Nonlinear interaction in a poling-free micro-racetrack resonator on thin film lithium niobate: Spontaneous quasi-phase matched second harmonic generation

Effective nonlinear coefficients in anisotropic crystals that depend on the direction of the wavevector provide an effective approach to the poling-free quasi-phase matched nonlinear process. Recent study shows that by designing the geometry and the orientation relative to the optics axis in a racetrack resonator on X-cut thin film lithium niobate, both the resonance and quasi phase-matching condition between fundamental and second-harmonic wave in TE-fundamental modes can be satisfied, thus the intracavity nonlinear frequency conversion process can be precisely controlled. Dual-resonant 37th- and 111th-order spontaneous quasi-phase matched second harmonic generation processes are experimentally demonstrated, and the results agree well with the theoretical prediction.