Integrated High- Q Crystalline AlN 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 (∼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 × 10⁶ and achieve a pure broadband Kerr comb with observable spectral lines ranging from ∼145 to 275 THz and a low parametric threshold of ∼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.