A new sensing approach based on the use of an optical Fabry-Pérot (FP) cavity in conjunction with surface plasmon resonance (SPR) is proposed and theoretically investigated. The impact of the SPR on the intensity and phase response of the proposed sensor structure is evaluated using a modified FP model that takes into account the SPR effect. Compared to the conventional optical-phase-detection-based Kretschmann configuration, the proposed sensing approach requires only the measurement of the output power spectrum over a narrow wavelength span of several nanometers to evaluate the phase responses of the sensor, making it more attractive for practical high-sensitivity sensing applications

Alameh, Kamal

Li, Guangyuan

Xiao, Feng

Xu, Anshi

Optics Letters

English

Research Online @ ECU

Edith Cowan University Research Online ECU Publications 2012 1-1-2012 Fabry-Perot-based surface plasmon resonance sensors Feng Xiao Guangyuan Li Kamal Alameh Edith Cowan University Anshi Xu Follow this and additional works at: https://ro.ecu.edu.au/ecuworks2012  Part of the Engineering Commons 10.1364/OL.37.004582 This is an Author's Accepted Manuscript of: Xiao, F. , Li, G., Alameh, K. , & Xu, A. (2012). Fabry-Perot-based surface plasmon resonance sensors. Optics Letters, 37(22), 4582-4584. Available here © 2012 Optical Society of America]. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modifications of the content of this paper are prohibited. This Journal Article is posted at Research Online. https://ro.ecu.edu.au/ecuworks2012/253 Fabry–Pérot-based surface plasmon resonance sensorsFeng Xiao,1,* Guangyuan Li,2,3 Kamal Alameh,1,4 and Anshi Xu21Electron Science Research Institute, Edith Cowan University, Joondalup 6027, Australia2Department of Electronics, Peking University, Beijing 100871, China3e-mail: gyli@pku.edu.cn4e-mail: k.alameh@ecu.edu.au*Corresponding author: f.xiao@ecu.edu.auReceived September 13, 2012; accepted September 24, 2012;posted September 28, 2012 (Doc. ID 176125); published November 6, 2012A new sensing approach based on the use of an optical Fabry–Pérot (FP) cavity in conjunction with surface plasmonresonance (SPR) is proposed and theoretically investigated. The impact of the SPR on the intensity and phase re-sponse of the proposed sensor structure is evaluated using a modified FP model that takes into account the SPReffect. Compared to the conventional optical-phase-detection-based Kretschmann configuration, the proposed sen-sing approach requires only the measurement of the output power spectrum over a narrow wavelength span ofseveral nanometers to evaluate the phase responses of the sensor, making it more attractive for practical high-sensitivity sensing applications. © 2012 Optical Society of AmericaOCIS codes: 280.4788, 240.6680.Surface plasmon resonance (SPR) has been used for thedetection of refractive index changes right above a metal-dielectric interface [1,2]. In the last two decades, SPR-based sensors have become one of the most importantlabel-free optical sensors for the study of biomolecularinteractions and the real-time detection and identifica-tion of chemical and biological analytes. Due to their highsensitivity and selectivity, SPR sensors have now beenused in a wide range of applications, including proteo-mics, medical diagnostics, drug designs, environmentalmonitoring, and food safety and security [3].The excitation of surface plasmons in SPR sensors re-sults in a change in one of the characteristics of the outputresponse, which can be detected through various opticaldetection approaches, thus enabling the measurement ofthe refractive index of the sample under investigation.Several SPR sensor configurations based on different sig-nal interrogation schemes have been reported, includingdetection of (1) the input optical beam angle at which re-sonance takes place [4], (2) the resonancewavelength [5],(3) the output optical signal intensity [6], (4) the outputoptical signal phase change [7], and (5) the output opticalsignal polarization change [8]. The first three types of sig-nal interrogation are being used in most of today’s SPRsensors, due to their simplicity, low cost, and high perfor-mance. The structures of the other types are complex, andhence they have not been widely used, although theirability to offer higher sensitivities has been demonstrated[8,9]. Recently, in order to improve the sensitivity andpracticality of SPR sensors and extend their applications,many research works have been focused on devising newconfigurations and mechanisms while retaining the sim-plicity of conventional SPR sensors [10,11]. Among them,plasmonic cavities have generally been used to enhancethe sensitivity of the SPR sensors [12,13].In this Letter, we propose and investigate a new sensingapproach based on the use of an optical Fabry–Pérot (FP)cavity in conjunction with SPR. Specifically, we incorpo-rate plasmonic structure within an FP cavity and theore-tically analyze the optical intensity and phase responsesresulting from the SPR effect. A practical sensing scenariois analyzed to demonstrate that the combination of the FPand SPR effects enables the measurement of the opticalphase response, and hence the evaluation of the refractiveindex of a sample, through themeasurement of the outputpower spectrum over a narrow wavelength span.The proposed sensing approach is shown in Fig. 1. Ametal layer of dielectric constant εm and thickness d iscoated on a prism. The refractive indexes of the prismand the sensed medium are assumed to be np and ns, re-spectively. In order to investigate the SPR effect on theFP interference response, a p-polarized incident beam islaunched normal to the prism curved entrance window,which is coated with a thin film of reflection coefficient r.The incident light then strikes the prism-metal interfaceat the center of the prism’s curved surface at a total-internal-reflection (TIR) angle of θ. This beam may excitesurface plasmons, thus losing a portion of its energy,while the remaining energy is reflected off the dielectric-metal interface and propagated toward the other exitcurved window of the prism, which is coated with thesame thin film as that of the entrance window. As a re-sult, the entrance and exit windows form an FP cavitywithin which SPR takes place. Note that this structurecan also be realized using optical fibers or waveguides,with an SPR sensing area being sandwiched between twoBragg gratings.The reflectance R of the prism-metal interface can beexpressed as follows:θPrism (np)Metal layer (εm )Sensed medium (ns)dInput beam Output beamReflection windowsr rrspr*exp(i )UtAirFig. 1. (Color online) Proposed sensing approach based onthe use of an optical FP cavity in conjunction with SPR. Thebeam entrance and exit windows are coated with a thin filmof reflection coefficient r.4582 OPTICS LETTERS / Vol. 37, No. 22 / November 15, 20120146-9592/12/224582-03$15.00/0 © 2012 Optical Society of AmericaR  rSPR  expiφ; (1)where rSPR is the amplitude reflectance and φ is the phasechange induced into the beam incident onto the prism-metal interface by the SPR. Assume that the electric-fieldamplitude of the incident light is A0, then after multiplereflections within the FP cavity, the successive opticalbeams emerging from the exit window differ in phase andamplitude, as they experience different path lengths andTIR-induced phase shifts and intensity changes. The elec-tric fields of the first n optical beams emerging from theexit window are given byU0  A0tRt0eiδ;U1  A0tRrRrRt0e3iδ  A0tR3r2t0e3iδ;U2  U1rRrRe2iδ  A0tR5r4t0e5iδ;U3  U2rRrRe2iδ  A0tR7r6t0e7iδ;↓Un  A0tR2n1r2n−1t0ei2n1δ; (2)where the coefficients r and t are the reflection and trans-mission of the entrance and exit windows, for light travel-ing from the prismmedium to the air, while r0 and t0 are thereflection and transmission coefficients for light travelingfrom the air to the prism medium, and δ is the round tripphase change caused by path length difference L withinthe prism medium.The overall output electric field, Ut is the super-position of all individual electric fields, Un withn  0; 1; 2;…, thereforeUt Xαn0Un: (3)Using Eqs. (2) and (3), the total transmitted intensity isgiven byI  UtUt A20r2SPR1 − r221 − r2r2SPR2  4r2r2SPR sin2δ φ; (4)where the relation tt0  1 − r2 was used. From Eq. (4), itcan be seen that (1) the expression of the output intensityis similar to that of FP interferometers, featuring periodictransmission peaks at optical frequencies separated bythe free spectral range, (2) the SPR-induced reflectanceat the prism-metal interface, rSPR which depends on thecoupling between the evanescent wave and the surfaceplasmon wave, affects the FP interference fringes, and(3) the phase change, φ, induced by the SPR has the si-milar effect of changing the FP cavity length, that is, itchanges the FP peak positions.In typical SPR sensors, the SPR curves of reflectanceexhibit a minimum reflectance at the resonance angle (orfrequency). This minimum reflectance, Rmin depends onmany parameters, such as the dielectric constants of themetal layer, prism, and the sensed medium; the reso-nance angle (or frequency); and the thickness of the me-tal layer. Most of the current SPR sensors are based onthe detection of reflectivity dip versus either the inci-dence angle or the wavelength, where r should be madelow enough to ensure that R is not affected by the FPeffects. For high r values, the I − θ curve does not resem-ble the conventional R − θ curve of the SPR, and hencethe intensity peak measurement does not exhibit the SPReffect explicitly.The phase shift in SPR has more impact than its am-plitude counterpart and has shown better sensitivity insensing and convenient fulfillment of SPR imaging [14].However, most of the current SPR sensors are based onangular or wavelength interrogation, mainly because thedetection of optical phase shift requires complex config-urations. The proposed SPR sensing approach allowsmonitoring shifts in interference fringes without the needfor detecting SPR-induced phase shifts, thus offering amuch easier solution compared with current phase de-tection schemes employing complex optical heterodyne,polarimetry, and interferometry. For high detection reso-lution the minimum reflectance, Rmin, is designed to be assmall as possible, and this yields a shallow interferencefringe when an FP cavity is used. And if Rmin is too small,the interference fringe I will eventually degenerate intothat of double-slit interference. Specifically, when Rmin 0, the FP fringe fails to recover the sensing informationcarried by the SPR. Note that in order to effectively de-tect the SPR-induced phase shift of the optical signal,Rmin can be relatively large, otherwise optical amplifica-tion may be used to improve the intensity, I, of the opticalsignal transmitted through the FP cavity.To numerically demonstrate the concept of the FP-based SPR sensor, a silver thin film of plasmon wave-length λp  1.2915 × 10−7 m, and the collisionwavelengthλc  5.4377 × 10−5 m, were chosen. The other sensorparameters used were θ  66.5°, d  50 nm, np  1.46,L  0.001 m, and r  0.9. Figures 2(a) and 2(b) show thereflectance spectrum, rSPR the phase shift,φ, and the trans-mitted intensity, I, for differentns. It is shown in Fig. 2 thatthe SPR excitation results in absorption dips in wave-length around 1.53 μm for ns  1.3302 and 1.54 μm forns  1.3303. It is also shown that at thesewavelength dipsthe phase shift φ changes dramatically because SPR isdominant but can easily be detected by monitoring thecorresponding fringe shift [inset of Fig. 2(c)]. As shownin the inset of Fig. 2(c), over the wavelength range wherethe phase shift is dramatic, the fringe shift is around 0.2 nmfor a refractive index change of 0.0001. Therefore, byusing an optical spectrum analyzer of resolution 0.01 nm,the resolution of the proposed sensor can be as high asΔnmin  5 × 10−6 refractive index unit (RIU). Note thatthe interference fringes have similar intensity dips asthose of rSPR, which are caused by the SPR. A large inten-sity dip of SPR reflectance results in a deep interferencefringe, making themeasurement of peak shift feasible andmore accurate; however, the trade-off is a small penalty insensitivity. Note that in the wavelength range where SPRis insignificant, the phase shift is mainly caused by TIR,leading to a negligible fringe shift.It is important to note from Eq. (4) that the SPR-induced phase shift φ can be compensated by adjustingthe FP-induced phase shift δ to keep the transmittedoutput, I, constant. A mature method to control theNovember 15, 2012 / Vol. 37, No. 22 / OPTICS LETTERS 4583FP-induced phase is to use a voltage-driven piezoelectrictransducer. In this case, the sensing system can befurther simplified by using a fixed-wavelength laser inconjunction with a phase modulator.It can also be seen from Eq. (4) that the sensitivity ofphase detection through the measurement of FP peakshifts also depends on the cavity length L. A long cavitylength leads to low phase detection sensitivity. It isimportant to mention here that waveguide-based SPRsensors have the inherent capability of achieving a short-length cavity, making them more advantageous and flex-ible than their prism-based counterparts wheneverphase-shift detection is the main method used for refrac-tive index measurements. FP-based SPR optical sensingoffers an effective, accurate, reliable, simple, and conve-nient approach for detecting optical phase shifts, com-pared to current techniques such as Mach–Zehnderinterference. More importantly, compared with conven-tional wavelength-interrogation SPR sensors that requirea super wideband light source and a spectrometer (whichis the main reason limiting their implementation usinginfrared light waves), our approach requires only anarrowband light source with a wavelength span of sev-eral nanometers for measuring the phase-induced SPR,making the proposed sensing scheme practical and costeffective.In conclusion, an FP-based SPR optical sensing ap-proach has been proposed and investigated. Theoreticalanalyses have shown that the phase change induced bySPR effect can be monitored through the use of an FPcavity. Therefore, this simple approach enables high-resolution sensing through the use of narrow-wavebandwavelength interrogation (or monitoring the output in-tensity of a fixed-wavelength laser signal) for measuringSPR-induced optical phase shift. The combination of SPRand FP effects has potential applications in biologicaland chemical sensing.The authors acknowledge the financial support of theNational Centre of Excellence in Desalination Australia,which is funded by the Australian Government throughtheWater for the Future initiative, and of theNational Nat-ural Science Foundation of China (NSFC) under GrantNo. 61107065.References1. B. Liedberg, C. Nylander, and I. Lundstrom, Sens. Actuators4, 299 (1983).2. J. R. Sambles, G. W. Bradbery, and F. Z. Yang, Contemp.Phys. 32, 173 (1991).3. J. Homola, Chem. Rev. 108, 462 (2008).4. S. Herminjard, L. Sirigu, H. P. Herzig, E. Studemann, A.Crottini, J. P. Pellaux, T. Gresch, M. Fischer, and J. Faist,Opt. Express 17, 293 (2009).5. J. T. Hastings, J. Guo, P. D. Keathley, P. B. Kumaresh,Y. Wei, S. Law, and L. G. Bachas, Opt. Express 15, 17661(2007).6. W. K. Kuo and C. H. Chang, Appl. Phys. A 104, 765 (2011).7. W. K. Kuo and C. H. Chang, Opt. Express 18, 19656 (2010).8. I. R. Hooper and J. R. Sambles, J. Appl. Phys. 96, 3004(2004).9. M. H. Chiu, S. F. Wang, and R. S. Chang, Opt. Lett. 30, 233(2005).10. L. Wu, H. S. Chu, W. S. Koh, and E. P. Li, Opt. Express 18,14395 (2010).11. C. Caucheteur, Y. Shevchenko, L. Y. Shao, M. Wuilpart, andJ. Albert, Opt. Express 19, 1656 (2011).12. G. W. Lu, B. L. Cheng, H. Shen, Y. L. Zhou, Z. H. Chen, G. Z.Yang, O. Tillement, S. Roux, and P. Perriat, Appl. Phys. Lett.89, 223904 (2006).13. R. Ameling, L. Langguth, M. Hentschel, M. Mesch, P. V.Braun, and H. Giessen, Appl. Phys. Lett. 97, 253116 (2010).14. A. R. Halpern, Y. Chen, R. M. Corn, and D. Kim, Anal. Chem.83, 2801 (2011).Fig. 2. (Color online) (a) SPR reflection rSPR, (b) phase shift φ,and (c) transmitted intensity I versus wavelength for the SPR-based optical FP sensor, for ns  1.3302 and 1.3303.4584 OPTICS LETTERS / Vol. 37, No. 22 / November 15, 2012

Fabry-Perot-based surface plasmon resonance sensors

A new sensing approach based on the use of an optical Fabry-Perot (FP) cavity in conjunction with surface plasmon resonance (SPR) is proposed and theoretically investigated. The impact of the SPR on the intensity and phase response of the proposed sensor structure is evaluated using a modified FP model that takes into account the SPR effect. Compared to the conventional optical-phase-detection-based Kretschmann configuration, the proposed sensing approach requires only the measurement of the output power spectrum over a narrow wavelength span of several nanometers to evaluate the phase responses of the sensor, making it more attractive for practical high-sensitivity sensing applications. (C) 2012 Optical Society of AmericaOpticsSCI(E)EI4ARTICLE224582-45843

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Fabry-Perot-based surface plasmon resonance sensors

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