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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 21 — Oct. 10, 2011
  • pp: 20691–20703
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Non-spectroscopic refractometric nanosensor based on a tilted slit-groove plasmonic interferometer

Xiaowei Li, Qiaofeng Tan, Benfeng Bai, and Guofan Jin  »View Author Affiliations


Optics Express, Vol. 19, Issue 21, pp. 20691-20703 (2011)
http://dx.doi.org/10.1364/OE.19.020691


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Abstract

Plasmonic nanosensors are promising for chip-based refractometric detections, most of which are based on spectroscopic monitoring of surface plasmon resonance. Here, we propose a simple non-spectroscopic refractometric sensing scheme based on a plasmonic interferometer integrating a metallic groove array and a tilted nanoslit. Owing to the interference of the directly transmitted light from the nanoslit and that mediated by the surface plasmon polaritons launched from the groove array, high-contrast intensity fringe can be detected under the illumination of monochromatic light. By inspecting the spatial shift of the interference fringe, the refractive index change of the cover analyte can be derived. In our experiment, the interferometer shows a sensitivity up to 5 × 103 μm/RIU and a figure of merit as high as 250. This sensor shows great potential for low-cost, portable, and high-throughput sensing applications due to its simple, robust, and non-spectroscopic scheme.

© 2011 OSA

1. Introduction

Surface plasmon polariton (SPP), the collective electromagnetic excitation of free electrons on a metal surface, is sensitive to the local refractive index change of the dielectric environment, due to the intrinsic strong confinement and localization property of SPPs on the metal-dielectric interface [1

1. H. Raether, Surface Plasmons on Smooth and Rough Surfaces and on Gratings (Springer, 1988).

]. This property has led to the development of label-free plasmonic sensors for diverse applications in areas such as medical diagnosis, environment monitoring, food safety screening, and threat detection. Most commercially available plasmonic sensors are surface plasmon resonance (SPR) sensors, such as the prism-coupler sensors based on attenuated total reflection (ATR) [2

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

]. Owning to the resonant photo-SPP coupling, these prism-based SPR sensors can reach an extremely small detection limit of 10−5 refractive index unit (RIU), which can be further improved by using phase sensitive interferometric scheme [3

3. P. P. Markowicz, W. C. Law, A. Baev, P. N. Prasad, S. Patskovsky, and A. Kabashin, “Phase-sensitive time-modulated surface plasmon resonance polarimetry for wide dynamic range biosensing,” Opt. Express 15(4), 1745–1754 (2007), http://www.opticsinfobase.org/abstract.cfm?URI=oe-15-4-1745. [CrossRef] [PubMed]

] or measuring the Goos-Hänchen shift [4

4. X. Yin and L. Hesselink, “Goos-Hänchen shift surface plasmon resonance sensor,” Appl. Phys. Lett. 89(26), 261108 (2006). [CrossRef]

]. However, such SPR sensors are typically bulky and expensive and require a large amount of sample solution, which are not suitable for small-volume, high-throughput, and chip-based detections. Nowadays, it is demanded to develop compact and low-cost sensors for robust, portable, rapid, and multiplexed measurements [5

5. 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]

]. Various plasmonic nanosensors have been exploited for these aims, such as metallic nanoparticles with localized SPR and metallic nanohole arrays with extraordinary optical transmission [5

5. 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]

,6

6. 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]

], although their sensing response are usually much lower than that of prism-based SPR sensors [2

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

]. Efforts have been taken to enhance the sensitivity and resolution of plasmonic nanosensors. For example, some recently emerged nanostructures, such as the plasmonic vertical Mach-Zehnder interferometer based on subwavelength double slits [7

7. Q. Gan, Y. Gao, and F. J. Bartoli, “Vertical plasmonic Mach-Zehnder interferometer for sensitive optical sensing,” Opt. Express 17(23), 20747–20755 (2009), http://www.opticsinfobase.org/abstract.cfm?URI=oe-17-23-20747. [CrossRef] [PubMed]

,8

8. 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]

] and the nanorod metamaterials [9

9. A. V. Kabashin, P. Evans, S. Pastkovsky, W. Hendren, G. A. Wurtz, R. Atkinson, R. Pollard, V. A. Podolskiy, and A. V. Zayats, “Plasmonic nanorod metamaterials for biosensing,” Nat. Mater. 8(11), 867–871 (2009). [CrossRef] [PubMed]

], show almost one order of magnitude sensitivity enhancement. Some novel physical mechanisms, such as electromagnetically induced transparency [10

10. 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]

], diffraction coupling between metal nanoparticles [11

11. V. G. Kravets, F. Schedin, and A. N. Grigorenko, “Extremely narrow plasmon resonances based on diffraction coupling of localized plasmons in arrays of metallic nanoparticles,” Phys. Rev. Lett. 101(8), 087403 (2008). [CrossRef] [PubMed]

], and cross polarization detection scheme [12

12. 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]

], have been proposed to narrow the resonance linewidth and thus increase its resolution.

Interferometry is one of the most sensitive optical interrogation methods and various interferometric schemes have been utilized in refractometric sensors, such as Mach-Zehnder interferometer [7

7. Q. Gan, Y. Gao, and F. J. Bartoli, “Vertical plasmonic Mach-Zehnder interferometer for sensitive optical sensing,” Opt. Express 17(23), 20747–20755 (2009), http://www.opticsinfobase.org/abstract.cfm?URI=oe-17-23-20747. [CrossRef] [PubMed]

,8

8. 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]

,13

13. F. Prieto, B. Sepúlveda, A. Calle, A. Llobera, C. Domínguez, A. Abad, A. Montoya, and L. M. Lechuga, “An integrated optical interferometric nanodevice based on silicon technology for biosensor applications,” Nanotechnology 14(8), 907–912 (2003). [CrossRef]

], Young interferometer [14

14. A. Ymeti, J. S. Kanger, J. Greve, P. V. Lambeck, R. Wijn, and R. G. Heideman, “Realization of a multichannel integrated Young interferometer chemical sensor,” Appl. Opt. 42(28), 5649–5660 (2003). [CrossRef] [PubMed]

], dual-polarization interferometer [15

15. M. J. Swann, L. L. Peel, S. Carrington, and N. J. Freeman, “Dual-polarization interferometry: an analytical technique to measure changes in protein structure in real time, to determine the stoichiometry of binding events, and to differentiate between specific and nonspecific interactions,” Anal. Biochem. 329(2), 190–198 (2004). [CrossRef] [PubMed]

], porous interferometer [16

16. Y. Y. Li, F. Cunin, J. R. Link, T. Gao, R. E. Betts, S. H. Reiver, V. Chin, S. N. Bhatia, and M. J. Sailor, “Polymer replicas of photonic porous silicon for sensing and drug delivery applications,” Science 299(5615), 2045–2047 (2003). [CrossRef] [PubMed]

], and back-scattering interferometer [17

17. D. J. Bornhop, J. C. Latham, A. Kussrow, D. A. Markov, R. D. Jones, and H. S. Sørensen, “Free-solution, label-free molecular interactions studied by back-scattering interferometry,” Science 317(5845), 1732–1736 (2007). [CrossRef] [PubMed]

]. Recently, it was reported that light transmission through a slit milled in a metal film can be enhanced or suppressed as a result of interference of the directly transmitted light with the SPPs launched by a nearby parallel groove [18

18. G. Gay, O. Alloschery, B. Viaris de Lesegno, C. O’Dwyer, J. Weiner, and H. J. Lezec, “The optical response of nanostructured surfaces and the composite diffracted evanescent wave model,” Nat. Phys. 2(4), 262–267 (2006). [CrossRef]

,19

19. P. Lalanne and J. P. Hugonin, “Interaction between optical nano-objects at metallo-dielectric interfaces,” Nat. Phys. 2(8), 551–556 (2006). [CrossRef]

]. The interference is sensitive to the characteristics of SPPs supported by the metal-dielectric interface, such as the effective refractive index and propagation loss of the SPPs [18

18. G. Gay, O. Alloschery, B. Viaris de Lesegno, C. O’Dwyer, J. Weiner, and H. J. Lezec, “The optical response of nanostructured surfaces and the composite diffracted evanescent wave model,” Nat. Phys. 2(4), 262–267 (2006). [CrossRef]

,19

19. P. Lalanne and J. P. Hugonin, “Interaction between optical nano-objects at metallo-dielectric interfaces,” Nat. Phys. 2(8), 551–556 (2006). [CrossRef]

]. This interferometric scheme has been applied in all-optical plasmonic switches and modulators [20

20. 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]

,21

21. D. Pacifici, H. J. Lezec, and H. A. Atwater, “All-optical modulation by plasmonic excitation of CdSe quantum dots,” Nat. Photonics 1(7), 402–406 (2007). [CrossRef]

] and is believed to be promising for sensitive label-free sensing applications [20

20. 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]

].

In this paper, we propose a non-spectroscopic refractometric sensing scheme based on a plasmonic interferometer integrating a metallic groove array and a tilted nanoslit, as shown in Fig. 1
Fig. 1 (a) Schematic of the proposed tilted nanoslit-groove interferometer. X is the auxiliary coordinate axis by rotating the x axis with a small angle α. (b) Cross-section view of the interferometer. (c) SEM image of a fabricated sample with N = 10, L0 = 15.8 μm, and α = 4°.
. Under normal incidence, SPPs can be excited by the groove array and propagate to the nanoslit. Due to the varied distance between the tilted nanoslit and the groove array at different x, the SPP-mediated transmission through the slit interferes constructively or destructively with the directly transmitted light, generating an intensity interference fringe along the nanoslit, which can be detected by, e.g., a charge coupled device (CCD). The variation of refractive index of the cover medium, which changes the wave vector (and therefore the accumulated phase shift) of the SPPs launched from the groove array, can be detected by inspecting the shift of the interference fringe.

2. Principle of the plasmonic interferometer

The proposed interferometer is schematically depicted in Fig. 1, in which an array of N shallow grooves with period p and a subwavelength nanoslit of width w are patterned in a 250 nm thick gold film deposited on a fused silica substrate with a 15 nm thick titanium adhesion layer in between. L0 is the distance between the center of the groove array and the nanoslit center at x = 0 (see Appendix A for the selection of L0). The small tilt angle of the nanoslit with respect to the grooves is denoted as α. A TM polarized light (whose magnetic field vector is parallel with the grooves) of wavelength λ = 1064 nm impinges on the structure from the upper side at normal incidence, for which the permittivities of gold and silica are −52.02 + 3.87i and 1.452, respectively. Since in experiment the incident light may deviate slightly from the normal direction, we denote the small angle between the incident wave vector k0 and the z axis in the yz plane as γ. For convenience of analysis, an auxiliary coordinate axis X is introduced, which is parallel with the nanoslit so that x = Xcosα. Due to the interference, the transmitted intensity I(X) along the nanoslit can be derived as (see Appendix B for details)
I(X)=Espp2+Edir2+2EsppEdircos[k0L(X)εmn2εm+n2+k0L(X)sinγ+φ0],
(1)
where Espp and Edir are the electric field amplitudes of the SPP-mediated and directly transmitted fields, respectively, ε'm is the real part of the permittivity of gold, n is the refractive index of the dielectric material on top of the metal surface, k0 = 2π/λ is the wave number of incident light in vacuum, L(X) = L0+Xsinα is the slit-groove distance (and also the propagation length of SPPs) at different X, and φ0 represents the initial phase shift of SPPs at excitation. In Eq. (1), the first term in the cosine function is the phase delay of SPPs when propagating from the groove array to the nanoslit and the second term is the phase difference induced by the non-normal incidence. Therefore, the period of the cosine-shaped interference fringe along the nanoslit is

d=(εmn2εm+n2+sinγ)1λsinα.
(2)

The structural parameters are optimized with the goal of keeping the intensities of the two arms as close as possible to each other so as to get the best contrast of interference. To begin with, we consider a simple case with air (n = 1.00) as the upper dielectric. The groove period p is chosen equal to the SPP wavelength (λspp = 1054 nm) according to the phase-matching condition of SPP excitation by a grating at normal incidence. For simplicity, the grooves are in rectangular profile with a depth of 50 nm and a duty cycle of 0.5. The other parameters are numerically optimized, with a commercial finite-element-method software COMSOL Multiphysics 3.5a, to achieve the best matched intensities of the two interference arms.

We first simulate the transmission through a single nanoslit (without the groove array nearby), as shown in Fig. 2(a)
Fig. 2 Simulated transmission from the tilted nanoslit-groove interferometer with air as the upper dielectric. (a) Normalized transmitted intensity from the nanoslit by illuminating only a single nanoslit with varied w (the solid line) or illuminating only the groove array in a slit-groove structure with w = 100 nm, p = 1054 nm, and different N (the dashed line). (b) Normalized total transmitted intensity from the nanoslit by illuminating the whole slit-groove structure with w = 100 nm, p = 1054 nm, and N = 11. (c1) and (c2) show the distributions of magnetic field component Hz in the slit-groove structure when L = 16.58 μm and 17.08 μm as indicated by arrows c1 and c2 in (b), respectively.
. The transmitted intensity varies slightly with respect to the change of slit width w (when w > 70 nm). Thus we simply choose w = 100 nm. Then we consider a slit-groove structure with L0 = 16 μm by illuminating only the groove array, in which case only the SPPs launched from the grooves contribute to the transmission. The calculated transmitted intensity from the nanoslit in terms of the number of grooves N is shown as the dashed curve in Fig. 2(a), which is almost equal in strength to the directly transmitted light when N = 11. Therefore, we choose N = 11 and w = 100 nm and simulate the total transmission intensity by illuminating the whole slit-groove structure, as shown in Fig. 2(b). As anticipated, the transmitted intensity shows a cosine-shaped interference curve with respect to L. The field distributions in Figs. 2(c1) and 2(c2) correspond to the minimum Imin and the maximum Imax points marked as c1 (with L = 16.58 μm) and c2 (L = 17.08 μm) in Fig. 2(b), respectively, in which the destructive and constructive interferences are evidently observed. We define an extinction ratio r = 10lg(Imax/Imin) to indicate the contrast of the interference curve. According to Fig. 2(b), we can derive r = 44.57 dB, which indeed shows very high contrast.

To validate the feasibility of the interferometer, a number of samples have been fabricated by focused ion beam milling (with FEI Nova 200 Nanolab), where the groove array and the nanoslit were milled in a 250 nm thick gold film deposited on a fused silica substrate with 15nm thick Ti adhesion layer in between. Figure 1(c) shows the scanning electronic microscope (SEM) image of a typical sample with N = 10, α = 4°, and L0 = 15.8 μm. In our experiment, as shown in Fig. 3
Fig. 3 Schematic and experimental setup of the optical characterization.
, the optical setup operates in a collinear transmission mode, i.e., the light source, the sample, and the detector are aligned along the same optical axis. A 1064 nm 200 mW solid state laser with spot size of 2 mm is used as the light source. After passing through a tunable attenuator and a polarizer, the TM polarized laser beam is incident on the sample. The transmitted light of the interference fringe from the nanoslit is collected by a long working-distance microscope objective (Nikon, 40×, NA = 0.6) and imaged by a CCD camera with 1280 × 1024 resolution and pixel size of 6.7 μm. The tunable attenuator is used to control the intensity of the incident light so that the detected CCD image has the best contrast (but is not saturated) with high signal-to-noise ratio. Two reflectors are used to fold the light path. The sample is attached to a three-dimensional translation stage. We found that the samples with N = 10 (instead of N = 11 in simulation) show the best contrast, probably due to the deviation of practical parameters such as the refractive index of the deposited gold film and the depth and width of the etched grooves. Figure 4
Fig. 4 Experimental characterization of the transmitted interference fringes in two samples with N = 10, pλspp = 1054 nm, w = 100 nm, L0 = 15.8 μm, and α = 2° or 4°. (a) Measured CCD image (false color image) of the interference fringe in a sample with α = 2°. (b) The same as (a) but for a sample with α = 4°. (c) Fringe profiles along the central lines of the nanoslits retrieved from (a) and (b) and fitted with Eq. (1).
shows the measured transmitted intensities of two samples with N = 10, pλspp = 1054 nm, w = 100 nm, L0 = 15.8 μm, nanoslit length of 60 μm, and α = 2° and 4°. The detected CCD images of the interference fringes can be clearly observed in Figs. 4(a) and 4(b). To retrieve the fringe period, we plot the intensity profiles along the central lines of the nanoslits and fit them with cosine functions according to Eq. (1), as shown in Fig. 4(c). The periods of the two interference curves are estimated to be 31.94 μm and 15.9 μm, which closely match the theoretical predictions of 30.19 μm and 15.10μm calculated by Eq. (2), respectively. The small discrepancies are probably caused by the small incident angle γ in our measurement, which are calculated to be 3.16° and 2.89° according to Eq. (2). This is possible in our measurement setup. With these experiments, the working principle of the proposed interferometer is proved.

3. Refractometric sensing experiment

Then we investigate the use of the interferometer as a plasmonic refractometric sensor. According to Eq. (1), the transmitted intensity from the nanoslit is sensitive to the refractive index n of the upper dielectric. Therefore, the refractive index change can be detected, e.g., by inspecting the intensity variation at a fixed point X on the slit. But this detection scheme is liable to be disturbed by the intensity fluctuation of the light source [22

22. 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]

]. Alternatively, we can inspect the shift of the whole interference fringe ∆X with respect to the cover refractive index change ∆n. Since we avoid the absolute intensity measurement, the scheme is robust to the source disturbance and allows high-precision detection. From Eq. (1), we can derive the sensitivity of the interferometer S = −∆X/∆n (see Appendix C for details). If we neglect the small influence by φ0 and γ, the sensitivity can be written in an elegant form
S=ΔXΔn=εmεmn+n3(L0sinα+X)εmεmn+n3L0sinα,
(3)
where the approximation is taken because L0/sinα >> X in general case.

Equation (3) shows that the sensitivity S can be improved by increasing the slit-groove spacing L0 or decreasing the tilt angle α. In principle, we can get infinitely large S by taking α close to 0. However, in practice this is not realistic. According to Eq. (2), smaller α leads to larger fringe period d, which makes the detection resolution (i.e., the minimum detectable fringe shift δX) worse. In other words, there is a trade-off between the sensitivity and the detection resolution, both of which are important for sensing applications. Therefore, to have a more meaningful measure of the practical sensing quality, we should also evaluate the figure of merit (FOM) of the device, which is defined as the ratio of sensitivity S to the full width at half maximum (FWHM) of the fringe [23

23. L. J. Sherry, R. Jin, C. A. Mirkin, G. C. Schatz, and R. P. Van Duyne, “Localized surface plasmon resonance spectroscopy of single silver triangular nanoprisms,” Nano Lett. 6(9), 2060–2065 (2006). [CrossRef] [PubMed]

]. Larger FOM means higher sensitivity for a fixed FWHM or narrower fringe for a fixed sensitivity. For our cosine-shaped interference fringe, the FWHM is half of the fringe period d. Thus, the theoretical FOM can be derived from Eqs. (2) and (3) as (if we still neglect the influence of γ)
FOM=Sd/2=2(εmεm+n2)32L0λ.
(4)
To demonstrate the sensing performance of the proposed interferometric sensor, we have performed proof-of-principle sensing experiments with NaCl aqueous solution of different concentrations (in wt %) as the analyte. The interferometric sensors were designed and fabricated in the same way as the air-covered samples studied above, by replacing the upper dielectric with the NaCl solution (whose refractive index is around 1.333). Several samples with different L0 between 67.7 μm to 127.7 μm and α between 1° and 2° were fabricated, with p = 786 nm, N = 20, w = 100 nm, and nanoslit length of 60 μm according to the numerical optimization. In the sensing experiment, a flow cell was overlaid on the sample chip to allow the access of NaCl solution to the interferometer. Note that the thermal effect is non-negligible for sensors because the refractive index of analyte is also dependent on the environment temperature. In the present work, we did not take special measure to precisely control the temperature. But all the experiments were performed in air-conditioned room temperature around 25°C. In each measurement, the liquid analyte was always illuminated by continuous laser beam for a while after its injection so that the temperature of the analyte can be stabilized. This can be observed from the stabilization of the interference fringe on the CCD camera. Then the CCD images were recorded as the measurement results. Our experimental results (as given later) also show that the temperature influence is quite small in this condition. But for the practical application of the sensors, the thermal effect must be thoroughly studied and the precise control of temperature should be taken into account, which will be the tasks of our future work.

Figure 5
Fig. 5 Measured interference fringes of two samples with NaCl solution of different concentrations as the upper dielectric: (a) and (c) for Sample I with L0 = 127.7 μm and α = 1°; and (b) and (d) for Sample II with L0 = 127.7 μm and α = 2°. (a) and (b) are the CCD false color images of the transmitted interference fringes. (c) and (d) are the intensity profiles along the central nanoslits and their cosine fitting curves. The curves are vertically displaced by 230 for clarity of demonstration. The dashed lines are visual guides to the valley positions of the interference fringes. The dashed circles show the position of a defect in the nanoslit.
shows the measured interference fringes in two typical samples (Sample I with L0 = 127.7 μm and α = 1°; and Sample II with L0 = 127.7 μm and α = 2°) covered with analytes of different NaCl concentrations. For both samples, we can clearly see the spatial shifts of the interference fringes to the left (as indicated by the dashed lines) with the increase of the analyte concentration. In Figs. 5(c) and 5(d), the intensity profiles along the center of the nanoslits, which are retrieved from the CCD images in Figs. 5(a) and 5(b), are plotted and fitted with cosine functions I = Acos(BX+C)+D, where A, B, C, and D are fitting parameters. It is seen that, except for the small fluctuation of the measurement data caused by the small fabrication defects of the nanoslits [such as those indicated by the dashed circles in Figs. 5(a) and 5(c)], the fringes are well fitted with the cosine profiles. Note that some other methods such as the fringe pattern matching method [24

24. Z. Wang, P. J. Bryanston-Cross, and D. J. Whitehouse, “Phase difference determination by fringe pattern matching,” Opt. Laser Technol. 28(6), 417–422 (1996). [CrossRef]

] and the fast Fourier transform method [17

17. D. J. Bornhop, J. C. Latham, A. Kussrow, D. A. Markov, R. D. Jones, and H. S. Sørensen, “Free-solution, label-free molecular interactions studied by back-scattering interferometry,” Science 317(5845), 1732–1736 (2007). [CrossRef] [PubMed]

] may also be used for the signal processing. However, since our interference fringe strictly follows the cosine function (as indicated in Section 2 and Appendix B), the simple cosine fitting is precise enough to determine the shift of the curves.

According to Fig. 6(b), the performance of the sensor mainly depends on the slit-groove distance L0 and the tilt angle α. By increasing L0, higher sensitivity and higher FOM can be achieved. However, in practice, this is limited by two factors. On one hand, the requirement for miniaturized nanosensors with small footprint for probing nanovolume analyte limits the increase of L0. On the other hand, L0 cannot be arbitrarily large due to the propagation loss of SPPs. For example, in our sensing experiment, the numerically optimized L0 is 62.43μm, in which case the two arms have the same intensity (Ispp = Idir) and the interference pattern has the best contrast V = 1. By increasing L0, if we want to keep the high contrast, the number of grooves N must be increased so as to maintain the intensity balance of the two arms (as discussed in Section 2). However, if all the other parameters are fixed and only L0 is increased, obviously the contrast will decrease due to the propagation loss of the SPPs. In other words, we may obtain higher sensitivity and higher FOM by paying the cost of losing the contrast of the interference fringe. In practical application, as long as the interference contrast is not too small and can still be resolved by the CCD, it is acceptable. We have used a tunable attenuator to control the incident light intensity to let the maximum fringe intensity always close to 256 (maximum pixel value on our 8-digit CCD camera). Then the minimum intensity of the fringe is 256−A, where A is the difference of the maximum and minimum intensities. For example, we may set up a constraint condition (which depends on the performance of the detection system): if the noise fluctuation of signal is less than A/4, the fringe signal can be resolved. According to our experimental results, the mean noise fluctuation of signal is around the magnitude of 25.43 [as estimated from the results in Fig. 5(c)]. Thus, with A/4 > 25.43 we can derive that the interference contrast should be V > 0.2479. Denote that L0 is increased by ∆L. Then Ispp is decreased due to the SPP propagation loss by a factor of exp(−2ksppL), where kspp is the imaginary part of the wave number of the SPPs and can be calculated from the SPP dispersion relation. Since Idir does not change, the interference contrast becomes
V=2IsppIdirIspp+Idir=2exp(2ksppΔL)exp(2ksppΔL)+1=0.2479,
(5)
from which we can get ∆L = 197.7μm. Thus the upper limit of the allowable L0 is 197.7 + 67.7 = 265.4μm. In our sensing experiment, we have fabricated eight samples with varied L0 (67.7μm, 87.7μm, 107.7μm, 127.7μm) and α (1° and 2°), whose interference contrasts are estimated and shown in Fig. 7
Fig. 7 Contrast of the interference fringes in the eight samples used in the sensing experiment with respect to the change of L0
. We can see that the interference contrast indeed gradually decreases with the increase of L0 but is not too low (> 0.65) because in this case L0 is much smaller than the allowable limit 265.4μm.

Reducing α is another effective way of enhancing the sensitivity, as seen from Eq. (3) and Fig. 6, where almost one-fold sensitivity enhancement can be achieved by changing α from 2° to 1° with the FOM almost unchanged. However, higher sensitivity makes the line width of the interference fringe broader (see Fig. 5), which degrades the detection resolution. Therefore, L0 and α should be properly chosen so as to make a reasonable compromise among the sensitivity S, FOM, and the detection resolution in practical sensing applications.

4. Conclusion

To conclude, we have proposed and experimentally demonstrated a non-spectroscopic refractometric sensing scheme based on a plasmonic interferometer integrating a metallic groove array and a tilted nanoslit. Owing to the interference of the directly transmitted light from the nanoslit and that mediated by the SPPs launched from the groove array, high-contrast intensity interference fringe with uniform cosine-shaped pattern can be generated under the illumination of monochromatic light. By inspecting the spatial shift of interference fringe, the refractive index change of the cover medium can be detected. Our sensing experiments with NaCl solutions as the analytes show that the sensitivity of the interferometric sensors can reach 5674 μm/RIU and the FOM is up to 250, which are among the highest performance of the reported plasmonic refractometric sensors.

Appendix A: On the selection of L0

As shown in Fig. 1, an array of N shallow grooves is used for SPP excitation. Since each groove can excite SPPs but has different distance with the nanoslit, the accumulated phase delay of the SPPs propagating from each groove to the nanoslit is also different, which yields different sensitivity according to Eq. (3). That is, the groove farthest from the nanoslit has the highest sensitivity, while the groove closest to the nanoslit has the lowest sensitivity. In our work, we selected L0 as the distance from the center of the groove array to the nanoslit. To show that this is a reasonable solution, we treat each groove as a SPP source and the groove period is chosen equal to the SPP wavelength at n = 1.333. In Fig. 8
Fig. 8 Calculated interference fringes contributed by each grooves (the blue curves) and the superposed total interference fringe (the red curve) in a slit-groove structure with L0 = 120μm and α = 1°.
, we show the calculated interference fringes along the nanoslit by considering the respective contributions of each grooves (the blue curves) as well as their superposition (the red curve). It’s seen that the superposed fringe is always in the same position as that contributed by the groove in the middle for the groove array. Therefore, our definition of L0 is reasonable, which is also validated by the good correspondence between experiment and theory in our sensing experiments (see Fig. 6).

Appendix B: Intensity expression of the interference fringe

When a TM polarized plane wave illuminates the tilted slit-groove interferometer at an angle γ with respect to the z axis, the grooves excite a fraction of the incident light into SPPs towards the nanoslit (whose propagation direction is indicated by dashed arrows in Fig. 1(a)). The SPPs scattered at the nanoslit reradiate into transmitted light whose electric field amplitude is denoted as Espp. The directly transmitted light through the nanoslit, whose amplitude is denoted as Edir, would interfere with the SPP-mediated transmitted light. Then the total transmitted intensity I(X) of the interference pattern along the nanoslit can be derived as
I(X)=Espp2+Edir2+2EsppEdircos[φspp(X)+φinc(X)+φ0],
(B1)
where
φspp(X)=ksppL(X)=k0(εmn2εm+n2)12L(X),
(B2)
is the phase delay of SPPs when propagating from the groove array to the slit,
φinc(X)=k0L(X)sinγ,
(B3)
is the phase difference induced by the non-normal incident angle γ, and φ0 is the additional initial phase of SPPs excited by the grooves.

Appendix C: Derivation of the sensitivity

The sensitivity of the interferometer is defined as S = −∆X/∆n, which can be obtained by putting the phase term in Eq. (B1) as a constant, i.e.,
φspp(X)+φinc(X)+φ0=const,
(C1)
and taking its derivative with respect to n. Therefore, with Eqs. (B2), (B3), and (C1), we have

S=dXdn=(εmεm+n2)32(L0+Xsinα)+dφ0dn[(εmn2εm+n2)12+sinγ]sinα,
(C2)

If we neglect the small influence by φ0 and γ, the sensitivity can be expressed in an elegant form as

S=εmεmn+n3(L0sinα+X),
(C3)

Since L0/sinα >> X, Eq. (C3) can be further simplified as

Sεmεmn+n3L0sinα.
(C4)

Acknowledgments

We thank Mr. Hao Zhu for measuring the actual refractive indices of the analytes with an Abbe refractometer. We acknowledge the support by the National Basic Research Program of China (Project No. 2007CB935303), the National Natural Science Foundation of China (Project No. 11004119), and the Academy of Finland (Project No. 128420).

References and links

1.

H. Raether, Surface Plasmons on Smooth and Rough Surfaces and on Gratings (Springer, 1988).

2.

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

3.

P. P. Markowicz, W. C. Law, A. Baev, P. N. Prasad, S. Patskovsky, and A. Kabashin, “Phase-sensitive time-modulated surface plasmon resonance polarimetry for wide dynamic range biosensing,” Opt. Express 15(4), 1745–1754 (2007), http://www.opticsinfobase.org/abstract.cfm?URI=oe-15-4-1745. [CrossRef] [PubMed]

4.

X. Yin and L. Hesselink, “Goos-Hänchen shift surface plasmon resonance sensor,” Appl. Phys. Lett. 89(26), 261108 (2006). [CrossRef]

5.

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]

6.

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]

7.

Q. Gan, Y. Gao, and F. J. Bartoli, “Vertical plasmonic Mach-Zehnder interferometer for sensitive optical sensing,” Opt. Express 17(23), 20747–20755 (2009), http://www.opticsinfobase.org/abstract.cfm?URI=oe-17-23-20747. [CrossRef] [PubMed]

8.

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]

9.

A. V. Kabashin, P. Evans, S. Pastkovsky, W. Hendren, G. A. Wurtz, R. Atkinson, R. Pollard, V. A. Podolskiy, and A. V. Zayats, “Plasmonic nanorod metamaterials for biosensing,” Nat. Mater. 8(11), 867–871 (2009). [CrossRef] [PubMed]

10.

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]

11.

V. G. Kravets, F. Schedin, and A. N. Grigorenko, “Extremely narrow plasmon resonances based on diffraction coupling of localized plasmons in arrays of metallic nanoparticles,” Phys. Rev. Lett. 101(8), 087403 (2008). [CrossRef] [PubMed]

12.

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]

13.

F. Prieto, B. Sepúlveda, A. Calle, A. Llobera, C. Domínguez, A. Abad, A. Montoya, and L. M. Lechuga, “An integrated optical interferometric nanodevice based on silicon technology for biosensor applications,” Nanotechnology 14(8), 907–912 (2003). [CrossRef]

14.

A. Ymeti, J. S. Kanger, J. Greve, P. V. Lambeck, R. Wijn, and R. G. Heideman, “Realization of a multichannel integrated Young interferometer chemical sensor,” Appl. Opt. 42(28), 5649–5660 (2003). [CrossRef] [PubMed]

15.

M. J. Swann, L. L. Peel, S. Carrington, and N. J. Freeman, “Dual-polarization interferometry: an analytical technique to measure changes in protein structure in real time, to determine the stoichiometry of binding events, and to differentiate between specific and nonspecific interactions,” Anal. Biochem. 329(2), 190–198 (2004). [CrossRef] [PubMed]

16.

Y. Y. Li, F. Cunin, J. R. Link, T. Gao, R. E. Betts, S. H. Reiver, V. Chin, S. N. Bhatia, and M. J. Sailor, “Polymer replicas of photonic porous silicon for sensing and drug delivery applications,” Science 299(5615), 2045–2047 (2003). [CrossRef] [PubMed]

17.

D. J. Bornhop, J. C. Latham, A. Kussrow, D. A. Markov, R. D. Jones, and H. S. Sørensen, “Free-solution, label-free molecular interactions studied by back-scattering interferometry,” Science 317(5845), 1732–1736 (2007). [CrossRef] [PubMed]

18.

G. Gay, O. Alloschery, B. Viaris de Lesegno, C. O’Dwyer, J. Weiner, and H. J. Lezec, “The optical response of nanostructured surfaces and the composite diffracted evanescent wave model,” Nat. Phys. 2(4), 262–267 (2006). [CrossRef]

19.

P. Lalanne and J. P. Hugonin, “Interaction between optical nano-objects at metallo-dielectric interfaces,” Nat. Phys. 2(8), 551–556 (2006). [CrossRef]

20.

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]

21.

D. Pacifici, H. J. Lezec, and H. A. Atwater, “All-optical modulation by plasmonic excitation of CdSe quantum dots,” Nat. Photonics 1(7), 402–406 (2007). [CrossRef]

22.

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]

23.

L. J. Sherry, R. Jin, C. A. Mirkin, G. C. Schatz, and R. P. Van Duyne, “Localized surface plasmon resonance spectroscopy of single silver triangular nanoprisms,” Nano Lett. 6(9), 2060–2065 (2006). [CrossRef] [PubMed]

24.

Z. Wang, P. J. Bryanston-Cross, and D. J. Whitehouse, “Phase difference determination by fringe pattern matching,” Opt. Laser Technol. 28(6), 417–422 (1996). [CrossRef]

25.

M. Svedendahl, S. Chen, A. Dmitriev, and M. Käll, “Refractometric sensing using propagating versus localized surface plasmons: a direct comparison,” Nano Lett. 9(12), 4428–4433 (2009). [CrossRef] [PubMed]

26.

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

OCIS Codes
(120.3180) Instrumentation, measurement, and metrology : Interferometry
(240.6680) Optics at surfaces : Surface plasmons
(280.4788) Remote sensing and sensors : Optical sensing and sensors

ToC Category:
Sensors

History
Original Manuscript: August 5, 2011
Revised Manuscript: September 2, 2011
Manuscript Accepted: September 22, 2011
Published: October 4, 2011

Citation
Xiaowei Li, Qiaofeng Tan, Benfeng Bai, and Guofan Jin, "Non-spectroscopic refractometric nanosensor based on a tilted slit-groove plasmonic interferometer," Opt. Express 19, 20691-20703 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-21-20691


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References

  1. H. Raether, Surface Plasmons on Smooth and Rough Surfaces and on Gratings (Springer, 1988).
  2. J. Homola, “Surface plasmon resonance sensors for detection of chemical and biological species,” Chem. Rev.108(2), 462–493 (2008). [CrossRef] [PubMed]
  3. P. P. Markowicz, W. C. Law, A. Baev, P. N. Prasad, S. Patskovsky, and A. Kabashin, “Phase-sensitive time-modulated surface plasmon resonance polarimetry for wide dynamic range biosensing,” Opt. Express15(4), 1745–1754 (2007), http://www.opticsinfobase.org/abstract.cfm?URI=oe-15-4-1745 . [CrossRef] [PubMed]
  4. X. Yin and L. Hesselink, “Goos-Hänchen shift surface plasmon resonance sensor,” Appl. Phys. Lett.89(26), 261108 (2006). [CrossRef]
  5. 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]
  6. 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]
  7. Q. Gan, Y. Gao, and F. J. Bartoli, “Vertical plasmonic Mach-Zehnder interferometer for sensitive optical sensing,” Opt. Express17(23), 20747–20755 (2009), http://www.opticsinfobase.org/abstract.cfm?URI=oe-17-23-20747 . [CrossRef] [PubMed]
  8. 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]
  9. A. V. Kabashin, P. Evans, S. Pastkovsky, W. Hendren, G. A. Wurtz, R. Atkinson, R. Pollard, V. A. Podolskiy, and A. V. Zayats, “Plasmonic nanorod metamaterials for biosensing,” Nat. Mater.8(11), 867–871 (2009). [CrossRef] [PubMed]
  10. 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]
  11. V. G. Kravets, F. Schedin, and A. N. Grigorenko, “Extremely narrow plasmon resonances based on diffraction coupling of localized plasmons in arrays of metallic nanoparticles,” Phys. Rev. Lett.101(8), 087403 (2008). [CrossRef] [PubMed]
  12. 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]
  13. F. Prieto, B. Sepúlveda, A. Calle, A. Llobera, C. Domínguez, A. Abad, A. Montoya, and L. M. Lechuga, “An integrated optical interferometric nanodevice based on silicon technology for biosensor applications,” Nanotechnology14(8), 907–912 (2003). [CrossRef]
  14. A. Ymeti, J. S. Kanger, J. Greve, P. V. Lambeck, R. Wijn, and R. G. Heideman, “Realization of a multichannel integrated Young interferometer chemical sensor,” Appl. Opt.42(28), 5649–5660 (2003). [CrossRef] [PubMed]
  15. M. J. Swann, L. L. Peel, S. Carrington, and N. J. Freeman, “Dual-polarization interferometry: an analytical technique to measure changes in protein structure in real time, to determine the stoichiometry of binding events, and to differentiate between specific and nonspecific interactions,” Anal. Biochem.329(2), 190–198 (2004). [CrossRef] [PubMed]
  16. Y. Y. Li, F. Cunin, J. R. Link, T. Gao, R. E. Betts, S. H. Reiver, V. Chin, S. N. Bhatia, and M. J. Sailor, “Polymer replicas of photonic porous silicon for sensing and drug delivery applications,” Science299(5615), 2045–2047 (2003). [CrossRef] [PubMed]
  17. D. J. Bornhop, J. C. Latham, A. Kussrow, D. A. Markov, R. D. Jones, and H. S. Sørensen, “Free-solution, label-free molecular interactions studied by back-scattering interferometry,” Science317(5845), 1732–1736 (2007). [CrossRef] [PubMed]
  18. G. Gay, O. Alloschery, B. Viaris de Lesegno, C. O’Dwyer, J. Weiner, and H. J. Lezec, “The optical response of nanostructured surfaces and the composite diffracted evanescent wave model,” Nat. Phys.2(4), 262–267 (2006). [CrossRef]
  19. P. Lalanne and J. P. Hugonin, “Interaction between optical nano-objects at metallo-dielectric interfaces,” Nat. Phys.2(8), 551–556 (2006). [CrossRef]
  20. 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]
  21. D. Pacifici, H. J. Lezec, and H. A. Atwater, “All-optical modulation by plasmonic excitation of CdSe quantum dots,” Nat. Photonics1(7), 402–406 (2007). [CrossRef]
  22. 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]
  23. L. J. Sherry, R. Jin, C. A. Mirkin, G. C. Schatz, and R. P. Van Duyne, “Localized surface plasmon resonance spectroscopy of single silver triangular nanoprisms,” Nano Lett.6(9), 2060–2065 (2006). [CrossRef] [PubMed]
  24. Z. Wang, P. J. Bryanston-Cross, and D. J. Whitehouse, “Phase difference determination by fringe pattern matching,” Opt. Laser Technol.28(6), 417–422 (1996). [CrossRef]
  25. M. Svedendahl, S. Chen, A. Dmitriev, and M. Käll, “Refractometric sensing using propagating versus localized surface plasmons: a direct comparison,” Nano Lett.9(12), 4428–4433 (2009). [CrossRef] [PubMed]
  26. J. Henzie, M. H. Lee, and T. W. Odom, “Multiscale patterning of plasmonic metamaterials,” Nat. Nanotechnol.2(9), 549–554 (2007). [CrossRef] [PubMed]

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