Quasi-Phase-Matched Second Harmonic Generation in Poling-Free Lithium Niobate Microrings with Surface Molecular Pattern

Thin film lithium niobate (TFLN) is a promising integrated photonic platform owing to its large quadratic nonlinearity enabling vital applications such as second-harmonic generation (SHG), optical parametric amplification, and spontaneous parametric down-conversion. However, to achieve high SH conversion efficiency, quasi-phase matching (QPM) through periodic electrical poling is typically required, which remains challenging in certain applications like multiple-ring coupled devices. Here, we demonstrate a surface molecule patterning strategy to enable QPM in a poling-free TFLN microring for SHG. An on-chip normalized conversion efficiency of approximately 11.5%/W is obtained in the poling-free TFLN microring with surface molecular pattern, showing an approximate 400-fold improvement compared to the same microring without surface molecular pattern. This approach offers a versatile route to engineer interfacial nonlinear interactions for nonlinear optics and quantum photonics applications and may be extended to multiple-ring coupled resonators and other platforms.