Brillouin Lasing in Ultra-High-Q Lithium Fluoride Disk Resonators

We report the fabrication of an ultra-high- $Q$ monolithic whispering gallery-mode resonator using a lithium fluoride crystal, with a quality factor above $10^{8}$ at 1550 nm. We show that when pumped above a certain threshold, we can excite single Stokes Brillouin lasing in this resonator. We thereby demonstrate Brillouin lasing in a monofluoride crystal for the very first time, to the best of our knowledge. We provide a detailed theoretical investigation of Brillouin scattering in this resonator, using a time-domain model which tracks the dynamics of the Stokes and pump waves. We also perform a stability analysis which enables us to analytically determine the threshold pump power. We expect such crystalline Brillouin lasers to find potential applications for ultra-pure microwave and multi-wavelength generation, as well as for compact sensors.