  Supercontinua are broadband spectra that are essential to optical
spectroscopy, sensing, imaging, and metrology. They are generated from
ultrashort laser pulses through nonlinear frequency conversion in fibers, bulk
media and chip-integrated waveguides. For any generating platform, balancing
the oppositional criteria of strong nonlinearity, transparency, and absence of
multiphoton absorption is a key challenge. Here, we explore supercontinuum
generation in integrated gallium nitride (GaN) waveguides, which combine a high
Kerr-nonlinearity, mid-infrared transparency and a large bandgap that prevents
two- and three-photon absorption in the technologically important telecom
C-band where compact erbium-based pump lasers exist. With such a laser, we
demonstrate tunable dispersive waves and gap-free spectra extending to almost 4
micron in wavelength, relevant to functional group chemical sensing. In
addition, also leveraging the material's second-order nonlinearity, we
implement on-chip f-2f interferometry to detect the pump laser's
carrier-envelope offset frequency, which enables precision metrology. These
results demonstrate the versatility of GaN-on-sapphire as a new platform for
broadband nonlinear photonics.
