Quasi-bound states in continuum in double-layer silicon gratings

Abstract Bound states in the continuum (BICs) are theoretically known to possess infinite lifetimes and Q factor. However, due to the difficulties in achieving it in reality, symmetry breaking is often introduced in the structure to transform symmetrically protected BICs into quasi-BICs (q-BICs) with extremely high Q factor. Therefore, q-BICs can be utilized to enhance the Q factor of optical sensors. In this paper, we propose the design of a double-layer composite one-dimensional grating with a high Q factor. The structure consists of a double-layers silicon (Si) grating on a silicon dioxide (SiO 2 ) substrate. By introducing a displacement in the upper-layer grating to break the symmetry, q-BICs are induced. The induced q-BICs achieve a Q factor of 2248 for transverse magnetic (TM) wave, enabling enhanced optical sensing capabilities. The proposed q-BICs sensor, exhibiting anisotropy for both TM and transverse electric wave (TE), holds great potential for narrowband polarizers and sensing applications.