Titanium dioxide-assisted metal focusing grating couplers for thin-film lithium niobate waveguides

Thin-film lithium niobate (TFLN)-based photonic integrated circuits (PICs) demand efficient, compact, and simply realizable grating couplers for optimal performance. This work introduces titanium dioxide (TiO₂)-assisted metal focusing grating couplers (TMFGCs) as a promising solution. The TMFGCs leverage a simplified fabrication process based on metal gratings. Moreover, they achieve a smaller footprint compared to previous one-dimensional metal grating couplers (GCs). Theoretical simulations predict a 1.3 dB enhancement in coupling efficiency (CE) due to the integrated TiO₂ structure. Experimental results demonstrate a maximum coupling efficiency of -4.88 dB, with the potential to reach -4.07 dB through refined fabrication techniques. The TMFGCs offer a simple yet effective approach to efficient light coupling in TFLN-based PICs.