Preventing and Reversing Vacuum-Induced Optical Losses in High-Finesse Tantalum (V) Oxide Mirror Coatings

TL;DR

This study investigates vacuum-induced optical losses in high-finesse Ta$_{2}$O$_{5}$ mirror coatings, demonstrating that oxygen exposure and laser illumination can reverse degradation, with surface passivation significantly reducing loss rates.

Contribution

It reveals the primary degradation mechanism as surface oxygen depletion and introduces effective methods for protecting and recovering mirror coatings.

Findings

Vacuum causes increased optical loss in Ta$_{2}$O$_{5}$ coatings

Oxygen filling and laser illumination reverse degradation

Surface passivation with SiO$_2$ greatly reduces loss rate

Abstract

We study the vacuum-induced degradation of high-finesse optical cavities with mirror coatings composed of SiO$_2$-Ta$_{2}$O$_{5}$ dielectric stacks, and present methods to protect these coatings and to recover their initial quality factor. For separate coatings with reflectivities centered at 370 nm and 422 nm, a vacuum-induced continuous increase in optical loss occurs if the surface-layer coating is made of Ta$_{2}$O$_{5}$, while it does not occur if it is made of SiO$_2$. The incurred optical loss can be reversed by filling the vacuum chamber with oxygen at atmospheric pressure, and the recovery rate can be strongly accelerated by continuous laser illumination at 422 nm. Both the degradation and the recovery processes depend strongly on temperature. We find that a 1 nm-thick layer of SiO$_2$ passivating the Ta$_{2}$O$_{5}$ surface layer is sufficient to reduce the degradation rate by more than a factor of 10, strongly supporting surface oxygen depletion as the primary degradation mechanism.