RESEARCH PAPER

Integrated optofluidic microsystem based on vertical high-orderone-dimensional silicon photonic crystals

G. Barillaro • S. Merlo • S. Surdo • L. M. Strambini •

F. Carpignano

Received: 22 July 2011 / Accepted: 19 September 2011 / Published online: 2 November 2011

� Springer-Verlag 2011

Abstract In this work, fabrication and testing of an op-

tofluidic microsystem exploiting high aspect-ratio, vertical,

silicon/air one-dimensional (1D) photonic crystals (PhC)

are reported. The microsystem is composed of an electro-

chemically micromachined silicon substrate integrating a

1D PhC featuring high-order bandgaps in the near-infrared

region, bonded to a glass cover provided with inlet/outlet

holes for liquid injection/extraction in/out the PhC-itself.

Wavelength shifts of the reflectivity spectrum of the pho-

tonic crystal, in the range 1.0–1.7 lm, induced by flow of

different liquids through the PhC air gaps are successfully

measured using an in-plane all-fibre setup, thanks to the

PhC high aspect-ratio value. Experimental results well

agree with theoretical predictions and highlight the good

linearity and high sensitivity of such an optofluidic mi-

crosystem in measuring refractive index changes. The

sensitivity value is estimated to be 1,049 nm/RIU around

1.55 lm, which is among the highest reported in the lit-

erature for integrated refractive index sensors, and

explained in terms of enhanced interaction between light

and liquid within the PhC.

Keywords Optofluidics � Microsystem �Photonic crystals � Refractive index

1 Introduction

Integrated optofluidics aims at manipulating fluids and

light at the microscale and exploiting their interaction to

create highly versatile microsystems (Monat et al. 2007; De

Stefano et al. 2007). Synergic combination of microfluidic

systems and microphotonic devices opens up a number of

potential applications that are mainly twofold: (a) fluids

can be used to carry substances to be analyzed through

highly sensitive microphotonic devices (biochemical

sensing); (b) fluids can be exploited to control micropho-

tonic devices, making them tunable, reconfigurable, and

adaptive (tunable photonics).

Photonic crystal (PhC) (Sakoda 2005) structures

obtained by periodic arrangement of air gaps in dielectric

materials provide a powerful building block for optoflui-

dics (Monat et al. 2008; Mogensen and Kutter 2009). On

one hand, PhC optical properties are very sensitive to tiny

changes of the dielectric constant and thickness of the

composing materials. On the other hand, injection of suit-

able fluids into air gaps allows PhC optical properties to be

effectively modified/tuned.

The generation of photonic crystals used so far for op-

tofluidic applications has been based on planar 1D/2D

structures with reduced value of the aspect-ratio (height-to-

width ratio about 1), mainly due to technological limita-

tions. A few representative examples of such a generation

of optofluidic PhCs can be found in the literature (Chow

et al. 2004; Lee et al. 2008; Grillet et al. 2004).

Despite the noteworthy theoretical and experimental

results reported in the literature on this subject, the reduced

aspect-ratio limits potentialities of PhCs as optofluidic

building blocks. Fluid injection into air gaps cannot be

easily performed through microfluidic channels integrated

on the same chip together with PhCs. Another substrate,

G. Barillaro (&) � S. Surdo � L. M. Strambini

Dipartimento di Ingegneria dell’Informazione: Elettronica,

Informatica, Telecomunicazioni, Universita di Pisa,

via G. Caruso 16, 56122 Pisa, Italy

e-mail: g.barillaro@iet.unipi.it

S. Merlo � F. Carpignano

Dipartimento di Elettronica, Universita degli Studi di Pavia,

Via Ferrata 1, 27100 Pavia, Italy

123

Microfluid Nanofluid (2012) 12:545–552

DOI 10.1007/s10404-011-0896-0

usually the chip cover, is provided with the necessary net of

channels for delivering fluids into PhC structures. Changes

in the optical properties induced by fluid injection into air

gaps are commonly investigated by reflection measure-

ments using an out-of-plane readout. An in-plane readout

beam cannot be easily exploited due to the reduced aspect-

ratio, thus reducing on-chip optical signal routing/processing

potential. Optical tuning is obtained by local modification of

the refractive index at the PhC surface through fluid injec-

tion, often limiting the PhC sensitivity due to the reduced

interaction length between light and fluid.

Based on advantages and limitations of the up-to-date

generation of PhCs, a new generation of 1D/2D PhC

structures featuring high aspect-ratio can be envisioned as

core element of future optofluidic microsystems and lab-

on-chip with advanced features. In principle, high aspect-

ratio PhCs allow both microfluidic channels and optical

paths to be effectively integrated together with optical

transducers (i.e. the PhC itself) on the same chip, thus

envisioning on-chip manipulation/routing/processing of the

optical signals before and after fluidic modification. They

also allow moving light-fluid interaction from surface to

bulk, so enabling deeper interaction between light and fluid

and, in turn, better tuning and higher sensitivity. In par-

ticular, vertical 1D PhCs with high aspect-ratio periodic

air-trenches are very appealing for the development of

optofluidic microsystems as they inherently feature inde-

pendent fluidic (in the direction of the air gaps) and optical

(perpendicular to air gaps) on-chip paths. Moreover,

besides mere optofluidics, vertical 1D PhCs are very

interesting as building block for label-free biosensors due

to the possibility of using reflections from multiple periodic

surfaces—instead of reflection from just a single surface as

it usually happens for 1D/2D planar structures—for the

detection of biological matter immobilized on the surfaces

themselves.

Despite the potential of high aspect-ratio 1D/2D PhC

structures for optofluidic/biosensing applications, only a

few cases have been reported in the literature so far

(Gruning and Lehmann 1996; Chelnokov et al. 2002;

Tolmachev et al. 2002; Saadany et al. 2006; Barillaro et al.

2006; Lipson and Yeatman 2007; Mandal et al. 2009),

mainly because of their more challenging fabrication with

respect to planar 1D/2D PhCs. Moreover, an even lower

number of papers have been published to date concerning

optofluidic application of high aspect-ratio, vertical 1D

PhCs (Nunes et al. 2008; Barillaro et al. 2009a). Nunes

et al. (2008) reported on the fabrication of vertical silicon

dioxide/air 1D PhCs for optofluidic applications by Deep

Reactive Ion Etching (DRIE) trenching of a 9-lm-thick

silicon dioxide layer grown by thermal oxidation of silicon

substrates for 21 days at 1,075�C. The higher value of the

aspect-ratio (about 3) of the fabricated PhC structure

allowed microfluidic channels and waveguides to be inte-

grated on the same chip together with the PhC itself. A

maximum sensitivity value of about 800 nm/Refractive

Index Unit (RIU) was claimed for such a microsystem,

which is higher than sensitivities achieved with planar PhC

structures (Chow et al. 2004; Lee et al. 2008; Grillet et al.

2004), thanks to the enhanced interaction between light and

fluid compared to evanescence-based devices.

Recently, we demonstrated that vertical silicon/air 1D

PhCs with high aspect-ratio (over 30), fabricated by elec-

trochemical micromachining (ECM) technology, featuring

high-order bandgaps in the near infra-red region (wave-

length range 1.0–1.7 lm), allow discriminating between

liquids (ethanol and isopropanol) with a refractive index

difference of the order of 10-2 through the 8 nm red-shift

of the reflectivity spectrum around 1.55 lm, in good

agreement with theoretical calculations (Barillaro et al.

2009a). ECM is a microstructuring technique based on the

electrochemical etching of n-type silicon in hydrofluoric

aqueous electrolytes (Barillaro et al. 2002a, b). This tech-

nique, which is commonly used for regular macropore

formation (Lehmann and Foll 1990; Lehmann 1993), only

recently has been assessed as a powerful method for fab-

ricating silicon microstructures. ECM represents today a

unique case of straightforward, low-cost wet-etching

technology with microfabrication flexibility typical of

complex, high-expensive dry-etching processes. Vertical

high aspect-ratio (about 50) silicon/air 1D PhCs with

optical high-quality surface (losses \ 1 dB) for optical

communication applications (bandgap or reflectivity notch

at 1.55 lm) fabricated by ECM have been also reported

(Barillaro et al. 2008, 2010).

In this work, we demonstrate the feasibility of an opto-

fluidic microsystem based on high-order, vertical silicon/air

1D PhCs fabricated by ECM. A micromachined silicon

substrate integrating a 1D PhC micromirror is anodically

bonded to a glass cover provided with inlet/outlet holes for

liquid injection in the PhC air gaps. The 1D PhC micro-

mirror is composed of 2.16-lm-thick silicon walls, sepa-

rated by 5.84-lm-wide air gaps, featuring a depth of 90 lm

(aspect-ratio over 40). After bonding, the microsystem is

equipped with two fitting ports for an easy-to-use fluidic

connection to a peristaltic pump, thus ensuring reproducible

filling and withdrawing sequences. Among the advantages

of such an optofluidic microsystem, with respect to planar

and non-planar devices reported in the literature, there are

the high uniformity of the silicon walls and the good optical

quality of the surfaces; moreover, efficient coupling with

the readout optical fibre is obtained in reflection. A further

advantage of our assembly is related to the relaxed, single-

side alignment procedure between the PhC structure and the

readout optical fibre thanks to the comparable size between

standard fibre diameter and silicon wall height.

546 Microfluid Nanofluid (2012) 12:545–552

123

Experiments are performed by pumping liquids (water,

ethanol, isopropanol) through the optofluidic microsystem

and measuring the reflectivity spectrum in the wavelength

range 1.0–1.7 lm (at normal incidence) with an optical fibre

setup. As theoretically expected, insertion of liquids with

increasing refractive index in the PhC structure causes a red-

shift of the reflectivity spectrum. Sensitivity, which is defined

as wavelength shift as a function of the refractive index var-

iation and evaluated for the reflectivity peak of different high-

order bandgaps, is found to be of 1,049 nm/RIU around

1.55 lm, which is of the same order of magnitude of the best

integrated refractive index sensors reported in the literature.

2 Design and fabrication of the optofluidic microsystem

The integrated optofluidic microsystem is realized by

anodically bonding a micromachined silicon substrate

integrating vertical, high aspect-ratio silicon/air 1D PhCs

with a borosilicate glass cover provided with suitable inlet/

outlet holes through which liquid insertion into the air-gaps

of the PhC can be performed.

2.1 Optofluidic section

Vertical silicon/air high-order 1D PhCs featuring a high-

order bandgap around a selected wavelength k can be

designed according to the so-called hybrid quarter-wave-

length stack and consist in periodic arrays of silicon layers

with thickness dSi and air gaps of width dAir that must

satisfy the conditions dSi = Mk/4nSi and dAir = Nk/4,

where M and N are odd integer, independent parameters,

nSi is the silicon refractive index at k (whereas air refrac-

tive index is 1), and n = (M ? N)/2 is the order of the nth

bandgap, which is centred at k (Barillaro et al. 2008). On

the other hand, by choosing even integer values for M and

N, 1D PhCs can be designed featuring a narrow notch,

corresponding to a reflectivity minimum between two

consecutive high-order bandgaps, which can be considered

as a resonance peak with high quality-factor (Q) at the

selected wavelength k (Barillaro et al. 2010).

In this work, vertical silicon/air 1D PhCs featuring a

number of high-order bandgaps in the near-infrared region

between 1.0 and 1.7 lm, among which one (namely the

17th) centred at the operation wavelength k = 1.55 lm,

are designed and fabricated. Design parameters are the

silicon refractive index value nSi = 3.48, the spatial period

p = 8 lm, and the porosity of the structure D = dAir/

(dAir ? dSi) = 0.73. The fabrication is performed by

means of the ECM technology according to the process

developed by Barillaro et al. (2008).

The starting material is a 675-lm-thick n-doped silicon

wafer (100) oriented, with a 3–8 X�cm resistivity, and with a

100-nm-thick thermally grown silicon dioxide layer on its

top. An array of 1-cm-long parallel straight lines, with a

width of 4 lm and a pitch of 8 lm, enclosed by a squared

frame is defined in the silicon-dioxide layer by means of a

standard lithographic step and subsequent buffered hydro-

fluoric acid (BHF) etching. A quarter of the silicon die after

the lithographic step and BHF etching is sketched in Fig. 1a.

A potassium hydroxide (KOH) etching is used to transfer the

pattern in the silicon substrate surface (Fig. 1b). The KOH

etching forms full V-grooves in the straight-line area, which

are used as initial seeds for silicon trenching by means of the

following electrochemical etching step (Barillaro et al.

2002b, 2008), and a microchannel having trapezoidal section

in the squared frame area, which surrounds the V-grooves

and is exploited in optofluidic applications for filling all the

trenches with suitable liquid. Electrochemical etching in a

HF-based solution (HF:H2O = 5%:95% by volume, with

the addition of 1,000 ppm of Sodium Lauryl Sulfate, SLS,

used as surfactant) is properly controlled to produce deep

regular trenches in the patterned substrate and, in turn, a

vertical, high aspect-ratio silicon/air PhC micromirror

(Fig. 1c). The etching voltage Vetch is set to a constant value

of 3 V for the entire etching process, while the etching

current Ietch is set to an initial value Ietch0 = 30.85 mA over

a circular etching area of 0.66 cm2 and properly reduced

during the etching in order to obtain a constant (nominal)

porosity value of 73% through the entire etching depth

(Barillaro et al. 2008). This porosity value corresponds to

silicon walls of 2.16 lm (M = 19 at k = 1.55 lm) sepa-

rated by 5.84-lm-thick air gaps (N = 15 at k = 1.55 lm).

The etching time tetch = 3,900 s is chosen to fully etch

90-lm-deep trenches. After the electrochemical etching, a

cleaning step in HF:H2O:EtOH (10%:10%:80% by volume)

aimed to the removal of the surfactant from the silicon

surface and a subsequent drying step aimed to the evapo-

ration of the liquid filling the trenches are performed.

A Scanning Electron Microscope view of a typical 1D PhC

fabricated according to the described technological process

is reported in Fig. 2. The high quality of the microfabricated

structure, in terms of uniformity (X, Y and Z direction) and

low surface (X–Y plane) roughness, is clearly highlighted.

As shown in Fig. 2, after the PhC fabrication process is

ended, micromachined silicon substrates are cut along the X-

direction, so as to obtain devices to be used as bottom part of

the microfluidic system and that are suitable for performing

optical measurements of the reflectivity spectrum on the X–

Y plane.

2.2 Glass cover

Two through-holes (1.2 mm diameter) are drilled, 4 mm

far apart, in a borosilicate glass slab (800 lm thick) to be

employed as cover. Anodic bonding is used to hermetically

Microfluid Nanofluid (2012) 12:545–552 547

123

seal the micromachined silicon substrate with the glass

cover and, in turn, allows insertion/extraction of liquids in/

out the photonic crystal structure through the inlet/outlet

holes to be performed (Fig. 1d). After bonding, the

microsystem is equipped with two fitting ports with inter-

nal/external diameter of 1.0/1.2 mm that allow easy-to-use

fluidic connections of the chip with an external setup to be

performed (Fig. 1e). As already mentioned, the trapezoi-

dal-section microchannel distributes the liquid inserted

through the fitting port in all the air trenches of the 1D PhC

(Fig. 1f). The microchannel is thus closed at each end with

epoxy glue in order to avoid liquid leakage during opto-

fluidic measurements. Figure 1g shows half-section of the

fabricated microsystem (not to scale) with a sketch of the

read-out optical fibre in front of the 1D PhC.

2.3 Microsystem assembly

Anodic bonding is a well-established, reliable, simple and

inexpensive silicon packaging technique (Knowles and van

Helvoort 2006). It is nowadays used for a range of material

combinations (i.e. glass/silicon, glass/glass, glass/metals,

etc.), device design (i.e. pressure sensors, micropumps,

lab-on-chip, etc.) and applications (i.e. automotive,

microfluidics, biomedical, etc.). Anodic bonding allows

silicon and glass to be hermetically joined together through

application of a suitable voltage (typically between 100 and

1,000 V) across the materials (silicon at the anode side)

after the materials are heated up to a temperature in the

range 300–500 �C. Flat (root mean square of the roughness

lower than 0.05 lm), smooth and clean surfaces of both

materials are required. Significantly higher values of sur-

face roughness, surface steps or foreign contaminant par-

ticles as well as lower temperature and voltage will all

prevent full-scale hermetic sealing.

The micromachined silicon substrate and glass cover are

cleaned before bonding by 15 min immersion in

H2SO4:H2O2 (1:1 vol), a chemical solution known as

Piranha etch, so as to remove organic contaminants off the

substrates. The silicon substrate is then rinsed in HF:EtOH

(1:1 vol) for 2 min to remove thin oxide layer from the

silicon surface. The glass cover is anodically bonded to the

micromachined silicon substrate using a voltage of 600 V

at the temperature of 400�C. An optical image showing the

photonic crystal microsystem after the bonding process is

reported in Fig. 3a, along with Scanning Electron

Fig. 1 Schematic perspective view of the silicon 1D PhC optofluidic

microsystem at different fabrication steps: a after the layout definition

on the top oxide layer, b after the KOH etching for seed points and

fluidic channel formation, c after the electrochemical etching for 1D

PhC fabrication, d after anodic bonding of the borosilicate glass

cover, e after connection of the fitting ports on the glass cover, f after

injection of liquid in the microsystem. In g, half-section of the 1D

PhC optofluidic microsystem (not to scale) with the read-out optical

fibre in front of the PhC is shown

Fig. 2 SEM bird-view (a) and YZ-section (b) of a typical high

aspect-ratio silicon/air 1D PhC fabricated according to the techno-

logical process reported in Fig. 1. The high quality of the microfab-

ricated structure, in terms of uniformity (X, Y and Z direction) and low

surface (X–Y plane) roughness, is clearly highlighted

548 Microfluid Nanofluid (2012) 12:545–552

123

Microscope images showing a detail of the inlet section

(Fig. 3b), further zoomed in Fig. 3c to resolve the fine

structure of the photonic crystal underneath the glass cover.

An optical image of the assembled optofluidic 1D PhC

microsystem equipped with fitting ports is shown in

Fig. 4a. Figure 4b reports an SEM image of the front-side

of the system highlighting the trapezoidal microfluidic

channels used to fill the 1D PhC air gaps with liquids. A

detail of one of the channels is reported in Fig. 4c.

3 Evaluation of functionality

The functionality of the optofluidic microsystem is investi-

gated by pumping different liquids (water, ethanol, and

isopropanol) through the 1D PhC and measuring the reflec-

tivity spectrum in a wide wavelength range (1.0–1.7 lm) at

normal incidence and room temperature, with angle accu-

racy of ±1� and wavelength-steps (spectral resolution) of

0.8 nm. The optical measuring setup incorporates standard

telecommunication single-mode optical fibres (SMR-28

9/125/250 lm core/cladding/coating diameter) (Barillaro

et al. 2009b). Radiation from a white light source emitting in

the selected wavelength range is launched into one port of a

50/50% coupler and shined onto the front face of the PhC.

Radiation back reflected from the PhC is coupled back to the

monochromator input of an Optical Spectrum Analyzer

(OSA) for signal detection. The readout fibre is mounted on a

precision micropositioner and alignment of the fibre termi-

nation in front of the PhC is monitored by means of a digital

camera with a telecentric lens, oriented at 45� with respect to

the fibre axis. Good optical coupling between fibre and PhC

is obtained using a tapered lensed termination characterized

by a working distance of 35 lm. The alignment procedure is

performed after flexible tubings are connected to the inlet/

outlet fittings port. The optical alignment is not very critical

since the spot diameter is 5 lm whereas the PhC is 90 lm in

height. Care is just required to avoid crashing the lensed fibre

tip against the PhC front face.

Fluids are pumped into and extracted from the micro-

system using a peristaltic pump. For each tested liquid,

reflected power spectra are acquired before liquid injection

(empty microsystem), when liquid has filled the PhC air

gaps between the silicon walls (filled microsystem), and

after liquid has been removed from the gaps. As an

example, Fig. 5 shows the power spectra measured in air

(before water injection and after water extraction) as well

as with water filling the PhC air gaps. It can be observed

that the optical spectra before liquid injection and after

liquid removal are very well superposed. The lineshape of

the spectrum changes quite evidently in the presence of

water, since the large increase of refractive index from air

to water raises the order of the bandgaps in the near

infrared region. Good reproducibility is obtained after

cycling water several times into the microsystem. Standard

deviation, which was estimated over six measurements by

taking the wavelength position of all the reflectivity peaks

in the measurement range, has a maximum value of

STD = 0.9 nm, which is in good agreement with the

spectral resolution (0.8 nm) used for measurements.

Experimental reflectivity spectra are obtained by nor-

malizing measured power spectra with respect to an ideal

reflector, as described by Barillaro et al (2009b). Typical

experimental reflectivity spectra for the three tested liquids

Fig. 3 Optical image showing the PhC optofluidic microsystem after

the anodic bonding process (a), and SEM images showing a detail of

the inlet section (b) further zoomed in (c) to resolve the fine structure

of the photonic crystal underneath the glass cover

Microfluid Nanofluid (2012) 12:545–552 549

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are reported in Fig. 6, along with theoretical reflectivity

spectra numerically calculated by using the characteristic

matrix method, which was properly modified to take into

account both non-idealities of the 1D PhC (i.e. roughness

in the X–Y plane, silicon/air thickness variation in the

Z direction) and limitations of the measuring setup (i.e.

resolution bandwidth, RB, of the optical spectrum analyzer;

spot diameter, SD, of the interrogating beam) (Barillaro

et al. 2009b). Theoretical spectra in Fig. 6 are calculated

through best-fitting of the experimental data, using porosity

mean value D and standard deviation rD as fitting

parameters, taking into account the limited resolution

bandwidth, RB = 10 nm, employed for the measurement

and the beam spot-size, SD = 5 lm. Under the hypothesis

that porosity follows a Gaussian statistical distribution with

mean value D and standard deviation rD, the mean value

D basically sets the spectral position of reflectivity band-

gaps (high-reflectivity regions) and the standard deviation

value rD essentially settles the extinction ratio between

high- and low-reflectivity regions (Barillaro et al. 2009b).

According to porosity definition, the mean value D can be

directly translated into silicon thickness, dSi = p(1 - D).

On the other hand, the porosity standard deviation rD can

be related to the peak-to-valley amplitude of the surface

roughness q = p3rD, with probability 0.997. The fitting

procedure yields D = 0.73 and rD = 1 9 10-4 for the

experimental reflectivity spectra of Fig. 6.

As expected, increasing the refractive index of the filling

medium, from water to ethanol and isopropanol, a shift of

the spectrum toward longer wavelengths is observed. In

view of the application of this optofluidic microsystem as

integrated refractive index sensor, we plot in Fig. 7 the

peak wavelength of different bandgaps, namely from the

18th (around 1.688 lm in water) up to the 30th (around

1.028 lm in water) order, as a function of the refractive

index value of the three liquids. The experimental position

of each reflectivity peak is obtained after application of a

numerical moving-average filter to the experimental data in

order to increase the signal-to-noise ratio and, in turn,

reduce the effect of measurement noise on the bandgap

peak position. Experimental results (dotted traces with

symbols) in Fig. 7a are in good agreement with theoretical

calculations also reported in the figure (solid gray traces,

Fig. 4 Optical image of the 1D PhC microsystem equipped with

fitting ports (a), and SEM images of the front-side of the system

highlighting the trapezoidal microfluidic channels used to fill the 1D

PhC air gaps with liquids (b), further zoomed in (c)

Fig. 5 Reflected power spectra acquired before water injection

(empty microsystem), when water has filled the PhC air gaps (filledmicrosystem), and after water has been removed from the PhC air

gaps (empty microsystem)

550 Microfluid Nanofluid (2012) 12:545–552

123

green online). By best fitting the experimental curve cor-

responding to the bandgap at 1.512 lm (in water) with a

linear relationship (solid dark gray trace in Fig. 7, red

online) we obtain a sensitivity (defined as wavelength shift

of the peak as a function of refractive index variation)

S = dk/dn = 1,049 nm/RIU, which is comparable to the

highest value reported in the literature for integrated

refractive index sensors, that is about 3,000 nm/RIU as

claimed by White and Fan (2008). The use of a linear

function for the experimental data fitting is fully supported

by theoretical predictions, as shown in Fig. 7. An estima-

tion of the actual limit of detection (LOD) of the proposed

PhC microsystem cannot be easily performed due to both

the limited spectral resolution (0.8 nm) and the high value

of the OSA resolution bandwidth (10 nm) required for

ensuring a sufficiently high signal-to-noise ratio and

dynamics in the measurements. A worst-case estimation

yields LODworst = 3 STD/S = 2.57 9 10-3 RIU for the

bandgap at 1.512 lm (in water), which is calculated using

the standard deviation maximum value STD = 0.9 nm

obtained from spectral measurements above reported. A

better estimation requires reducing both the spectral reso-

lution and the resolution bandwidth, for example by using

an amplified spontaneous emission source and limiting the

spectral measurement to a narrow wavelength interval

around the bandgap of interest.

4 Conclusions

In conclusion, in this work the feasibility and functional-

ity of an integrated optofluidic microsystem based on

Fig. 6 Experimental reflectivity spectra (black traces) in air and for

the three tested liquids (water, ethanol, isopropanol), best fitted to

theoretical reflectivity spectra (gray traces, green online) (D = 0.73

and rD = 1 9 10-4, RB = 10 nm, SD = 5 lm)Fig. 7 a Experimental (dotted traces with symbols) and theoretical

(solid traces) wavelength position of reflectivity peak for different

bandgaps (from 18th up to 30th) as a function of the refractive index

of the liquid filling the PhC air gaps, in the wavelength range

1.0–1.7 lm. b Detail of (a), highlighting the experimental peak

position wavelength of the bandgap at 1.512 lm (in water) as a

function of the refractive index of the three tested liquids (blacksymbols), together with the linear curve best fitting experimental data

(solid dark gray trace, red online)

Microfluid Nanofluid (2012) 12:545–552 551

123

high-order, vertical silicon/air 1D PhCs, fabricated by

electrochemical micromachining are for the first time

demonstrated. In-plane read-out of the PhC spectral

reflectivity is successfully performed for different liquids

flowing within the microsystem, using a low coupling-loss

optical-fibre setup and straightforward alignment procedure

thanks to the high aspect-ratio of the PhC. Experimental

wavelength shifts of PhC reflectivity spectra as a function

of the refractive index of the different liquids injected in

the microsystem yield a good linearity and a high sensi-

tivity, in agreement with theoretical predictions. A sensi-

tivity value of 1,049 nm/RIU is measured around 1.55 lm,

which is comparable to the top state-of-the-art integrated

refractive index sensors. A LOD worst-case of the order of

10-3 RIU is estimated from spectral measurements, though

limited by the specific parameters used for optical mea-

surement. Further tests will be aimed at performing an

exhaustive characterization of the PhC microsystem using

a larger set of refractive index values as well as estimating

the actual LOD of the microsystem. Future work will be

performed to obtain an optimized design of the PhC aimed

at improving the microsystem sensitivity while reducing

the LOD, as well as a better layout allowing read-out paths

to be integrated in the silicon substrate together with the

PhC structure and the fluidic channels.

It is our opinion that the proposed microsystem repre-

sents a first step towards the development of a new-concept

PhC optofluidic building block opening new horizons for

both optofluidic microsystems and Lab-on-Chip.

Acknowledgments This research was partially supported by PRIN-

MIUR and CARIPLO Foundation.

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