  The microscale integration of mid- and longwave-infrared photonics could
enable the development of fieldable, robust chemical sensors, as well as highly
efficient infrared frequency converters. However, such technology would be
defined by the choice of material platform, which immediately determines the
strength and types of optical nonlinearities available, the optical
transparency window, modal confinement, and physical robustness. In this work,
we demonstrate a new platform, suspended AlGaAs waveguides integrated on
silicon, providing excellent performance in all of these metrics. We
demonstrate low propagation losses within a span of nearly two octaves (1.26 to
4.6 $\mu$m) with exemplary performance of 0.45 dB/cm at $\lambda = 2.4$ $\mu$m.
We exploit the high nonlinearity of this platform to demonstrate 1560 nm-pumped
second-harmonic generation and octave-spanning supercontinuum reaching out to
2.3 $\mu$m with 3.4 pJ pump pulse energy. With mid-IR pumping, we generate
supercontinuum spanning from 2.3 to 6.5 $\mu$m. Finally, we demonstrate the
versatility of the platform with mid-infrared passive devices such as low-loss
10 $\mu$m-radius bends, compact power splitters with 96 $\pm$ 1% efficiency and
edge couplers with 3.0 $\pm$ 0.1 dB loss. This platform has strong potential
for multi-functional integrated photonic systems in the mid-IR.
