Stokes and anti-Stokes Raman scatterings from frequency comb lines in poly-crystalline aluminum nitride microring resonators

In optical microresonators, stimulated Raman scattering (SRS) competes with four-wave mixing process and impact Kerr comb generation. Here, we demonstrate Raman frequency combs in poly-crystalline aluminum nitride (AlN) microring resonators. The Raman shifts at transverse-electric (TE) and transverse-magnetic (TM) polarizations are characterized from AlN straight waveguides using backscattering geometries. In polycrystalline AlN microring resonators, the frequency matching of cavity resonances with broad Raman gain enhances the SRS and leads to Raman-assisted frequency combs. As a result, comb lines near Raman scattering regions of AlN are generated.