Shallow etched low-loss thin film lithium niobate waveguides with bound states in the continuum

Thin film lithium niobate (TFLN) features a small photonic device footprint and excellent optical properties. Despite the recent progress on plasma-etched TFLN waveguides, scattering loss induced by the sidewall roughness still limits the performance of the integrated photonic devices. Here, a shallow-etched structure that can mitigate the scattering loss along the waveguide sidewall is proposed and investigated, while the consequent coupling to the continuous slab mode can be easily eliminated by properly adjusting the waveguide width, achieving a low-loss bound state in the continuum. Our proposed scheme would facilitate low-loss and high performance TFLN photonic devices in the future.