AlN nonlinear optics and integrated photonics

The commercial success of flat panel displays, Blu-ray lasers, and light-emitting diodes has launched III-nitrides as the second most widely used semiconductor across the globe. Aluminum nitride (AlN) in particular is unique among common semiconductors as it possesses both quadratic and cubic nonlinearities as well as an enormous bandgap of 6.2 eV. This chapter reviews the recent advances in leveraging crystalline AlN for nonlinear optical frequency conversions ranging from ultraviolet to near-infrared regimes in a nanophotonic platform. We begin with applications of quadratic nonlinearities such as high-efficiency second-harmonic generation and the first nanophotonic optical parametric oscillator. We then move to cubic nonlinearities for realizing Raman lasing and Kerr microcomb formation. Finally, we highlight applications employing simultaneous quadratic and cubic nonlinearities for access to near-visible and ultraviolet microcombs. These achievements pose epitaxial AlN as a serious contender to conventional nonlinear materials such as silicon nitride and lithium niobate for chip-scale nonlinear optics.