title: Mid-infrared ultra-high-Q resonators based on fluoride crystalline materials.
abstract: 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 BaF2, CaF2, MgF2and SrF2microresonators. We show that MgF2is limited by multiphonon absorption by studying the temperature dependence of the Q factor. In contrast, in SrF2and BaF2the lower multiphonon absorption leads to ultra-high Q factors at 4.5 μm. These values correspond to an optical finesse of , the highest value achieved for any type of mid-infrared resonator to date.
authors.affiliation: École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
authors.forename: C
authors.initials: C
authors.lastname: Lecaplain
authors.affiliation: École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
authors.forename: C
authors.initials: C
authors.lastname: Javerzac-Galy
authors.affiliation: Russian Quantum Center, 143025 Skolkovo, Russia.|Faculty of Physics, M.V. Lomonosov Moscow State University, 119991 Moscow, Russia.
authors.forename: M L
authors.initials: ML
authors.lastname: Gorodetsky
authors.affiliation: École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
authors.forename: T J
authors.initials: TJ
authors.lastname: Kippenberg