Efficient Second-Harmonic Emission with Strong Modal Overlap in a Single-Resonant Lithium Niobate Nanocavity

Integrated nonlinear optical devices are essential for next-generation photonic technologies, yet achieving high-efficiency second-harmonic generation (SHG) in compact systems remains a significant challenge. Here, we demonstrate a high- Q single-resonant photonic crystal nanobeam cavity on a polymer-loaded lithium niobate on insulator platform that achieves a record-high normalized SHG conversion efficiency of 163%/W. This efficiency outperforms previous LN-based photonic crystal cavities by over 3 orders of magnitude, primarily driven by strong modal overlap. The device generates visible SH light at 768.77 nm with a single-lobe radiation pattern and precise spectral alignment between fundamental and second-harmonic modes. Importantly, the SHG efficiency remains stable across a temperature range of up to 20 °C, addressing a common limitation in multiresonant systems. Our work establishes a new benchmark for SHG in lithium niobate microcavities and provides a scalable, thermally stable platform for integrated nonlinear photonics, with applications in quantum optics and chip-scale interconnects.