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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Temperature dynamics of laser cooling of solids with Yb3+ions

Galina A. Nemova, Raman Kashyap

Published Feb 19, 2014• 1 citation
Publisher

Abstract

We consider a theoretical model for laser cooling with anti-Stokes fluorescence of a Yb3+:YAG sample. We have estimated the fluorescence power density removed from the system with spontaneous emission, the power density radiated with stimulated emission as well as the heat power density generated in the system by non-radiative decays from the impurities in the host material. We have investigated the influence of each of these power densities on the cooling process in Yb3+:YAG system. It has been shown how the temperature dependences of the different parameters of the system as well as the concentration of the impurities in the host influence the final temperature of the cooled sample.

DOI
10.1117/12.2038447

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We consider a theoretical model for laser cooling with anti-Stokes fluorescence of a Yb3+:YAG sample. We have estimated the fluorescence power density removed from the system with spontaneous emission, the power density radiated with stimulated emission as well as the heat power density generated in the system by non-radiative decays from the impurities in the host material. We have investigated the influence of each of these power densities on the cooling process in Yb3+:YAG system. It has been shown how the temperature dependences of the different parameters of the system as well as the concentration of the impurities in the host influence the final temperature of the cooled sample.
We develop a detailed theoretical model for laser cooling in Yb3+:YAG. The expressions for the fluorescence power density removed from the system with spontaneous emission, the power density radiated with stimulated emission, as well as the heat power density generated in the system by nonradiative decays on the impurities of the host material have been calculated. The influence of each of these power densities on the cooling process has been analyzed. We show, for the first time to our knowledge, how the temperature dependences of the different parameters of the system as well as the concentration of the impurities in the host influence the final temperature of the cooled sample.
Laser cooling of solids is also called anti-Stokes fluorescent cooling, it is a new optical cooling technology in recent years. We propose a two-level model to analyze the absorption and stimulated-emission processes between the Yb3+2F7/2 ground-state manifold and the 2F5/2 excited-state manifold, and discuss several parameters that influence the cooling power, and find some ways to improve the cooling power. The influences of the doped concentration, pumping power and the effective pump-spot area on cooling are particularly analyzed. At the same time, we make computer simulation about the cooling process and obtain the temperature as a function of the cooling time, which is similar to the experimental results. So this shows that our model is reasonable.
In this paper, quantum mechanical calculations of cooling characteristics for the Yb3+: YLF system with use of the vibronic model of laser cooling are presented. Dynamics of the laser cooling process for the seven-level system of an Yb ion is described by the density-matrix formalism. Dependences of the cooling characteristics on the pump intensity are obtained for various temperatures and absorption coefficients of impurity ions. It is shown that the pump intensity, at which the net cooling power has a maximum, depends on temperature. Thus, choosing the intensities, which correspond to the net cooling power maximum over the entire temperature range, we achieve a lower sample temperature at a shorter time than in the case of using a constant intensity throughout the cooling process. Calculations are performed for the parameters of the Yb3+:YLF system.
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.
Laser cooling of solids with anti-Stokes fluorescence is currently attracting widespread attention because of the wide range of its applications, including all-optical cryocooling for airborne and space-based applications, heat suppression in high-power lasers, and cooling of nanoparticles for biological and mesoscopic physics. This laser cooling process has typically very low (only a few%) efficiency and its enhancement is therefore highly desirable. In this work, we propose to leverage Mie resonance to enhance anti-Stokes fluorescence cooling in RE-doped nanoparticles. As an example, we consider an Yb3+:YAG nanosphere pumped at the long wavelength tail of the Yb3+ absorption spectrum, at 1030 nm. We show that if the radius of the nanosphere is adjusted to the pump wavelength in such a manner that the pump excites some of the Mie resonant modes of the sample, the cooling power density generated in the sample is considerably enhanced and the temperature of the sample is consequently appreciably decreased.
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.