Refractive index sensing by asymmetric dielectric gratings with both bound states in the continuum and guided mode resonances

Guided mode resonances (GMRs) and bounded states in the continuum (BICs), both supported by dielectric gratings, can realize ultrahigh Q -factors and strong localized field enhancements, beneficial to high-performance sensing applications. In this paper, based on GMR theory and numerical simulations, we systematically investigate the relationship between different order GMRs and BICs/quasi-BICs in Si-based dielectric gratings with symmetric, singly, and doubly asymmetric profiles. The introduction of broken-symmetry in adjacent gaps or Si nanobeams brings about new GRM and symmetry-protected BIC and can transform the fundamental BIC into a resonant state with finite Q -factor as high diffraction orders. A Friedric-Wintgen BIC is also achieved under normal incidence by breaking symmetries of both gaps and Si nanobeams. Further, the asymmetric dielectric gratings with high Q -factor quasi-BICs are designed as a refractive index sensor. Although the Q -factor and localized electric field penetrating into the vacuum are greatly improved with the decreasing asymmetry parameter, the sensitivity is almost unchanged while the FOM demonstrates an inverse square dependence on the asymmetry parameter. To further improve the sensitivity, we construct an asymmetric dielectric grating with a low fill factor and a big period, which manifests an excellent sensing performance with a near theoretical sensitivity limit of ∼1506 nm/RIU and an ultrahigh FOM of ∼5000.