Demonstration of reverse symmetry waveguide sensing in aqueous solutions

A reverse symmetry waveguide is presented for evanescent wave sensing in aqueous solutions. The waveguide consists of a thin polystyrene film, supported by a thicker substrate layer of nanoporous silica on glass. The nanoporous substrate layer has a refractive index of $n_S\text{=1.193,}$ hence, with an aqueous cover refractive index of $n_C\text{=1.331,}$ a reverse symmetry ${\mathrm{(n}}_S{\mathrm{<n}}_C)$ is obtained. A surface relief grating formed in the polystyrene film is used as a coupling element. Sensitivities considerably higher than for a conventional waveguide sensor are demonstrated.