Fluoride microresonators for mid-IR applications

We study crystalline fluoride microresonators for mid-infrared applications. Whispering gallery mode resonators were fabricated with BaF$_2$, CaF$_2$ and MgF$_2$ crystals. The quality factors were measured at wavelengths of 1.56 {\mu}m and 4.58 {\mu}m. The impacts of fabrication technique, impurities, multiphonon absorption and surface water are investigated. It is found that MgF2 resonators have room temperature Q factor of $8.3\times 10^6$ at wavelength of 4.58 {\mu}m, limited by multiphonon absorption.