Optical refrigeration of Tm³⁺ doped indium fluoride glass from room temperature

Optical refrigeration has significant promise for vibration-free cryogenic cooling and radiation-balanced fiber lasers. In this paper, we report theoretical and experimental investigations of optical refrigeration in thulium (Tm³⁺) doped indium fluoride glass (Tm³⁺:InF₃) from room temperature. Comprehensive spectroscopic studies of Tm³⁺:InF₃ were conducted, and its cooling with 1875, 1900, and 1950 nm pump lasers at different power levels has been measured. Maximum cooling of 2.4 K was achieved with 4.4 W from a 1900 nm laser. The experimental results agree with our theoretical expectations. Our experiment has demonstrated that Tm³⁺:InF₃ has better cooling efficiency than Tm³⁺ doped ZrF₄-BaF₂-LaF₃-AlF₃-NaF glass due to Tm³⁺:InF₃'s lower maximum phonon energy, which reduces non-radiative decay.