Mid-infrared ultra-high-Q resonators based on fluoride crystalline materials

The unavailability of highly transparent materials in the mid-infrared has been the main limitation in the development of ultra-sensitive molecular sensors or cavity-based spectroscopy applications. Whispering gallery mode microresonators have attained ultra-high-quality (Q) factor resonances in the near-infrared and visible. Here we report ultra-high Q factors in the mid-infrared using polished alkaline earth metal fluoride crystals. Using an uncoated chalcogenide tapered fibre as a high-ideality coupler in the mid-infrared, we study via cavity ringdown technique the losses of BaF(2), CaF(2), MgF(2) and SrF(2) microresonators. We show that MgF(2) is limited by multiphonon absorption by studying the temperature dependence of the Q factor. In contrast, in SrF(2) and BaF(2) the lower multiphonon absorption leads to ultra-high Q factors at 4.5 μm. These values correspond to an optical finesse of [Image: see text], the highest value achieved for any type of mid-infrared resonator to date.