High-efficiency scalable hybrid silicon grating couplers for thin-film lithium niobate photonics

In this work, we report a high-efficiency hybrid silicon-on-thin-film lithium niobate (Si-on-TFLN) grating coupler platform. By utilizing a silicon overlay to enhance diffraction contrast, a simulated peak coupling efficiency of −0.76 dB (84%) at 775 nm and −0.9 dB (81%) at 1552 nm using apodized structures is achieved. Experimental verification shows a measured efficiency of −2.1 dB using a 780 nm narrow linewidth source (Rb absorption line) and −1.24 dB at 1520 nm, representing the highest reported values for metal-unassisted transverse electric grating couplers on TFLN to date. The platform exhibits a 3 dB bandwidth of 40 nm at 1520 nm. This hybrid architecture eliminates the need for complex metal reflectors and deep TFLN etching, providing a scalable solution for system-on-chip integration in quantum sensing, medical diagnostics, and optical communications.