Brillouin- and Raman-Kerr soliton combs in a z-cut magnesium fluoride microresonator

We investigate multi-nonlinearity comb formation in a high-Q z-cut magnesium fluoride (MgF₂) whispering-gallery-mode microresonator featuring a broadened Brillouin gain. This exceptional property brings the thresholds for stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), and Kerr nonlinearity into close proximity. By selectively exciting distinct transverse mode families with a single external continuous-wave pump laser, we reveal a series of nonlinear regimes, including Brillouin- and Raman-Kerr combs and their controlled coexistence within a single crystalline cavity. Depending on the excited mode, either a Brillouin-Kerr comb or a soliton Raman-Kerr comb dominates, and a coexistence regime emerges where both combs are simultaneously sustained but spectrally separated. We further observe center-shifted and single-soliton Brillouin-Kerr states, evidencing recoil dynamics and deterministic soliton formation. These observations demonstrate the z-cut MgF₂ resonator as a versatile platform for studying coupled Brillouin–Raman–Kerr dynamics and for realizing compact, multi-band coherent light sources with potential applications in dual-comb spectroscopy and low-noise microwave photonics.