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Virtual Journal for Biomedical Optics

Virtual Journal for Biomedical Optics

| EXPLORING THE INTERFACE OF LIGHT AND BIOMEDICINE

  • Editors: Andrew Dunn and Anthony Durkin
  • Vol. 8, Iss. 4 — May. 22, 2013
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Plasmonic interferometers for label-free multiplexed sensing

Yongkang Gao, Zheming Xin, Qiaoqiang Gan, Xuanhong Cheng, and Filbert J. Bartoli  »View Author Affiliations


Optics Express, Vol. 21, Issue 5, pp. 5859-5871 (2013)
http://dx.doi.org/10.1364/OE.21.005859


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Abstract

We report a plasmonic interferometric biosensor based on a simple slit-groove metallic nanostructure that monitors the phase changes of surface plasmon polaritons resulting from biomolecular adsorptions. The proposed sensing scheme integrates the strengths of miniaturized plasmonic architectures with sensitive optical interferometry techniques. Sensing peak linewidths as narrow as 7 nm and refractive index resolutions of 1 × 10−5 RIU were experimentally measured from a miniaturized sensing area of 10 × 30 µm2 using a collinear transmission setup and a low-cost compact spectrometer. A high-density array of such interferometric sensors was also fabricated to demonstrate its potential for real-time multiplexed sensing using a CCD camera for intensity interrogation. A self-referencing method was introduced to increase the sensitivity and reduce sensor noise for multiplexing measurements. The enhanced sensing performance, small sensor footprint, and simple instrumentation and optical alignment suggest promise to integrate this platform into low-cost label-free biosensing devices with high multiplexing capabilities.

© 2013 OSA

1. Introduction

Following Ebbesen’s report on extraordinary optical transmission (EOT) in 1998, metallic nanostructures have been the focus of intense research due to their unique optical properties [1

1. C. Genet and T. W. Ebbesen, “Light in tiny holes,” Nature 445(7123), 39–46 (2007). [CrossRef] [PubMed]

3

3. S. A. Maier, Plasmonics: Fundamental and Applications (Springer, 2007).

]. Surface plasmon polaritons (SPPs) are electromagnetic waves coupled to coherent charge oscillations at a metal-dielectric interface, and the excitation of SPPs can generate large field enhancements within nanoscale volumes [2

2. J. A. Schuller, E. S. Barnard, W. Cai, Y. C. Jun, J. S. White, and M. L. Brongersma, “Plasmonics for extreme light concentration and manipulation,” Nat. Mater. 9(3), 193–204 (2010). [CrossRef] [PubMed]

]. The unprecedented light concentration and resulting strong light-matter interactions in plasmonic nanostructures open up many opportunities in chemical and biological sensing, fluorescence, Raman scattering, lithography, and photovoltaics [3

3. S. A. Maier, Plasmonics: Fundamental and Applications (Springer, 2007).

]. Among these applications, plasmonic sensing has been the subject of particularly intense study. Indeed, commercial surface plasmon resonance (SPR) sensors have become the gold standard for label-free biomolecular sensing, and have found utility in wide applications ranging from biomedical diagnostics and immunoassays, to environmental sensing and food safety monitoring [4

4. J. Homola, “Surface plasmon resonance sensors for detection of chemical and biological species,” Chem. Rev. 108(2), 462–493 (2008). [CrossRef] [PubMed]

].

Most commercial SPR systems rely on the prism-based Kretschmann configuration to couple light into SPPs propagating on a flat metal film. While this approach results in relatively high sensitivities, it is not conducive to system miniaturization and low-cost production, because of its optical complexity and bulky instrumentation [5

5. A. De Leebeeck, L. K. S. Kumar, V. de Lange, D. Sinton, R. Gordon, and A. G. Brolo, “On-chip surface-based detection with nanohole arrays,” Anal. Chem. 79(11), 4094–4100 (2007). [CrossRef] [PubMed]

,6

6. A. Cattoni, P. Ghenuche, A. M. Haghiri-Gosnet, D. Decanini, J. Chen, J. L. Pelouard, and S. Collin, “λ³/1000 Plasmonic Nanocavities for Biosensing Fabricated by Soft UV Nanoimprint Lithography,” Nano Lett. 11(9), 3557–3563 (2011). [CrossRef] [PubMed]

]. It is also a significant challenge to extend the sensing capability of current SPR systems to high-throughput multiplexed assays. SPR imaging (SPRi) is the most commonly employed approach to address this need, typically employing a CCD camera to monitor the intensity distribution of light reflected from an SPR surface containing multiple sensing spots [7

7. G. Spoto and M. Minunni, “Surface plasmon resonance imaging: what next?” J. Phys. Chem. Lett. 3(18), 2682–2691 (2012). [CrossRef]

]. However, the Kretschmann configuration employed in SPRi results in a tilted image plane, leading to image defocusing and optical aberrations [8

8. N. C. Lindquist, A. Lesuffleur, H. Im, and S. H. Oh, “Sub-micron resolution surface plasmon resonance imaging enabled by nanohole arrays with surrounding Bragg mirrors for enhanced sensitivity and isolation,” Lab Chip 9(3), 382–387 (2009). [CrossRef] [PubMed]

]. The prism-based coupling scheme also prohibits the use of high numerical aperture (NA) imaging systems to increase spatial resolution [9

9. K. A. Tetz, L. Pang, and Y. Fainman, “High-resolution surface plasmon resonance sensor based on linewidth-optimized nanohole array transmittance,” Opt. Lett. 31(10), 1528–1530 (2006). [CrossRef] [PubMed]

,10

10. J. C. Yang, J. Ji, J. M. Hogle, and D. N. Larson, “Multiplexed plasmonic sensing based on small-dimension nanohole arrays and intensity interrogation,” Biosens. Bioelectron. 24(8), 2334–2338 (2009). [CrossRef] [PubMed]

]. The relatively large sensing spot size of commercial SPR imagers (with a typical diameter of 200 µm) [11

11. C. T. Campbell and G. Kim, “SPR microscopy and its applications to high-throughput analyses of biomolecular binding events and their kinetics,” Biomaterials 28(15), 2380–2392 (2007). [CrossRef] [PubMed]

] limits their effectiveness for probing nanovolumes and single cells and for high-density microarray applications.

In the present work, we propose a new plasmonic interferometric sensing platform, operating in a simple collinear transmission configuration, for real-time, sensitive, and multiplexed sensing applications. This interferometric sensor employs a compact slit-groove nanostructure, and its output is analyzed using a low-cost fiber-optic spectrometer. Sensing peak linewidths as narrow as 7 nm and refractive index resolutions of 1 × 10−5 refractive index units (RIU) were experimentally measured for this miniaturized sensor, which has a sensing area of 30 × 10 µm2. Analytical models were used to predict sensing characteristics and possible ways to further improve the sensor performance. The collinear transmission geometry, narrow sensing peaks, and small sensor footprint suggest promise for sensitive multiplexed sensing using intensity interrogation. As a proof-of-concept demonstration, an array of the proposed interferometers was fabricated on a sensor chip with a packing density of 4 × 104 sensors per cm2 for real-time multiplexed sensing using a CCD camera and a narrow band light source. A self-referencing method, which will be discussed in detail in Section 5, was also introduced to approximately double the sensitivity and reduce the sensor noise. With demonstrated sensing performance and the possibility for further improvement, this multiplexed sensing platform shows promise for future integration into low-cost, miniatuarized, high-throughput biosensing devices.

2. Slit-groove plasmonic interferometer

The solid curves in Fig. 2
Fig. 2 Experimental interference patterns for slit-groove plasmonic interferometers in an air environment with L = 5.1 and 9.0 µm.
are experimental spectra measured in an air environment using two interferometers with slit-groove separations of 5.1 and 9.0 µm. Data were normalized to the transmission spectrum of an identical reference nanoslit milled on the same sample. One can see obvious spectral oscillations with narrow peaks and valleys resulting from the constructive and destructive interference between the light transmitted directly through the slit and SPPs propagating between the grooves and the slit. The interference pattern of the interferometer with the larger L of 9.0 µm exhibits faster spectral oscillations with decreased contrast. To better interpret these measurements, theoretical interference patterns can be expressed as [32

32. V. V. Temnov, U. Woggon, J. Dintinger, E. Devaux, and T. W. Ebbesen, “Surface plasmon interferometry: measuring group velocity of surface plasmons,” Opt. Lett. 32(10), 1235–1237 (2007). [CrossRef] [PubMed]

]:
II0=1+Espp2Efree2+2EsppEfreecos(4πLλnspp+φ0).
(1)
Here Efree and Espp are the field amplitudes of directly transmitted light and SPP modes, respectively. nspp(λ) = Re((εmn2/(εm + n2))1/2) is the effective refractive index of SPPs at the metal/dielectric interface, εm is the metal permittivity [33

33. E. D. Palik, Handbook of Optical Constants of Solids (Academic Press, 1998).

], n is the refractive index of the dielectric material on top of the metal surface, and φ0 is an additional phase shift [32

32. V. V. Temnov, U. Woggon, J. Dintinger, E. Devaux, and T. W. Ebbesen, “Surface plasmon interferometry: measuring group velocity of surface plasmons,” Opt. Lett. 32(10), 1235–1237 (2007). [CrossRef] [PubMed]

] due to SPP reflection at the grooves and scattering at the slit. According to this equation, the intensity of the transmitted light at a specific wavelength depends on the phase difference between SPPs and free-space light through the term (4πLnspp/λ + φ0), which can be modulated by bulk refractive index changes or biomolecule adsorptions at the upper sensor surface between the central slit and two grooves. For broadband illumination, a change in refractive index causes a spectral shift of the interference pattern, providing the basis of the proposed sensing scheme.

3. Refractive index sensing experiment

To experimentally demonstrate the theoretically predicted sensor performance, we integrated our sample with a polydimethylsiloxane (PDMS) microfluidic flow cell and injected a series of glycerol-water solutions with different refractive index. As shown in Fig. 3(a)
Fig. 3 (a) Measured interference patterns of nanoplasmonic interferometers for water and 3, 6, and 9% glycerol-water solutions. Black curves imposed on the raw data are guides to the eye. The directions of the arrows indicate the red-shifts of the interference patterns. (b) The monitored peak positions for two interferometers as a function of time. The response of the interferometer with L = 5.1 µm was vertically displaced by 2 nm for clarity. The upper inset indicates the sensor noise level and the lower inset shows the spectral positions of the interference peak versus liquid refractive index.
, the interference patterns of interferometers with two different L both red-shift as the liquid refractive index increases. The peak positions were extracted using a Lorentzian fitting method [4

4. J. Homola, “Surface plasmon resonance sensors for detection of chemical and biological species,” Chem. Rev. 108(2), 462–493 (2008). [CrossRef] [PubMed]

] and plotted in Fig. 3(b) as a function of time. For clarity, the sensor response of the interferometer with L = 5.1 µm was vertically displaced by 2 nm in this plot. As seen in Fig. 3(b), both interferometers exhibit stable peak wavelengths at each glycerol concentration and the peak shifts were approximately proportional to the increase in glycerol concentration. The sensing peaks return to their initial spectral positions for both interferometers with the final DI water injection. The lower inset of Fig. 3(b) shows the peak positions as a function of the liquid refractive index. The solid lines are the linear fits to the experimental data, providing sensitivities of the two sensors. For interferometers with L = 5.1 and 9.0 µm, the measured sensitivities are 488.7 and 469.1 nm/RIU, respectively, with peak linewidths of 13.9 and 7.0 nm, and FOMs of 35.2 and 67.0, respectively, all in good agreement with the theoretical predictions (see Table 1

Table 1. Experimental and Calculated Sensing Performances for Interferometers with Two different values of L.a

table-icon
View This Table
).

Further optimization of this interferometric sensor is primarily limited by the relatively low interference contrast caused by the unbalanced intensities of interfering SPPs and light. We now discuss reasons for such intensity imbalance and possible ways to optimize it. According to Lalanne’s theory [35

35. P. Lalanne, J. P. Hugonin, and J. C. Rodier, “Theory of surface plasmon generation at nanoslit apertures,” Phys. Rev. Lett. 95(26), 263902 (2005). [CrossRef] [PubMed]

] and our following experimental validation [30

30. Q. Gan, Y. Gao, Q. Wang, L. Zhu, and F. J. Bartoli, “Surface plasmon waves generated by nanogrooves through spectral interference,” Phys. Rev. B 81(8), 085443 (2010). [CrossRef]

], a large portion of the slit-guided mode (up to ~40%) can be coupled to propagating SPPs (with the rest 60% scattered into free-space light) when the slit width is around 20% of the incident wavelength. As a result, we employed an optimized slit width of 100 nm in this work (660 nm × 20% / 1.33 = 99 nm, where the width is scaled by 1.33 as the device is in a water environment). Under this optimized condition, the intensity of the generated SPPs is comparable to interfering free-space light. Therefore, the relatively low modulation depth results mainly from the SPP reflection loss at the nanogrooves and propagation loss at the sensor surface. Here we mention several potentially important improvements that could reduce these losses. First, SPP reflection efficiency at the two grooves can be enhanced by improving the quality of the fabricated grooves (e.g., using Ag-Al double metal layers with the bottom Al layer as a slow etch rate FIB stop to precisely and uniformly control the fabricated groove depth [36

36. J. S. Q. Liu, R. A. Pala, F. Afshinmanesh, W. Cai, and M. L. Brongersma, “A submicron plasmonic dichroic splitter,” Nat Commun 2, 525 (2011). [CrossRef] [PubMed]

]). Second, instead of using single grooves as reflectors, groove or ridge arrays can be designed to function as Bragg mirrors, which have been shown to provide reflection efficiencies larger than 90% after structural optimization [37

37. M. U. Gonzalez, J. C. Weeber, A. L. Baudrion, A. Dereux, A. L. Stepanov, J. R. Krenn, E. Devaux, and T. W. Ebbesen, “Design, near-field characterization, and modeling of 45° surface-plasmon Bragg mirrors,” Phys. Rev. B 73(15), 155416 (2006). [CrossRef]

,38

38. J. A. Sanchez-Gil and A. A. Maradudin, “Surface-plasmon polariton scattering from a finite array of nanogrooves/ridges: Efficient mirrors,” Appl. Phys. Lett. 86(25), 251106 (2005). [CrossRef]

]. Note that a greater number of grooves or ridges make this interferometer less suitable for broadband sensor operation, but do not affect narrow-band intensity-interrogated sensing as will be discussed in Section 5. Third, SPP propagation loss can be reduced by employing ultrasmooth metal films obtained by template stripping [15

15. K.-L. Lee, P.-W. Chen, S.-H. Wu, J.-B. Huang, S.-Y. Yang, and P.-K. Wei, “Enhancing surface plasmon detection using template-stripped gold nanoslit arrays on plastic films,” ACS Nano 6(4), 2931–2939 (2012). [CrossRef] [PubMed]

,39

39. P. Nagpal, N. C. Lindquist, S. H. Oh, and D. J. Norris, “Ultrasmooth patterned metals for plasmonics and metamaterials,” Science 325(5940), 594–597 (2009). [CrossRef] [PubMed]

]. Fourth, the sensor noise level could be further reduced by using a more advanced spectrometer with higher saturation level and lower dark noise [29

29. A. B. Dahlin, S. Chen, M. P. Jonsson, L. Gunnarsson, M. Käll, and F. Höök, “High-resolution microspectroscopy of plasmonic nanostructures for miniaturized biosensing,” Anal. Chem. 81(16), 6572–6580 (2009). [CrossRef] [PubMed]

].

4. Biosensing using plasmonic interferometer

To demonstrate the feasibility of this sensing platform to detect biomolecular binding events, we monitored the specific binding between BSA and anti-BSA molecules. The microfluidic channel was first injected with a 10 mM HEPES buffer for 25 min to rinse the sensor chip and define the baseline of the experiment. A 500 μg/mL BSA solution in HEPES buffer was then introduced into the channel to functionalize the metal surface with a BSA monolayer. This leads to a 0.9 nm shift of the peak wavelength (see the first signal change at the time of 1200 s in Fig. 4
Fig. 4 Real-time sensor response upon BSA adsorption to the sensor surface and subsequent specific protein binding between BSA and anti-BSA. The arrows indicate the injections of analytes and buffer solutions. The upper inset shows a schematic of anti-BSA binding to BSA immobilized on the sensor surface.
). A subsequent 25 min buffer rinse had little effect on the peak wavelength. Then, a 42 µg/mL anti-BSA solution was injected into the channel and followed by a buffer rinse to wash out the unbound anti-BSA molecules. The small spikes observed at time t = 3000 s and 4500 s are measurement artifacts caused by exchanging syringes. The specific binding between BSA and anti-BSA corresponds to a peak wavelength shift of 1 nm. We can now calculate the effective protein layer thickness d and the sensor resolution in terms of protein surface concentration by using a well-established quantitative formalism: Δλ = S(nl-nb)(1-e−2d/l) [40

40. L. S. Jung, C. T. Campbell, T. M. Chinowsky, M. N. Mar, and S. S. Yee, “Quantitative interpretation of the response of surface plasmon resonance sensors to adsorbed films,” Langmuir 14(19), 5636–5648 (1998). [CrossRef]

]. Here S is the bulk refractive index sensitivity obtained in the calibration experiment; nl and nb are the refractive indices of protein layer and buffer solution, respectively; and l is the SPP decay length. Assuming the refractive index of pure BSA is 1.57 [40

40. L. S. Jung, C. T. Campbell, T. M. Chinowsky, M. N. Mar, and S. S. Yee, “Quantitative interpretation of the response of surface plasmon resonance sensors to adsorbed films,” Langmuir 14(19), 5636–5648 (1998). [CrossRef]

], the observed 0.9 nm peak shift, Δλ, corresponds to an effective thickness of the pure BSA layer, d, of 0.92 nm. By use of the density of BSA (1.3 g/cm3) [40

40. L. S. Jung, C. T. Campbell, T. M. Chinowsky, M. N. Mar, and S. S. Yee, “Quantitative interpretation of the response of surface plasmon resonance sensors to adsorbed films,” Langmuir 14(19), 5636–5648 (1998). [CrossRef]

,41

41. S. Sjölander and C. Urbaniczky, “Integrated fluid handling system for biomolecular interaction analysis,” Anal. Chem. 63(20), 2338–2345 (1991). [CrossRef] [PubMed]

], the surface concentration of this saturated protein monolayer is calculated to be 1.2 × 10−7 g/cm2 (i.e., 1.3 g/cm3 × 0.92 nm). As this saturated BSA layer is detected with a signal-to-noise ratio of 180, the resolution of the biosensor can thus be calculated as 6.6 pg/mm2 in terms of protein surface concentration. This sensor resolution improves when detecting larger biomolecules and can be further optimized through the methods discussed in Section 3.

5. Multiplexed sensing experiment

While the capability of this interferometer platform for biosensing has been demonstrated, the spectral measurement approach is not suitable for dynamic, highly multiplexed sensing, which requires simultaneous determination of light intensities transmitted through multiple sensing elements. In this section, we further perform real-time, multiplexed sensing experiments using a CCD camera and a narrow band light source for intensity interrogation. As a proof-of-concept demonstration, we fabricated a 4 × 3 array of the slit-groove plasmonic interferometer (Fig. 5(a)
Fig. 5 (a) A bright-field microscope image of the fabricated plasmonic interferometer array. Scale bar: 10 µm. The interferometers are fabricated with two different L: 5.1 (the first and third column) and 5.2 µm (the second and fourth column) (b) An SEM image of the 4 × 3 microarray. Scale bar: 10 µm. (c) A CCD image of one of the interferometers.
). Interferometers in the first and third (second and fourth) column of the microarray have a groove-slit distance, L, of 5.1 (5.2) µm. Figure 5(b) and 5(c) show an SEM image of the fabricated sensor array and a CCD image of one of the plasmonic interferometers, respectively. Each interferometer has a footprint of around 300 µm2 with the center-to-center distance between each sensing element of 50 µm, giving a potential packing density of 4 × 104 sensors per cm2. The dense packing capability of the proposed sensing scheme illustrates its promise for high-throughput microarray applications. Further improvement in this packing density is still possible by fabricating groove/ridge arrays as Bragg mirrors instead of single grooves to enhance SPP reflection efficiency and decrease crosstalk between sensor elements [8

8. N. C. Lindquist, A. Lesuffleur, H. Im, and S. H. Oh, “Sub-micron resolution surface plasmon resonance imaging enabled by nanohole arrays with surrounding Bragg mirrors for enhanced sensitivity and isolation,” Lab Chip 9(3), 382–387 (2009). [CrossRef] [PubMed]

].

For the imaging experiment, the fabricated sensor array was illuminated through the substrate using a white light source passing through an optical band-pass filter centered at 655 nm with a 12 nm bandwidth. The transmitted light from 12 interferometers was then collected simultaneously by a 40 × microscope objective and imaged onto a CCD camera. The intensity change of the transmitted light from each interferometer is determined by two factors: one is the peak shift resulting from refractive index changes and the other is the slope of the spectrum at the illumination wavelength. Accordingly, to achieve optimal sensing performance, the slit-groove distance needs to be carefully designed to locate the high-slope region of the interference pattern at the illumination wavelength. Plasmonic interferometers with L of 5.1 and 5.2 µm are used in this measurement, and their transmission spectra are shown in Fig. 6(a)
Fig. 6 (a) Transmission spectra of interferometers with two different values of L: 5.1 and 5.2 µm. The yellow regions indicate the spectral range of the filtered white light source. As the dielectric refractive index increases, the transmitted intensity can either decrease (L = 5.1 µm) or increase (L = 5.2 µm). (b) The blue and green dots shown in the inset indicate the real-time measurements of the normalized transmitted intensities from two interferometers. The black dots are the experimental results obtained after using a self-referencing method.
, respectively. The yellow regions indicate the spectral range of the incident light. Both interference patterns red-shift with the increase in the liquid refractive index, and the transmitted light intensities either increase or decrease, depending on the negative or positive slope of the spectrum. Green and blue dots in the inset of Fig. 6(b) present real-time experimental measurements of the transmitted intensities from these two interferometers. As a series of glycerol-water solutions with increasing refractive index are injected into the channel, the transmission intensity of the interferometer either decrease (for L = 5.1 µm) or increase (for L = 5.2 µm), in agreement with predictions. Following the 6% glycerol test, DI water was again introduced into the channel, returning the transmitted intensities to their initial levels and validating the reliability of the sensing performance.

The standard deviation of the measured light intensity determines the noise level and detection resolution of this intensity-interrogated sensor. To improve the sensor performance, background fluctuations needs to be subtracted, including noise from mechanical vibrations and light intensity fluctuations. Here a self-referencing method was introduced to reduce the influence of these effects and approximately doubles the sensor sensitivity. As shown in Fig. 6(a), interferometers carefully designed with two different L exhibit similar initial transmitted intensities but have positive and negative intensity-change sensitivities. To design such two sensors, the interference pattern of the second interferometer needs to be spectrally shifted by half of an interference period from the first one. The theoretical expression of ΔL (the difference between L of two sensors) can be easily derived from Eq. (1) as: ∆L = λ/ 4nspp, where λ is the sensor working wavelength. According to this equation, two interferometers with L of 5.1 and 5.2 µm were designed and fabricated for measurements. When performing the experiment, a signal arising from the refractive index change shifts the transmitted intensities of these two sensors in two different directions (that is transmission increase or decrease), while unwanted signal from light intensity fluctuations and mechanical vibrations change two transmitted intensities in the same direction. Monitoring the intensity difference between the two interferometers results in a near two-fold improvement in sensor sensitivity, and also subtracts the background noises (see the black dots in Fig. 6(b)). The resulting intensity sensitivity is 684%/RIU with a sensor noise level of 0.033%. Here the transmission intensity through single interferometer in water serves as the reference intensity. The sensor resolution is calculated to be 5 × 10−5 RIU (0.033%/ 684%/RIU), 6 times smaller than that of a single interferometer (3 × 10−4 RIU for the interferometer with L = 5.1 µm).

6. Conclusions

In conclusion, we have demonstrated real-time, multiplexed sensing using plasmonic interferometric sensors. This new group of sensors combines plasmonic architectures with interferometry techniques to monitor the SPP phase changes induced by surface biomolecular adsorptions. The SPP-light interference results in narrow sensing peak linewidths of 7 nm and provides a new route for plasmon line shape engineering by tailoring interferometer structures as well as SPP amplitude and phase properties in two interfering channels. The easy collinear transmission setup and simple, compact slit-groove nanostructure also show promise for future sensor miniaturization and low-cost production. A refractive index resolution of 1 × 10−5 RIU has been measured from a miniaturized sensing area of 30 × 10 µm2 using a low-cost spectrometer. This sensor resolution can be further improved by fabricating large-area interferometer arrays, employing an advanced spectrometer, or using multiple groove/ridges to increase SPP reflection efficiencies at the groove reflectors. We have also demonstrated the real-time, multiplexed sensing capability of this small-footprint sensor by using a CCD camera and a narrow band light source for intensity interrogation. The demonstrated sensing and multiplexing performance and possible further improvements suggest that this novel class of plasmonic interferometric sensors have exciting promise to be integrated into multiplexed, miniaturized sensing devices for label-free biochemical applications.

Appendix: Experimental details

Sample fabrication. Standard glass microscope slides (Fisher Scientific) were first cleaned thoroughly with acetone, isopropyl alcohol (IPA) and deionized (DI) water in an ultrasonic cleaner for 20 min each, and subsequently blow-dried with nitrogen. A 350 nm silver film was then deposited by e-beam evaporation (Indel system) onto glass slides at a deposition rate of 1 Å/s. FIB (FEI Dual-Beam system 235) milling (Ga+ ions, 30 kV, 30 pA) was used to fabricate a series of groove-slit-groove nanostructures with varying slit-groove distance L. The groove depth is around 70 nm, measured using AFM (NT-MDT Solver NEXT). Another identical single nanoslit was also milled to serve as a reference for spectrum normalization. After the FIB milling, plasma-enhanced chemical vapor deposition was used to deposit a 5 nm thick silicon dioxide film on top of the silver surface. This dielectric film functions as a protection layer to improve biocompatibility and chemical stability of the silver-based device, particularly in aqueous solutions [44

44. H. Im, S. H. Lee, N. J. Wittenberg, T. W. Johnson, N. C. Lindquist, P. Nagpal, D. J. Norris, and S. H. Oh, “Template-stripped smooth Ag nanohole arrays with silica shells for surface plasmon resonance biosensing,” ACS Nano 5(8), 6244–6253 (2011). [CrossRef] [PubMed]

]. The prepared sample surface was then cleaned and activated by oxygen plasma and bonded to a microfluidic flow cell.

Optical measurements. White light beam from a 100 W halogen lamp was focused onto the sample from the substrate side through the microscope condenser of an Olympus IX81 inverted microscope. The far-field transmitted light from the interferometer was collected by a 40× microscope objective with numerical aperture NA = 0.6. The collected light could be coupled into a portable fiber-optic spectrometer (Ocean optics USB 4000) for spectral measurements or focused on a CCD camera (Cooke sensicam qe) for multiplexed sening experiments using intensity interrogation. For spectral measurements, the CCD camera was also employed to record the positions of the slit-groove interferometers. Under identical experimental conditions, the fabricated reference single nanoslit was then moved to the recorded positions of the interferometers and the collected transmission spectrum was used as a reference for spectra normalization.

Refractive index sensing and biosensing measurements. To calibrate the sensor performance for refractive index sensing, a series of glycerol-water solutions with glycerol volume concentrations of 0%, 3%, 6% and 9% was prepared. An ellipsometer (J. A. Wollam, V-VASE) was used to measure their refractive indices at 650 nm wavelength, ranging from n = 1.3312 to n = 1.3450. The HEPES buffer and anti-BSA used in the biosensing experiment were purchased from Sigma-Aldrich. BSA was purchased from Thermal-Scientific. Solutions were injected into the microfluidic channel (50 µm deep, 4 mm wide) using a syringe pump (Harvard Apparatus) at a flow rate of 20 µL/min. For both real-time refractive index sensing and biosensing experiments, transmission spectra were continuously recorded (100 ms integration time) and averaged over 100 acquisitions as solutions flowed over the sample surface. The temporal resolution for both spectral sensing experiments was 10 s.

Multiplexed sensing experiments using CCD imaging. In multiplexed sensing experiments, the fabricated interferometer array was illuminated through the substrate using a 100 W halogen lamp passing through the microscope condenser and an optical band-pass filter (Semrock) at 655 nm (bandwidth 12 nm). Images were captured every 1 s with an exposure time of 90 ms using a CCD camera (Cooke sensicam qe). A custom made MATLABTM image processing program was used to integrate the transmission intensity over the slit region of each interferometer as the real-time sensor output. The detector dark noise was further removed by subtracting the transmitted intensity integrated over the same area on the sample surface without any nanostructure. CCD images were continuously captured and averaged over 20 images to increase the sensor signal-to-noise ratio. Collecting 20 images with 1 s interval corresponds to a sensor temporal resolution of 20 s.

Acknowledgments

This work was supported by the National Science Foundation (Award # CBET-1014957). Q. Gan acknowledges financial support from National Science Foundation (Award # ECCS-1128086).

References and links

1.

C. Genet and T. W. Ebbesen, “Light in tiny holes,” Nature 445(7123), 39–46 (2007). [CrossRef] [PubMed]

2.

J. A. Schuller, E. S. Barnard, W. Cai, Y. C. Jun, J. S. White, and M. L. Brongersma, “Plasmonics for extreme light concentration and manipulation,” Nat. Mater. 9(3), 193–204 (2010). [CrossRef] [PubMed]

3.

S. A. Maier, Plasmonics: Fundamental and Applications (Springer, 2007).

4.

J. Homola, “Surface plasmon resonance sensors for detection of chemical and biological species,” Chem. Rev. 108(2), 462–493 (2008). [CrossRef] [PubMed]

5.

A. De Leebeeck, L. K. S. Kumar, V. de Lange, D. Sinton, R. Gordon, and A. G. Brolo, “On-chip surface-based detection with nanohole arrays,” Anal. Chem. 79(11), 4094–4100 (2007). [CrossRef] [PubMed]

6.

A. Cattoni, P. Ghenuche, A. M. Haghiri-Gosnet, D. Decanini, J. Chen, J. L. Pelouard, and S. Collin, “λ³/1000 Plasmonic Nanocavities for Biosensing Fabricated by Soft UV Nanoimprint Lithography,” Nano Lett. 11(9), 3557–3563 (2011). [CrossRef] [PubMed]

7.

G. Spoto and M. Minunni, “Surface plasmon resonance imaging: what next?” J. Phys. Chem. Lett. 3(18), 2682–2691 (2012). [CrossRef]

8.

N. C. Lindquist, A. Lesuffleur, H. Im, and S. H. Oh, “Sub-micron resolution surface plasmon resonance imaging enabled by nanohole arrays with surrounding Bragg mirrors for enhanced sensitivity and isolation,” Lab Chip 9(3), 382–387 (2009). [CrossRef] [PubMed]

9.

K. A. Tetz, L. Pang, and Y. Fainman, “High-resolution surface plasmon resonance sensor based on linewidth-optimized nanohole array transmittance,” Opt. Lett. 31(10), 1528–1530 (2006). [CrossRef] [PubMed]

10.

J. C. Yang, J. Ji, J. M. Hogle, and D. N. Larson, “Multiplexed plasmonic sensing based on small-dimension nanohole arrays and intensity interrogation,” Biosens. Bioelectron. 24(8), 2334–2338 (2009). [CrossRef] [PubMed]

11.

C. T. Campbell and G. Kim, “SPR microscopy and its applications to high-throughput analyses of biomolecular binding events and their kinetics,” Biomaterials 28(15), 2380–2392 (2007). [CrossRef] [PubMed]

12.

M. E. Stewart, C. R. Anderton, L. B. Thompson, J. Maria, S. K. Gray, J. A. Rogers, and R. G. Nuzzo, “Nanostructured plasmonic sensors,” Chem. Rev. 108(2), 494–521 (2008). [CrossRef] [PubMed]

13.

J. N. Anker, W. P. Hall, O. Lyandres, N. C. Shah, J. Zhao, and R. P. Van Duyne, “Biosensing with plasmonic nanosensors,” Nat. Mater. 7(6), 442–453 (2008). [CrossRef] [PubMed]

14.

K. M. Mayer and J. H. Hafner, “Localized surface plasmon resonance sensors,” Chem. Rev. 111(6), 3828–3857 (2011). [CrossRef] [PubMed]

15.

K.-L. Lee, P.-W. Chen, S.-H. Wu, J.-B. Huang, S.-Y. Yang, and P.-K. Wei, “Enhancing surface plasmon detection using template-stripped gold nanoslit arrays on plastic films,” ACS Nano 6(4), 2931–2939 (2012). [CrossRef] [PubMed]

16.

A. A. Yanik, A. E. Cetin, M. Huang, A. Artar, S. H. Mousavi, A. Khanikaev, J. H. Connor, G. Shvets, and H. Altug, “Seeing protein monolayers with naked eye through plasmonic Fano resonances,” Proc. Natl. Acad. Sci. U.S.A. 108(29), 11784–11789 (2011). [CrossRef] [PubMed]

17.

H. Im, A. Lesuffleur, N. C. Lindquist, and S. H. Oh, “Plasmonic nanoholes in a multichannel microarray format for parallel kinetic assays and differential sensing,” Anal. Chem. 81(8), 2854–2859 (2009). [CrossRef] [PubMed]

18.

N. Verellen, P. Van Dorpe, C. J. Huang, K. Lodewijks, G. A. E. Vandenbosch, L. Lagae, and V. V. Moshchalkov, “Plasmon line shaping using nanocrosses for high sensitivity localized surface plasmon resonance sensing,” Nano Lett. 11(2), 391–397 (2011). [CrossRef] [PubMed]

19.

S. Zhang, K. Bao, N. J. Halas, H. Xu, and P. Nordlander, “Substrate-induced Fano resonances of a plasmonic nanocube: a route to increased-sensitivity localized surface plasmon resonance sensors revealed,” Nano Lett. 11(4), 1657–1663 (2011). [CrossRef] [PubMed]

20.

N. Liu, T. Weiss, M. Mesch, L. Langguth, U. Eigenthaler, M. Hirscher, C. Sönnichsen, and H. Giessen, “Planar metamaterial analogue of electromagnetically induced transparency for plasmonic sensing,” Nano Lett. 10(4), 1103–1107 (2010). [CrossRef] [PubMed]

21.

B. Auguié and W. L. Barnes, “Collective resonances in gold nanoparticle arrays,” Phys. Rev. Lett. 101(14), 143902 (2008). [CrossRef] [PubMed]

22.

Y. Gao, Q. Gan, Z. Xin, X. Cheng, and F. J. Bartoli, “Plasmonic Mach-Zehnder interferometer for ultrasensitive on-chip biosensing,” ACS Nano 5(12), 9836–9844 (2011). [CrossRef] [PubMed]

23.

Q. Gan, Y. Gao, and F. J. Bartoli, “Vertical plasmonic Mach-Zehnder interferometer for sensitive optical sensing,” Opt. Express 17(23), 20747–20755 (2009). [CrossRef] [PubMed]

24.

X. Wu, J. Zhang, J. Chen, C. Zhao, and Q. Gong, “Refractive index sensor based on surface-plasmon interference,” Opt. Lett. 34(3), 392–394 (2009). [CrossRef] [PubMed]

25.

J. Feng, V. S. Siu, A. Roelke, V. Mehta, S. Y. Rhieu, G. T. R. Palmore, and D. Pacifici, “Nanoscale plasmonic interferometers for multispectral, high-throughput biochemical sensing,” Nano Lett. 12(2), 602–609 (2012). [CrossRef] [PubMed]

26.

X. Li, Q. Tan, B. Bai, and G. Jin, “Non-spectroscopic refractometric nanosensor based on a tilted slit-groove plasmonic interferometer,” Opt. Express 19(21), 20691–20703 (2011). [CrossRef] [PubMed]

27.

O. Yavas and C. Kocabas, “Plasmon interferometers for high-throughput sensing,” Opt. Lett. 37(16), 3396–3398 (2012). [CrossRef]

28.

J. Bravo-Abad, L. Martín-Moreno, and F. J. García-Vidal, “Transmission properties of a single metallic slit: From the subwavelength regime to the geometrical-optics limit,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 69(2), 026601 (2004). [CrossRef] [PubMed]

29.

A. B. Dahlin, S. Chen, M. P. Jonsson, L. Gunnarsson, M. Käll, and F. Höök, “High-resolution microspectroscopy of plasmonic nanostructures for miniaturized biosensing,” Anal. Chem. 81(16), 6572–6580 (2009). [CrossRef] [PubMed]

30.

Q. Gan, Y. Gao, Q. Wang, L. Zhu, and F. J. Bartoli, “Surface plasmon waves generated by nanogrooves through spectral interference,” Phys. Rev. B 81(8), 085443 (2010). [CrossRef]

31.

M. E. Stewart, N. H. Mack, V. Malyarchuk, J. A. Soares, T. W. Lee, S. K. Gray, R. G. Nuzzo, and J. A. Rogers, “Quantitative multispectral biosensing and 1D imaging using quasi-3D plasmonic crystals,” Proc. Natl. Acad. Sci. U.S.A. 103(46), 17143–17148 (2006). [CrossRef] [PubMed]

32.

V. V. Temnov, U. Woggon, J. Dintinger, E. Devaux, and T. W. Ebbesen, “Surface plasmon interferometry: measuring group velocity of surface plasmons,” Opt. Lett. 32(10), 1235–1237 (2007). [CrossRef] [PubMed]

33.

E. D. Palik, Handbook of Optical Constants of Solids (Academic Press, 1998).

34.

J. Henzie, M. H. Lee, and T. W. Odom, “Multiscale patterning of plasmonic metamaterials,” Nat. Nanotechnol. 2(9), 549–554 (2007). [CrossRef] [PubMed]

35.

P. Lalanne, J. P. Hugonin, and J. C. Rodier, “Theory of surface plasmon generation at nanoslit apertures,” Phys. Rev. Lett. 95(26), 263902 (2005). [CrossRef] [PubMed]

36.

J. S. Q. Liu, R. A. Pala, F. Afshinmanesh, W. Cai, and M. L. Brongersma, “A submicron plasmonic dichroic splitter,” Nat Commun 2, 525 (2011). [CrossRef] [PubMed]

37.

M. U. Gonzalez, J. C. Weeber, A. L. Baudrion, A. Dereux, A. L. Stepanov, J. R. Krenn, E. Devaux, and T. W. Ebbesen, “Design, near-field characterization, and modeling of 45° surface-plasmon Bragg mirrors,” Phys. Rev. B 73(15), 155416 (2006). [CrossRef]

38.

J. A. Sanchez-Gil and A. A. Maradudin, “Surface-plasmon polariton scattering from a finite array of nanogrooves/ridges: Efficient mirrors,” Appl. Phys. Lett. 86(25), 251106 (2005). [CrossRef]

39.

P. Nagpal, N. C. Lindquist, S. H. Oh, and D. J. Norris, “Ultrasmooth patterned metals for plasmonics and metamaterials,” Science 325(5940), 594–597 (2009). [CrossRef] [PubMed]

40.

L. S. Jung, C. T. Campbell, T. M. Chinowsky, M. N. Mar, and S. S. Yee, “Quantitative interpretation of the response of surface plasmon resonance sensors to adsorbed films,” Langmuir 14(19), 5636–5648 (1998). [CrossRef]

41.

S. Sjölander and C. Urbaniczky, “Integrated fluid handling system for biomolecular interaction analysis,” Anal. Chem. 63(20), 2338–2345 (1991). [CrossRef] [PubMed]

42.

C. Escobedo, S. Vincent, A. I. K. Choudhury, J. Campbell, A. G. Brolo, D. Sinton, and R. Gordon, “Integrated nanohole array surface plasmon resonance sensing device using a dual-wavelength source,” J. Micromech. Microeng. 21(11), 115001 (2011). [CrossRef]

43.

N. C. Lindquist, T. W. Johnson, D. J. Norris, and S. H. Oh, “Monolithic integration of continuously tunable plasmonic nanostructures,” Nano Lett. 11(9), 3526–3530 (2011). [CrossRef] [PubMed]

44.

H. Im, S. H. Lee, N. J. Wittenberg, T. W. Johnson, N. C. Lindquist, P. Nagpal, D. J. Norris, and S. H. Oh, “Template-stripped smooth Ag nanohole arrays with silica shells for surface plasmon resonance biosensing,” ACS Nano 5(8), 6244–6253 (2011). [CrossRef] [PubMed]

OCIS Codes
(120.3180) Instrumentation, measurement, and metrology : Interferometry
(130.6010) Integrated optics : Sensors
(240.6680) Optics at surfaces : Surface plasmons
(220.4241) Optical design and fabrication : Nanostructure fabrication
(250.5403) Optoelectronics : Plasmonics

ToC Category:
Sensors

History
Original Manuscript: December 19, 2012
Revised Manuscript: January 30, 2013
Manuscript Accepted: January 30, 2013
Published: March 1, 2013

Virtual Issues
Vol. 8, Iss. 4 Virtual Journal for Biomedical Optics

Citation
Yongkang Gao, Zheming Xin, Qiaoqiang Gan, Xuanhong Cheng, and Filbert J. Bartoli, "Plasmonic interferometers for label-free multiplexed sensing," Opt. Express 21, 5859-5871 (2013)
http://www.opticsinfobase.org/vjbo/abstract.cfm?URI=oe-21-5-5859


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References

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  2. J. A. Schuller, E. S. Barnard, W. Cai, Y. C. Jun, J. S. White, and M. L. Brongersma, “Plasmonics for extreme light concentration and manipulation,” Nat. Mater.9(3), 193–204 (2010). [CrossRef] [PubMed]
  3. S. A. Maier, Plasmonics: Fundamental and Applications (Springer, 2007).
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  6. A. Cattoni, P. Ghenuche, A. M. Haghiri-Gosnet, D. Decanini, J. Chen, J. L. Pelouard, and S. Collin, “λ³/1000 Plasmonic Nanocavities for Biosensing Fabricated by Soft UV Nanoimprint Lithography,” Nano Lett.11(9), 3557–3563 (2011). [CrossRef] [PubMed]
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  9. K. A. Tetz, L. Pang, and Y. Fainman, “High-resolution surface plasmon resonance sensor based on linewidth-optimized nanohole array transmittance,” Opt. Lett.31(10), 1528–1530 (2006). [CrossRef] [PubMed]
  10. J. C. Yang, J. Ji, J. M. Hogle, and D. N. Larson, “Multiplexed plasmonic sensing based on small-dimension nanohole arrays and intensity interrogation,” Biosens. Bioelectron.24(8), 2334–2338 (2009). [CrossRef] [PubMed]
  11. C. T. Campbell and G. Kim, “SPR microscopy and its applications to high-throughput analyses of biomolecular binding events and their kinetics,” Biomaterials28(15), 2380–2392 (2007). [CrossRef] [PubMed]
  12. M. E. Stewart, C. R. Anderton, L. B. Thompson, J. Maria, S. K. Gray, J. A. Rogers, and R. G. Nuzzo, “Nanostructured plasmonic sensors,” Chem. Rev.108(2), 494–521 (2008). [CrossRef] [PubMed]
  13. J. N. Anker, W. P. Hall, O. Lyandres, N. C. Shah, J. Zhao, and R. P. Van Duyne, “Biosensing with plasmonic nanosensors,” Nat. Mater.7(6), 442–453 (2008). [CrossRef] [PubMed]
  14. K. M. Mayer and J. H. Hafner, “Localized surface plasmon resonance sensors,” Chem. Rev.111(6), 3828–3857 (2011). [CrossRef] [PubMed]
  15. K.-L. Lee, P.-W. Chen, S.-H. Wu, J.-B. Huang, S.-Y. Yang, and P.-K. Wei, “Enhancing surface plasmon detection using template-stripped gold nanoslit arrays on plastic films,” ACS Nano6(4), 2931–2939 (2012). [CrossRef] [PubMed]
  16. A. A. Yanik, A. E. Cetin, M. Huang, A. Artar, S. H. Mousavi, A. Khanikaev, J. H. Connor, G. Shvets, and H. Altug, “Seeing protein monolayers with naked eye through plasmonic Fano resonances,” Proc. Natl. Acad. Sci. U.S.A.108(29), 11784–11789 (2011). [CrossRef] [PubMed]
  17. H. Im, A. Lesuffleur, N. C. Lindquist, and S. H. Oh, “Plasmonic nanoholes in a multichannel microarray format for parallel kinetic assays and differential sensing,” Anal. Chem.81(8), 2854–2859 (2009). [CrossRef] [PubMed]
  18. N. Verellen, P. Van Dorpe, C. J. Huang, K. Lodewijks, G. A. E. Vandenbosch, L. Lagae, and V. V. Moshchalkov, “Plasmon line shaping using nanocrosses for high sensitivity localized surface plasmon resonance sensing,” Nano Lett.11(2), 391–397 (2011). [CrossRef] [PubMed]
  19. S. Zhang, K. Bao, N. J. Halas, H. Xu, and P. Nordlander, “Substrate-induced Fano resonances of a plasmonic nanocube: a route to increased-sensitivity localized surface plasmon resonance sensors revealed,” Nano Lett.11(4), 1657–1663 (2011). [CrossRef] [PubMed]
  20. N. Liu, T. Weiss, M. Mesch, L. Langguth, U. Eigenthaler, M. Hirscher, C. Sönnichsen, and H. Giessen, “Planar metamaterial analogue of electromagnetically induced transparency for plasmonic sensing,” Nano Lett.10(4), 1103–1107 (2010). [CrossRef] [PubMed]
  21. B. Auguié and W. L. Barnes, “Collective resonances in gold nanoparticle arrays,” Phys. Rev. Lett.101(14), 143902 (2008). [CrossRef] [PubMed]
  22. Y. Gao, Q. Gan, Z. Xin, X. Cheng, and F. J. Bartoli, “Plasmonic Mach-Zehnder interferometer for ultrasensitive on-chip biosensing,” ACS Nano5(12), 9836–9844 (2011). [CrossRef] [PubMed]
  23. Q. Gan, Y. Gao, and F. J. Bartoli, “Vertical plasmonic Mach-Zehnder interferometer for sensitive optical sensing,” Opt. Express17(23), 20747–20755 (2009). [CrossRef] [PubMed]
  24. X. Wu, J. Zhang, J. Chen, C. Zhao, and Q. Gong, “Refractive index sensor based on surface-plasmon interference,” Opt. Lett.34(3), 392–394 (2009). [CrossRef] [PubMed]
  25. J. Feng, V. S. Siu, A. Roelke, V. Mehta, S. Y. Rhieu, G. T. R. Palmore, and D. Pacifici, “Nanoscale plasmonic interferometers for multispectral, high-throughput biochemical sensing,” Nano Lett.12(2), 602–609 (2012). [CrossRef] [PubMed]
  26. X. Li, Q. Tan, B. Bai, and G. Jin, “Non-spectroscopic refractometric nanosensor based on a tilted slit-groove plasmonic interferometer,” Opt. Express19(21), 20691–20703 (2011). [CrossRef] [PubMed]
  27. O. Yavas and C. Kocabas, “Plasmon interferometers for high-throughput sensing,” Opt. Lett.37(16), 3396–3398 (2012). [CrossRef]
  28. J. Bravo-Abad, L. Martín-Moreno, and F. J. García-Vidal, “Transmission properties of a single metallic slit: From the subwavelength regime to the geometrical-optics limit,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys.69(2), 026601 (2004). [CrossRef] [PubMed]
  29. A. B. Dahlin, S. Chen, M. P. Jonsson, L. Gunnarsson, M. Käll, and F. Höök, “High-resolution microspectroscopy of plasmonic nanostructures for miniaturized biosensing,” Anal. Chem.81(16), 6572–6580 (2009). [CrossRef] [PubMed]
  30. Q. Gan, Y. Gao, Q. Wang, L. Zhu, and F. J. Bartoli, “Surface plasmon waves generated by nanogrooves through spectral interference,” Phys. Rev. B81(8), 085443 (2010). [CrossRef]
  31. M. E. Stewart, N. H. Mack, V. Malyarchuk, J. A. Soares, T. W. Lee, S. K. Gray, R. G. Nuzzo, and J. A. Rogers, “Quantitative multispectral biosensing and 1D imaging using quasi-3D plasmonic crystals,” Proc. Natl. Acad. Sci. U.S.A.103(46), 17143–17148 (2006). [CrossRef] [PubMed]
  32. V. V. Temnov, U. Woggon, J. Dintinger, E. Devaux, and T. W. Ebbesen, “Surface plasmon interferometry: measuring group velocity of surface plasmons,” Opt. Lett.32(10), 1235–1237 (2007). [CrossRef] [PubMed]
  33. E. D. Palik, Handbook of Optical Constants of Solids (Academic Press, 1998).
  34. J. Henzie, M. H. Lee, and T. W. Odom, “Multiscale patterning of plasmonic metamaterials,” Nat. Nanotechnol.2(9), 549–554 (2007). [CrossRef] [PubMed]
  35. P. Lalanne, J. P. Hugonin, and J. C. Rodier, “Theory of surface plasmon generation at nanoslit apertures,” Phys. Rev. Lett.95(26), 263902 (2005). [CrossRef] [PubMed]
  36. J. S. Q. Liu, R. A. Pala, F. Afshinmanesh, W. Cai, and M. L. Brongersma, “A submicron plasmonic dichroic splitter,” Nat Commun2, 525 (2011). [CrossRef] [PubMed]
  37. M. U. Gonzalez, J. C. Weeber, A. L. Baudrion, A. Dereux, A. L. Stepanov, J. R. Krenn, E. Devaux, and T. W. Ebbesen, “Design, near-field characterization, and modeling of 45° surface-plasmon Bragg mirrors,” Phys. Rev. B73(15), 155416 (2006). [CrossRef]
  38. J. A. Sanchez-Gil and A. A. Maradudin, “Surface-plasmon polariton scattering from a finite array of nanogrooves/ridges: Efficient mirrors,” Appl. Phys. Lett.86(25), 251106 (2005). [CrossRef]
  39. P. Nagpal, N. C. Lindquist, S. H. Oh, and D. J. Norris, “Ultrasmooth patterned metals for plasmonics and metamaterials,” Science325(5940), 594–597 (2009). [CrossRef] [PubMed]
  40. L. S. Jung, C. T. Campbell, T. M. Chinowsky, M. N. Mar, and S. S. Yee, “Quantitative interpretation of the response of surface plasmon resonance sensors to adsorbed films,” Langmuir14(19), 5636–5648 (1998). [CrossRef]
  41. S. Sjölander and C. Urbaniczky, “Integrated fluid handling system for biomolecular interaction analysis,” Anal. Chem.63(20), 2338–2345 (1991). [CrossRef] [PubMed]
  42. C. Escobedo, S. Vincent, A. I. K. Choudhury, J. Campbell, A. G. Brolo, D. Sinton, and R. Gordon, “Integrated nanohole array surface plasmon resonance sensing device using a dual-wavelength source,” J. Micromech. Microeng.21(11), 115001 (2011). [CrossRef]
  43. N. C. Lindquist, T. W. Johnson, D. J. Norris, and S. H. Oh, “Monolithic integration of continuously tunable plasmonic nanostructures,” Nano Lett.11(9), 3526–3530 (2011). [CrossRef] [PubMed]
  44. H. Im, S. H. Lee, N. J. Wittenberg, T. W. Johnson, N. C. Lindquist, P. Nagpal, D. J. Norris, and S. H. Oh, “Template-stripped smooth Ag nanohole arrays with silica shells for surface plasmon resonance biosensing,” ACS Nano5(8), 6244–6253 (2011). [CrossRef] [PubMed]

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