Effective Orthogonal TE-TM Mode Conversion and Sum-Frequency Generation in Poling-Free Lithium Niobate Microring

Abstract

In nonlinear integrated photonics based on thin-film lithium niobate (TFLN), controlling nonlinear interactions between orthogonal transverse electric (TE) and transverse magnetic (TM) modes is critical for optical sensing, signal processing, and polarization-division multiplexing. However, realizing effective TE-TM nonlinear coupling and frequency conversion via conventional birefringent phase matching (BPM) or quasi-phase-matching (QPM) remains challenging, as they require strict prerequisites or complex fabrication. This work addresses this challenge by using intrinsic TE-TM mode hybridization in a poling-free X-cut TFLN microring resonator. Through waveguide-geometry design, we observed pronounced TE-TM mode hybridization, manifested as polarization conversion, and further observed sum-frequency generation (SFG) involving TE- and TM-like modal components in the same poling-free X-cut TFLN microring. These results connect a linear TE-TM hybridized mode with a second-order nonlinear frequency-conversion process and are interpreted using a qualitative coupled-mode analysis that is consistent with the experimental observations. To the best of our knowledge, this is the first experimental demonstration of SFG enabled by intrinsic TE-TM mode hybridization in a single poling-free X-cut TFLN microring, providing a pathway toward polarization-engineered nonlinear integrated photonics for sensing, signal processing, and polarization-division multiplexing.