Abstract

A novel structure with long-period grating in a metal clad ridge waveguide (MCRW) as a highly sensitive ambient refractive index (ARI) sensor is proposed suitable for biosensing applications. We have exploited the coupling between the fundamental mode and a higher order mode via the long period grating written in the photosensitive core of the MCRW. Utilizing quasi-transverse electric (TE) modes of the MCRW structure, it is shown that ARI sensitivity of ∼ ${{\text{100}}}\,{\mu} {\text{m/}}$ refractive index unit (RIU) can be achieved by using dual resonance near dispersion turn around point. This ultrahigh sensitivity can be attributed to the metal under-cladding which enhances the evanescent field in the ambient medium more prominently for the higher order modes as compared to the fundamental mode. It is shown that device compactness is also increased significantly due to the metal under-cladding. Further, the propagation lengths of quasi-TE modes in the proposed structure are about 35 to 185 times higher than that of quasi-transverse magnetic modes due to which, no in-line polarizer is required in the proposed device.

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