Barium fluoride whispering-gallery-mode disk-resonator with one billion quality-factor

TL;DR

This paper reports the fabrication of a monolithic barium fluoride whispering-gallery-mode resonator achieving an ultra-high quality factor of over one billion, demonstrating potential for advanced optical and particle detection applications.

Contribution

First demonstration of a monolithic BaF$_2$ resonator with a $Q$-factor exceeding one billion through precision polishing of a relatively soft crystal.

Findings

Achieved $Q$-factor above $10^9$ at 1550 nm

Surface roughness of 2 nm on mm-sized disk

Observed complex thermo-optical dynamics

Abstract

We demonstrate a monolithic optical whispering gallery mode resonator fabricated with barium fluoride (BaF$_2$) with an ultra-high quality ($Q$) factor above $10^9$ at $1550$ nm, and measured with both the linewidth and cavity-ring-down methods. Vertical scanning optical profilometry shows that the root mean square surface roughness of $2$ nm is achieved for our mm-size disk. To the best of our knowledge, we show for the first time that one billion $Q$-factor is achievable by precision polishing in relatively soft crystals with mohs hardness of ~$3$. We show that complex thermo-optical dynamics can take place in these resonators. Beside usual applications in nonlinear optics and microwave photonics, high energy particle scintillation detection utilizing monolithic BaF$_2$ resonators potentially becomes feasible.