15, a big ratio of Au is presenting in the thin film, which is
caused by the gold coating layer on top of the sample for SEM imaging. Also presented is
some component of Si, which is because the coverslip substrate is made of SiO2.
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Figure 3-15. EDX characterization of thin films fabricated with the different scanning
speed of (a) 2 mm/min, (b) 40 mm/min with the Ag weight of 54.10% and 31.74%,
respectively.
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Comparing the amount of silver of 57.1 % and 46.64 % for two different samples
fabricated at a low speed of 2 mm/min and 10 mm/min, respectively and it indicates a higher
silver ratio for the first case as it is shown in Figure 3-16 (i-ii); which in turn confirms a
higher filling ratio in that sample providing a higher conductivity. As a result, with an
increasing scanning speed, the silver weight in the thin film decreases because the size and
the filling ratio of silver NPs become smaller and the NPs become less connected. Hence, the
silver ratio of 43.17 % and 32.52 % has been achieved for the samples with the scanning
speed of 20 mm/min in Figure 3-16 (iii) and 40 mm/min Figure 3-16 (iv), as shown in Figure
3-16. Therefore, the low the scanning speed, the higher the conductivity of the formed film.
Figure 3-16. Ag weight of different thin films fabricated with various scanning speeds of (i) 2 mm/min, (ii) 10mm/min, (iii) 20 mm/min, (iv) 40 mm/min.
3.3.2 Power dependency
Another key parameter in the laser-based fabrication is the exposure power. To optimize the
fabrication parameters, we need to find the power dependency on the conductivity and the
surface roughness, which are the most significant factors to achieve plasmonic-based
structures. In this Section, several thin films made by different powers have been
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characterized and presented according to different aspects to realize the potential of the
material.
Thin films are made with the scanning speed of 40 mm/sec and various powers of 70, 80,
90 and 120 mW as it is shown in Figure 3-17. The same method has been used to measure the
conductivity of the material depending on the laser power. It is investigated that the thin film
fabricated with the power of less than 30 mW gives in ultrahigh resistivity because there is no
proper connection of the NPs to form a continues surface for the current flow. Since the thin
film is made from the overlapped lines, at low power the linewidth is very thin so they are not
overlapping well and the proper connection between lines is lost. As a result, current does not
flow appropriately through the structure. But by tuning the laser power, a promising range of
conductivity of the thin film can be found. At a low laser power of 70 mW, a sheet resistance
of 121.65 Ω/square has been achieved. When the power increased to 80 mW, the resistance
decreased to 72.65 Ω/square. Further increasing the power to 90 mW can further reduce the
resistance to 45.61 Ω/square. Eventually, at 120 mW, the ultralow resistance of 3.3 Ω/square
can be achieved, which is comparable to the thermally evaporated metal film. It has been
found that high power helps the aggregation of the NPs forming a densely packed film, which
could significantly improve the conductivity. Therefore, by adjusting the laser parameters, it
is possible to control the fabrication products very well.
Analyzing the SEM images of these samples provides us more insight on light
interaction with the material under different laser powers. As is shown in Figure 3-18 (a), at a
low power of 10 mW, NPs cannot be created properly. Increasing the laser power to 70 mW
and 80 mW, small NPs start to generate and aggregate in a proper way according to the SEM
images in Figure 3-18 (b, c). However, the packing density of the NPs is still low leading to
space between particles, which in turns affects the conductivity and also the surface
roughness of the thin film.
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Figure 3-17. The sheet resistance of thin films made at various laser powers.
When increasing the laser power to 120 mW, polygonal NPs are formed leading to a
significant increase in the particle packing density. Therefore, the surface roughness
decreases and electrical conductivity increase as is shown in Figure 3-18 (d). Calculating the
size of the NPs using high magnification SEM images confirms that changing the laser power
tunes the size of NPs. As is shown in Figure 3-18, the size of NPs increased slightly from
132.86 nm at the power of 60 mW to 137.31 nm at the power of 70 mW and reaches about
149 nm for the power of 80-90 mW.
The size of NPs gradually increases to about 176 nm with the power of 120 mW
according to the graph of Figure 3-19. Considering these results, we can conclude that
optimizing the structure based on the morphologically is for achieving the optimized
electrical and optical properties of the structure.
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Figure 3-18. SEM images of laser reduced NPs made by different powers of (a) 10 mW, (b) 70 mW, (c) 80 mW and (d) 120 mW.
To further understand the conductivity of the resulted thin film, EDX measurement was
conducted. It has been confirmed that increasing the laser power increases the amount of the
silver in the thin film with increasing the NPs size as shown in Figure 3-20.
The amount of silver weight is increased from about 5% to the power of 10 mW to
almost 15% at the power of 70 mW and 24% at the power of 100 mW. Eventually, the weight
of just above 32.5% was achieved with the power of 120 mW. According to these results, it
can be concluded that controlling the laser power is another effective way controlling the
structure properties.
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Figure 3-19. The size of the NPs in the thin film versus laser power.
Through characterizing the particle size, roughness, and conductivity versus the laser
fabrication conditions, we are able to control the thin film properties during the fabrication
process. It has been confirmed that the laser reduced metallic structures have low surface
roughness, high conductivity, which are suitable for fabricating plasmonics structures for
different applications
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Figure 3-20. Silver weight in the reduced structure versus laser power.
Potential applications
According to the fabrication results with our optimized material by introduced UV setup in
this Chapter, it can be realized that the fabrication of lines with the resolution in a range of
couple of micrometres in a large scale sample opens the gate to a cost-effective, fast and very
simple technique for the fabrication of functional applications [162].
Microfluidic devices in their simple versions are proper candidates for this case,
however, according to the discussion in Section 2.5.3. they need to be of high resolution in a
complex structure.
Moreover, a low sheet resistance of 3 Ω/square for the fabricated silver nanowires makes
them excellent applicants for circuitry and electronics interconnection applications such as a
heating circuit because according to our explanation in the Section 2.5.2. they need high
conductivity as well.
As a result, fabrication of highly smooth and conductive structure with our material
through UV laser setup provides us with a great potential for fabricating many optical and
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electrical applications, including fast prototyping of electrical circuitry and lab-on-a-chip
devices.
Conclusion
In this Chapter, we presented the material characterizations that provide us the useful
information on the UV light reduction of metal ions and the fabrication results dependence on
the different light intensity and scanning speed.
The fabricated large-scale thin films (4×4 mm2) via the single-photon photoreduction
method were presented. We have also comprehensively characterized the thin film from
different aspects such as conductivity, smoothness, the NPs distribution, shape, and size
depending on the laser power as well as the scanning speed of the stage.
From the point of view of materials chemical properties, characterisation of the thin film
made by the UV lamp have indicated that the appropriate NDSS ratio in the reduction
solution should be 0.016 M, through which a very low sheet resistance of 1.55 Ω/ square, a
high reflection efficiency of 89% comparable to the commercial mirror and a high surface
smoothness of 54.74 nm have been achieved.
Fixing the NDSS ratio of the material at 0.016 M and characterizing the thin film made
by a UV laser confirmed that at a scanning speed lower than 40 mm/min and a laser power
higher than 120 mW, we are able to achieve high-quality thin films with a very low sheet
resistance of about 3 Ω/square and a smooth surface with an average roughness of 54.43 nm,
which is promising for many optical applications with significantly reduced scattering loss.
As discussed earlier in this Chapter, both the sizes and distribution of the formed NPs
determine the silver weight on the created thin film, which in turn define the surface
smoothness and the conductivity of the thin film. Therefore, these optimized fabrication
conditions are important to achieve the appropriate electrical and optical properties of the
final plasmonic micro/nanopattern.
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Functional optical resonators fabricated via the
2PR process
Introduction to the 2PR fabrication process
As mentioned in the previous chapters in this thesis, compared to the two-step templating
methods, the single-step photoreduction method can directly fabricate multi-dimensional
metallic micro/nanostructures from solution with great simplicity [20, 22, 24, 40, 51, 56, 62,
79, 92, 163]. Nevertheless, it is still challenging to realize functional plasmonic
nanostructures with resonances in the optical regime due to the large linewidth of the
fabricated metallic structures, very rough surfaces, and the subsequent poor electrical
conductivity.
In this chapter through increasing the sensitivity of the materials and optimizing the
fabrication conditions, we demonstrate the realization of plasmonic silver nanostructure
arrays with smooth surfaces and high conductivity using the direct laser writing (DLW)
reduction technique [22]. The fabricated metallic nanostructure arrays show pronounced
plasmonic resonances in the near-infrared (NIR) with polarization sensitivity [59]. We will
also present the results investigating that with tuning the parameters of the silver resonator
arrays, the plasmonic frequency can be tuned across a large dynamic range offering a viable
cost-effective fabrication solution for plasmonic nanostructures.
In summary, we are going to answer the following questions in this Chapter: How high-
quality metallic nanostructures can be fabricated by the femtosecond laser induced
photoreduction process; what are the optimized fabrication parameters for the line and c-
shaped structures; how the optical functionality of c-shaped array resonators is designed,
fabricated and characterized; how to realize the flexibility and tenability of the fabricated
silver nanostructures through two-photon photoreduction (2PR).
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Experimental process of 2PR for the fabrication of c-shape array
In the single-step 2PR fabrication process, a tightly focused near-infrared femtosecond laser
beam is used to reduce silver ions from the solution into silver seeds in the focal region of the
laser according to Figure 2-3 (a). As is mentioned in Section 2.2, silver seeds grow in size to
nanoparticles and the accumulated nanoparticles eventually form silver patterns following the
movement of a scanning stage. After fabrication, the samples are rinsed with deionized water
and dried in air.
To improve the fabrication conditions for functional plasmonic structures, it is worth to
achieve high quality in both electrical and morphological aspects. To this end, in addition to
preparing the photoreduction solution with the optimized ratio of each chemical, we use 532
nm wavelength pulses to provide elevated photon energy outside the single photon absorption
regime instead of the conventional 800 nm laser pulses, as shown in Figure 3-1 (b). This has
significantly improved the photosensitivity and therefore enhances the fabrication resolution
[22].
Since for plasmonic structures, there are two main introduced losses as the ohmic loss
and the scattering loss, it is important to figure out the sources for those two kinds of losses
and optimize the fabrication conditions based on minimizing them to achieve a high-quality
structure. As studied in Chapter 3, it is important to control the level of the surfactant n-
decanoyl sarcosine sodium (NDSS) that largely affects both the grain size of the particles and
the ability to conduct charge between neighboring particles, through which the amount of
losses will decrease. Based on the optimized results in Chapter 3, the NDSS condition was
fixed at 0.016 M level, which provides us a balanced particle size (surface roughness) and the
conductivity.
4.2.1 Fabrication setup
The DLW setup is similar to the one used in our previous experiment [9]. In brief, the
photoreduction precursor was mounted on a 3D scanning stage and was illuminated by a
femtosecond (270 fs) pulsed laser at 532 nm focused using a high NA objective (100×,
NA=1.4). The laser light was introduced into the objective lens and tightly focused into the
ion solution through a cover slip according to Figure 2-3 (a).
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4.2.2 2PR fabrication of c-shape array
In order to achieve plasmonic resonances in the optical region, it is required to fabricate
metallic structures with sub 200 nm feature sizes. To this end, we explored the minimum
achievable structure size as a function of the laser power, as shown in Figure 4-1. It is found
that the fabricated structure size follows a nonlinear dependence on the laser power similar to
those observed in the previous two-photon polymerization experiments [9]. A threshold
power exists at 0.15 mW, where the smallest linewidth with a smooth surface can be
achieved at 160 nm. The linewidth increases with increasing laser power and saturates at
approximately 400 nm where the lines are no longer smooth due to the ablation effect from
the high laser power.
Figure 4-1. Plot of the reduced silver linewidth as a function of the incident power. Insets are the SEM images of the fabricated lines for 5 different incident powers.
To prove the fabricated structure is silver, we performed Energy-dispersive X-ray
spectroscopy (EDX) during the SEM measurement of the fabricated structures. Ag species
can be clearly seen from the measurement in Figure 4-2, while the other species are from the
Figure 4-2. Energy-dispersive X-ray spectroscopy of the reduced silver (Ag) lines on a silica substrate.
The high conductivity and smooth surface of the photo-reduced nanostructures allow us
to construct high-quality plasmonic structures. As it has been discussed in Section 2.5.4, split
ring resonators (SRRs) are known as popular planar plasmonic and metamaterial structures
that have attracted huge attentions from various research groups due to their broad range of
applications [2, 7, 79, 164-169]. SRRs are introduced by Pendry et al. [170] for the first time,
that can produce negative permeability near its resonant frequency [171]. Moreover, SRRs
have also been used as constituents for supporting magnetoinductive (MI) waves via a
periodic array of SRRs[172], which makes SRRs practical for various application such as
slow-light structures, artificial delay lines, and MI lenses at optical frequencies. In addition to
SRRs optical properties, their 3D fabrication is very challenging and different research
groups are working on optimizing the fabrication process [173, 174] considering its time,
cost, simplicity, and flexibility. Here, in this Section we have explored the fabrication of c-
shaped array as a kind of SRRs using the two-photon photoreduction process based on direct
laser reduction method with the optimized precursor solution. The fabrication conditions are
optimized regarding the laser power and the focal plane to achieve the best surface
smoothness. The dependence of the transmission characteristic on the dimension of the c-
shaped array such as gap size, radius and periodicity are then analysed.
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Figure 4-3 (a-b) shows the schematics of the c-shape array fabrication via the 2PR
method.
Figure 4-3. (a) Schematics of the c-shape arrays fabrication via the two-photon reduction method. (b) The c-shape design with all defined parameters; r: radius, a: periodicity, w: gap, t: thickness.
4.2.3 Characterization of the structure
Finding the best conditions of laser characteristics for fabrication of high-quality c-shape
arrays was the first step. Since c-shape is more complicated than simple metallic lines, it is
important to find out the best power of the laser beam as well as to determine the correct
interface of the coverslip and the metal ion solution. Figure 4-4 shows the scanning electron
microscopy (SEM) images of fabricated c-shape arrays with different powers at the same
scanning speed of 10 µm/s.
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Figure 4-4. SEM images of fabricated c-shape arrays with different power of (a) 0.10 mW, (b) 0.15 mW, (c) 0.20 mW, (d) 0.25 mW, (e) 0.30 mW and (f) 0.35 mW at the scanning speed of 10 µm/s.
According to the Figure 4-4 (a) a low power of 0.10 mW is not enough to form
continuous metallic structures and it can just aggregate nanoparticles (NPs) at the focal
region. When increasing the power to 0.15 mW, the c-shape patterns start to be formed
continuously, although it seems the power is still not enough to aggregate all the NPs around
the pattern and it can be seen Figure 4-4 (b) that there are non-continues line around the
central c-shapes which are almost gone with increasing the power to 0.20 mW in Figure 4-4
(c). High quality continues c-shape array has been fabricated in this case and the line around
each metamolecule completely disappears with the power of 0.25 mW (Figure 4-4 (d)).
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Increasing the power further to be about 0.30 mW escalates the height of the c-shape as is
clear from Figure 4-4 (e). Some of the c-shapes started to be destroyed and lose their
uniformity in some part. It is investigated that the size of the fabricated structure saturated at
this power. While increasing the fabrication power to 0.35 mW damages the c-shapes and
affects the continuity of the metamolecules as is shown in Figure 4-4 (f). It is also obvious
that the more power increases the more NPs are formed from the scattered light around the
focal region, which declines the functionality of the plasmonic structures. We will discuss
that how we can deal with this issue in the following section.
According to the SEM images of the nanostructures, there are many NPs surrounding the
fabricated nanostructure which are created due to a weak photoreduction process by the
scattered light around the focal region during the fabrication. Those NPs might affect the
depth of resonance and degrade its intensity. The idea to remove those NPs after fabrication
without damaging the nanostructure is: coating substrate (coverslip) with a thin layer of
diluted SU-8 of some hundreds nm thickness -which is a transparent layer and would not
create problem of imaging on charge-coupled device (CCD) camera during fabrication- then
locate the aqueous solution on top of the SU-8 of 2010 series (supplied from MicroChem)
layer and continue with the fabrication as a normal case. After the fabrication, we need to
heat the sample on a hot plate for 1 minute and then develop it with the SU8 developer.
Finally rinsing the sample with water could clear up all the NPs generated on this layer, while
not affecting the fabricated metallic structures. Eventually, a clean structure with no NPs
around is formed as is shown in Figure 4-5 (a). From Figure 4-5 (a) and its inset it is very
clear how good the NPs surrounding the c-shapes can be removed completely leaving the
nice and clean structure.
During the laser fabrication process, it is important to find the correct interface between
the coverslip and the metal ion solution since otherwise the fabricated structures will not be
able to stick well to the substrate and would be washed away in the post-development
process.
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Figure 4-5. (a) SEM images of C-shaped array fabrication after removing the NPs by using a thin layer of SU-8 film.
On the other hand, due to the refractive index mismatch between the coverslip and the
aqueous solution, the correct interface gives the best focus for an appropriate fabrication
condition, at which the fabricated structures are nice and clear. Otherwise, the structures
would suffer from an aberration, which deteriorates the final focal spot and degrades the
fabricated structures.
We experimentally tested the laser focus positions relevant to the cover glass according
to the results shown in Figure 4-7. As it is shown in Figure 4-6, focus condition can be
changed when the laser beam is moving inside the material at different heights. When the
focus is close to the interface of the coverslip and the material (Figure 4-6 a), the formed c-
shape could not be continuous and complete. Moving the focal spot completely inside the
material (Figure 4-6 c) results in the formation of a continues and neat c-shape until
defocusing happen again far away from the interface of coverslip and material in Figure 4-6 f.
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Figure 4-6. Schematics of the focus condition in different height inside the material.
C-shape structures are fabricated far away from the surface of the substrate according to
schematically shown in Figure 4.6 (a), in which there is not proper intensity of light allocated
to the material to generate nice c-shape due to bad focus on the fabricated surface (Figure 4-7
(a)), while we can see from Figure 4-7 (b) that when the laser beam is moved towards the
material and incident light is started to focus better and the c-shape is going to be formed and
solidify. Moving the objective towards the material shows that the focus is promising and
almost a good c-shape is produced (Figure 4.7. (c)), although there is a line of NPs around the
pattern that declares focus can be better to get rid of that. Finally, moving objective inside the
material could form a nice and proper focus resulting in a good c-shape (Figure 4-7 (c,d)) and
it is clear that the beam has focused is in the best situation to fabricate a complete and
acceptable structure for our goal.
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Figure 4-7. SEM images of fabricated c-shape arrays at different positions relevant to the cover glass surface.
Further movement of the laser beam inside the material resulted in the creation of
defocusing again as it is shown in Figure 4-7 (e), as it is clear from the SEM image another c-
shape is forming around original c-shape; when the movement height inside the material is
increased and the structure does not look like in a good quality. So achieving the right focal
surface is one of the important parameters for the fabrication inside the solution via
femtosecond laser.
Eventually, we have conducted the experiment based on all the optimized material
properties and the fabrication conditions. According to our results in Chapter 3, the best
properties for the material can be achieved by mixing silver nitrate, ammonia, NDSS, 2-
hydroxy-4’-(2-hydroxyethoxy)-2-methylpropiophenone (HHMP) with the concentration of
1.2 M, 2.4 M, 0.016 M and 0.027 M respectively, in deionized water; and according to the
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studies earlier in this Chapter, the best fabrication conditions can be obtained with the laser
power of 0.25 mW and the scanning speed of 10 µm/s at the right focal plane. Performing our
experiment with all these parameters, we have investigated high-quality c-shape array
fabrication as shown in the SEM images in Figure 4-8 (a-c).
Figure 4-8. (a-b) SEM images of fabricated c-shape array showing a 200 nm linewidth and excellent smoothness.
It is evident from the enlarged SEM image in Figure 4-8 (a-c) inset that each individual
c-shape is well defined with a smooth surface, which minimizes the scatting losses. The
optical functionalities of the c-shape arrays have been characterized by unpolarized Fourier
transform infrared spectroscopy (FTIR) spectroscopy and compared to polarized numerical
simulations. Figure 4-9 shows the transmission spectrum of one typical laser reduced silver
SRR array with t=0.2 µm, r=0.5 µm, w=0.35 µm and a=2.25 µm. Two transmission dips are
observed in our experimental measurement, at wavelengths of 3 µm and 4 µm. As shown in
the insets of Figure 4-9 (i, ii). The first resonance at 3 µm wavelength is a Mie-type
resonance corresponding to the surface plasmon mode oscillating in the vertical side of the c-
shape, and the second resonance at 4 µm wavelength corresponds to the electric dipoles
oscillating in the two horizontal c-shape arms [168]. Together, the two resonances combine to
form the transmission dips in the experimental transmission spectra, confirming the high
quality of our laser reduced SRR structures, which are smooth and conductive enough to
support surface plasmon resonance that is sensitive to the excitation polarization [164, 166,
175].
(a) (b) (c)
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Figure 4-9. (a) Transmission spectrum of a c-shape array with parameters listed in the figure showing two distinct dips in the transmission that are attributed to the two Mie-type resonances when the polarization is (i) parallel to the gap and (ii) perpendicular to the gap. (b)The C-shape design with all the parameters; w: gap, r: radius, a: periodicity, t: thickness.
Although such polarization responsive resonances have been reported in SRR structures
fabricated by the E-beam lithography method [7, 49], this is the first observation in laser-
reduced SRRs. To confirm that these two resonances corresponded to surface plasmon
resonances of the c-shape resonator arrays, we conducted numerical simulation using a
commercial software package (CST Microwave Studio) and geometry of the resonators taken
from the SEM images. A plane wave source was applied to simulate the light incident on the
SRRs, and periodic boundary conditions (PBCs) were used at the lateral boundaries of the
simulation model to mimic the semi-infinite periodic arrays of SRRs in the experiment. Since
the electrical conductivity of the photo-reduced silver is close to that of bulk silver, we
directly use the experimental dielectric functions of silver from the Rakic [176]. Our
simulation confirms that the two resonance dips correspond to two distinct electric Mie-type
resonances for the two linear polarization states of the incident field.
Based on the operating principle of the SRRs, the resonance frequencies are readily
tunable with the split gap distance w, the radius r and the periodicity a of the SRRs. We
(a) (b)
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investigate the influence of these parameters on the SRR resonances for both polarizations in
the following of this chapter.
TPR method has already shown the flexibility for different design fabrication. To
approve this ability, SEM images of high-quality c-shaped arrays fabricated with different
gaps is presented in Figure 4-10 (a-c). As discussed previously, modifying the geometry of
the structure can tune the working wavelength of the devices, which in turn enables different
applications.
The SRR is known as a resonator, which couples the incident light [164] into the
resonant modes. The transmission characteristics mainly depend on the geometry and
dimensions of the SRRs. We have demonstrated here that, either a change in gap size, radius
or periodicity of SRRs resulted in tuning the transmission properties and in turn enables
different applications [2, 177].
Figure 4-11 shows a plot of experimentally measured and a numerically simulated
resonant wavelength for the Mie resonances as the c-shape gap size is increased. The
measured resonant wavelength exhibits a monotonic decrease for both polarizations.
Figure 4-10. SEM images of c-shaped arrays fabricated with the power of 0.3 mW, the periodicity of 2 µm, the radius of 0.5 µm and different gaps of (a) 4.03 µm, (b) 4.12 µm, (c) 4.33 µm.
This shift can be attributed to the reduction in the horizontal and vertical c-shape size
that subsequently blue-shifts the localized surface plasmon dipole moment.
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Figure 4-11. Dependence of the resonant wavelength on the gap size of the c-shape.
The experimentally measured resonant wavelengths match remarkably well with the
numerical simulations, indicating the excellent quality of the reduced c-shapes. Figure 4-12
(a) presents the resonant wavelength dependence on the radius of the c-shape SRR. Through
changing the radii from 400 nm to 600 nm, the resonant wavelengths are shifted significantly
from 2.5 µm to 3.75 µm for the vertical polarization and 3.3 µm to approximately 5 µm for
the parallel polarization, respectively, offering a large dynamic tuning range. Again the
experimental results reproduce the theoretical predictions. The resonant wavelength also
shows a gradual red shift when increasing the periodicity, as predicted by the simulation
results in Figure 4-12 (b).
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Figure 4-12. (a) and (b) Dependence of the resonant wavelength on the c-shape radius and array periodicity, respectively.
Conclusion
In summary, using a simple and low-cost 2PR method, we have demonstrated the successful
fabrication of smooth surfaced, highly conductive and high-resolution silver c-shape arrays,
which afford sufficient quality forming surface plasmonic resonances in the optical regime.
These functional resonant structures are responsive to external light polarizations and
readily tunable in a large dynamic range by simply changing the laser fabrication parameters.
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We envisage combining this versatile direct laser reduction method with the
superresolution [14] and parallel writing [21] methods will lead to a new laser-based
fabrication platform enabling multidimensional functional artificial material fabrication in
future.
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Flexible structure fabrication and investigation on
multifocal and 3D structure fabrication
Introduction to the fabrication of out-of-plane structure
Design and fabrication of three-dimensional (3D) metallic micro/nanostructures have gained
great attention due to their great potential in achieving various device architectures for
diverse applications such as micro-electromechanical systems (MEMS), biosensors,
plasmonics, and metamaterials [12, 24, 119, 174, 178, 179]. Among the current applications
for 3D micro/nanostructures, metamaterials as artificial materials allow to achieving
magnetism at the optical frequencies [141]. Hence, metamaterials can be used for their
unique property of negative refractive index [153]. Introducing chirality has been one of the
exceptional aspects of the emerging field of metamaterials [156, 158], which is inherently a
3D phenomenon that naturally occurs in DNA, cholesteric liquid crystals, screws, and
circular metal helices [19] as one of the fundamental geometries [180]. As discussed
extensively in Section 2.7, in a chiral optical material such as a circular metal helix, the
magnetic and electric fields mix with each other in order to induce the magnetic dipoles by
electric field and vice versa, when either the induced magnetic dipoles or electric dipoles
become parallel to the exciting electric or magnetic field, respectively. Under this condition, a
pure chirality can occur and the eigenpolarizations correspond to circular polarization.
Therefore, a chiral metallic metamaterial can create a giant gyrotropy [137] and circular
dichroism [19, 44, 149, 151, 155, 181]. Therefore, there are two different responses of a
chiral material for a left-handed circular polarization (LCP) and a right-handed circular
polarization (RCP) of the electromagnetic wave because of the structure asymmetry [171].
These two properties, circular dichroism and different absorption of left and right circularly
polarized light, are very useful to introduce new modulation mechanism for light [182].
As discussed in Section 4.2.2, split ring resonators (SRRs) are known as a kind of
metamaterial that introduces the negative refractive index. We chose the popular c-shape
SRR design in this thesis for plasmonic applications and investigated the structure
experimentally and theoretically in Chapter 4. Here, we present our preliminary exploration
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on extending the c-shape fabrication from planar design to out-of-plane geometry towards
chiral functionality.
The current fabrication method of 3D metallic micro/nanostructures suffers from the
high-cost and the complexity of the process. The two-photon photoreduction (2PR) method
demonstrated in this thesis provides a low-cost and simply option for potential 3D structure
fabrication. It has been shown in the previous chapters that the single-step photoreduction
method can directly fabricate flexible metallic micro/nanostructures from solution with a high
level of simplicity [20, 22, 24, 40, 51, 56, 62, 79, 92, 163]. In this Chapter, we explore the
possibility of fabricating out-of-plane arbitrary structures using the 2PR technique [22] via
two approaches: firstly, the standard process of scanning of the focal spot inside the
photoreduction solution to make a metallic structure pattern; secondly, generation the c-shape
structure directly by modulating the fabrication beam through a spatial light modulator
(SLM) and fabricate the metallic structure with a single exposure.
In summary, we are going to answer the following questions in this chapter: Is it possible
to fabricate complex out-of-plane structure with this material via the 2PR technique using the
scanning of the focal spot process; what are the optimized fabrication parameters for the
fabrication of out-of-plane c-shape structure using the scanning method; how flexible is 2PR
method to fabricate various types of structures; is it possible to fabricate complex out-of-
plane structure with this material via the 2PR technique with a single laser shot using a SLM;
what are the optimized fabrication parameters for fabricating the out-of-plane c-shape
structure with a single shot using a SLM.
Fabrication of out-of-plane sliver c-shape via scanning process
Being aware of the possible material challenges for 3D fabrication, we have started our
investigation on the material threshold for the fabrication of out-of-plane c-shape with
slightly increasing the height of one arm of our previous c-shape design.
As our out-of-plane c-shape structure is a simple version of a 3D helical structure, the
effective parameters in the functionality of the structure for the helix are valid for this case as
well. According to the schematic of the out-of-plane c-shape in Figure 5-1 (a, b), which show
the top view and side view of the structure, respectively; the effective parameters for this
structure is the diameter of the wire (DW), the diameter of the helix (DH), the length of the
helix (LH), the spacing in grid (SG) and the number of turns.
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Figure 5-1. Schematics of the out-of-plane c-shape structure, (a) top view, (b) side view; DW: diameter of the wire, DH: the diameter of the helix, LH: the length of the helix-period, SG: the spacing of the grid.
For the first step, out-of-plane silver c-shape structure is fabricated via scanning the focal
with different laser powers to realize the threshold and linewidth changes. Thickness
measurement of the top arm of the structure that is fabricated with different powers confirms
the principle that linewidth can be modified by tuning the fabrication power. Considering the
graph in Figure 5-2, modifying the fabrication power affects the linewidth and increasing the
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laser power from 0.08 mW to 0.35 mW raises the thickness from 230 nm to 680 nm,
respectively.
Figure 5-2 SEM images of the power dependency of out-of-plane c-shape on the thickness with different laser powers of (a) 0.08 mW, (b) 0.1 mW, (c) 0.15 mW, (d) 0.2 mW, (e) 0.5 mW and (f) 0.35 mW.
SEM images of the out-of-plane c-shape arrays are fabricated with different powers as
shown in Figure 5-3 (a-f). It can be seen that at the laser power of 0.08 mW (Figure 5-3 (a))
the structure is of high quality; there is noticeable shadow beneath of the structure in this
SEM image, which confirms the out-of-plane arm of the structure. It seems with increasing
the laser power to 0.1 mW in Figure 5-3 (b), more NPs attach to each other so the surface is
smoother, and also the thickness is increased in this case. When the power reaches to 0.15
mW in Figure 5-3 (c) and causes NP formation around the main structure but the structure is
still pretty good. While, increasing the laser power to 0.20 mW and above according to
Figure 5-3 (d-f), resulted in increased scattering light around the focal spot and in turn more
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ions grow to NPs so the structure cannot remain neat anymore and as is shown in Figure 5-3
(d-f). Inside part of the structure, it is filled with the created NPs which might affect the
functionality of the structure, intensely.
Figure 5-3. SEM images of out-of-plane c-shape arrays fabricated with different fabrication powers of (a) 0.08 mW, (b) 0.1 mW, (c) 0.15 mW, (d) 0.2 mW, (e) 0.25 mW and (f) 0.35 mW.
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As a result, by controlling the fabrication power we can not only control the linewidth
but also the number of formed NPs to make a high quality and neat out-of-plane structure. To
remove the created NPs, the same fabrication strategy with a thin layer of coated SU-8 on the
substrate as mentioned in Section 4.2.3 will work. Here we are not going to use this method,
since it is critical for us to realize how different parameters of the laser beam affect the
creation of the NPs during the fabrication of the structure.
So far, we have explored the possibility of fabricating a single-helix in this Chapter. As
mentioned earlier in this thesis, Gansel et al. [19] developed broadband circular polarizers
with the circular dichroism in the region of 3-6 µm and a single-to-noise ratio (S/N) of ~ 10
dB by means of single-helical metamaterials fabricated via the inversion process due to its
high intensity conversion between RCP and LCP of 10%. Circular polarizers made with the
helical metamaterial have attracted enormous attention from different research groups due to
their unique properties of broad wavelength ranges and also their compact structures that
make them an appropriate candidate for integration with other optical devices.[183-185]
After this great success, different research groups have proposed a kind of circular polarizer
based on double-helical and multi-helical nanowires (Figure 5-4 (a-d)) in order to either
increase the operation band to more than 50% [184] or increase the average S/N to about two
orders higher to ~ 35 dB [183, 185-187]. The important parameters for characteristics of
multi-helical structures are known as the diameter of the wire, the number of the helix-
periods, the spacing of the grid, the length of the helix-period, and the diameter of the helix.
Tunning each parameter can change the characteristics of the circular polarizer.
However, all the research has been conducted based on simulations and theoretically,
while very limited research has been reported for the fabrication and experimentally
characterization of the structure due to a very challenging procedure for the fabrication of
such a complex design. In this Section we are exploring the fabrication of double-helical
nanowire with a small height, that can be considered as a double-out-of-plane c-shape via
2PR. We have modified the design slightly to make the structure clear for imaging purposes.
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Figure 5-4. Schematic diagram of the optical circular polarizers using helical metamaterials; (a) single-helical structure; DW: the diameter of the wire, DH: the diameter of the helix, LH: the length of the helix-period, (b) double-helical structure, (c) three-helical structure, (d) four-
helical structure [185].
According to Figure 5-5 (a-b), we have fabricated different configurations of special
design, which includes two helices inside each other in the same direction and in opposite
direction, respectively. It is clear from the scanning electron microscopy (SEM) images that
fabrication of helix has been started from the bottom part attached to the surface of the
substrate that is darker in the image and the structure has been built towards the top end
coming out of the plane, which is brighter in the SEM images.
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Figure 5-5. SEM images of the out-of-plane c-shape fabrication via a scanning process, (a) double-out-of-plane c-shape in the same directions, (b) double-out-of-plane c-shape in the opposite directions.
To examine the design flexibility and tunability of this method, we fabricated structures
with different radii of inner helices as shown in Figure 5-6 and Figure 5-7. All structures
fabricated with 0.2 mW of laser power, the scanning speed of 10 µm/s and the gap of 𝜋
8 in
Figure 5-6. The size of the outer helix is 1 µm, which is kept the same here in all designs;
however, the radius of the inner helix is changed from (a) 0.35 µm to (b) 0.5 µm, (c) 0.75
µm. Gap for inner helices is measured about (a) 102.7 nm, (b) 306.4 nm, (c) 519.8 nm.
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Figure 5-6. SEM images of a double-out-of-plane c-shape structure fabricated with the different radius of (a) 0.35 µm, (b) 0.5 µm, (c) 0.75 µm for the inner structure.
In addition to the above designs, we have indicated fabrication possibility of another
complex design including two helices in opposite directions as it is shown in Figure 5-7. All
structures were fabricated with a laser power of 0.2 mW, a scanning speed of 10 µm/s and a
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gap of 𝜋
8 in the design. These structures include one outer helix with radius of 2 µm and inner
helix with different radiuses of (a) 0.35 µm, (b) 0.5 µm, (c) 0.75 µm, and the same gap size of
inner helices as Figure 5-7 ( (a) 102.7 nm, (b) 306.4 nm, (c) 519.8 nm). As a result, by
controlling the size of radius and gap of both helices, we are able to fabricate two separate
helices with the right thickness.
Figure 5-7. SEM images of a double-out-of-plane structure fabricated with the different radii of (a) 0.35 µm, (b) 0.5 µm, (c) 0.75 µm for the inner structures.
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In Figure 5-8 (a-d), we characterized the structures with a 3D optical profiler (model). It
can be clearly seen that the double c-shapes are clearly out-of-plane, matching our design
very well. However, it is hard to recognize the difference in height of the two ends for the
inner helices because of a small aspect ratio of the structures.
Figure 5-8. Optical profiler results of double-out-of-plane c-shape structure from the different point of views (a-d) that confirm the elongation of the structure in the z-direction.
So according to the results presented in this Section, the fabrication of double out-of-
plane metallic c-shape structure via the scanning process based on 2PR technique has been
successfully confirmed. Following this fabrication, the next step is the fabrication of the out-
of-plane c-shape structure via single exposure incident with an SLM that will be discussed in
the next Section.
Fabrication of out-of-plane sliver c-shape via a single exposure using
an SLM
In this section, we present briefly why multifocal fabrication is required and how we can
achieve it. We also present the generation of the out-of-plane c-shaped and helix structure
through the superposition of two concentric circularly polarized beams using one-step direct
laser writing (DLW) fabrication method via 2PR. Our experimental demonstration with an
SLM confirms that an array of out-of-plane c-shape can be fabricated in the metallic solution.
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Due to the high speed of processing [16], high power efficiency, good control of the
intensity and the shape of each focal spot [188], and high-throughput [189, 190] multifocal
array generation technique applying an SLM has achieved great attention in the
micro/nanofabrication technology [191-193]. It is also worth noting that obtaining parallel
design by means of loading computer-generated hologram (CGH) on the SLM, provides us
with the opportunity to run a cost-effective diffraction-limited process without the necessity
of diffractive optical components and also enables us to generate arbitrary patterns [194]. As
a unique highlight for using SLM is also its property in adjusting the intensity distribution in
the focal plane depend on altering the phase of the incident beam, in addition to the potential
to control the amplitude and also the polarization of the incident beam [195]. Lin et al. in
2013 proposed highly uniform generation of multifocal array by SLM and moreover,
applying a vectorial PSF engineering method -depend on the positions and topological
charges of the phase vortices- with a circularly polarized vortex beam they could form each
focal spot as a complex pattern, through which a multifocal array can be fabricated by using
one single shot.
We believe, applying one single shot using SLM for the fabrication of our out-of-plane
c-shape structure helps us to greatly increase the fabrication resolution due to simultaneously
forming nanoparticles (NPs) at the same focal region to achieve the whole pattern and also
highly increase the fabrication uniformity owing from consistent energy distribution among
all focal region; rather than the scanning process of a huge number of points -we experienced
in our planar c-shape fabrication in Chapter 4, through which the scanning process of each
point causes heat distribution to the surrounded area of the scanned point and affects the
uniformity of the previous and the next target points in the material and reduce the quality of
the structure.
To generate our multifocal pattern, we have used diffraction-limited non-Airy multifocal
arrays based on vectorial Debye diffraction theory, which has been conducted by Lin et al.
[16] for highly efficient parallel laser fabrication: The general form of the Debye integral is
where 𝑝(𝜃) = 𝑝(𝑅)√𝑐𝑜𝑠𝜃 is the apodization function of the objective. The objective
with high NA of 1.4 has been used in our setup; R and φ correspond to the polar coordinates
on the back aperture; k is the wave number and 𝜃 is the convergence angle of the objective;
r2, 𝜓, z2 correspond to the cylindrical coordinates in the focal plane and i,j,k are the unit
vectors along the x, y, z directions.
According to the equation 5-2, a phase vortex beam can be determined with vortices
located at different positions. Since a phase modulation 𝛷(𝑥, 𝑦) consists of N phase vortices
situating at the positions 𝑥𝑘 = 𝑅𝑘𝑐𝑜𝑠𝜑𝑘, 𝑦𝑘 = 𝑅𝑘𝑠𝑖𝑛𝜑𝑘 (1<k<N) in the back aperture plane,
which can be defined as
Φ(𝑥, 𝑦) = ∑ 𝑙𝑘𝑠𝑖𝑔𝑛(𝑦 − 𝑦𝑘) × 𝑎𝑟𝑐𝑐𝑜𝑠 [𝑥−𝑥𝑘
√(𝑥−𝑥𝑘)2+(𝑦−𝑦𝑘)2]𝑁
𝑘=1 (5.3)
the phase when 𝑦 < 𝑦𝑘 , can be corrected by 𝑠𝑖𝑔𝑛(𝑦 − 𝑦𝑘). A phase ramp of 2/π
around a point in the transverse plane is known as a phase vortex, and 𝑙 is known as a
topological charge. The sign of 𝑙 pronounces the handedness of the phase vortices, namely
positive sign corresponds to the left- handed phase and negative sign corresponds to the right-
handed phase. If 𝑙=±1, then the out-of-plane c-shape would have just one turn; increasing the
quantity of the topological charge would increase the number of turns for the final structure;
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which leads to broaden the applications area. Therefore, tuning the topological charge helps
us to control the handedness and also the number of turns in the helical structure [196]. We
just consider 𝑙=-1 in this thesis.
5.3.1 Experimental setup
Our optical setup here is almost the same as our previous setup[195, 196] as is shown in Figure
5-9. A 532 nm femtosecond (fs) laser beam is expanded and illuminated on an SLM (Holoeye
Pluto) via 4f system in our setup which helps the imaging process and transfers the phase
modulation (PM) from the SLM to the aperture of the high NA objective (Olympus, 100,
1.40 NA). An out-of-plane c-shaped pattern is generated at the focal plane of the objective. A
CCD camera is used here to check and control the fabrication process in-situ. The rest of
fabrication process is similar to the scanning technique; the preparation of the sample is done
as the previous fabrications procedures. Locating the sample on the scanning stage with the
limitation of 300300 µm and controlling all the hardware through the computer, an out of
plane c-shaped is fabricated with one single exposure.
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Figure 5-9. Experimental setup for fabrication by TPR based on DLW method using SLM.
Calibration of PM in terms of gray level for SLM is the first priority before putting on
any phase patterns on SLM. In this method, the intensity modulation measurement is used to
calibrate the PM of the SLM stated as below [195, 197]
∅ = 2𝑎𝑟𝑐𝑠𝑖𝑛√𝐼−𝐼𝑚𝑖𝑛
𝐼𝑚𝑎𝑥−𝐼𝑚𝑖𝑛, (5.4)
where I is the measured average intensity of a certain gray level; 𝐼𝑚𝑎𝑥 𝑎𝑛𝑑 𝐼𝑚𝑖𝑛 are the
measured maximum and minimum of the average intensities.
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5.3.2 Phase pattern generation
Phase pattern for an out-of-plane c-shape structure ( Figure 5-10 (a)) and the related field
distribution in the focal plane ( Figure 5-10 (b)) are calculated based on the Deby-theory
[195, 196] considering the SLM consisting of 19201080 (256 gray level) pixels.
Figure 5-10. (a) phase profile of a spatially-shifted vortex beam. (b) Corresponding intensity distribution.
Using circularly polarized vortex beam which consists three different vortices that
spatially are shifted along the radial direction in the back aperture and considering the
structure as a doughnut with removing zero intensity, we are able to create phase pattern and
field distribution of c-shape in the x-y plane here as is shown in Figure 5-10 (b). The
spatially-shifted vortex beam is produced with the combination of three vortices with the
topological charge of lk=-1 and R1=0, R2=0.25R0, R3=0.65R0, and R0 is the radius of the back
aperture.
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5.3.3 Experimental fabrication of out-of-plane c-shape with a single exposure
We fabricated the out-of-plane c-shape arrays using the SLM as shown in Figure 5-11 (a)
with the laser power of 2 mW, the scanning speed of 10 μm/s and exposure time of 10 ms.
The results show the correct PM, which is similar to its polymer version reported by Han Lin
et al. [195]. However, it is very challenging to deal with our metallic material for this kind of
fabrication; since through 2PR method, the aggregation of NPs creates the final structure and
it might be clear from SEM image in Figure 5-11 (b) that our material is not like solid
polymer. It seems that the structure is kind of falling and causes the structure flatter shape
rather than nicely standing on the surface.
Figure 5-11. (a) SEM image of the fabricated structure with the incident power of 2 mW, green coloured c-shape structure on the top right side of the image makes the structure clear. (b) enlarged image of a single out-of-plane c-shape structure.
It might result that in the single-shot fabrication all the spots of the pattern exposed and
heated simultaneously such as patterning with mold. However, in the case of scanning
process each spot is exposed and is heated by the incident light and then cooled down, and
the scanning of the next spot affects this area, which results in reducing the uniformity of the
structure. It is very promising because the metallic out-of-plane c-shape pattern could be
fabricated here with an SLM for the first time to best of our knowledge. However, the
achieved results in Figure 5-11 indicate that our material needs more improvement regarding
its mechanical strength in order to fabricate a real metallic 3D helical structure.
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Fabrication of out-of-plane c-shape structure with different incident power shows a very
high sensitivity of the material in response to a higher power in the focal spot. According to
the SEM images in Figure 5-12, the fabricated structure with the laser power of 2 mW in
Figure 5-12 (a) presents a very nice and clear fabrication. One structure among all in each
image is highlighted with the green color to make the structure clearer.
As soon as the power is increased to 3 mW in Figure 5-12 (b), NPs in side lobe of the c-
shapes start to create and surrounding the structure, a sudden growth in amount of formed
NPs in the case of 4 mW laser power makes the structure messy even they could still remain
after the washing process. These results confirm that higher laser power would not help to
develop the functionality of the structure since those impurities at the background decrease
the structure quality. However, the structure itself is becoming clearer with higher thickness
and linewidth in terms of high laser powers.
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Figure 5-12. SEM images of fabricated out-of-plane c-shape structure with the different incident powers of (a) 2 mW, (b) 3 mW and (c) 4 mW. Green coloured c-shape structures on the top right side of the images make the structure clear and more recognizable.
For further study of the material properties in the SLM assisted fabrication, we
fabricated an out-of-the-plane c-shape structure on different focal surfaces. Two sets of
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fabrication with the same incident power of 5 mW, scanning speed of 10 μm/s, exposure time
of 10 ms and the different focal position of z=149 nm for the Figure 5-13(a) and z=150 nm
for Figure 5-13 (b). As it is seen from SEM images in Figure 5-13 (a, b), slightly changing
the focal position shows a big difference in the structure especially in the continuity of the
structure. It is obvious from the inset of Figure 5-13 (a), that if the surface is not in the right
focus, NPs cannot be trapped properly with the incident light in the focal spot, so they cannot
form a nice and continuous surface. However, the correct focal plane inside the material
results in a continuous and solid structure as is shown in Figure 5-13 (b).
Figure 5-13. SEM images of fabricated structure on the different surface of (a) z=149 nm and (b) z=150 nm. Green coloured c-shape structures on the top right side of the images make the structure clear and more recognizable.
Conclusion
In summary, the fabrication of the out-of-plane c-shapes with both single and double
structures are explored with our material through two different techniques based on 2PR by
both scanning the focal spot on the focal plane inside the material to create the pattern and
also the single exposure process with a SLM produced patterns by modifying the phase and
the shape of the beam.
In both processes, the out-of-plane c-shapes are successfully fabricated, however, the
final profile is slightly different based on the different writing techniques. The material has
shown a promising feedback in the first step of the fabrication towards a 3D helix structure
and could be reduced and be solidified out of the plane, moreover, the fabricated structure has
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resisted in the washing process against the surface tension of water. In addition, even more
potential of the material has confirmed during the fabrication of double-c-shape structures;
the material could stand against massive heat distribution due to the double fabrication and
eventually produced a neat structure.
Although, the material shows good potential for the fabrication of the out-of-plane c-
shape with more turn with higher height. To achieve a real 3D helical structure with our
material via the 2PR technique, it is essential to improve the material mechanical strength to
support a larger height of the structure. It might be also useful if the carbon-chain length of
the material can be increased and the material properties can be further studied in future.
130
Conclusion and future work
Thesis conclusion
The research work presented in this thesis investigates the electrical, morphological and
optical properties of the fabricated structure with the introduced optimized material solution
via a two-photon photoreduction (2PR) method. The metal-ion aqueous solution includes the
silver nitrate as a silver resource, a nitrogen-atom containing alkyl carboxylate (NDSS) as the
surfactant and 2-hydroxy-4’-(2-hydroxyethoxy)-2-methylpropiophenone (HHMP) as
photosensitiser. The resolution of features in different structural and material conditions
through theoretical and numerical calculations is also investigated. This work also has
confirmed the possibility of the fabrication of nanostructures with this material via two
different approaches, both single-photon photoreduction, and 2PR processes. Fabrication of
functional structure in the optical region with a high level of conductivity and surface
smoothness is also approved through 2PR based on the direct laser writing (DLW) fabrication
technique.
In this work, the potential of the 2PR-DLW method for fabrication of functional metallic
micro/nanostructures has been confirmed as a simple and low-cost approach.
A wide range of analysis has been conducted in Chapter 3 on the interaction between the
UV light and the material regarding various effects and changes. A large square with the area
of 4×4 mm2 is fabricated by an ultraviolet (UV) laser source and also a UV lamp. Thin films
that have been made via these two techniques, presented a low sheet resistance of 1-2
Ω/square, which is comparable to the sheet resistance of bulk silver. The structure also shows
a low average surface roughness of around 50 nm. Moreover, according to the study on the
properties of nanoparticles (NPs) forming thin films, it is obvious that the properties of the
material can be controlled through the fabrication parameters. The high electrical
conductivity and surface smoothness make the material as a competent candidate for
fabricating a variety structures using single-photon fabrication method for large-area
structures such as circuitry designs, microfluidic channels or lab-on-a-chip (LoC) device.
Optically functional silver nanoresonators could be fabricated via 2PR and the final
structure has been characterized optically by Fourier transform infrared spectroscopy (FTIR)
131
in Chapter 4. The transmission spectrum of one typical laser reduced planar silver c-shape
array with t=0.2 µm, r=0.5 µm, w=0.35 µm and a=2.25 µm shows two transmission dips in
the experimental measurement, at wavelengths of 3 µm and 4 µm. The first resonance at 3
µm wavelength is a Mie-type resonance corresponding to surface plasmon mode oscillating
in the vertical side of the c-shape. The second resonance at 4 µm wavelength corresponds to
electric dipoles oscillating in the two horizontal c-shape arms [168].
To achieve functional devices in the visible region, the fabricated structures need to
possess a smaller sized structure in order to host resonance at below one micrometer
wavelength, it would be helpful to fabricate with superresolution[13, 14] technique and also
regarding the material aspect increasing carbon-chain length might be helpful [92]. This
needs further research to study the material properties performing this technique.
In this section, we tested the potential fabrication of the material for the out-of-plane c-
shape structure via two methods in Chapter 5, scanning the focal spot inside the material and
single exposure incident process with created phase pattern on the spatial light modulator
(SLM). The results show the positive response of the material to both methods, which
confirms appropriate fabrication conditions for the fabrication of double out-of-plane c-
shaped structure, which broadens the application range in this fabrication field. However, the
material did not respond properly to the fabrication of out-of-plane structure with two turns
and higher height due to its low mechanical strength, which needs further study and
modifications of the material to make it mechanically stable and strong enough for a three
dimensional (3D) and self-supported structures such as a helix.
Outlook and future work
The research conducted in this thesis can be further studied to open new opportunities for the
following aspects to increase the potential of 2PR method for the fabrication with our
introduced material for broad range of applications.
6.2.1 Multifocal high-quality fabrication
For 2PR based on the DLW, the fabrication throughput is a general concern for
nanofabrication of a 3D structure. To this end, parallel writing could noticeably increase the
throughput of the 2PR method and make 2PR one of the most promising future
nanofabrication technologies [16, 17, 21]. In principle, it is possible to employ an SLM to
produce multifocal array to fabricate different arbitrary structures at different focal points
132
simultaneously [198]. According to the literature multifocal fabrication of polymer split ring
array structure is possible with phase design and the SLM enables laser fabrication of
microstructures SR array with a single exposure process as shown in Figure 6-1 [196].
Figure 6-1. Experimental setup for the DLP nanofabrication system. The arrow indicates the polarization direction of the laser beam. Inset: the displayed phase modulation.[196]
Using this method, the fabrication of diffraction-limited split ring resonator array or any
other building blocks of nano-devices could be introduced through multifocal fabrication
based on the 2PR method. It is also worth exploring 3D functional metallic structures that
could be fabricated via 2PR. So far dots, lines, pyramid, u-shape and c-shape structures have
been demonstrated by the 2PR method. The structure design demonstrated a high level of
flexibility. Although so many structures have been fabricated, very limited functionality has
been demonstrated [20, 51, 199]. In order to achieve functionality, the nanoparticles (NPs) in
the fabricated structure should be packed densely enough to resist surface tension during the
washing process. However, in most of the cases, the structure is quite rough due to the
fabrication based on scanning in the z-direction. Since during scanning of each point either
over exposing or extra thermal distribution affecting the previous spot and degrade the
uniformity of the size, shape, and spacing of NPs. So the final structure is less controlled in
the physical properties and leads to no functionality. This situation can be avoided by
fabricating based on one the SLM assisted one single shot direct laser writing fabrication
rather than scanning point by point [16, 196], it might be possible to fabricate highly uniform
3D structures with a proper continuity that lead to good functionality. In principle, it is
possible to shape the beam in the focal region by using an SLM and fabricate an arbitrary
structure by one single shot.
Successfully fabrication of out-of-plane c-shape structure with SLM presented in
Chapter 5 of this thesis has confirmed the high potential of our material for the fabrication of
133
a metallic structure via 2PR, through which it can be concluded that the parallel fabrication of
high-quality metallic structure is promising with our introduced fabrication conditions here
and it is very promising field for further research in future.
6.2.2 Flexible structures
Although, the fabrication of different metallic structures via 2PR have been presented in
literature, very limited functionality has been achieved. Moreover, tacking the advantage of
this method and using an SLM open the opportunities to fabricate a broad range of arbitrary
structures useful for different applications.
Effectively fabrication of complex planar and out-of-plane metallic structures presented
in Chapter 4 and Chapter 5 of this thesis, has already shown the significant potential of our
method for the fabrication of multi-dimensional functional metallic structures. By improving
the electrical and morphological properties of the material through optimization of all the
chemical ratio and also the optical fabrication parameters lead to the achievement of a great
conductivity and surface smoothness of the eventual structure towards the device
functionality. In this regard, the initial characterization of the material is critical in order to
develop the optical properties of the final structure based on different applications.
6.2.3 Superresolution
Apart from all the current methods to improve the feature resolution in 2PR fabrication,
performing superresolution fabrication technique can also lead to resolution enhancement
impressively. Gan et al. [13] reported 3D optical beam lithography fabrication with 9 nm
feature size as is shown in Figure 1-1 (b) and 52 nm two-line resolution in a developed non-
metallic two-photon absorption resin with high mechanical strength. According to the
superresolution method, the dual optical beam lithography has the advantage of fabricating
3D arbitrary geometry with a nanometer feature size and resolution comparable to electron-
beam lithography (EBL) by the photo inhibition strategy. If fabricating metallic structure
with this method would be possible, DLW based on 2PR could be the champion in the
competition with the EBL technique due to its ability in cost-effectiveness and simple
fabrication capability of 3D arbitrary geometry with nanometer feature size. Utilizing this
method can bring a revolution in metallic nanofabrication technology. However, there are
still a couple of challenges to be solved towards making this approach possible.
134
Basically, in this method there are two laser beams of different wavelengths that should
overlap at the focal spot; one laser beam starts the writing process as the excitation beam and
another beam suppresses the material excitation as an inhibiting beam. On one hand, to
understand the interaction of the material with two beams simultaneously is challenging. On
the other hand, the mechanical strength of the material is very critical at this point in order to
achieve the highest resolution potential of the photoinhibition photoreduction [14, 200].
6.2.4 Applications of the metallic nanostructures
Extensive research is required to investigate the possible improvement in mechanical strength
of the material, which could help to fabricate a real functional 3D self-supported structure
such as a helix. It might be also useful to increase the carbon-chain length of the surfactant
(NDSS) of the solution to explore whether the quality of the eventual structure would change.
With a proper development in material properties, combining direct laser reduction method
with the super-resolution and parallel writing techniques will lead to a new laser-based
fabrication approach to making artificial material and devices with exceptional functionalities
in future. It is also promising to use SLM, which helps to generate any arbitrary patterns by
shaping the beam and fabricating through 2PR in order to broaden the application range for
metallic nanostructures in future.
In summary, using a simple and low-cost 2PR method, functional multidimensional
metallic micro/nanostructures with sufficient quality can be formed for variety type of
applications such as nanowires, surface-enhanced Raman scattering (SERS), microfluidic
chips, LoC, plasmonics, and metamaterials. However, both superresolution and parallel
writing advances require material development, we believe combining this versatile direct
laser reduction technique with the superresolution and parallel writing methods will lead to a
new laser-based fabrication platform enabling multidimensional functional artificial material
fabrication in future.
135
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Publications by Author
Journal Papers
1. Sahar Tabrizi, Yaoyu Cao, Benjamin P. Cumming, Baohua Jia, and Min Gu.
Functional Optical Plasmonic Resonators Fabricated via Highly Photosensitive Direct
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