Surface plasmon resonance prism coupler for gas sensing based on Stokes polarimetry

A surface plasmon resonance prism coupler comprising a half-ball lens, an isotropic Cr–Au layer and a Ta2O5 anisotropic layer is proposed for gas sensing applications. In the proposed approach, the coupler is illuminated with polarized light and a Stokes polarimetry approach is used to measure the change in six effective ellipsometric parameters as the coupler is exposed to gas samples with different refractive indices. The theoretical analysis is performed to investigate the sensitivity of the effective ellipsometric parameters to the refractive index of the detected gases. The validity of the proposed method is confirmed by comparing the experimental results for the sensitivity of the effective ellipsometric parameters with the analytical results. The results show that the coupler has a sensitivity of 7.5 × 104 deg/RIU (refractive index units) and a sensing resolution of 2 × 10−7 RIU in the indices dynamic range of 1.0–1.001. In general, the results presented in this study confirm that the proposed surface plasmon resonance coupler provides a simple and reliable solution for gas sensing using a Stokes polarimetry approach.