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We report the first observation of laser cooling in Yb

We explore candidate materials for solid-state laser cooling on electric-dipole allowed transitions that could accelerate refrigeration. We analyze the required external quantum efficiencies and tolerable impurity absorption required to achieve net cooling, and examine limitations imposed by charge transfer and excited state absorption in Yb2+:SrF2, Ce3+:LiCAF and Ti3+:Al2O3.

Crystal-field parameters are extracted from ab initio calculations of the energy level structure of the 4 f 2 and 4 f 5 d configurations for Pr 3 + as a free ion and doped into Ca F 2 and Lu 2 O 3 crystals. The parameters are compared to those derived from direct phenomenological fits to experimental levels. The ab initio crystal-field parameters are in good agreement with experiment. For the ab initio calculations the F 2 Coulomb parameter is consistently larger, and the electron correlation configuration interaction parameter α is up to a factor of three larger, than the experimental parameters. This is consistent with previous calculations that suggest that significant contributions to electron correlation arise from continuum states.

We report the first observation of laser cooling in Yb3+:KYW and validate the results by comparison with experiments in the well-studied material Yb3+:YAG. Radiation from a single-mode Ti:Al2O3 laser was used to achieve cooling of 1.5 K/W in 1% Yb:KYW at 1025 nm, comparing well with the reference material 3% Yb:YAG which cooled by 3.5 K/W at 1030 nm under open lab conditions. Experimental results for KYW crystals mounted on aerogels and doped with 1-20% Yb were in excellent agreement with the theoretical dependence of cooling power on the Yb absorption spectrum. Elimination of thermal conduction through the sample support structure was found to permit the attainment of lower temperatures and to simplify modeling of radiation balance conditions in self-cooled lasers with longitudinal thermal gradients. Contrary to the notion that more coolant ions yield higher cooling power, concentrations of Yb over 1% caused re-absorption of luminescence in KYW crystals, leading to a progressive red shift in the optimal cooling wavelength and the prevention of laser cooling altogether in a 20% sample at room temperature. The prospect of attaining radiation-balanced lasing in commercially-available tungstate crystals is evaluated.

We demonstrate a direct measurement of the energy levels of impurity-trapped excitons in CaF${}_{2}$:Yb${}^{2+}$. The radically different radiative decay rates of the lowest exciton state and higher excited states enable the generation of a transient photoluminescence enhancement measured via a two-step excitation process. We observe sharp transitions arising from changes of state of localized electrons, broad bands associated with changes of state of delocalized electrons, and broad bands arising from trap liberation.

Calculated energy levels from recent \emph{ab initio} studies of the electronic structure of SrCl$_{2}$:Yb$^{2+}$ and CsCaBr$_{3}$:Yb$^{2+}$ are fitted with a semi-empirical "crystal-field" Hamiltonian, which acts within the model space $4f^{14} + 4f^{13}5d + 4f^{13}6s$. Parameters are obtained for the minima of the potential-energy curves for each energy level and also for a range of anion-cation separations. The parameters are compared with published results parameters fitted to experimental data and to atomic calculations. The states with significant $4f^{13}6s$ character give a good approximation of the impurity-trapped exciton states that appear in the \emph{ab initio} calculations., Comment: Minor revision