Quasi-phase-matching with Spontaneous Domain Inversion in an Integrated Lithium Niobate Micro-racetrack Resonator

Quasi-phase-matching (QPM) technology is the most popular and significant method to achieve efficient nonlinear frequency conversion. The realization of periodically poling to achieve QPM in photonic integrated circuits (PICs) is a challenging issue for the requirement of CMOS compatible and large-scale fabrication. Here we realize a spontaneous periodical domain inversion without poling but by dispersion engineering and designing the orientation of the crystal due to the circular propagation of light waves in an integrated lithium niobate micro-racetrack resonator (MRR). The QPM second harmonic generation (SHG) with a normalized conversion efficiency of 2.25$\%$/W (169th-order QPM) has been achieved in the high-quality factor resonator of $\sim 10^{8}$ with the straight waveguide (TE$_{00}$ mode) of ultra-low propagation loss of 0.0022dB/cm. The efficiency can be further enhanced by using a first-order QPM, and the bandwidth can be made broader by employing a shorter interaction length for photonics and quantum optics. The configurable spontaneous quasi-phase-matching lithium niobate MRR on X-cut thin-film lithium niobate on insulator (LNOI) provides a significant on-chip integrated platform for other optical parametric processes