Chip-scale spontaneous quasi-phase matched second harmonic generation in a micro-racetrack resonator

In this paper, we demonstrate efficient spontaneous quasi-phase matched (SQPM) second harmonic generation (SHG) in a micro-racetrack resonator on X-cut thin film lithium niobate. Our approach does not involve poling, but exploits the anisotropy of the crystals to allow the phase-matching condition to be fulfilled spontaneously as the TE-polarized light circulates in a specifically designed racetrack resonator. In experiment, normalized on-chip conversion efficiencies of 1.01 × 10−4/W and 0.43 × 10−4/W are achieved by 37th-order and 111th-order SQPM, respectively. The configurable SQPM will benefit the application of nonlinear frequency conversion and quantum source generation in chip-scale integrated photonics compatible with standard CMOS fabrication processes.