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Radiation-balanced lasing and thermal profiling is reported in two Yb-doped laser crystals at room temperature. In 3% Yb3+:YAG a record efficiency of 30.5% was achieved by frequency-locking the laser cavity to the input light. Both the average excursion and axial temperature gradient of the gain medium were maintained within 0.1 K of room temperature at the radiation balance point. By including saturation of background impurity absorption in the analysis, quantitative agreement was obtained between theory and the experimentally measured laser threshold, radiation balance condition, output wavelength, and laser efficiency with only one free parameter. Radiation-balanced lasing was also achieved in 2% Yb3+:KYW with an efficiency of 2.2% despite high background impurity absorption, losses from Brewster end faces that were not parallel, and non-optimal output coupling. Our results confirm that relatively impure gain media can be operated as radiation-balanced lasers, contrary to earlier predictions which ignored background impurity properties.

Background absorption has been studied for an YLF:10%Yb3+ crystal at different intensities at room temperature. The cooling efficiency was measured by both the DLT (Differential Luminescence Thermometry) and TLS (Thermal Lens Spectroscopy) methods. Results show that background and coolant absorption saturate differently at room temperature. A cw Ti:sapphire pump beam was tuned from 920 nm to 1040 nm with an intensity range from 100 W/cm2 to 20,000 W/cm2. Changes of temperature and thermal strength were measured by DLT and TLS methods, respectively. The cooling efficiencies with these approaches at different wavelengths were then compared based on theoretical fits to the experimental results. The cooling efficiency at 1000 nm was found to be independent of pump intensity. There, the saturation of intensity of background absorption had the same value as that of Ytterbium. The cooling efficiency below 1000 nm dropped at elevated intensity. In this range, Ytterbium absorption saturated easily, reducing the cooling ion absorption, while the absorption of background impurities did not saturate as much as Ytterbium. Consequently the cooling efficiency was lowered. For wavelengths above 1000 nm, increases in the pump intensity led to improved cooling efficiency. In this range, background absorption saturated more easily than absorption of coolant ions and parasitic heating was reduced, leading to higher cooling efficiency. Thus we have devised a method of measuring differential absorption saturation and determined its effect on laser cooling at room temperature. Saturation effects of this kind have important consequences in the heat equation for radiation-balanced lasers.

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.

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.