  Tremendous scientific progress has been achieved through the development of
nonlinear integrated photonics. Prominent examples are Kerr-frequency-comb
generation in micro-resonators, and supercontinuum generation and frequency
conversion in nonlinear photonic waveguides. High conversion efficiency is
enabling for applications of nonlinear optics, including such broad directions
as high-speed optical signal processing, metrology, and quantum communication
and computation. In this work, we demonstrate a gallium-arsenide-on-insulator
(GaAs) platform for nonlinear photonics. GaAs has among the highest second- and
third-order nonlinear optical coefficients, and use of a silica cladding
results in waveguides with a large refractive index contrast and low
propagation loss for expanded design of nonlinear processes. By harnessing
these properties and developing nanofabrication with GaAs, we report a record
normalized second-harmonic efficiency of 13,000% W-1cm-2 at a fundamental
wavelength of 2 um. This work paves the way for high performance nonlinear
photonic integrated circuits (PICs), which not only can transition advanced
functionalities outside the lab through fundamentally reduced power consumption
and footprint, but also enables future optical sources and detectors.
