High-Q crystalline germanium microresonators for Mid-IR

We report on the whispering gallery modes microresonators fabrication from crystalline germanium. The resulting Q-factor exceeding $10^{7}$ is the highest ever recorded for $\mathbf{G e}$ microresonators measured at the $\mathbf{2 . 6 8} \mu \mathrm{m}$ pump wavelength. Self-injection locking of a laser diode to germanium microresonator, which is one of the ways to stabilize the laser frequency is achieved. The obtained results pave the way for new opportunities in applications beyond $2 \mu \mathrm{m}$ in development of the low-cost, compact, high efficiency devices for various fields, including laser stabilization, nonlinear optics, spectroscopy, ets.