Laser Cooling of Silica Glass

TL;DR

This paper demonstrates laser cooling of high-purity Yb-doped silica glass, a widely used optical material, by measuring cooling efficiency and quantum efficiency, opening new possibilities for applications in quantum and high-power laser technologies.

Contribution

First successful demonstration of laser cooling in silica glass, with detailed efficiency measurements and quantum efficiency estimation for potential technological applications.

Findings

Net cooling achieved in Yb-doped silica glass

High quantum efficiency due to low impurity levels

Cooling efficiency varies with laser wavelength

Abstract

Laser cooling of a solid is achieved when a coherent laser illuminates the material in the red tail of its absorption spectrum, and the heat is carried out by anti-Stokes fluorescence of the blue-shifted photons. Solid-state laser cooling has been successfully demonstrated in several materials, including rare-earth-doped crystals and glasses. Silica glass, being the most widely used optical material, has so far evaded all laser cooling attempts. In addition to its fundamental importance, many potential applications can be conceived for anti-Stokes fluorescence cooling of silica. These potential applications range from the substrate cooling of optical circuits for quantum information processing and cryogenic cooling of mirrors in high-sensitivity interferometers for gravitational wave detection to the heating reduction in high-power fiber lasers and amplifiers. Here we report the net cooling of high-purity Yb-doped silica glass samples that are primarily developed for high-power fiber laser applications, where special care has been taken in the fabrication process to reduce their impurities and lower their parasitic background loss. The non-radiative decay rate of the excited state in Yb ions is very small in these glasses due to the low level of impurities, resulting in near-unity quantum efficiency. We report the measurement of the cooling efficiency as a function of the laser wavelength, from which the quantum efficiency of the silica glass is calculated.