SPR Sensing with Bimetallic Layers in Optical Fibers and Phase Interrogation H. Moayyed a,b , I. T. Leite a,b , L. Coelho a,b , J. L. Santos a, b , D. Viegas a,c a INESC Porto, Rua do Campo Alegre, 687, 4169-007 Porto, Portugal b Faculdade de Ciências da Universidade do Porto, Rua do Campo Alegre, 687, 4169-007 Porto, Portugal c Cabelte, SA, Rua Espírito Santo, Arcozelo, 4410-420 Vila Nova de Gaia, Portugal. ABSTRACT An analytical model based on geometrical optics and multilayer transfer matrix method is applied to determine the sensing properties of tapered optical fiber based SPR sensors incorporating bimetallic (Gold and Silver) layers, particularly when phase interrogation is considered. Phase interrogation is studied as a methodology to attain enhanced sensitivities. The performance of the sensing heads as function of the bimetallic layers and taper parameters is analyzed. It is shown the bimetallic combination is capable to provide larger values of sensitivity compared with the single layer approach. The results derived from this study are guiding the experimental study of these structures. Keywords: Surface plasmon resonance, optical fiber sensors, tapered fibers, phase interrogation, bimetallic combination. 1. INTRODUCTION Plasmonics, which deals with surface electromagnetic excitations in metallic structures, is nowadays one of the most active areas of Photonics. The central motivation comes from the opportunities associated with the fact that with surface plasmons, which are electromagnetic excitations coupled to collective motion of conduction electrons near a metal surface, the problem of light manipulation can be reduced from three to two dimensions, enabling interactions, signal conditioning and processing on the sub-wavelength scale. This recent activity burst in Plasmonics also opened new possibilities in sensing. During the years the technique of Surface Plasmon Resonance (SPR) has been widely used for an highly accurate detection of several physical, chemical, and biochemical parameters [1, 2]. Indeed, ever since the introduction of various optical methods in the excitation of the surface plasmon resonance at a metal-dielectric interface [3], it has been widely recognized that such an excitation can be utilized to achieve sensing or monitoring of various interfacial phenomena with ultrahigh sensitivity. These include, for example, chemical and biological sensing [4], film-thickness sensing [5], temperature sensing [6], and angular measurement [7]. Recently, it has been demonstrated that the SPR technique applied to chemical and biological sensing can achieve refractive index resolutions down to 10 -7 , values not accessible to others optical sensing techniques [8]. The surface plasmons are accompanied by a longitudinal (TM- or p-polarized) electric field which decays exponentially in metal as well as in dielectric medium. The electric field has its maximum at metal-dielectric interface. The TM- polarization and exponential decay of electric field are found by solving the Maxwell equation for semi-infinite media of metal and dielectric with an interface of metal-dielectric. The propagation constant (K SP ) of the surface plasmon wave propagating along the metal-dielectric interface is given by [4] m s SP m s K c εε ω ε ε = + (1) where ε m and ε s are the dielectric constants of metal and the dielectric medium, respectively, ω is the frequency of incident light, and c is the velocity of light [9]. In common practice, there are at least four choices of parameters that one can monitor to accomplish the SPR sensing process, these are (i) the change of the resonant angle (“angular 8th Iberoamerican Optics Meeting and 11th Latin American Meeting on Optics, Lasers, and Applications, edited by Manuel Filipe P. C. Martins Costa, Proc. of SPIE Vol. 8785, 8785FK © 2013 SPIE · CCC code: 0277-786X/13/$18 · doi: 10.1117/12.2027614 Proc. of SPIE Vol. 8785 8785FK-1 Downloaded From: http://spiedigitallibrary.org/ on 11/19/2013 Terms of Use: http://spiedl.org/terms