  The combination of nonlinear and integrated photonics has recently seen a
surge with Kerr frequency comb generation in micro-resonators as the most
significant achievement. Efficient nonlinear photonic chips have myriad
applications including high speed optical signal processing, on-chip
multi-wavelength lasers, metrology, molecular spectroscopy, and quantum
information science. Aluminium gallium arsenide (AlGaAs) exhibits very high
material nonlinearity and low nonlinear loss when operated below half its
bandgap energy. However, difficulties in device processing and low device
effective nonlinearity made Kerr frequency comb generation elusive. Here, we
demonstrate AlGaAs-on-insulator as a nonlinear platform at telecom wavelengths.
Using newly developed fabrication processes, we show high-quality-factor
(Q>100,000) micro-resonators with integrated bus waveguides in a planar circuit
where optical parametric oscillation is achieved with a record low threshold
power of 3 mW and a frequency comb spanning 350 nm is obtained. Our
demonstration shows the huge potential of the AlGaAs-on-insulator platform in
integrated nonlinear photonics.
