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Acta Physica Sinica

Research on several parameters influencing on laser cooling of solids

Jia You-Hua, Ji Xian-Ming, Yin Jian-Ping, (1)光谱学与波谱学教育部重点实验室，华东师范大学物理系，上海 200062; (2)光谱学与波谱学教育部重点实验室，华东师范大学物理系，上海 200062；南通大学理学院，南通 226007

Published Jan 1, 2007• 1 citation
Publisher

Abstract

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.

DOI
10.7498/aps.56.1770

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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.
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.
Publisher Summary This chapter focuses on the concept of laser cooling of a solid, which may occur when the average energy of the photons emitted by the solid is larger than the energy of the ones it absorbs. More formally, the anti-Stokes emission, which occurs at frequencies larger than that of the pump laser, must dominate the Stokes emission that occurs at smaller frequencies. A crucial additional requirement is that the nonradiative decay rates of the laser-pumped states be negligible in comparison to their radiative decay rates. For the sake of specificity, these states will be taken to be those of a set of isolated ions embedded within an insulating host, although in general they could equally well be those of gas-, liquid-, or solid-phase neutral atoms or molecules, or even those of the energy bands of intrinsic semiconductors. Outside the realm of impurity-doped insulators, recent experiments with semiconductors make net anti-Stokes fluorescence cooling of these materials appear achievable. A long-term idea for improving laser cooling of solids takes advantage of photonic band gap materials. The idea is to fabricate a refractive-index structure within the cooling element that would establish a three-dimensional photonic gap immediately below the pump frequency. In this way, emission at Stokes frequencies falling within the gap would be inhibited, thereby enhancing the cooling power derived from the anti-Stokes emission.
Since the successful realization of laser cooling of solids in 19951, the optical cooling techniques2-5 have made fast development, and laser cooling experiments have obtained some important progress. However, how to improve the absorption of pump laser and enhance the temperature drops so as to realize the practical optical refrigeration, it has become a hot research point in applied physics. As early as 2002, the Heeg et. al proposed an extraor intra-cavity enhancement scheme to improve the laser cooling effect of solids2, and experimentally demonstrated their intra-cavity enhancement scheme in 2004, and found that there is no large temperature drops due to the experimental parameters were not optimized3. In 2003, Hoyt proposed a multiple-pass scheme to enhance the absorption of pump laser and did a series of experimental researches about the multiple-pass scheme4. As expected, a greater temperature drop was obtained. However, due to some limitation in the early work, it is difficulty to cool the sample to the cryogenics temperature, such as low absorption of cooling materials5, the coating of the sample surfaces3, the purity of the sample6, the load from the ambient1,7, etc. Therefore, people have done much work to improve the record of temperature drop. For example, to avoid the coating absorption deposited on the sample surface, they cut the sample with a Brewster angle along with the cavity axis8, or improve the purity of cooling material9-11, and so on. Mansoor. Seletskiy used the multi-pass scheme to do some meaningful experiments and obtain a milestone progress, that is, they cool a high pure Yb3+-doped YLF to cryogenic temperature12 in this year. To obtain an efficient absorption, another scheme was proposed to insert a Brewster-cut sample into a resonant cavity and make the cavity to be resonated with the pump laser, and then the maximum absorption of the pump laser can be achieved 2, 13-18. Seletskiy demonstrated this scheme in the experiment13. However, this scheme depends on the power of the pump laser. Fortunately, the intra-cavity scheme proposed by Heeg can solve this problem in the case of low absorption2. He also suggested that inserting a cooling sample into the resonant cavity of a laser with a gain (so-called intra-cavity enhancement scheme) to realize the maximum absorption of the pump laser2, 14, 19 and carried out it in the experiment3. Besides of enhancement laser cooling of bulk solids using resonant cavity, we propose another new scheme to optically cool some film materials (i.e., filmier solids) by using a high fineness planar waveguide, which is similar to the cavity enhanced cooling scheme. In our laser cooling scheme, the solid film as a high-index gap waveguide is separated from a prism by a low-index gap one, and an incident laser beam is resonantly coupled into a
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, also known as optical refrigeration, is an area of optical science investigating the interaction of light with condensed matter to remove thermal energy of a solid through the interaction of the pump photons and phonons in a solid. Apart from being of fundamental scientific interest, this topic addresses a number of important practical issues such as the development of all solid state optical cryo-coolers, and biological applications. A short history of laser cooling as well as latest achievement of optical refrigeration in rare-earth (RE) doped macro-samples are presented and discussed in the paper. The main technique of laser cooling of RE doped solids based on anti-Stokes fluorescence is presented in this paper. The new approach to optical refrigeration based on the Raman cooling is also considered. It is shown that the future prospects of the research are connected with laser cooling of μm- and nm-sized samples, are in their applications in biophysics in the fundamental studies of low-temperature physics.