  Aluminum gallium arsenide (AlGaAs) and related III-V semiconductors have
excellent optoelectronic properties. They also possess strong material
nonlinearity as well as high refractive indices. In view of these properties,
AlGaAs is a promising candidate for integrated photonics, including both linear
and nonlinear devices, passive and active devices, and associated applications.
For integrated photonics low propagation loss is essential, particularly in
nonlinear applications. However, achieving low-loss and high-confinement AlGaAs
photonic integrated circuits poses a challenge. Here we show an effective
reduction of surface-roughness-induced scattering loss in fully etched
high-confinement AlGaAs-on-insulator nanowaveguides, by using a heterogeneous
wafer-bonding approach and optimizing fabrication techniques. We demonstrate
ultrahigh-quality AlGaAs microring resonators and realize quality factors up to
3.52E6 and finesses as high as 1.4E4. We also show ultra-efficient frequency
comb generations in those resonators and achieve record-low threshold powers on
the order of ~20 uW and ~120 uW for the resonators with 1 THz and 90 GHz
free-spectral ranges, respectively. Our result paves the way for the
implementation of AlGaAs as a novel integrated material platform specifically
for nonlinear photonics, and opens a new window for chip-based
efficiency-demanding practical applications.
