  Recent advances in nonlinear optics have revolutionized the area of
integrated photonics, providing on-chip solutions to a wide range of new
applications. Currently, the state of the art integrated nonlinear photonic
devices are mainly based on dielectric material platforms, such as Si3N4 and
SiO2. While semiconductor materials hold much higher nonlinear coefficients and
convenience in active integration, they suffered in the past from high
waveguide losses that prevented the realization of highly efficient nonlinear
processes on-chip. Here we challenge this status quo and demonstrate an
ultra-low loss AlGaAs-on-insulator (AlGaAsOI) platform with anomalous
dispersion and quality (Q) factors beyond 1.5*10^6. Such a high quality factor,
combined with the high nonlinear coefficient and the small mode volume, enabled
us to demonstrate a record low Kerr frequency comb generation threshold of ~36
uW for a resonator with a 1 THz free spectral range (FSR), ~100 times lower
compared to that in previous semiconductor platform. Combs with >250 nm broad
span have been generated under a pump power lower than the threshold power of
state of the art dielectric micro combs. A soliton-step transition has also
been observed for the first time from an AlGaAs resonator. This work is an
important step towards ultra-efficient semiconductor-based nonlinear photonics
and will lead to fully integrated nonlinear photonic integrated circuits (PICs)
in near future.
