Optical frequency comb and spectroscopy with crystalline resonators in MIR

High-Q whispering gallery mode resonators have been mostly studied in the visible and near-IR wavelength regions for optical frequency comb and spectroscopy. With crystalline materials, their use can be extended to the mid-IR beyond 2 μm where molecular gases not only have very rich characteristic spectral lines but also very large absorption cross sections. In this paper, we describe our continued efforts of pushing whispering gallery mode resonator applications in the mid-IR wavelength region for spectroscopic applications including Kerr comb generation and molecular absorption spectroscopy. With a variety of mid-IR transmitting crystalline materials, we investigate their Q and limiting factors, dispersion and spectral engineering, parametric oscillation and comb generation. We have also explored the utility and limitation of using high Q resonators for ringdown molecular absorption measurements.