Figure 4: Mid-IR swept frequency ringdowns of BaF2 and SrF2 microresonators.
From: Mid-infrared ultra-high-Q resonators based on fluoride crystalline materials
(a) Thermal distortions of a resonance in the BaF2 crystalline microresonator. Thermal–optical dynamics prevent measuring the linewidth and the transmission accurately using standard frequency modulation spectroscopy method. (b) Using a cavity swept laser ringdown method, we observe modal coupling between two ultra-high-Q resonances (red shading). (c,e) Transmission spectra of the ringdown signal of an ultra-high-Q resonance in mid-IR at 4.4 μm using BaF2 (c) and SrF2 (e). (d,f) Exponential fits (red lines) of the minima and maxima data sets (blue circles) extracted from c,e. In BaF2 (d), the measured amplitude lifetime of 
results in a linewidth of 
and a corresponding Q factor of ∼1.4 × 108. In SrF2 (f), the measured amplitude lifetime of 
results in a linewidth of 
and a corresponding Q factor of ∼1.6 × 108.
