Advances in III-nitride-based microresonator optical frequency combs (Invited)

Chip-scale optical frequency combs based on microresonators have great potentials in spectroscopy, microwave photonics, optical atomic clocks and coherent optical communications. The non-centrosymmetric wurtzite crystal structure of aluminum nitride (AlN) and gallium nitride (GaN) allows them to exhibit both second- and third-order nonlinear optical coefficients, together with wide transparency window and large refractive index contrast against sapphire substrate, making III-nitrides an attractive platform for nonlinear photonics. The basic properties of AlN and GaN microresonators as well as recent advances in III-nitride-based microresonator frequency combs are presented, including broadband frequency comb generation and optical parametric oscillation in AlN microresonators, and soliton microcomb generation in GaN microresonators.