Anti‐Stokes cooling of Yb-doped KYF₄ single crystals

Truly vibration-free, optical cooling of solids, relying on annihilation of lattice phonons via anti-Stokes fluorescence, is a rapidly advancing technology with a huge potential impact on a variety of space and terrestrial applications. In this paper, we focus on investigation of active media and we report on the first observation of anti-Stokes cooling in a novel material, a Yb-doped KYF₄ single-crystal, promising in relation to its multi-site structure and low phonon cut-off energy. The cooling efficiency of the sample was measured, external quantum efficiency and background absorption were evaluated and compared to other fluoride active media. Bulk cooling by ~ 1.8 °C was observed under ~ 1.5 W laser excitation, tuned close to 1025 nm, in single-pass configuration.