Properties of fluoride microresonators for mid-IR applications

We study crystalline fluoride microresonators for mid-infrared (mid-IR) applications. Whispering gallery mode resonators were fabricated with BaF₂, CaF₂, and MgF₂ crystals. The quality factors were measured at wavelengths of 1.56 and 4.58 μm. The impacts of post-fabrication processing, impurities, and surface water are highlighted. The mid-IR optical losses due to multiphoton absorption are measured. It is found that MgF₂ resonators have a room temperature Q-factor of 8.3×10⁶ at a wavelength of 4.58 μm, limited by multiphoton absorption.