Please verify that (1) all pages are present, (2) all figures are correct, (3) all fonts and special characters are correct, and (4) all text and figures fit within the red margin lines shown on this review document. Complete formatting information is available at http://SPIE.org/manuscripts Return to the Manage Active Submissions page at http://spie.org/app/submissions/tasks.aspx and approve or disapprove this submission. Your manuscript will not be published without this approval. Please contact author_help@spie.org with any questions or concerns. Analysis of Phase Interrogated SPR Fiber Optic Sensors with Different Bimetallic Combinations H. Moayyedb,a*, I. T. Leite b,a, L. Coelhob,a, J. L. Santosb, a, A. Guerreirob,a, D. Viegasa,c a INESC TEC (coordinated by INESC Porto), Rua do Campo Alegre 687, 4169-007 Porto, Portugal Faculdade de Ciências da Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal c INL- International Iberian Nanotechnology Laboratory, Avenida Mestre José Veiga s/n, 4715-330 Braga Portugal. b ABSTRACT Optical fiber sensors based on the phenomenon of plasmonic resonance can be interrogated applying different methods, the most common one being the spectral approach where the measurand information is derived from the reading of the wavelength resonance dip. In principle, a far better performance can be achieved considering the reading of the phase of the light at a specific wavelength located within the spectral plasmonic resonance. This approach is investigated in this work for surface plasmon based fiber optic sensors with overlays which are combinations of bimetallic layers, permitting not only to tune the wavelength of the plasmon resonance but also the sensitivity associated with the phase interrogation of the sensors. The metals considered for the present analysis are silver, gold, copper, and aluminum. Keywords: Plasmonics, optical fibers, sensors, sensor interrogation, phase reading, sensitivity, bimetallic combinations. 1. INTRODUCTION The effect of surface plasmon resonance (SPR) has been widely used for highly sensitive and selective detection of several physical, chemical, and biochemical parameters.1 Indeed, ever since the introduction of various optical methods in the excitation of the SPR at a metal-dielectric interface,2 it has been widely recognized that such an excitation can be used to achieve sensing or monitoring of various interfacial phenomena with ultrahigh sensitivity. These include, for example, chemical and biological sensing,3 film-thickness sensing,4 temperature sensing,5 and angular measurement.6 Recently, it has been demonstrated that the SPR technique applied to chemical and biological sensing can achieve resolutions down to 10-7 refractive index units (RIUs), values not accessible to others optical sensing techniques.7 Phase interrogation has not yet been widely explored in the context of SPR sensors. Surely, this methodology is somewhat more complex to implement than most traditional ones, requiring deeper know-how on optical components, modulation instruments and signal processing techniques, but eventually the main reason has to be with the scientific domains that historically were the driving force for this sensing technology, mostly situated in the chemical and biochemical fields where the application of spectroscopic characterization techniques is the standard. This is changing since plasmonics is moving to areas other than sensing, such as imaging and data storage, attracting to the subject researchers with more diverse backgrounds. A consequence of this dynamics is a clear improvement of the resolution values obtained when considering phase interrogation. Most of these works were performed with classical sensing heads based on a prism coupling system in Kretschmann’s configuration. Under appropriate conditions the SPR induced phase change would be greatly amplified if the light undergoes multiple attenuated total reflections, as happens when the sensing platform is an optical fiber. Therefore, the investigation of SPR based optical fiber sensors with phase interrogation has attracted interest in recent years.8 During the years, the SPR technique has been widely used for a quick and accurate detection of several physical, chemical, and biochemical parameters. The metal-dielectric interface supports an electromagnetic wave with characteristics highly sensitive to the outer medium and, in general, silver and gold are the two main metals that are used for SPR sensor applications.9 Silver-based sensors are known for their narrow spectral width, but are chemically very *hmoayyed@fc.up.pt; phone +351 22 60 82 601; fax +351 22 60 82 679 AOP100 - 222 V. 1 (p.1 of 4) / Color: No / Format: A4 / Date: 4/30/2014 6:09:32 AM SPIE USE: ____ DB Check, ____ Prod Check, Notes: Please verify that (1) all pages are present, (2) all figures are correct, (3) all fonts and special characters are correct, and (4) all text and figures fit within the red margin lines shown on this review document. Complete formatting information is available at http://SPIE.org/manuscripts Return to the Manage Active Submissions page at http://spie.org/app/submissions/tasks.aspx and approve or disapprove this submission. Your manuscript will not be published without this approval. Please contact author_help@spie.org with any questions or concerns. unstable and are highly vulnerable to oxidation when used in liquid or gaseous environments. On the other hand Goldbased sensors are lesser accurate than Silver-based ones but, in contrast, are chemically very stable. It was shown that the bimetallic layers based sensors not only displayed a high shift of resonance angle as a Au-based sensor, but also showed narrower resonance curve as Ag-based sensors, plus the additional advantage of protecting silver against oxidation. 9 Such appealing characteristics of the bimetallic arrangement for SPR based sensing motivated us to study it further, which is the focus of this work. The properties of different bimetallic combinations to be used in a fiber optic sensor based on the SPR technique, particularly when phase interrogation is considered, are studied here. The metals chosen for the present analysis are silver, gold, copper, and aluminum. The performance of these sensing structures is assessed, particularly when considering phase interrogation. 2. SENSING STRUCTURE AND ANALYSIS The structure of the sensing head under analysis is schematically illustrated in Fig. 1. It consists of a fiber layout with two metal layers. Figure 1 – Schematic of the SPR-based optical fibre sensing probe under analysis. The length of the sensing region is assumed to be L, ρ0 the radius of the core and θ0 the angle of incidence of the optical ray with respect to the normal to the core-cladding interface. The normalized transmitted power through the sensing region of the fiber is given by 8 ⁄ trans ref = , ⁄ (1) where Rp is the reflectivity of the multilayer structure and θcr = sin-1(n2/n1) is the critical angle for the light confinement inside the fiber (n1 and n2 are respectively the refractive indices of the core and the cladding of the fiber). Nref ( ) is the number of reflections the optical ray with θ0 undergoes in the sensing region, and is given by = . (2) ref tan Moreover, sin = cos (3) cos is the intensity distribution between the continuum of guided modes. In order to access the performance of phase interrogation of SPR based sensors supported by fiber structures, it is convenient to express the amplitude reflection coefficients rs and rp in the polar form as =| | , = . (4) Then the phase difference variation δϕp,s resulting from a single reflection at a given incident angle between the p and the s polarization components is = − , (5) , and the total phase difference variation considering all the reflections inside the sensing region is given by: ∆ , = ref , . AOP100 - 222 V. 1 (p.2 of 4) / Color: No / Format: A4 / Date: 4/30/2014 6:09:32 AM SPIE USE: ____ DB Check, ____ Prod Check, Notes: (6) Please verify that (1) all pages are present, (2) all figures are correct, (3) all fonts and special characters are correct, and (4) all text and figures fit within the red margin lines shown on this review document. Complete formatting information is available at http://SPIE.org/manuscripts Return to the Manage Active Submissions page at http://spie.org/app/submissions/tasks.aspx and approve or disapprove this submission. Your manuscript will not be published without this approval. Please contact author_help@spie.org with any questions or concerns. Figure 2 – (a) Phase difference (between s and p polarizations) and (b) phase sensitivity as function of the refractive index of the surrounding medium, for a sensing structure comprised a 50 nm thickness metal layer. The considered interrogation wavelength is 632.8 nm. At this stage, it is useful to define the sensitivity of the sensing structure to the refractive index ns of the surrounding medium, which can be regarded as the basic parameter quantifying the performance of the sensor. A change in the refractive index ns = (εs)1/2 of the surrounding dielectric produces a significant variation in the propagation constant of the surface plasmon, resulting in the modification of the SPR coupling condition. This can be observed as a change in one or more of the properties of the light transmitted through the sensing structure. The sensitivity can be defined as: S , = (7) where δξ is the change in a given property of light (e.g. intensity, resonance wavelength or phase) due to a variation of δns in the refractive index of the surrounding medium. We will be particularly concerned with the case of phase interrogation, where the property of light of interest is the phase difference Δ p,s between the p and s polarization components. In this case, the phase sensitivity to refractive index variations is then defined as Sn,ϕ = δ p,s/δns. 3. RESULTS AND DISCUSSION In our analysis, we have considered a step-index multimode silica fiber, with core diameter of 100 µm and 0.24 numerical aperture, and a 1 cm long sensing region. The metals considered for the present analysis are Au, Ag, Cu, and Al. The refractive indices for these metals were obtained from experimental data available in the literature.10 The study was performed considering an incident light with wavelength of 632 nm. Figure 2(a) shows phase difference between s and p polarizations and Fig. 2(b) shows the corresponding phase sensitivities for different single metal layers with thickness of 50 nm. From these figures, we can see Au and Ag have better phase sensitivity and maximum phase sensitivity for Cu is close to the refractive index of water (ns = 1.333 at 632.8 nm). On the other hand a single layer of Al does not show resonance in phase interrogation. In Fig. 3 we show phase difference between s and p polarizations and maximum phase sensitivity as function of different bimetallic combinations. The configuration contains a combination of two metallic layers with the thickness of 40 nm and 10 nm for inner and outer layers, respectively. Figure 3(a) shows phase difference and sensitivity detection when inner layer of metal combination is Ag. Regarding to the Fig. 2, the point of maximum phase sensitivity for a single metal layer of Ag is in 1.354 RIU, therefore by coating with a layer of Au and Cu this maximum shifts to 1.333 RIU and 1.347 RIU, respectively. Maximum phase sensitivity for the single layer of Au is -3.5×104 degrees/RIU that happens on refractive index of 1.32 RIU. Figure 3(b) shows by coating with a layer of Ag to the single layer of Au, maximum phase sensitivity reaches the value of -6.7×104 degrees/RIU, besides the tuning of this maximum to the refractive index of 1.34. The inclusion of bimetallic combination has also the benefit of shifting and increasing the maximum phase sensitivity for Cu, as illustrated in Fig. 3(c). It can be seen that with coating of Au and Ag phase sensitivity rises and shifts to newer refractive index values. It can also be observed that combination of metals with Al does not show relevant results when addressing phase interrogation. AOP100 - 222 V. 1 (p.3 of 4) / Color: No / Format: A4 / Date: 4/30/2014 6:09:32 AM SPIE USE: ____ DB Check, ____ Prod Check, Notes: Please verify that (1) all pages are present, (2) all figures are correct, (3) all fonts and special characters are correct, and (4) all text and figures fit within the red margin lines shown on this review document. Complete formatting information is available at http://SPIE.org/manuscripts Return to the Manage Active Submissions page at http://spie.org/app/submissions/tasks.aspx and approve or disapprove this submission. Your manuscript will not be published without this approval. Please contact author_help@spie.org with any questions or concerns. Figure 3 - Phase difference (between s and p polarizations) and phase sensitivity as function of the refractive index of the surrounding medium, for a sensing structure comprised of 40 nm and 10 nm for inner and outer metallic layers, respectively. The considered interrogation wavelength is 632.8 nm. Figures report three different combinations when (a) Ag (b) Au and (c) Al are the inner layers. 4. CONCLUSION In this work we studied the characteristics of fiber optic SPR sensors incorporating the bimetallic combination in what concerns the sensitivity to variations of the surrounding refractive index when considering phase interrogation. These results suggest that the bimetallic sensing structure can be particularly advantageous in the context of phase interrogation since it substantially increases the phase sensitivity to refractive index variations. ACKNOWLEDGMENTS This research was performed in the framework of project PTDC/FIS/119027/2010 (Plasmonics Based Fiber Optic Sensing with Enhanced Performance) funded by National Funds through FCT – Fundação para a Ciência e a Tecnologia. L. Coelho acknowledges the support from FCT grant SFRH/BD/78149/2011. REFERENCES [1] Liedberg, B., Nylander, C, Lundström, I., “Surface plasmon resonance for gas detection and biosensing,” Sens. Actuators 4, 299-304 (1983). [2] Raether, H., [Surface Plasmons on Smooth and Rough Surfaces and on Gratings], Springer-Verlag (1988). [3] Homola et al., “Surface plasmon resonance sensors: Review,” Sens. Actuators B Chem. 54, 3-15 (1999). [4] Suzuki, H., Sugimoto, M., Matsui, Y., Kondoh, J., “Effects of gold film thickness on spectrum profile and sensitivity of a multimode-optical-fiber SPR sensor,” Sens. Actuators B Chem. 132, 26-33 (2008). [5] Ozdemir, S.K., Turhan-Sayan, G., “Temperature effects on surface plasmon resonance: Design considerations for an optical temperature sensor,” J. Lightwave Technol. 21, 805-814 (2003). [6] Gwon, H.R., Lee, S.H., “Spectral and angular responses of surface plasmon resonance based on the Kretschmann prism configuration,” Mater. Trans. 51, 1150-1155 (2010). [7] Naraoka, R., Kajikawa, K., “Phase detection of surface plasmon resonance using rotating analyzer method,” Sens. Actuators B Chem. 107, 952-956 (2005). [8] Sharma, A.K., Jha, R., Gupta, B.D., “Fiber-optic sensors based on surface plasmon resonance: A comprehensive review,” IEEE Sens. J. 7, 1118-1129 (2007). [9] Sharma, A.K., Gupta, B. D., “On the sensitivity and signal to noise ratio of a step-index fiber optic surface plasmon resonance sensor with bimetallic layers,” Opt. Commun. 245, 159 (2005). [10] Winsemius, P., van Kampen, F. F., Lengkeek, H. P. and van Went, C. G., “Temperature dependence of the optical properties of Au, Ag and Cu,” J. Phys. F: Metal Phys. 6(8), 1583-1606 (1976). AOP100 - 222 V. 1 (p.4 of 4) / Color: No / Format: A4 / Date: 4/30/2014 6:09:32 AM SPIE USE: ____ DB Check, ____ Prod Check, Notes: