Integrated Photonic Nanofences: Combining Subwavelength

Integrated Photonic Nanofences: Combining Subwavelength Waveguides with an Enhanced Evanescent Field for Sensing Applications Victor J. Cadarso,*,† Andreu Llobera,‡ Mar Puyol,§ and Helmut Schift† †

Laboratory for Micro- and Nanotechnology, Paul Scherrer Institut (PSI), 5232 Villigen PSI, Switzerland Chemical Transducers Group, Institut de Microelectrònica de Barcelona (IMB-CNM, CSIC), 08193 Bellaterra, Spain § Sensors & Biosensors Group, Department of Chemistry, Autonomous University of Barcelona, Edifici Cn, 08193 Bellaterra, Spain ‡

ABSTRACT: Photonic nanofences consisting of high aspect ratio polymeric optical subwavelength waveguides have been developed for their application into photonic sensing devices. They are up to millimeter long arrays of 250 nm wide and 6 μm high ridges produced by an advanced lithography process on a silicon substrate enabling their straightforward integration into complex photonic circuits. Both simulations and experimental results show that the overlap of the evanescent fields propagating from each photonic nanofence allows for the formation of an effective waveguide that confines the overall evanescent field within its limits. This permits a high interaction with the surrounding medium which can be larger than 90% of the total guided light intensity (approximately 20000 times larger than the evanescent field of a standard waveguide with equivalent dimensions). In this work, we not only investigate the photonic properties of these structures but also demonstrate their successful integration into a photonic sensor. An absorbance-based sensor for the determination of lead in water samples is therefore achieved by the combination of the photonic nanofences with an ion-sensitive optical membrane. The experimental results for lead detection in water show a sensitivity of 0.102 AU/decade, and a linear range between 10−6 M and 10−2 M Pb(II). A detection limit as low as 7.3 nM has been calculated according to IUPAC for a signal-to-noise ratio of 3. KEYWORDS: evanescent field, integrated waveguides, photonic nanowires, subwavelength optics, light guiding, sensing applications

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different photonic sensors here cited, those exploiting the interaction of the analyte with an evanescent field (EF)8 have so far achieved the highest sensitivity and sophistication.9−12 However, the inherently weak interaction between the EF and the analyte requires large interaction lengths, which demands the use of complex technologies to achieve devices with homogeneous thicknesses over long distances restricting the fabrication of compact, cost-efficient photonic sensors. Photonic nanostructures are envisioned to be the next generation high-sensitivity sensors due to their small footprint and the strong light−analyte interaction that can be achieved in comparison to conventional photonic structures. In this context, the formation of electric field hot spots in nanoplasmonics13,14 or in slot waveguides15,16 is the most studied concept. However, even though very high sensitivities have been reported for nanoplasmonic structures, the use of external optical tools is still required. In slot-waveguides, the hot spot is

he development of compact, cost-efficient, and highly sensitive sensing devices is still a major research field despite the advances achieved during the last decades.1 It has been shown before that photonic sensing approaches provide extremely high sensitivities and low limits of detection (LoD).2 The most common approaches, however, use photonic and microfluidic devices in combination with bulky external tools such as confocal microscopes. This strategy, though valid from a research point of view, does not allow for miniaturization into portable devices for sensing applications in more complex environments. Also, this does not allow for the sensing setup mass production at low cost. Numerous devices and strategies have been devised to overcome such drawbacks,3 such as the use of optical sensors based on biodoped sol−gel glasses for the detection of chemical substances,4 integrated silicon microflow cytometers for cell counting,5 absorbance photonic sensors with integrated microfluidic networks,6 or Mach−Zehnder interferometers (MZI) for label-free biochemical sensing.7 It is clear that the enhancement of the light−analyte interaction has been the main goal in the development of all the aforementioned devices, and among the © XXXX American Chemical Society

Received: September 17, 2015 Accepted: November 30, 2015

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DOI: 10.1021/acsnano.5b05864 ACS Nano XXXX, XXX, XXX−XXX

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Figure 1. (a) Schematic representation of a single PNF connected to input and output waveguides. (b) Similar configuration but containing an array of PNFs. Schematic cross sections of the (c) single PNF and (d) the array, on top of an antiresonant reflective substrate and the numerically simulated light power distribution of the EF for the TE00 mode. Identical cross sections showing the simulated (e) TE01 mode for the single PNF and (f) the TE11 mode for the array. Schemas are at scale considering 200 nm wide nanowires.

achieved by splitting the waveguide into two17 or more cores18 by means of a subwavelength gap. This approach has been used successfully in highly sensitive devices16 even though it demands the presence of the analyte in the subwavelength gap, thus reducing the range of applications in which slotwaveguides can be used, and increasing the integration complexity, especially when used with microfluidic components. Alternatively, the use of integrated waveguides with small cross-sectional dimensions has been proposed as photonic nanosensor elements.19 Though the achieved EF (around 10% of the total field) is still relatively small with this approach, the geometrical cross section can be further decreased below the working wavelength to define photonic nanowires.20 This is highly advantageous for optical sensing, since in this configuration the EF can be up to 80% of the total field coupled to the nanowire.21 Nonetheless, until now, the standard photonic nanowire sensor consists of a single nanoribbon or nanofiber that has been individually fabricated and usually individually manipulated and positioned.21 Clearly, not only the reproducibility and production capabilities are limited with this fabrication approach but also the sensor’s integration potential within a complex system is drastically reduced. In this work, we alternatively present an integrated photonic nanosensor with which a very large light−environment interaction can be achieved. In here, photonic nanofences (PNFs) that have been fabricated directly on a substrate have been developed as the basic elements of this highly robust system. The PNFs are defined as narrow nanoridges with rectangular cross sections and have one lateral dimension smaller than the working wavelength (λ) and high aspect ratios

(AR, from 3:1 up to 40:1) and are fabricated as a single nanostructure or in an array configuration (two or more PNFs with equal distances). We demonstrate with simulations and by experimental results that such structures allow the generation and confinement of a large EF within the PNF system. As a proof of concept, this capability has been exploited for the straightforward development of an absorbance sensor for lead ion detection in water. The sensor consists in the simple integration of PNFs with an optode containing a lead (Pb(II)) sensitive ionophore. The PNFs are fabricated by advanced nanolithography methods directly on a substrate. Hence, it is not necessary to individually manipulate every single nanostructure to interconnect them with other components. This enables parallel mass production, e.g. by nanoimprint lithography,22−24 and the possibility to develop integrated devices based on PNFs.

RESULTS AND DISCUSSION PNF concept and simulation. A single PNF (Figure 1a) and an array (Figure 1b) of PNFs directly connected to input and output waveguides with square cross sections have been simulated. The schemas represent typical PNFs with a 200 nm width. The coupling waveguides are 10 μm wide. In this configuration, the light confined at the input waveguide is directly coupled with the PNFs by direct mode matching between the input waveguide modes and the PNFs modes. Since the effective index of both the single and the PNF arrays are smaller than the substrate, an antiresonant reflective multilayer25 is included to minimize the light transfer to the substrate. The cross section of the single and the PNF array is schematically shown in Figures 1c and 1d along with the numerical simulation of the light power distribution (modulus B


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ACS Nano of the Poynting vector) corresponding to the fundamental transverse electric (TE) mode. This simulation has been done by defining a refractive index (n) of the PNF material to nPNF = 1.555, air as the surrounding media (nm = 1.0), and a working wavelength of 635 nm. In this simulation only TE polarized modes have been considered, since the antiresonant reflecting multilayer used in the substrate (2 layers: bottom SiO2, n = 1.465, 240 nm and top Si3N4, n = 2.001, 100 nm) will induce higher losses in the transverse magnetic (TM) modes.26 In case of the single PNF, light is coupled to the solid core of the structure that exhibits simultaneously an EF equivalent to 46% of the total light intensity coupled to the TE00 mode. In the PNF array, light is not coupled individually to each PNF, but rather forms a mode obtained by the collective contribution resulting from each individual PNF and modulated by the modal profiles (TE00 mode in Figure 1d) of an effective waveguide formed by the PNF array. Furthermore, a large percentage of light (47% in this concrete case for the TE00) is not confined inside the PNFs but it is guided within the medium surrounding them. Thus, it is possible to consider that the PNF array exhibits an EF spatially distributed within the limits of the effective waveguide. This effect is due to the superposition of the EF of each PNF that individually behaves as a subwavelength structure. It should be noted that this effect is not limited to the fundamental mode of the effective waveguide formed by the PNF array. For the selected working wavelength, λ of 635 nm, and with the considered materials’ refractive index n, the PNFs are single mode in the horizontal (subwavelength) direction for widths below 410 nm.27 In the vertical direction, the PNFs have higher order modes as can be seen in Figure 1e for the mode TE11. Such multimode behavior can be filtered out by the antiresonant structure underneath the PNF, being then possible to achieve single mode behavior for a thickness of a few microns.25 Conversely, for the effective waveguide formed by the PNF array, additional lateral modes can be found, as shown in Figure 1f for the TE11 mode. They are due to the waveguide extension in the lateral direction and cannot be easily filtered out as it does in the vertical direction by an antiresonant structure. Thus, it can be concluded that single PNFs exhibit single mode behavior, whereas uniformly arrayed PNFs are multimode. By including this multimode behavior in the proposed PNF array configurations, the light interacting with the environment can reach values higher than 90% of the total light intensity coupled with the effective waveguide. As it will be discussed below, this value is several orders of magnitude larger than the corresponding one for a solid waveguide of equivalent dimensions (above 20000 times in the presented example). In Figures 2a−d, the simulated light power distribution on the horizontal direction for different numbers of PNFs with different widths (100 nm, 200 and 500 nm) is presented. As it can be seen for the fundamental mode (TE00) in Figure 2a, due to their subwavelength dimensions, single PNFs have an enhanced evanescent field, the magnitude of which increases when the PNF width is reduced. Such enhanced evanescent fields are intrinsic to subwavelength photonic structures.27 When several PNFs are placed next to each other in an array configuration, their individual EFs overlap. As a result, an effective waveguide is formed, where light remains confined within the limits of the array, even though most of it is propagating as an EF through the medium surrounding the PNFs. This apparently contradictory behavior is better understood with the help of Figure 2b for 10 PNFs uniformly

Figure 2. Numerically simulated light power cross-section distribution (modulus of Poynting vector) of the TE00 mode for (a) 1, (b) 10, and (c) 20 PNF arrays and (d) the TE11 mode for a 20 PNF array. The PNFs have widths of 100, 200, and 500 nm. (e) Effective neff and percentage of the light intensity coupled to the PNFs evanescent fields at the TE00 mode versus the photonic nanofences width for a single PNF and an array of 20 PNFs.

distributed along 10 μm. Even though the overlap between the evanescent fields from each individual PNF is minimal, the overall light distribution clearly follows the TE00 mode of the effective waveguide. As the number of PNFs increases, this effect is even more noticeable, as can be seen for the case of 20 PNF, i.e. the TE00 mode (Figure 2c) and the TE10 (Figure 2d). In this last example, the 500 nm-width PNFs are sufficiently wide so that they reach each other. For comparison purposes a standard full solid waveguide has also been presented. From these results it can be confirmed that light can be guided by a single PNF as a subwavelength waveguide structure or by an array formed by multiple PNFs. Their geometry and distribution directly affect the coupling efficiency and the percentage of light intensity in the EF. In order to quantify both, the confinement efficiency of a single PNF and an array of such elements (i.e., containing 20 distributed on 10 μm), the effective refractive index (nef f) and the percentage of EF are presented in Figure 2e versus the width of the PNFs for the TE00 mode. As it can be seen, both variables need to be optimized in order to ensure a certain coupling degree within the PNFs and still obtain a high EF (finally achieving the sought strong light-sensing element interaction). In this work we have decided to develop PNFs with widths varying between 200 and 250 nm to be in an intermediate range for both C


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ACS Nano effective refractive index neff and light intensity coupled to the PNFs evanescent fields. Nonetheless, it is clear that other parameters could be selected depending on whether light confinement or high degree of interaction are the most important for a particular application. Fabrication of the 2D polymeric nanofences. The photonic structures were fabricated by a combination of two lithography techniques, direct laser writing (DLW)28,29 for the PNFs and proximity photolithography for the micrometer-sized input/output waveguides. The latter enables the easy design of extended input/output structures in integrated devices. This approach allows for the direct integration of complex optical components and devices based on PNFs. Furthermore, this technology will make possible the fabrication of the devices using a myriad of different polymeric materials as long as they are photopatternable. Consequently, the use of optically optimal materials, biocompatibility, or even doping of such is possible with this approach. Figure 3a shows a SEM picture of

Figure 4. (a) Optical picture of a chip containing the nanofences, the input and output waveguides, and the optical fiber coupling system. (b) Microscope top view image of the PNFs region for different number of nanofences; the light is coupled from the left side and propagates to the right.

the expected effective waveguide and thus validating the results presented in the previous section. The intensity decay as the light propagates along the PNFs has also been quantitatively characterized for 250-nm-wide PNFs, distributed uniformly along a width of 10 μm. This corresponds to the width of the input and output waveguides in arrays with filling factors between 0 (no PNFs) and 50% (20 PNFs) and for lengths (LPNF) ranging from 50 to 1000 μm. Although the losses for a single PNF can be considered rather large (55.7 ± 0.8 dB/mm), they are comparable to other similar subwavelength structures such as semiconductor nanowires.30 The propagation losses can be drastically reduced to values of only 12 ± 2 dB/mm for arrays with 20 PNFs. The measured coupling losses from the standard waveguides to the PNFs prove that both optical structures have been effectively combined, allowing high coupling efficiency with only (2.67 ± 0.01) dB of coupling losses for the single PNF configuration and reducing these losses to (1.39 ± 0.03) dB for arrays containing 20 PNFs. Having demonstrated light guiding by the use of PNFs with relatively low losses, the next step was to acquire their modal profile. Near field images obtained at the PNF’s array cleaved cross section for arrays with different number of PNFs are shown in Figure 5. Dotted squares flag the position of each individual PNF. As expected, a large percentage of the light, which is coupled into the effective waveguide, is actually not confined inside the solid PNFs and propagates in the form of an EF. It is also clear that the effective waveguides composed by the PNF arrays are multimode, and for this reason, the light distribution does not correspond to the TE00 mode presented in Figure 2. Concretely, for a single PNF (Figure 5a) 80% ± 3% of the total light intensity is confined within the EF. This value is comparable to that observed in a single nanowire of similar characteristics.27 This fact proves that, although only one of the lateral dimensions of the proposed structures is subwavelength, the behavior is the same as that observed in nanowires. The case of 2 PNFs (Figure 5b) may appear similar to the one observed in slot waveguides.15,31 However, in contrast to a slot waveguide with its submicrometer slot, a PNF array is composed from elements with inverse dimensions (i.e., the subwavelength structure is the core of the waveguides), and the separation between them is a large gap rather than a slot. In this concrete case, the gap is 5 μm, which is much larger than the wavelength. Even with this large separation, the EF of the PNFs

Figure 3. SEM photographs of (a) a 5 nanofences array in an optically transparent polymer and (b) the detailed image of the PNFs with a width of ∼250 nm and a height of 10 μm (AR of 40).

fabricated PNF arrays already connected to the input and output waveguides. As can be seen in Figure 3b, the width of the obtained nanofences was 250 nm and it was possible to obtain structures with a height of up to 10 μm, resulting in ARs of up to 40:1. The interface between the input/output waveguides and the PNFs shows smooth tapered transitions in the lateral direction that effectively serve to reduce mechanical stress, thus making these structures more robust and stable even at such high ARs. However, in order to further increase their robustness the final PNFs were fabricated with heights of 6 μm, resulting in an AR of 24:1. Optical characterization. Light was directly coupled to the input waveguide, guided through the PNFs, and then coupled into the output waveguide, as shown in Figure 4a. Figure 4b shows top-view images of light coupled into the PNFs. For a single PNF (N = 1), although light is coupled into the subwavelength structure, the light intensity decreasing with the propagation distance can be observed. This phenomenon is also observed when the light is coupled into two PNFs (N = 2), but in this case, the intensity decline is not so evident. The addition of more PNFs further reduces the losses, as can be seen for N = 5 and N = 10. Additionally, it is also clear that in all cases the light remains guided in the PNFs region, forming D


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light propagating out of the solid sections, obtaining simultaneously low propagation losses, high coupling efficiency and extremely high interaction between the light coupled in the PNFs and the environment. Characterization as an absorbance sensor. In order to provide a proof of concept of the sensing capabilities of the proposed PNFs due to their high interaction with the surrounding area, an ion-selective optode was used to embed a 500 μm long, 7 PNF array waveguide. This array was selected since it provides a good compromise between total intrinsic losses (14 mm). These results demonstrate not only the sensing capabilities of the proposed PNFs but also their capability to be straightforwardly integrated into complex devices. E


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isopropanol. At this point, a single drop of IP-Dip (Nanoscribe GmbH, Germany) photoresist was directly casted onto the PNF array region. The PNFs were then fabricated using DLW, by means of a nonlinear two-photon absorption process using a 780 nm wavelength laser (Photonic Professional GT, Nanoscribe GmbH). After writing and cross-linking the PNFs, a second development step was carried out in PGMEA and the samples were rinsed and stored in isopropanol. In order to prevent the collapse of the high AR structures during drying of the isopropanol, the samples were dried in a critical point dryer chamber (K850WM Emitech, Quorum Technologies Ltd., UK). Once dried, the input and output waveguides were cleaved using a diamond pen in order to obtain optical quality facets. Optical losses characterization. Light was coupled into the input waveguides using single mode glass fiber optics with a core diameter of 4 μm. This fiber was directly connected to a Fabry−Perot Laser Source (S1FC635, Thorlabs Inc., US) with a working wavelength of 635 nm. The cleaved end of the fiber was aligned to the input waveguides by means of a X-Y-Z stepper motor micropositioner and piezo system (MAX341, Thorlabs Inc.). The light arising from the output waveguide was directly collected by a cleaved multimode fiber with a diameter of 50 μm connected to a photodiode power sensor (SC150C, Thorlabs Inc.). This fiber was aligned to the output waveguide by means of an X-Y-Z stepper motor (MAX343, Thorlabs Inc.). The alignment process and PNFs were observed using a CCD camera (DCU223, Thorlabs Inc.) connected to a long-working distance microscope. Evanescent field measurement. Light was coupled into the input waveguides as described above. For this characterization the structures were directly cleaved at the end of the PNFs before reaching the output waveguide. The fiber previously used to collect the light was substituted by a long-working distance microscope objective (Mitutoyo 10x), acquiring the near field images at the end of the PNFs. These images were recorded using a CCD camera (DCU223, Thorlabs Inc.) and analyzed using the software I+Solex 2.15 (Solex, Spain) in order to calculate the total amount of light coupled into the PNFs and the corresponding EF. This was done by integrating the intensity over the area of the CCD images with and without the area corresponding to the PNFs and comparing both values. Optode and absorbance sensor reagents. All reagents were of analytical grade. Lead standard solutions were prepared by direct weighting and dilution of a lead(II) nitrate salt (Pb(NO3)2) stock (Merck, Germany) into a buffer solution, which consisted on 0.01 M magnesium acetate (CH3COOMg) prepared in doubly distilled water and adjusted to pH 4 with nitric acid (HNO3). The optode was prepared using poly(vinyl chloride) (PVC high molecular weight) as the polymer matrix, tris(2-ethylhexyl)phosphate (TOPh) as the plasticizer and tetrahydrofuran (THF) as the solvent. Additionally potassium tetrakis (4-clorophenyl)borate (KTpClPB) was used as the lipophilic anionic additive and tert-butylcalix[4]arene-tetrakis(N,Ndimethylthioacetamide) (Lead Ionophore (IV), Sigma-Aldrich) was used as the lead ionophore. The used acidochromic dye, synthesized in the Sensors & Biosensors group, was ketocyanine 5ce.35 Optode preparation. The optode consisted of 62.8 wt % TOPh, 28.3 wt % PVC, 7 wt % lead ionophore, 1 wt % KTpClPB and 0.6 wt % ketocyanine 5ce, which were weighed and dissolved in 1.5 mL of THF.36 Prior to deposition in the PNF array region, the optode was further diluted in THF with a ratio 1:2.5 (optode dilution v: v THF) in order to reduce the viscosity. Then 50 μL of the membrane solution were deposited over the PNFs by direct casting using a micropipette, after 5 min drying a second deposition of 50 μL was performed to fully cover the PNF array region. After complete evaporation of the THF the sensors were ready to be used. Pb(II) sensor characterization. Light was coupled to the sensor as described above. In this case a fiber-coupled LED (M780F2, Thorlabs Inc.) with the emission peak at 780 nm was used as a light source. Detection was done using a spectrometer (CSS200, Thorlabs Inc.) to record the light intensity at 770 nm. Initially the optode was conditioned by adding 20 μL of the buffer solution onto the membrane to protonate the dye and achieve the desired initial conditions of maximal absorbance. After that, increasing concen-

CONCLUSIONS Both simulations and experiments demonstrate the unique behavior of PNFs. An effective waveguide behavior has been demonstrated, when multiple PNFs are combined. This allows for a large percentage of light to interact with the environment in the form of an EF confined within the effective waveguide. The individual PNFs have single mode behavior in the direction perpendicular to their elongated side (i.e., in the direction in which the subwavelength structures are defined horizontal in this work), exhibiting an EF containing 80% of the total amount of coupled light. If PNFs are arranged in an array with distances up to several micrometers (distances up to 5 μm are demonstrated in this work), their EFs overlap and couple with the neighboring PNFs, forming an effective waveguide with an enhanced EF that is confined between the limits of the array but is still accessible and interacting with the environment. Effective optical waveguides with micrometric sizes (10 × 6 μm) and filling factors of only 25% have been achieved in this work. This, in turn, enables a high degree of interaction between the light coupled to the PNFs and the medium (up to above 90%). In contrast, the use of a standard optical waveguide with the same dimensions will exhibit an evanescent field of only 0.004%. Hence, the use of the proposed photonic nanofences allows us to generate an EF 20000 times larger than in the standard waveguide, with the corresponding increase of their sensitivity. This effect has been effectively applied in this work to develop a proof of concept absorbance sensor for lead(II) detection in water. The sensor consisting of a PNF array embedded on an optical membrane has been characterized and validates the proposed approach, which can be furthermore applied for a myriad of sensing applications. MATERIALS AND METHODS Optical simulations. Numerical simulations were carried out using the commercial software from PhotonDesign (modules FIMMWAVE and FIMMPROP) using the Film Mode Matching (FMM) solver33,34 to determine the eigenmodes of the individual PNF and the PNF arrays. This allows calculating the light distribution for the different modes in both single and array configurations of PNFs. Furthermore, it is also possible to simulate the light distribution and the EF in single and multiple PNFs structures after coupling the light from a micrometer-size waveguide. In order to do so, multimode behavior was considered including more than 300 high resolution modes in each simulation. The power distribution of the PNF arrays is integrated along one cross-sectional line with and without the light inside the solid PNFs. Both values are compared to obtain the percentage of light corresponding to the EF. Materials and fabrication processes. The antireflecting multilayer consisted on a 100 nm-thick silicon nitride (Si3N4, nSi3N4 = 2) layer deposited by plasma enhanced chemical vapor deposition (PECVD) on top of 240 nm-thick silicon oxide (SiO2, nSiO2 = 1.465) layer grown on a silicon wafer by thermal wet oxidization. Two different negative-tone polymeric materials were used for the optical structures: (i) Ormocomp with niow = 1.55 for the input and output waveguides and (ii) IP-Dip optimized to achieve high resolution in 2photon polymerization DLW and transparent in the working and sensing wavelengths (@635 nm and @770 nm) nPNF = 1.555 for the PNFs. For the microsized waveguides fabrication, the surface was activated using an O2 plasma process at 500 W for 7 min and immediately after a layer of Ormocomp (micro resist technologies GmbH, Germany) was spin-coated. The Ormocomp was exposed to UV light (SUSS MA 6 mask aligner, i-line) with a dose of 1200 mJ/ cm2 in proximity mode using a chromium mask to fabricate the input and output standard microsized waveguides. Afterward the non-crosslinked Ormocomp was removed by development in propylene glycol methyl ether acetate (PGMEA) and the wafer was rinsed in F


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AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors thank R. Kirchner and C. Donnelly (PSI) for assistance with the two-photon lithography process, J. Brugger (EPFL) for assistance with numerical simulations software, S. Stutz (PSI) for assistance with the experimental setup, and C. David (PSI) for his advice and help in preparing the manuscript. This work is partially funded by the Swiss National Science Foundation (SNF) Ambizione project (n° PZ00P2_142511) granted to V.J.C. and the European Research Council under the European Community’s seventh Framework Program (FP7/2007-2013)/ERC grant agreement no. 209243. ABBREVIATIONS LoD, limit of detection; MZI, Mach−Zehnder interferometer; EF, evanescent field; PNF, photonic nanofence; AR, aspect ratio; TE, transverse electric; TM, transverse magnetic; nef f, effective refractive index REFERENCES (1) El-Ali, J.; Sorger, P. K.; Jensen, K. F. Cells on Chips. Nature 2006, 442, 403−411. (2) Lechuga, L. M. In New frontiers in optical biosensing; European Conference on Integrated Optics, Copenhagen, Denmark, Copenhagen, Denmark, 2007; p FPT2. (3) Kuswandi, B.; Nuriman; Huskens, J.; Verboom, W. Optical Sensing Systems for Microfluidic Devices: A Review. Anal. Chim. Acta 2007, 601, 141−155. (4) Llobera, A.; Cadarso, V. J.; Darder, M.; Domínguez, C.; Fernández-Sánchez, C. Full-Field Photonic Biosensors Based on Tunable Bio-Doped Sol-Gel Glasses. Lab Chip 2008, 8, 1185−1190. (5) Bernini, R.; De Nuccio, E.; Brescia, F.; Minardo, A.; Zeni, L.; Sarro, P. M.; Palumbo, R.; Scarfi, M. R. Development and Characterization of an Integrated Silicon Micro Flow Cytometer. Anal. Bioanal. Chem. 2006, 386, 1267−1272. (6) Llobera, A.; Demming, S.; Wilke, R.; Büttgenbach, S. Multiple Internal Reflection Poly(Dimethylsiloxane) Systems for Optical Sensing. Lab Chip 2007, 7, 1560−1566. (7) Blanco, F. J.; Agirregabiria, M.; Berganzo, J.; Mayora, K.; Elizalde, J.; Calle, A.; Dominguez, C.; Lechuga, L. M. Microfluidic-Optical Integrated CMOS Compatible Devices for Label-Free Biochemical Sensing. J. Micromech. Microeng. 2006, 16, 1006−1016. (8) Lambeck, P. V. Integrated Optical Sensors for the Chemical Domain. Meas. Sci. Technol. 2006, 17, R93−R116. (9) Lapsley, M. I.; Chiang, I. K.; Zheng, Y. B.; Ding, X.; Mao, X.; Huang, T. J. A Single-Layer, Planar, Optofluidic Mach-Zehnder Interferometer for Label-Free Detection. Lab Chip 2011, 11, 1795− 1800. (10) Grist, S. M.; Schmidt, S. A.; Flueckiger, J.; Donzella, V.; Shi, W.; Talebi Fard, S.; Kirk, J. T.; Ratner, D. M.; Cheung, K. C.; Chrostowski, L. Silicon Photonic Micro-Disk Resonators for Label-Free Biosensing. Opt. Express 2013, 21, 7994−8006. (11) Mayer, K. M.; Hafner, J. H. Localized Surface Plasmon Resonance Sensors. Chem. Rev. 2011, 111, 3828−3857. (12) Biagioni, P.; Huang, J. S.; Hecht, B. Nanoantennas for Visible and Infrared Radiation. Rep. Prog. Phys. 2012, 75, 02440210.1088/ 0034-4885/75/2/024402 G


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