High efficiency and ultra-broadband suspended grating coupler on ultra-thin lithium niobate on insulator.

We propose and demonstrate a high-performance suspended grating coupler on ultra-thin X-cut lithium niobate on insulator (LNOI) for fully-etched photonic devices. The optimized design achieves exceptional coupling efficiency (-3.5 dB at 1546.24 nm), a broad bandwidth (1 dB bandwidth of 70 nm and 3 dB bandwidth exceeding 90 nm), and robust fabrication tolerance. This work addresses a critical challenge in integrated photonics by enabling efficient grating couplers on ultra-thin LNOI platforms, thereby providing a practical and scalable solution for coupling light into photonic crystal waveguides and other suspended photonic devices.