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 ∼108 with
the straight waveguide (TE00​ 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