Fabrication-Friendly Shape Optimization for Lithium Niobate Nanophotonics Based on Levelset Method

Lithium niobate on insulator (LNOI) has received much attention in recent years due to its excellent material properties, such as low optical loss, strong nonlinearities and high index contrast, possessing great potential in future nanophotonic devices and systems. However, most of current LNOI devices are designed by traditional physical models and inefficient parameter sweeps, which limit their performance and functionality. In this paper, we propose an inverse design methodology that introduces the levelset method to achieve ultra-compact, high-performance, and readily manufacturable LNOI devices. The levelset method offers a flexible and natural representation of device contours. In addition, it also yields simply-connected device structures (no holes, islands and gaps) that are robust to fabrication errors. We also factor in crystal anisotropy, ridge feature and sidewall angle of LN waveguide to substantially improve simulation accuracy. Using this methodology, we design and fabricate a waveguide crossing and a mode multiplexer, both of which exhibit ultra-compact footprints, simply-connected device contours and consistency between simulation and experimental results. Our design method can be applied to a wide range of LNOI devices and nanophotonic platforms with similar material properties and fabrication constraints, paving the way toward future integrated optical chips and nonlinear technologies.