Integrated High-Q Crystalline AIN Microresonators for Broadband Kerr and Raman Frequency Combs

Development of planar-integrated microresonators with high quality factors (Q's) is crucial for nonlinear photonics in a robust chip. Compared with silicon and silicon nitride, aluminum nitride (AlN) features intrinsic quadratic and cubic susceptibilities as well as an enormous band gap (similar to 6.2 eV), making it ideal for nonlinear optical interactions. However, sputtered polycrystalline AlN is susceptible to scattering and defect-related absorption losses, thereby inducing limited Q-factors. Here, we demonstrate single-crystalline AlN epitaxially grown on sapphire as a novel nonlinear platform for broadband chip-scale frequency comb generation. We fabricate an AlN-on-sapphire microring with a high loaded Q-factor of 1.1 x 10(6) and achieve a pure broadband Kerr comb with observable spectral lines ranging from similar to 145 to 275 THz and a low parametric threshold of similar to 25 mW. As crystalline AlN exhibits strong Raman gain, we further investigate the influence of stimulated Raman scattering (SRS) on four-wave mixing (FWM) by comparing the nonlinear process in AlN chips with distinct geometries. By locating the pump in a normal dispersion regime, we attain a wideband Raman comb via Raman-assisted FWM and observe a sharp subcomb formation via avoided mode crossing. The interplay between FWM and SRS observed in AlN is also applicable to other crystalline platforms.