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Applied Optics

Stability of a high-finesse optical cavity at 493 nm in vacuum for cavity QED with barium ions

Diptaranjan Das, Ezra Kassa, Hiroki Takahashi

Published Apr 27, 2026• 2 citations
Publisher

Abstract

We explore the stability of a high-finesse optical cavity at 493 nm in vacuum for cavity QED with barium ions. A high-finesse Fabry-Perot cavity is built using mirrors with high-reflectivity (HR) coatings that are implemented by stacking multiple thin films of low-loss dielectrics on substrates. Applications of such HR mirrors in the near ultraviolet (UV) range have been hampered by degradation of coatings in vacuum. Here, we explore the degradation of mirrors with HR coatings at 493 nm in vacuum. We study both vacuum-induced and laser-induced effects on oxide-coated cavity mirrors by probing changes in cavity loss using cavity lifetime measurements. We investigate the role of circulating power in the rate of increase in cavity loss and demonstrate methods of reversal of cavity degradation. While we observe no degradation without long exposure or with short exposures at lower circulating powers, we find evidence of degradation on long exposure to high circulating powers. We discuss potential causes and conclude that laser-induced deposition is the likely cause while ruling out thermally activated processes due to laser-induced heating.

DOI
10.1364/ao.596113

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Acta Physica Sinica•Jan 1, 2013•8 citations
We explore the stability of a high-finesse optical cavity at 493 nm in vacuum for cavity QED with barium ions. A high-finesse Fabry-Perot cavity is built using mirrors with high-reflectivity (HR) coatings that are implemented by stacking multiple thin films of low-loss dielectrics on substrates. Applications of such HR mirrors in the near ultraviolet (UV) range have been hampered by degradation of coatings in vacuum. Here, we explore the degradation of mirrors with HR coatings at 493 nm in vacuum. We study both vacuum-induced and laser-induced effects on oxide-coated cavity mirrors by probing changes in cavity loss using cavity lifetime measurements. We investigate the role of circulating power in the rate of increase in cavity loss and demonstrate methods of reversal of cavity degradation. While we observe no degradation without long exposure or with short exposures at lower circulating powers, we find evidence of degradation on long exposure to high circulating powers. We discuss potential causes and conclude that laser-induced deposition is the likely cause while ruling out thermally activated processes due to laser-induced heating.
We explore the stability of a high-finesse optical cavity at 493 nm in vacuum for cavity QED with Barium ions. A high-finesse Fabry-Perot cavity is built using mirrors with high-reflectivity (HR) coatings that are implemented by stacking multiple thin films of low-loss dielectrics on substrates. Applications of such HR mirrors in the near ultraviolet (UV) range have been hampered by degradation of coatings in vacuum. Here, we explore the degradation of mirrors with HR coatings at 493 nm in vacuum. We study both vacuum-induced and laser-induced effects on oxide-coated cavity mirrors by probing changes in cavity loss using cavity lifetime measurements. We investigate the role of circulating power in the rate of increase in cavity loss and demonstrate methods of reversal of cavity degradation. While we observe no degradation without long exposure or with short exposures at lower circulating powers, we find evidence of degradation on long exposure to high circulating powers. We discuss potential causes and conclude that laser-induced deposition is the likely cause while ruling out thermally activated processes due to laser-induced heating.
The long-term performance of high-finesse optical cavities depends on the stability of their mirror coatings under vacuum and under continuous exposure to high circulating power. In cavities supporting multiple wavelengths, a central question is whether degradation at one wavelength compromises performance at another. Understanding this interplay is essential for those cavity quantum electrodynamics experiments that need diverse optical fields for cavity stabilization, trapping, and light--matter coupling. Here, we investigate wavelength-dependent degradation and recovery of a bichromatic cavity with high-reflectivity coating at 455 nm and 671 nm in vacuum. Cavity ring-down measurements reveal a decrease in the 455 nm finesse from 7.7*10^3 to 1.9*10^3 after 11 hours of continuous exposure to high circulating power. Simultaneous measurements show no measurable degradation at 671 nm. Continuous operation at 671 nm, instead, produces no measurable degradation at either wavelength under the tested conditions. Comparison of oxygen depletion and Avrami-type loss models indicates kinetics compatible with nucleation and growth, without uniquely identifying the microscopic mechanism. We demonstrate partial recovery of the finesse with illumination at 455 nm in air. Our results indicate a wavelength dependence of laser-induced optical damage in bichromatic cavities and provide an estimate of the blue light intensity that can be safely used in such cavities.
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.
Transparent conductive oxides (TCO) enable confinement of charge-sensitive ions and Rydberg atoms proximal to dielectric structures including waveguides and photon detectors. However, optical loss precludes the use of TCOs within high-finesse optical micro-resonators. Here we characterize a ZnO-based TCO that markedly reduces optical absorption. At 1650\text{ nm} we observe a 22,000 finesse in a Fabry-Pérot optical cavity coated with a 30\text{ nm} ZnO layer. This is a 5000 times reduction relative to indium tin oxide (ITO) at this wavelength. The same ZnO film exhibits 0.01\text{ \ensuremathΩ\ensuremath{\cdot}cm} surface resistivity at DC. We anticipate a step change in cavity-QED systems incorporating ultra-low loss TCOs like ZnO.
Ultra-high finesse micro-resonator plays an important role in realizing the interaction between atoms and cavity field in the study of cavity quantum electrodynamics (QED) system, weak optical nonlinear effects and micro-optic devices. By measuring basic parameters of the microcavity, the atom-cavity coupling coefficient and the cavity decay rate can be determined precisely. It is also useful for exploring the dynamic characteristics of the system. However, it has difficulty in determining resonate frequency and effective cavity length due to the structure of the ultra-high finesse optical microcavity itself and the characteristics of multilayer coating. In this paper, we demonstrate the precision measurement of effective cavity length under different resonant frequencies which our cavity mirror is coated with 37 layers of dielectric film. The theoretical expectation when using the revised model of the multilayer coating agrees well with that of the experiment; and the measurement error for longitudinal mode interval is below 0.004 nm which is two orders of magnitude better than that obtained in previous unrevised model. The tiny depths into mirror coatings that the standing-wave light field inside the cavity penetrates are given for different mode numbers. This method may be applied to other micro resonator in the precision measurement.