\u003ctitle\u003ereconfigurable optofluidic silicon-based photonic crystal...

$: \u003ctitle\u003eReconfigurable optofluidic silicon-based photonic crystal components\u003c/title\u003e$
Reconfigurable optofluidic silicon-based photonic crystal components

Christian Karnutsch1*, Uwe Bog1, Cameron LC Smith1, Snjezana Tomljenovic-Hanic1, Christian Grillet1, Christelle Monat1, Liam O’Faolain2, Tom White2, Thomas F Krauss2, Ross McPhedran1,

Benjamin J Eggleton1 1Institute of Photonics and Optical Science (IPOS), Centre for Ultrahigh-bandwidth Devices for

Optical Systems (CUDOS), School of Physics, University of Sydney, NSW 2006, Australia 2School of Physics and Astronomy, University of St Andrews, St Andrews, Fife, Scotland

ABSTRACT

We report reconfigurable optofluidic photonic crystal components in silicon-based membranes by controllably infiltrating and removing fluid from holes of the photonic crystal lattice. Systematic characterizations of our fluidically-defined microcavities are presented, corresponding with the capability to increase or decrease the span of the fluid-filled regions and thus alter their optical properties. We show initial images of single-pore fluid infiltration for holes of diameter 265 nm. Furthermore, the infiltration process may employ a large range of optical fluids, adding more flexibility to engineer device functionality. We discuss the great potential offered by this optofluidic scheme for integrated optofluidic circuits, sensing, fluorescence and plasmonic applications. Keywords: Microfluidics, microphotonics, optofluidics, tuneability, photonic integration, microcavity, sensors

1. INTRODUCTION

Photonic crystals (PhCs) represent a class of materials that display a periodic arrangement of dielectric constants [1]. In contrast to a homogeneous medium, the dielectric modulation causes the dispersion of a PhC to be highly frequency dependent. The abrupt spectral variations in the associated photonic band structure imply that moderate shifts in the refractive index – accomplished through an external perturbation – can substantially modify the optical properties of the PhC at a particular frequency. This offers the potential for creating flexible and dynamic optical functionalities, which could be favorably used in applications such as optical information processing (switching, routing, buffering), quantum electrodynamics experiments and highly sensitive and localized optical sensing [2]. Many of the relevant properties of PhCs require a large refractive index contrast, which is provided e.g. by air ↔ silicon interfaces. A variety of materials have been introduced in PhCs, such as liquids [3-6], organic liquids [7], liquid crystals [8-10], polymers [11, 12], nanoparticle-based composites [13], colloidal quantum dots [14, 15] and fluorescent organic dyes [16-20]. Because there exists a range of liquid materials featuring a wide array of optical properties, PhC infiltration opens up many different opportunities associated with the particular characteristics of the infused material [21, 22]. The idea of PhC infiltration has recently been expanded through the concept of selective fluid filling. Introducing liquid crystals into individual air pores of a planar PhC can potentially create various tunable photonic elements (Y-junctions, bends, waveguide intersections and beam splitters) integrated in a PhC circuit [23]. Planar PhCs can confine light in three dimensions by combining a 2D PhC lattice and a step index waveguide (e.g. a thin silicon slab) [24, 25]. Hence their fabrication is compatible with the mature microelectronic manufacturing techniques, while they provide a suitable platform for creating a variety of optofluidic devices that can be readily integrated onto a single chip [26]. Planar PhC components (e.g. waveguides and cavities) are realized by introducing a local ‘defect’ in the periodic lattice. While these defects generally consist of removed or displaced air holes, they can alternatively be created through the selective infiltration of air holes with a liquid. Light can be effectively routed along these fluidically defined paths [27]. Although the resulting light confinement is shallower due to the reduction of the PhC index contrast, effective light guiding is possible even around tight bends [27]. Fig. 1 shows a schematic vision of such an integrated optofluidic photonic circuit, where

* [email protected]; phone: +61-2-9351 3958; fax: +61-2-9036 7158

Silicon Photonics IV, edited by Joel A. Kubby, Graham T. Reed, Proc. of SPIE Vol. 7220, 72200K · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.811083

Proc. of SPIE Vol. 7220 72200K-1

infiltrated holes act as waveguides, microcavities, light sources, filters or switches. The different components of this integrated photonic circuit could not only be modulated based on the tunable properties of the fluid, but also completely erased and subsequently rewritten. This flexible platform will therefore provide a dynamic control over the guiding of light as well as full reconfigurability of the resulting photonic integrated circuit.

Figure 1 Schematic of a microfluidic reconfigurable photonic circuit made of a planar silicon photonic crystal. The device integrates various components – all achieved through selective infiltration of air holes – onto the same platform.

Among the attractive properties of PhC structures lies the ability to tightly confine light within highly compact microcavities [28, 29]. Planar PhC microcavities represent a versatile platform for realizing various small-scale optical components, such as low threshold lasers [30, 31], optical switches [32, 33], narrow-band filters [34], and slow-light structures [35]. For this wide range of applications, design rules generally aim at generating high quality factors, Q and small modal volumes (V<λ3) to trap light for a long time and in a small space [36, 37].

In the context of optofluidics [38, 39], the strong optical confinement within planar PhC microcavities can enhance the interaction between light and the fluid (gas or liquid) that is infiltrated into their air pores. Additionally, they can be designed to maximize the modal intensity overlapping with the infused fluid in the pores [40, 41]. These properties along with the device compactness have driven an entire research field dedicated to the use of PhC microcavities for chemical and bio-sensing applications [42]. The cavity resonance is highly sensitive to the properties of the surrounding environment, providing the basis for detection in many sensor schemes [43]. While a large liquid-light interaction and high Q-factors are both required for improving the sensitivity and the detection limit of the sensor, there is generally a trade-off between these two parameters [43]. The cavity Q-factor tends to decrease after the infiltration step due to the reduction of the PhC index contrast [3, 5, 41, 44]. Additionally, the liquid may not be lossless, thereby degrading the Q-factor and hence the enhancement generated by the cavity for sensing performance [43].

There have recently been strong efforts to integrate many compact PhC sensors onto a single chip in order to increase the device throughput, through parallel and multi-analyte detection schemes. This is particularly relevant for lab-on-a-chip technology, in which many analytical functions are miniaturized and integrated onto the one platform for both diagnostic and bio-chemical detection purposes. In brief, generic microfluidic PhC circuits offer a dual application for (i) chemical and bio-sensing in the broader context of the lab-on-a-chip concept, and (ii) dynamic and reconfigurable complex photonic chips. Note that both applications face the same issues, which are inherent to their hybrid microfluidic-photonic nature. In particular, increasing the light-liquid interaction through PhC design optimization in the context of sensing is also relevant for producing widely tunable photonic functions. In addition, the mentioned advances on PhC sensors related to photonic dense integration and integration with microfluidic networks are also significant for reconfigurable microfluidic-photonic circuits.

The following sections will be focused on the theoretical and experimental works carried out on a particular class of planar PhC Microcavities: microfluidic double-heterostructures. Besides the versatility of planar PhC microcavities in general, these components possess unique properties that are attractive to optofluidics. In particular, we show that a PhC resonator can be directly created by infusing a liquid into any section of a uniform PhC waveguide [45-47]. This self-aligned approach that exploits the microfluidic equivalent of the double-heterostructure (DH) concept [48], relaxes the constraint on both the fabrication and infiltration accuracies while ensuring the interaction between the confined light and the infused liquid. As such, we believe that these components will play a central part in achieving the photonic circuit depicted in Fig. 1, and represent one of the first milestones towards the realization of reconfigurable PhC circuits using optofluidics.

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2. Design of high-Q optofluidic double-heterostructure cavities

A PhC slab cavity is usually formed in one of two ways: forming a point cavity or forming a double-heterostructure (DH) cavity. Double-heterostructures combine PhC regions whose geometries contain slightly different lattice parameters in a single slab (see Fig. 2). These structures can be created in many different ways, but they all rely on an increase of the average refractive index within the central region PhC2, which shifts the bandstructure features to lower frequencies. Hence, a waveguide introduced across the PhC slab has a lower-lying dispersion curve within PhC2 than in the surrounding PhC1. Both waveguide modes are within the same photonic bandgap (PBG), but there is a gap between them. If the resonant frequency of the DH cavity falls within this mode-gap the mode can propagate in the PhC2 waveguide and is evanescent in the PhC1 waveguide. The waveguide section within PhC2 then acts as a cavity due to the mode-gap effect [48]. The highest measured quality factors in PhC slab cavities were achieved using this type of cavity [49, 50]. In these concepts, the PhC2 region is formed by displacing holes, such that the air filling factor decreases, thus increasing the average refractive index. However these designs need to be finalized at the fabrication stage and they rely on extremely precise control of the size and position of the holes. We have proposed another double-heterostructure concept, which takes advantage of post-processing techniques that do not require any change in the geometry of the PhC structure [51, 52]. We induce the refractive index change via air-hole infiltration of the central part of a homogenous PhC structure [53] (see section 3 below).

Figure 2 Schematic of a PhC slab with a W1 waveguide in the Γ-K (x) direction and refractive index distribution in the plane of the structures.

Numerical model and methods

We assume a PhC slab composed of a triangular array of cylindrical air holes in a silicon slab, as illustrated in Fig. 2a. The structure has holes of radius R, a is the lattice constant and h is the thickness of the slab. Across the PhC slab is a waveguide in the Γ-K direction. A W1 waveguide, formed by omitting one row of air holes, is used in all numerical simulations presented here unless stated otherwise. To optimize the experiment we briefly consider the effect of using a narrower waveguide. We start with a homogeneous slab (Fig. 2a) and design the double-heterostructures by changing the holes’ refractive index in the central region of the slab (indicated by the darker circles in Figures 2b and c). Firstly, we consider a silicon-based (n=3.4) PhC slab that is infinite in-plane to obtain the PBGs and associated eigenstates of a waveguide introduced in the Γ–K direction. As the second design step, a finite PhC slab (with 25a in the x-direction and 25a in the z-direction) is considered with the cavity in the centre. For both structures the hole radius is R=0.29a and the thickness of the slab is h=0.6a. We consider structures with varying cavity lengths L, with L=ma+2R, m is an integer. Two numerical methods are used: (i) 3D plane wave expansion method for the PBG calculations and associated eigenstates of the photonic crystal waveguide, and (ii) 3D finite-difference time-domain (FDTD) method – combined with techniques of fast harmonic analysis – for the quality factor calculations. In most calculations, the PML width is 2a and the height of the computational window is 4a. The grid size that provides satisfactory convergence depends on the quality factor. For Q=105, 28 points per period suffice, whereas 32 points per period are needed when Q=106.

The cavity concept in double-heterostructures relies on the mode-gap effect [48]. Therefore we first examine whether there is a sufficient mode-gap between structures with materials other than air in the holes. Fig. 3a shows the dispersion curves for the regular structure (PhC1) and infiltrated structure (PhC2). Both structures have two guided modes below the light line in the lowest PBG, one in the middle of the bandgap and the other one in the lower part of the bandgap. The lower mode is the mode of interest because it provides the potential for high-Q cavities [53]. The dispersion curves of this mode for the regular structure (PhC1), and PhC2 where air holes are infiltrated with material having a refractive index n=1.5, are plotted in Fig. 3a. The lower band edge is indicated for the regular and modified structure. In practice, high-Q cavity modes that originate in the mode-gap can be excited by evanescent coupling from a tapered fiber directly to the cavity [54, 55]. Consequently, we plot a dispersion curve of the tapered fiber used to excite the cavity modes in our experiments along with the numerical results.

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Clearly, filling the holes with a material of higher refractive index than air increases the refractive index of the entire structure, which as a consequence brings the dispersion curve down to lower frequencies. The gap between the dispersion curves, measured at the edge of the Brillouin zone, is ω~Δ =3·10-3, where ca πωω 2/~ = . There is another important factor for the design of high-Q cavities, which is the relative position of the mode-gap within the PBG [53]. The mode-gap should not be too close to the PhC band edge as it is the case for the W1 waveguide. We consider a waveguide with the two PhC sections to either side of the PhC waveguide shifted closer together with the waveguide width 0.9 times the width of a W1 waveguide, a so-called W0.9 waveguide. As shown in Fig. 3b both dispersion curves move upwards in the PBG as the waveguide width is reduced. This modification increases the frequency range between the fundamental waveguide mode and the low-frequency edge of the PhC bandgap. The mode-gap corresponding to the infiltrated W0.9 is not too close to the PBG edge and furthermore allows for a broad selection of fluid indices to configure our devices. In our experiments we noticed that evanescent coupling to the structure is improved by using waveguides narrower than W1.

Cavity design

We now combine PhC1 and PhC2 in a single structure and evaluate the properties of the resulting cavity using the FDTD method. We start with calculating the quality factors for the structures shown in Fig. 2b. The refractive index of the material in the holes of PhC2 is varied between n=1.1 and n=1.7. The results for the quality factors and modal volumes of the resonant modes are plotted in Fig. 4a. The maximum quality factor of Q = 2.5·105 is found for n=1.4. As the holes’ refractive index is increased, the average refractive index of the structure increases. This results in improved out-of-plane confinement and therefore reduced out-of-plane losses, increasing the Q factor. However, at n=1.4 the Q starts to decrease. This is because the dispersion curves for higher refractive indices shift to lower frequencies whilst the lower band edge for PhC1 remains unchanged. Consequently, with increasing index the dispersion curve of PhC2 approaches the lower bandgap edge of PhC1. This further confirms that the relative position of the mode-gap within the PBG is an important factor when designing high-Q cavities. Even with a non-optimal refractive index it is still possible to engineer a favorable position by adjusting the waveguide width. This additional degree of freedom allows for the use of refractive indices larger than n=1.4. However, the Q-factor can be significantly reduced for narrow waveguides. As an example, we calculate the quality factors of a W1 and W0.7-based cavity with otherwise identical parameters (nholes=1.3, m=4). In this scenario, the quality factor decreases from Q = 7.6·105 for the W1-based cavity to Q = 5.2·104 for the W0.7-based cavity. Note that there is a large range of refractive indices, n=1.25-1.6, where the quality factors are of the order of 105. This coincides with the refractive indices of many liquids [45, 56], polymer materials [57], liquid crystals [40], and nanoporous silica [58].

The results for modal volumes of these resonances, expressed in (λ/n)3 with n=3.4, are also plotted in Fig. 4a. As the refractive index in the central holes increases the modal volume decreases from V = 2.11·(λ/n)3 to V = 1.17·(λ/n)3. This is expected behavior, as the resonant mode becomes more confined with the increased refractive index difference between the two PhC regions. The ratio Q/V, important for many applications, still has a maximum at n=1.4.

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Next we investigate the effect of the cavity length on the quality factor and modal volume. Filling more holes changes the cavity length, as illustrated in Fig. 2c. We calculate quality factors for cavity lengths L=ma+2R, where m=1, 2,…, 5. The results for a fixed refractive index of n=1.4 are shown in Fig. 4b. Up to m=4, increasing the cavity length increases the quality factor with the maximum value exceeding Q=6·105. The drop in the quality factor in Fig. 4b is due to the decrease of the in-plane component, which can be changed by increasing the size of the PhC slab in the waveguide direction. The modal volume does not change significantly with m, as the electric field is mainly concentrated in the central part of the cavity. The resonant frequencies are plotted in Fig. 4b, with the mode-gap edges indicated by horizontal dotted lines. As the refractive index is fixed, the mode-gap (ranging from =ω~ 0.2607 to =ω~ 0.2636) does not change with m. The resonant frequency for m=1 occurs just below the upper mode-gap edge. As m increases, the resonant frequency crosses over the mode-gap region nearly linearly, passing the mid mode-gap close to m=3. Consequently, for longer cavities there is a possibility of guiding more than one mode within the mode-gap, which we indeed observe in our experiments (see section 3, Figures 8-10).

In many applications, e.g. for biomedical nanosensing applications, it is important to have cavities with high Q’s and a large overlap of the electric field and the analyte [59]. In our geometry the analyte would reside within the holes. In Fig. 5 we plot the major electric field component, Ex, at the centre of the PhC slab for m=4 and nholes=1.25. The electric field is symmetric in y-direction and anti-symmetric in x- and z-directions. It is mainly concentrated in the high refractive index region. This is not surprising as the mode is close to the lower band gap edge, the so-called dielectric band, where the field is mainly localized within the dielectric [1].

Figure 5 Cross-sectional view of the major electric field component amplitude, Ex, of the resonant lower mode-gap mode. Circles indicate the holes. The infiltrated region (m=4 and nholes=1.25) is marked by the vertical lines.

In summary, optofluidic double-heterostructure cavities formed by air-hole infiltration enable ultrahigh quality factors Q of the order of 106. This is accomplished by operation in the mode-gap regime that relies on refractive index perturbation. A key advantage of our design is that it does not require changes in the PhC geometry with nanometer precision. The process of air-hole infiltration can be performed at any time after fabrication. If the holes are filled with liquid crystals or electro-optic/nonlinear polymers, there is the possibility for externally tuning these cavities.

3. Optofluidic photonic crystal components

In the following section, we introduce our micro-infiltration method to fill air pores of the PhCs. We explain the evanescent coupling technique used for the optical characterization of the fabricated optofluidic PhC components, and present experimental results of optofluidic double-heterostructure cavities.

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Infiltrating PhC air pores with typical diameters of less than 300 nm is an enormous challenge. At these small dimensions, interface forces such as surface tension and capillary forces are the dominating factors governing the infiltration of liquids into the holes. Hence, care has to be taken in the choice of combination of substrate material and liquid and their respective wetting properties. If the liquid does not wet the surface it will sit on top of the holes without infiltrating. If the liquid does wet the surface but has a low viscosity, it will flood the structure, and no controlled infiltration process will be achievable. It is possible to change and control the wetting properties of the surface and liquid to optimize the infiltration process, for example by applying an oxygen or nitrogen plasma treatment to the surface or adding a surfactant to the liquid [60]. Our micro-infiltration technique (Fig. 6) uses a tapered glass microtip with an apex diameter of ∅ ≈ 220 nm.

Figure 6 Schematic of our micro-infiltration process: a glass microtip is immersed into a liquid and is then drawn across a PhC structure to create a microfluidic optical component, in this example a microfluidic DH cavity.

The movement of the microtip during the infiltration is controlled by a high-precision translation stage with a positioning accuracy of ±20 nm. To begin the infiltration, the microtip is inserted within a meniscus of the infiltration liquid. Both polar liquids (such as water, acetone, etc.) and non-polar liquids (such as microscopy immersion oil or toluene) can be infiltrated into the structure, which offers a wide range of refractive indices and wetting properties. When the microtip is withdrawn from the meniscus, droplets remain attached along its length due to adhesive forces between the glass and the liquid. These droplets are then deposited onto the substrate in close proximity to the PhC structure of interest; this process is monitored with a microscope. Lastly, the microtip is used to draw a chosen droplet across the PhC area to create infiltrated regions where the liquid enters the holes by capillary action. We note that it is possible to fill single holes using a slightly modified technique, whereby the microtip is not drawn across the PhC but is brought in contact with the intended hole to infiltrate.

Evanescent coupling technique

Coupling to PhC optical devices – such as waveguides or microcavities – is a challenging task due to the very small mode field dimensions of these components. To facilitate an optical characterization of the fabricated microfluidic PhC components, we employ an evanescent coupling technique [61-67]. For the coupling we use a silica fiber that has a tapered region where its diameter has been reduced to less than 1.5 µm. Due to the reduced dimensions of the tapered fiber, the electromagnetic field of the propagating mode extends significantly beyond the boundary of the fiber, allowing its evanescent field to interact with the PhC structure. Coupling between the tapered fiber and the PhC modes can occur when phase matching is achieved [55, 61]. Examples of resulting transmission spectra are displayed in the experimental results section below. In our experimental setup (Fig. 7), light from a broadband source is launched into a single mode silica fiber connected to the tapered region. The transmission spectrum through the tapered fiber is recorded with an optical spectrum analyzer. The tapered fiber is aligned to the PhC structure using a nanopositioning setup and an imaging system.

Figure 7 Schematic of the evanescent coupling setup. A polarization controller and polarizer select TE-like light from the broadband light source. The tapered fiber couples light evanescently to the photonic crystal (PhC) sample and it is connected to an optical spectrum analyzer (OSA) for monitoring the transmission signal.

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In this section, we show that PhC microcavities can indeed experimentally be created by infusing a liquid into a uniform PhC waveguide region. For our experiments, we use suspended silicon membranes with a slab thickness of 220 nm, into which a triangular PhC lattice with period of a = 410 nm and hole diameter of 2R = 265 nm has been etched. The PhC structures are 34 periods (14 µm) wide and 61 periods (25 µm) long. The fabrication process of these PhCs is detailed in reference [68]. We investigated a W0.9 waveguide geometry, i.e. a W1 waveguide – formed by omitting a single row of holes in the Γ-K direction – where the PhC structure has been shifted inwards such that the waveguide width is only 90% of a regular W1. Fig. 8 displays measured transmission spectra of various PhC structures. It is important to note that the tapered fiber was not in contact with the PhC structure. Firstly, we took a reference transmission spectrum of the waveguide before infiltration (Fig. 8a). We then started with infiltrating a small DH cavity (originally 2 µm long) and incrementally increased the infiltrated region on the same PhC waveguide structure in steps of ~2 µm (see Fig. 8b). We imaged the infiltrated cavities with a microscope objective (150×, NA 0.9) using a color temperature conversion filter to improve resolution. The resulting images are displayed in the insets of Fig. 8. The reference spectrum of the uninfiltrated waveguide has a spectral feature at 1392 nm that is associated with coupling to the fundamental TE-like waveguide mode (Fig. 8a). The discrete features are attributed to Fresnel reflections at the open ends of the PhC waveguide. After the first infiltration (Fig. 8b), the spectral features appear at longer wavelengths. This is due to the increased effective refractive index of the guided modes caused by the presence of the fluid. The observed fringe spectra are attributed to Fabry-Pérot (FP) resonances sustained by the microfluidic cavity. As revealed by the envelope to the transmission dips of Fig. 8b, the coupling strength between the cavity resonances and the tapered fiber is at a maximum when the phase-matching is optimum [55, 61].

Figure 8 Normalized transmission through the tapered fiber when probing PhC waveguide structures. (a) Reference spectrum of the uninfiltrated waveguide; (b) Spectra for various infiltrated cavity lengths; (c) Spectrum of the reconfigured (cleaned) structure. Insets are 150× microscope images of the corresponding waveguide/cavities.

It can be seen that, as the cavity length increases from 2 µm to 17.5 µm, the fringe spacing, Δλ becomes smaller, which is consistent with an increased spectral density of modes for larger cavities. We note that the experimentally investigated cavity lengths are larger than the ones considered in the theory section above; hence we observe several modes in our experiments. Additionally, the fringe spacing within a particular spectrum becomes smaller at longer wavelengths for all investigated cavity lengths, which results from the dispersive nature of the PhC waveguide. In a final infiltration step, we completely filled the PhC region with liquid. In this case, the fringes associated with the FP resonances disappear as the mode-gap effect no longer exists. Now we only couple to the fluid-filled fundamental PhC waveguide mode, which displays a spectral signature similar to the uninfiltrated case of Fig. 8a but shifted to longer wavelengths. Complete reconfigurability of optofluidic circuits is highly desirable, as it enables the creation and tuning of optical functional elements from the same homogeneous PhC platform. One approach to achieve this is to remove the infiltrated liquid


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Quality factors Q of optofluidic DH cavities

Fig. 9 shows the trend of the intrinsic Q-factor1 measured for two different cavity lengths (5.3 µm and 16.8 µm) with increasing frequency. For both cavities, we plot their spectral signature (Figures 9a and 9b) and the corresponding Q-factors for each of the resonances evident in the spectra (Figures 9c and 9d). The Q-factors increase with decreasing frequency, and we note that this trend is representative for all investigated cavity lengths. This is expected behavior [69], because the guided modes at lower frequencies experience a higher effective refractive index and thus a better vertical confinement within the slab, reducing the out-of-plane losses. The intrinsic quality factors obtained in this set of experiments have values up to Qintrinsic = 3.5·104, but we note that the measurements were limited by the resolution of the optical spectrum analyzer (OSA) used in this particular experiment.

In order to gain an insight into the full potential of optofluidic DH cavities, we repeated our initial experiments using a high-resolution OSA (Ando AQ6317B). We investigated cavity lengths of 3.3 µm and 16 µm as typical representatives for a short and a long cavity. Fig. 10a shows the normalized transmission spectrum associated with a 3.3 µm long microfluidic DH cavity when measured with the high-resolution OSA. The resonances exhibit measured Q-factors ranging from Qmeasured = 19,300 for resonance (1) up to Qmeasured = 36,300 for resonance (4). This short 3.3 µm PhC cavity corresponds to a modal volume of only ~1.5·(λ/n)3 [53], which highlights the potential for generating high quality factors in very compact microfluidic devices. Fig. 10b shows the normalized transmission spectrum when probing a longer 16 µm cavity. The associated Q-factors are higher than for the short cavity, showing that the loss is dominated by the reflection losses at the interfaces between the infiltrated and uninfiltrated regions. For example, resonances (5) and (6) in Fig. 10b reveal measured Q-factors of Qmeasured = 45,740 and Qmeasured = 52,050, respectively. The derived intrinsic Q-factors for these two resonances are Qintrinsic = 50,430 and Qintrinsic = 57,080.

Figure 9 Normalized transmission spectra and intrinsic quality factor Qintrinsic as a function of normalized frequency. (a) and (b) represent spectra from 5.3 µm and 16.8 µm cavity lengths. Insets are close-ups of the resonances exhibiting the highest Q-factors. (c), (d) display the Q-factors associated with respective FP-resonances in the spectra from (a), (b).

In contrast with earlier demonstrations of liquid-infiltrated PhC cavities, where the Q-factor was usually degraded after the infiltration [3], the high Q-factors presented here demonstrate that the DH cavity can be applied as a highly sensitive microfluidic sensor. PhC cavity sensors typically exploit the resonance shift Δλ that occurs when the refractive index of the analyte in the PhC holes changes by a value Δn. The shift of the PhC bandstructure induced by the fluid infiltration (calculated by a plane wave expansion method) allows us to estimate a potential sensitivity of Δλ/Δn = 60 nm/RIU. The

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sensitivity is limited by the relatively small overlap of the electric field with the air holes - approximately 6% (estimated from first-order approximation perturbation theory [59]). Nevertheless, considering the full-width-half-maximum of the cavity resonance as the limit, a minimum refractive index resolution of δnanalyte = 4.5·10-4 could be achieved by exploiting the high-Q resonance (6) in Fig. 10b. This compares favorably with the values (δnanalyte = 2·10-3) demonstrated in previous work on passive PhC based sensors [70].

Figure 10 Normalized transmission spectra while probing a microfluidic DH cavity of (a) 3.3 µm length. Cavity mode (4) exhibits a measured Q-factor of 36,300. (b) 16 µm length. The measured Q-factors for resonances (5) and (6) are Qmeasured = 45,740 and Qmeasured = 52,050, respectively. The inset shows a close-up view of resonance (6).

Single hole infiltration

In this section we present preliminary results pertaining to the infiltration of fluid within an individual PhC pore, as shown in Fig. 11. Previous sections discussed microfluidic DH cavities, defined by a fluid-filled region. Here, the infiltration of a single pore opens up the possibility to arbitrarily define sophisticated geometries and systems using this optofluidic scheme. The precision of the infiltration process is mainly limited by the amount of fluid residing on the microtip. As such, the prerequisite for realizing fluid penetration for individual holes is the ability to precisely control the fluid volume. We can clearly observe that it is indeed possible to create single-hole point defects and line defects with varying lengths, constituting the basic building blocks for optofluidic circuits.

Figure 11 Microscope image of a PhC waveguide with infiltrated point defects and line defects of varying length.

4. Conclusions and outlook

Merging microfluidics with photonics is a promising route to tune and reconfigure photonic components. Planar PhC double-heterostructure cavities possess the unique advantage of confining light at the exact location where the liquid that defines the cavity is introduced. Increasing the light-liquid interaction within these microcavities will be crucial for creating dynamic and tunable functions as well as sensitive detectors. By adding an active material – such as colloidal quantum dots – into the infiltration liquid, the optofluidic cavities could be exploited to generate reconfigurable light sources. Additionally, nonlinear processes of the infiltrated liquid within the double-heterostructure cavities will be significantly enhanced by the resonant electric fields. The presented microfluidic cavity is just one basic building block that will provide the starting point to realize more complex functions, all of which could be integrated within a reconfigurable photonic circuit. In the future, we intent to realize more complex functionalities, for instance combining several cavities to realize coupled resonator systems - a promising contender for controlling the speed of light [35].

2x point defect

2 holes line defect

5 holes line defect

(a) (b)


5. Acknowledgements

The support of the Australian Research Council through its Federation Fellow, Centre of Excellence and Discovery Grant programs is gratefully acknowledged. Additional acknowledgement is given to the support of the School of Physics, University of Sydney, through its Denison Foundation and the International Science Linkages program through the ISL DEST grant. The silicon samples were fabricated in the framework of the EU-FP6 funded ePIXnet Nanostructuring Platform for Photonic Integration (www.nanophotonics.eu).

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