High Quality Factor Unidirectional Guided Resonances in Etchless Lithium Niobate Metagratings for Polarization Modulation

Unidirectional guided resonances (UGRs), as distinctive resonant eigenstates in planar photonic lattices, exhibit unique capability of emitting light in a single direction. In this work, UGRs with high-Q factor and infinite proximity to the Γ-point infinitely using etchless lithium niobate (LN) metagratings are proposed and investigated numerically. By adjusting the parameters of metagraings, the Q-factor and asymmetric radiation ratio of UGRs can be flexibly tuned, and the wavelength center of UGRs respect will move with respect to the wave vector along the Γ-X direction. Accompanied by the optimizing of asymmetric radiation ratio, the evolution of two dispersion curves from avoided crossing to crossing can be observed. Furthermore, leveraging the polarization sensitivity of UGRs, we achieve a broadband linear-to-circular polarization conversion with a high polarization extinction ratio. This work advances the fundamental understanding of UGRs while potentially offering promising applications in metagratings-based surface-emitting lasers, beam steering, and refractive index sensors.