Preventing and reversing vacuum-induced optical losses in high-finesse tantalum (V) oxide mirror coatings

Abstract

High-finesse optical cavities placed under vacuum are foundational platforms in quantum information science with photons and atoms. We study the vacuum-induced degradation of high-finesse optical cavities with mirror coatings composed of SiO₂-Ta₂O₅ dielectric stacks, and present methods to protect these coatings and to recover their initial low loss levels. 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 Ta₂O₅, while it does not occur if it is made of SiO₂. 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 SiO₂ passivating the Ta₂O₅ 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.

Strong Purcell Effect on a Neutral Atom Trapped in an Open Fiber Cavity

“…The initial decay corresponds to an exponential increase in losses L of the form L(t) = L0+∆L · exp(t/τ1) (red curve) yielding a time constant of τ1 = 14.5 days. With constant UV illumination the finesse quickly recovers (sudden positive slope) and the decay process slows down, following the curve predicted by the oxygen depletion model[43] given by L(t) = L0+∆L · (1−exp(−t/τ2)) (blue) with a time constant of τ2 = 1670 days. (b) Effective coupling of an atomic ensemble for an increasing number of atoms.…”

“…We observe that the finesse can be recovered (up to 80% of the initial value) when flushing the vacuum apparatus with pure oxygen. The recovery process takes place at rates much faster than the ones predicted by the oxygen depletion model [42,43]. It remains unclear why UV radiation slows down the degradation process in vacuum, and if it affects both mirrors equally (the surface layer of the mirrors coating is composed of SiO 2 and Ta 2 O 5 for the LT and HT mirrors, respectively).…”

Ultraviolet Fabry-Perot cavity with stable finesse under ultrahigh vacuum conditions

“…A linear model yields a variation of (0.9 ± 3.5) · 10 −3 ppm/h, compatible with zero. For comparison, the smallest degradation rate reported in [14] for the same wavelength was (12.3 ± 4.3) · 10 −3 ppm/h, however, with a Ta 2 O 5 top layer of the mirror coating. The other data point from the literature we can compare to is the rate measured in [11] of (230 ± 20) · 10 −3 ppm/h, which is considerably larger.…”

Cavity-enhanced deep ultraviolet laser for two-photon cooling of atomic hydrogen

“…Second, short wavelength optics are known to degrade when exposed to high power due to surface oxygen depletion [20] and hydrocarbon contamination [21]. In the UV [20], and extreme ultraviolet [22] degradation due to both effects has been mitigated by admitting O 2 at the location of the mirrors -albeit at a lower average power than what is required for two-photon laser cooling.…”