1 Institute of Photonics and Quantum Sciences, Heriot-Watt University, SUPA, Edinburgh, Scotland, EH14 4AS, UK 2 Dept. of Electrical and Computer Engineering, Birck Nanotechnology Center, Purdue University, West Lafayette, IN, 47907, USA 3 Dept. of Physics & Astronomy, Birck Nanotechnology Center, Purdue University, West Lafayette, IN, 47907, USA *Corresponding author: [email protected]
Received XX Month XXXX; revised XX Month, XXXX; accepted XX Month XXXX; posted XX Month XXXX (Doc. ID XXXXX); published XX Month XXXX
OCIS codes: (130.3120) Integrated optics devices; (160.1245) Artificially engineered materials; (220.1080) Active or adaptive optics; (320.7080) Ultrafast devices.
http://dx.doi.org/10.1364/AO.99.099999
1. INTRODUCTION Despite the fact that their first theoretical representation was
initially proposed in late 1952 [1], plasmon polaritons started becoming fundamental elements in integrated photonics roughly one decade ago when, for the first time, metallic nano-devices were proposed for manipulating light on a deep subwavelength scale [2]. Of course, the real boom of plasmonic applications had to wait for some time until the full development of novel technologies, aiming at the fabrication and manipulation of nano-objects, was accomplished. Initially, the capability of coupling electromagnetic radiation with the oscillation of the electron plasma at a metal-dielectric interface, plus the related subwavelength mode confinement, was the initial driver for plasmonics, which provided a potential solution to the problem of the diffraction limit of light. Since then, plasmonics has come a long way with a myriad of ingenious solutions for implementing many fundamental functionalities on a scale unreachable by purely photonic components. Among the unique capabilities enabled by metamaterials and plasmonic devices, we can list enhanced Raman spectroscopy and bio-sensing [3-4], super-resolution imaging [5], flat optics [6], nano-lasers [7], enhanced nonlinear optics [8], metatronics [9], supercoupling channels [10], electric levitation [11], ultra-compact
optoelectronics [12], enhanced quantum optics [13-14], ultra-dense information storage [15], efficient and cost effective thermal photovoltaic [16], Surface plasmon assisted catalysis [17], and many others. Some of these exciting applications are still partially confined in the theoretical realm, while others are on the way to become solid/applicable technologies, and a few have already been proven to be practical and effective.
Most of the currently explored applications of plasmonics are static in nature, meaning that the optical properties of the overall structure are engineered during the fabrication processes and remain unchanged during operation. The present review aims at collecting the most recent and relevant results in plasmonics and metamaterials where they pertain to the dynamic changes of the optical properties of the metallic inclusions. Of course, in many cases nano-devices exploit the dynamic change of occurring in the dielectric constituent, however most of these mechanisms have been largely investigated in semiconductor physics and nonlinear optics, and a broad literature is already available. In addition to this, our attention will be solely focused on experimental demonstrations to provide a better feeling about the current status of tunable plasmonic technologies in applied physics, which is one of the main target of the present review.
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From pioneering works such as K.F. MacDonald and co-workers [40] (see Fig.3(a)) or by D. Pacifici et al. [41], until today, remarkable results have been attained towards the realization of all-optical dynamic nano-devices. However, while many strategies and solutions have been proposed in theory, the number of experimental demonstrations is still quite limited. In the following paragraph we will give a brief overview of few selected works that in our opinion stand out in terms of originality and/or the absolute magnitude of the result achieved. Because of the limited literature available, we focused our attention non only on results pertaining fully finished devices but we also included those works dealing with novel active materials.
From a material point of view, one of the most promising approaches for ultra-fast modulation of plasmonic signals relays on the photo-carrier excitation in transparent conducting oxides (TCOs). This is a class of highly doped and large bandgap materials with a crossover point of the dielectric permittivity typically in the near infrared spectral region. These materials can be synthesized at ambient conditions and by means of many standard fabrication techniques (CVD, PVD, PLD, ect.) achieving high quality/electronic-grade thin films. In addition to this, TCOs allow one to operate inside the epsilon-near-zero regime which has proven to be a winning strategy for maximizing material nonlinearities [42-43] and exploiting alternative guiding mechanisms [44]. For more information about fabrication strategies and structural properties of TCOs we suggest the reader to consult the recent review by X. Yu and co-authors [45]. In the domain of nano-photonics exploiting TCOs materials, it is important recalling the research activities carried on by the group of O. L. Muskens employing ITO-based devices [46,47]. However, only in very few cases ITO is used as an active substitute of the metallic constituents. Most commonly ITO is employed as active substrate on top of which plasmonic nanoantennas are fabricated.
Very recently, towards the realization of optical active devices, P. Guo and co-workers have observed a large redshift of the absorption peak in intraband pumped ITO-based metasurfaces. The device consists of ITO nanorod antennas with NIR absorption at 1.5µm associated to the transverse resonance of the nanostructures (see Fig.3(b)). The device shows ultrafast sub-picosecond response but is limited in bandwidth (due to its resonant nature) while also being polarization sensitive. One very important peculiarity of this scheme deals with the intraband pumping of the nanostructures that leads to a large plasma frequency modulation and induced absorption [48].
ITO has been the first choice for the initial study on TCOs in photonics due to its very well established fabrication processes for industrial applications. However, the need for lower losses, larger static/dynamic tunability of the optical properties, and for materials employing abundant and safe chemical elements, has pushed material scientists to work for alternative solutions. In this sense, inside the class of TCOs great attention has been given to Al- and Ga-doped zinc oxide [49]. These two compounds possess large electron mobility and allow for very broad tunability of the crossover point in a wavelength range embracing part of the visible region plus the near infrared up to 1.6µm. In addition to this, novel fabrication processes have been recently developed in order to largely reduce the recombination time of photo-generated carriers, a fundamental feature for nanophotonic applications that require ultra-fast ON/OFF capabilities [50].
Looking for other alternative plasmonic materials with good optic tunability, M. B. Price and co-workers showed how metal-halide perovskites can exhibit appreciable dynamic change of the complex dielectric function on short time scale [51]. Their work is particularly interesting because it describes the carrier cooling dynamic and sub-bandgap transient absorption in CH3NH3PbI3. a better understanding of these processes in perovskites could lead to a noticeable increase in
the efficiency of solar photovoltaic devices based on these materials which represent the fastest-advancing solar technology to date.
Full optical control is typically performed when the goal is maximizing the device performance in terms of speed. However, this is a very general statement that does not encompass all the possible cases. For instance, important results have been achieved towards the design of all-optical plasmonic nano-memories. In this direction we should mention about the phase changing memory based on a periodic array of Au nanodisks fabricated on a thin VO2 film deposited on an SiO2 substrate. The device was demonstrated by D. Y. Lei and co-workers and it exploits the coupling between the localized surface plasmon resonance of nanoelements (excited by means of a UV pump beam) and the characteristic insulator-to-metal phase transition of the underneath VO2 layer. One very interesting characteristic of the demonstrated heterostructure is that the memory effect is encoded in the plasmon resonance and that the entire phase space can be optically addressed by thermal initialization [52].
As previously stated the main peculiarity of the phase-transition approach is the possibility of latching a targeted physical state. The related temporal transition is relatively slow while the energy required to complete the transition is normally quite conspicuous. In the direction of mitigating these two issues we wish to remind the work by J. M. Gaskell and co-workers [53] where VO2 films were optimized by atmospheric pressure chemical vapor deposition, thus pushing the transition time of VO2 in the picosecond regime.
On the quest for very large nonlinearities, a novel and efficient stratagem deals with the concept of reconfigurable metamaterials which are artificial structured materials capable of altering their geometry under excitation [54]. In the most typical case, resonant metallic nano-elements are fabricated on top of suspended dielectric nano-strips (see Fig. 3(c)). When light strikes upon such a structure, the resultant thermal and electromagnetic forces induce a reversible mechanical deformation of the material, thereby strongly altering its optical properties [55]. In order to enlarge the limited operational bandwidth (due to the resonant effect) without losing efficacy, other configurations employ a simple array of suspended bilayer stripes of metal-on-insulator supplied with a constant current flow. Under this condition, the modulation can be achieved by applying an external magnetic field that will induce a strong mechanical deformation because of the induced Lorentz force [56].
All the results shown so far make use of a change in the device properties in order to achieve the desired temporal variation of the optical properties. However, it might be useful to keep in mind that analogue effects could be in principle attained by operating on the signal properties. In other words, we could think to encode the modulation by “marking” in some way the input signal while the device is predesigned in order to act differently on different signals. This is the case of the plasmonic switch based on a SOI waveguide coupled to a nano-antenna realized by R. Bruck and co-workers. In this device the modulation function is achieved by constructive or destructive interference of two counter-propagating beams whose interference profile is tuned in and out of resonance with a plasmonic antenna [57]. Of course, the device is characterized by the typical pros and cons of a phase modulator, for which high extinction ratio, moderately high speed, and limited scalability of the overall device are defining features.
4. Mechanical Actuation Light-matter interactions, in plasmonics and metamaterials, are a
function of the physical structure of the material as well as its optical properties; therefore, mechanical tunability of the structures is an effective technique to achieve active optical functionalities of plasmonic based devices. Different approaches have been implemented to obtain reversible mechanical deformation of
Marcello Ferrera
plasmonic structures. Structural reconfigurability is accomplished either by action on individual plasmonic elements or by collective reconfiguration of arrays. In the following sections, efforts exploiting these approaches will be covered.
The approach of modifying the whole plasmonic array/structure is practically easier than reshaping individual plasmonic elements. Most efforts investigated in this regard are mainly obtained through using stretchable materials or through using micro-electro-mechanical system (MEMS) structures. Stretchable films [58-66] have been developed in order to enable flexible shaping of the substrate. They are specifically important for their potential to develop wearable and personal optical devices.
Tunable optical devices can also be achieved by building plasmonic metasurfaces on top of flexible materials. These metasurfaces exhibit variable periodicity upon stretching and relaxation of the substrate. By building a beam bending metasurface on top of a stretchable substrate, beam steering devices can be achieved [67, 68]. Similarly, axial scanning can be realized through stretching a plasmonic meta-lens. Successful implementations of tunable optical operations using stretchable materials have been demonstrated. Zhu et al. has successfully demonstrated tunable coloration using flexible metasurfaces [69]. Another reconfigurable chiral structure has been demonstrated by Kim et al., where an S-like plasmonic chain of gold nanoparticles is being stretched and relaxed causing differential response with respect to circular polarizations [70].
The alternative approach of MEMS has been efficiently implemented either through mechanical movement of the whole plasmonic structure, or through inter-structural deformations. Stark et al. implemented a mid IR plasmonic spectrometer, where a gold film with subwavelength holes exhibiting extraordinary transmission is displaced using MEMS electrostatic actuation above a gold reflector to form an interferometric cavity with a modulated length from 1.7μm to 21.67μm [71]. Another application of a plasmonic biosensor has been demonstrated by Zhu et al. where molecular sensing is enhanced by a graphene enabled opto-mechanical device based on a silicon nitride membrane [72]. In the work by Lapine et al., relative displacement between multi-layered arrays of split-ring resonators caused tunability of the absorption in microwave region [73]. Similar techniques have been successfully applied to THz waves [74]. Successful implementation of MEMS based single-layered optical devices have been demonstrated by Ou et al. [75]. In their work, electrostatic actuation is utilized to attract gold strings patterned on top of a silicon nitride membrane. This causes an 8% modulation of of the transmission at MHz modulation speeds. Ou et al. have demonstrated thermally controlled reconfigurable structures through utilizing bilayer materials with a large thermal expansion coefficient mismatch that bends in response to temperature changes [76].
All the previous techniques of mechanical reconfigurability rely more or less on modulating the spacing between plasmonic inclusions. There is an alternative technique of modifying the structure of individual plasmonic elements. In these regards, there have been successful demonstrations of metasurfaces based on reconfigurable plasmonic nanoantennas [77,78]. Reconfigurable microfluidic metasurfaces are also obtained either by utilizing channels of liquid polymer [79] or liquid metal [80-82]. Antennas can be reconfigured in a way that is alternative to changing the periodicity of the substrate leading to beam steering and other dynamic optical responses.
Other morphological nanostructures such as kirigami structures [83-88] can also be used and they provide much larger dynamic range than stretchable films. Kirigami is the ancient Japanese art of obtaining a 3D structure out of a single and properly cut 2D sheet. This strategy
Fig. 4. Demonstration of some mechanically reconfigurable plasmonic structures: (a) using MEMS to obtain plasmonic biosensor [72]; (b) using microfluidic plasmonic nano-antennas with liquid metals to obtain meta-lens [82]; (c,d) using kirigami structures [87]. has recently been implemented at the nanoscale. By appropriately slicing some cuts inside a thin layer of a material, complex structures are obtained which are extremely flexible, and can achieve modulation lengths exceeding those attainable by stretchable substrates. Fig. 4 demonstrates some examples of mechanically reconfigurable plasmonic kirigami along with other examples of reconfigurable plasmonic structures.
5. Chemical Control
Phase changing materials are prominent candidates for switchable photonics, particularly those that exhibit metal-to-insulator or insulator-to-metal phase transitions. The application of heat, electricity, and/or light are the predominant methods for engendering these electronic and structural changes. Alternatively, the introduction of a chemical reagent, sometimes along with a catalyst, can induce phase changes which are strong, stable, and reversible.
Many conventional plasmonic materials are either chemically inert or react uncontrollably with ambient environments. The ideal plasmonic material suitable for chemical control would show a pronounced reversible phase transition to a particular reagent while remaining stable to other external chemicals. A particular method which has shown great promise over the past 5 years has been the hydrogenation of plasmonic metals. In 2014, N. Strohfeldt et al. exposed an array of yttrium dihydride (YH2) nanoparticles to an environment of 5% H2 in N2 and observed a drop in the extinction spectrum over a period of 50 seconds. When the antennas were removed from the hydrogen environment, the extinction spectrum returned, indicating the YH2 nanoparticles had reestablished a metallic phase [89]. F. Sterl et al. demonstrated a similar effect by exposing metallic Mg nanodisks to hydrogen gas, resulting in the formation of the dielectric, MgH2; a cap of palladium facilitates the diffusion of hydrogen into the Mg nanoparticle. The transition was observed to be reversible and repeatable for several cycles before stress-induced damages degraded the performance of the nanodisks [90]. C. Wadell et al. showed that by alloying palladium and gold, the hysteresis observed in previous hydrogen-based studies could be completely removed, thereby enhancing the sensitivity of the plasmonic particle [91]. As a practical demonstration, X. Duan et al. fabricated chiral Mg and Au nanostructures and found a strong hydrogen-regulated chiroptical response across the visible spectrum [92].
Fig. 5. a) Hydrogenization of metallic (M) nanoparticles on
a substrate: (1) the particles are initially in their metallic phase; (2) exposure to hydrogen gas allows for the diffusion of hydrogen ions into the nanoparticles, forming the dielectric compound MHx; and (3) particles return to metallic phase after removal from hydrogen environment. b) prototypical extinction curves corresponding to each of the three phases of the nanoparticles.
In conclusion, plasmonics and metamaterials have granted
us full access to the entire optical space ( , ), and enabled us to mold the flow of light at will. This fact has triggered an explosion of research activities around these two fascinating subjects. However, despite the large available literature we believe that these domains still remain largely underexploited. From our perspective, the key for a full development of plasmonic technologies has to be found in the realm of material science. More specifically, novel CMOS compatible compounds capable of largely altering their optical response (if not radically changing their nature for metallic to dielectric and vice versa) on a sub-picosecond time scale and with minimal energy consumption, could be the solution to loosen fabrication constrains, simplify device designs, and make tunable plasmonics a reality. All this said, we will keep enjoying all the ingenious and creative ideas, even outside material engineering, that will be published in the years to come in the direction of dynamic nanophotonics.
Funding sources and acknowledgments. M.F. acknowledges
support from the People Program (Marie Curie Actions) International Outgoing Fellowship (ATOMIC) under REA grant agreement n° [329346]. V.S. and A.B. would like to acknowledge the funding support for this work from AFOSR MURI Grant FA9550-14-1-0389, AFOSR Grant FA9550-14-1-0138, ARO Grant w911NF-13-1-0226, and NSF MRSEC Grant DMR-1120923.
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