Title: Optical sensors based on symmetric waveguide gratings

Abstract

We demonstrate the fabrication and characterization of polymer based symmetric optical waveguide gratings. The experimental results are verified using rigorous coupled-wave analysis (RCWA). Typically, Waveguide gratings consist of a grating on top of a single-mode slab waveguide. Changing the waveguide’s refractive index will alter the filling factor of the grating with respect to the mode intensity profile. This results in a propagation length of the mode that is slightly sensitive to refractive index changes. In this work we investigate whether this sensitivity can be increased by using symmetric waveguide gratings. We observed that both maximized propagation lengths and sensitivities with respect to refractive index changes can be achieved when the grating is placed at the position of intensity nodes of higher-order modes. For odd modes this condition is always fulfilled in symmetric optical waveguide gratings. The increased sensitivity may enable novel kinds of sensors and filters based on tunable waveguide gratings. While node alignment leads to a minimized coupling of the mode leading to a maximized but finite radiative quality factor, symmetric waveguide gratings can even support waveguide modes of infinite radiative quality factor. Introducing a silver grating with mirror symmetry under glide operations, hybrid BICs can be formed in the waveguide grating. We present a theoretical description and experimental prove for hybrid BICs. The sensing performance (intensity change per permittivity change) of hybrid BICs is theoretically analyzed in different sensing geometries. The results are compared to classical dielectric BICs and node-aligned waveguide grating modes of finite radiative quality factor.