Spectroscopic Investigation of Yb-Doped Silica Glass for Solid-State Optical Refrigeration

We argue that a high-purity Yb-doped silica glass can potentially be cooled via anti-Stokes-fluorescence optical refrigeration. In order to achieve net solid-state optical refrigeration, it is necessary for the cooling efficiency to be positive. This requires the pump wavelength to be greater than the mean fluorescence wavelength and the internal quantum efficiency as well as the absorption efficiency to be near unity. Our conclusion is reached by showing, using reasonable assumptions for the host-material properties, that the nonradiative decay rate of Yb ions can be made substantially smaller than the radiative decay rate. Therefore, an internal quantum efficiency of near unity can be obtained. Moreover, the background absorption coefficient in high-quality silica glass lies in an acceptable range to guarantee a near-unity absorption efficiency at room temperature. Using spectral measurements of the fluorescence from a Yb-doped silica optical fiber at a range of temperatures, we estimate the minimum achievable temperature in Yb-doped silica glass for different values of the internal quantum efficiency.