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Applied Physics B

Comparison of Yb3+: YAP with Yb3+: YAG cooling crystals

Chaoyu Wang, Jianping Yin

Published Apr 27, 2026• 1 citation
Publisher

Abstract

Abstract YAlO 3 (YAP) shows lower phonon energy than Y 3 Al 5 O 12 (YAG), which implies a lower temperature can be obtained in Yb 3+ -doped YAP according to the single-frequency phonon model. Here, a 2% Yb 3+ : YAP crystal was refrigerated by anti-Stokes fluorescence and compared to YAG crystal with the same doping concentration in experiments. Yb 3+ exhibited a broad spectral cooling range in YAP from 1020nm to 1070nm, which was broader than in YAG from 1020 nm to 1055 nm. A minimum achievable temperature (MAT) dropped to ~ 249.7 K at 1040 nm for Yb 3+ : YAP, which was lower than that of ~ 259.3 K obtained in 2% Yb 3+ : YAG. Further analysis predicted that the cooling limit of two crystals can reach ~167 K for Yb 3+ : YAP and ~171 K for Yb 3+ : YAG from room temperature. This work exhibited the excellent cooling properties of Yb 3+ : YAP compared with Yb 3+ : YAG in experiment and provided a unique perspective for the practical application of high-power Radiation balance lasers (RBLs).

DOI
10.1007/s00340-026-08668-3

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•Jan 1, 2023•0 citations
Abstract YAlO 3 (YAP) shows lower phonon energy than Y 3 Al 5 O 12 (YAG), which implies a lower temperature can be obtained in Yb 3+ -doped YAP according to the single-frequency phonon model. Here, a 2% Yb 3+ : YAP crystal was refrigerated by anti-Stokes fluorescence and compared to YAG crystal with the same doping concentration in experiments. Yb 3+ exhibited a broad spectral cooling range in YAP from 1020nm to 1070nm, which was broader than in YAG from 1020 nm to 1055 nm. A minimum achievable temperature (MAT) dropped to ~ 249.7 K at 1040 nm for Yb 3+ : YAP, which was lower than that of ~ 259.3 K obtained in 2% Yb 3+ : YAG. Further analysis predicted that the cooling limit of two crystals can reach ~167 K for Yb 3+ : YAP and ~171 K for Yb 3+ : YAG from room temperature. This work exhibited the excellent cooling properties of Yb 3+ : YAP compared with Yb 3+ : YAG in experiment and provided a unique perspective for the practical application of high-power Radiation balance lasers (RBLs).
Yttrium aluminum perovskite (YAP) is a host material that can provide a strong emission from a rare-earth dopant and it has a lower phonon energy than yttrium aluminum garnet (YAG). Therefore, YAP is a promising material for optical refrigeration and radiation balanced laser. We measured the photoluminescence (PL) spectra of ytterbium (Yb)-doped YAP and compared them with those of Yb-doped YAG. The estimated ideal laser cooling efficiencies of Yb-doped YAP and Yb-doped YAG were comparable at 300 K. Based on the temperature-dependent anti-Stokes PL, we found that the laser cooling power of Yb-doped YAP at 470 K is 14.3 times higher than that at 200 K. This enhancement at higher temperatures is 3.2 times larger than that observed for Yb-doped YAG. We attributed the higher laser cooling power of Yb-doped YAP to a lower multi-phonon relaxation rate (and/or a higher energy transfer rate) and an antenna effect caused by the energy transfer from Yb ions that are located at the Y-site to Yb ions that are surrounded by an inhomogeneous alloy structure. The calculated small signal gain of (Yb:Y)AP is 3.5 times larger than that of (Yb:Y)AG. The larger small signal gain of (Yb:Y)AP arises from its strong absorbance and small Stark splitting width.
Anti-Stokes cooling has been observed in an Yb3+-doped BaY2F8 single crystal. Single crystals have been grown by the Czochralski technique. The absorption spectra and the emission properties have been measured at room temperature and at 10K. The energy positions of the Stark sublevels of the ground and the excited state manifolds have been determined and separated from the vibronic substructure. The intrinsic decay time of the F5∕22 level has been measured taking care of avoiding the effect of multiple reabsorption processes. The theoretical and experimental cooling efficiencies of Yb:BaY2F8 are evaluated and compared with respect to those of the most frequently investigated materials for laser cooling. A temperature drop of almost 4K was measured by pumping the crystal with 3W of laser radiation at ∼1025nm in single pass configuration with a cooling efficiency of ∼3%.
Laser cooling and trapping of Yb3+:YAG nano-crystals has been theoretically investigated. It has been shown that in the system of Yb3+:YAG nano-crystals pumped at 1030 nm in the long wavelength tail of the Yb3+ absorption spectrum both internal and translational laser cooling processes take place. The internal laser cooling process based on anti-Stokes fluorescence serves as a supplementary process to translational cooling of rare-earth (RE) doped nano-crystals. The internal laser cooling process can help to reach lower equilibrium translational temperatures of RE-doped nano-crystals.
A vibronic model of laser cooling for crystals doped with Yb3+ ions is proposed. The model involves the stimulated Raman adiabatic passage pumping through dipole-allowed 4f–5d transitions, electron transitions between Stark-split sublevels due to vibronic interaction, and anti-Stokes fluorescence from the excited ion state. Dynamics of the cooling process are described by the density matrix formalism. It is shown that the cooling power in the case of vibronic interaction is higher at least by a factor of 3.4 compared to the case of electron–phonon interaction. In the proposed model, an increase in the cooling power yields deeper and faster refrigeration of a crystal doped with Yb3+ ions. Estimations show that, on cooling by the stimulated Raman adiabatic passage pumping, the temperature of a crystal with an irradiated volume of 2.12×10−4 cm3 can reach 64 K in a time of 2 s. In the proposed model, the cooling temperature is 30 K lower and the cooling process is 100 times faster compared to the traditional model of direct pumping. The estimates are applicable to present-day materials of technologically attainable purity.
The role of saturation of background absorption by unintended impurities has been investigated for the first time theoretically and experimentally in the context of laser cooling of crystalline solids and the operation of self-cooled or radiation-balanced lasers. In this thesis it is shown that when saturation of the background takes place at an intensity lower than the saturation intensity of dopant ions responsible for cooling, as in Yb3+:LiYF4, cooling efficiency can be improved by using elevated input intensity. With the same input power, the measured cooling efficiency in 10% Yb3+:LiYF4 crystal was doubled at 1020 nm when the beam radius was reduced by a factor of 10, by partially saturating the background impurity absorption. This is effective not only as a post-growth method of enhancing cooling efficiency but offers a new way of reaching lower temperatures than were previously possible with anti-Stokes fluorescent (ASF) refrigeration. Prior to this work, cooling efficiency was thought to be independent of intensity and crystal purification was considered as the only way to improve the cooling efficiency. It is further demonstrated that background saturation plays a key role in the operation of self-cooled or radiation-balanced lasers. Because radiation-balanced lasers operate under saturated conditions it is essential to include limitations imposed by impurity content of the gain medium. When this is done, quantitative predictions of the required conditions for radiation balance become possible. Laser operation can then be made more efficient and can be extended to new materials which may be relatively impure and uncommon but are well-suited to efficient operation under radiation-balanced conditions by virtue of a special combination of physical characteristics. Radiation-balanced laser operation is achieved in 3% Yb:YAG (1 × 1 × 10 mm3) and is found to result in exceptionally uniform temperature distributions using high resolution thermal imagery. Threshold levels, output power, and input power at the radiation balance point are all shown to be in close agreement with theory. This laser achieved 30.5% laser efficiency (agreed with predictions within 2%), which is the highest efficiency level of any self-cooled laser operated to date. Radiation-balanced lasing is also achieved for the first time in the tungstate crystal 2% Yb3+:KYW (0.9×1.2×10 cm3), despite having very high impurity content as the result of being an uncommon gain medium. The achieved laser efficiency was only 2.2%, as the result of non-optimal optical components, high Yb concentration, high impurity content, crystal polish, excitation wavelength, and output coupling. However, this is the first demonstration of an Yb3+ radiation-balanced laser in the tungstate host. Correction of these deficiencies should enable high quality Gaussian output at the 5 W level with 18% efficiency in existing commercial 1% Yb:KYW samples. Even better performance of this new self-cooled laser could result from improved crystal growth efforts.