Propagating quasi-bound states in the continuum in silicon waveguides with sidewall gratings.

Photonic bound states in the continuum (BICs) have emerged as an attractive platform for integrated photonic devices owing to their ability to support strongly confined optical modes with high quality factors, enabling applications in optical filtering, sensing, nonlinear optics, and laser technologies. Here, we present a fully planar quasi-BIC waveguide implemented on a 220-nm silicon-on-insulator (SOI) platform that is fully compatible with standard complementary metal-oxide-semiconductor (CMOS) fabrication processes. The device consists of a central strip waveguide, longitudinally offset sidewall gratings, and a semi-infinite slab that provides the continuum of leaky modes. The quasi-BIC state is realized by optimizing the waveguide geometry to induce destructive interference between the radiative pathways coupled to the slab-supported mode. The proposed design exhibits a narrowband, high-transmission response near the 1550-nm telecommunications wavelength, with an experimentally measured quality factor of 191, corresponding to a 3-dB bandwidth of 8 nm for a device comprising 2000 grating periods.