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Physica Scripta

Multi-Watt cavity for 266 nm light in vacuum

Christian Brand, Christian Knobloch, Ksenija Simonović, Markus Arndt

Published Jun 29, 2023• 1 citation
Publisher

Abstract

Abstract Intense coherent ultraviolet radiation is gaining increasing importance in advanced quantum technologies—from optical clocks and quantum computers to matter-wave interferometry—as well as in photochemistry, life sciences, semiconductor industry, and space applications. Since the preparation of multi-Watt light sources is still an open challenge for many ultraviolet wavelengths, resonant enhancement in a cavity is an attractive alternative. However, many experiments with atoms, molecules or nanoparticles require isolation in high vacuum where UV optics often show fast degradation. Here, we present stable performance of a cavity for 266 nm light with several Watt of intra-cavity power in high vacuum despite the presence of hydrocarbons. Comparing two sets of cavity mirrors indicates that this feat is connected to the micro-chemical environment at the topmost coating layer. Our study emphasizes the need for further developments in this direction to facilitate robust, compact, and high-performing devices employing UV radiation.

DOI
10.1088/1402-4896/ace300

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Abstract Intense coherent ultraviolet radiation is gaining increasing importance in advanced quantum technologies—from optical clocks and quantum computers to matter-wave interferometry—as well as in photochemistry, life sciences, semiconductor industry, and space applications. Since the preparation of multi-Watt light sources is still an open challenge for many ultraviolet wavelengths, resonant enhancement in a cavity is an attractive alternative. However, many experiments with atoms, molecules or nanoparticles require isolation in high vacuum where UV optics often show fast degradation. Here, we present stable performance of a cavity for 266 nm light with several Watt of intra-cavity power in high vacuum despite the presence of hydrocarbons. Comparing two sets of cavity mirrors indicates that this feat is connected to the micro-chemical environment at the topmost coating layer. Our study emphasizes the need for further developments in this direction to facilitate robust, compact, and high-performing devices employing UV radiation.
Non-classical squeezed states of light are becoming increasingly important to a range of metrology and other quantum optics applications in cryptography, quantum computation and biophysics. Applications such as improving the sensitivity of advanced gravitational wave detectors and the development of space-based metrology and quantum networks will require robust deployable vacuum-compatible sources. To date non-linear photonics devices operated under high vacuum have been simple single pass systems, testing harmonic generation and the production of classically correlated photon pairs for space-based applications. Here we demonstrate the production under high-vacuum conditions of non-classical squeezed light with an observed 8.6 dB of quantum noise reduction down to 10 Hz. Demonstration of a resonant non-linear optical device, for the generation of squeezed light under vacuum, paves the way to fully exploit the advantages of in-vacuum operations, adapting this technology for deployment into new extreme environments.
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.
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.
Optical cavity/molecule strong coupling offers attractive opportunities to modulate photochemical or photophysical processes. When atoms or molecules are placed in an optical cavity, they can coherently exchange photonic energy with optical cavity vacuum fields, entering the strong coupling interaction regime. Recent work suggests that the thermodynamic and kinetic properties of molecules can be significantly changed by strong coupling, resulting in the emergence of intriguing photochemical and photophysical phenomena. As more and more physico-chemical systems are studied under strong coupling conditions, optical cavities have also advanced in their sophistication, responsiveness, and (multi)functionality. In this review, we highlight some of these recent developments, particularly focusing on Fabry-Perot microcavities.
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.