  Due to their capacity for non-classical light generation, high-efficiency
second-order nonlinear parametric processes play an important role in quantum
photonic technology, and chip-scale realization of these processes is
recognized as the key to building efficient light sources for integrated
quantum photonic circuits. To achieve ultra-high nonlinear conversion
efficiency, traditional method uses quasi-phase matching (QPM) technology.
However, QPM requires electric field poling, which is incompatible with the
CMOS fabrication process, and this hinders the wafer-scale production of
integrated photonic circuits. In this paper, we demonstrate efficient
spontaneous quasi-phase matched (SQPM) frequency conversion in a
micro-racetrack resonator. Our approach does not involve poling, but exploits
the anisotropy of the ferroelectric crystals to allow the phase-matching
condition to be fulfilled spontaneously as the TE-polarized light circulates in
a specifically designed racetrack resonator. SQPM second harmonic generation is
observed with a normalized intracavity conversion efficiency of 0.85%/W,
corresponding to the 111st-order QPM. This could theoretically reach 186,000%/W
by first-order QPM. In this case such high intracavity conversion efficiency
can be implemented in practice with an optimized outward coupling. Our
configurable SQPM approach will benefit the application of nonlinear frequency
conversion in chip-scale integrated photonics with CMOS-compatible fabrication
processes, and is applicable to other on-chip nonlinear processes such as
quantum frequency conversion or frequency-comb generation.
