In order to meet the demands for applications of optical refrigerators in the fields of spaceflight, aviation, space science, and detection, a 2 wt. % Yb3+-doped LuLiF4 crystal, as a new laser cooling material, was prepared and demonstrated by using a 178 mW diode laser centered at 1015 nm and a resonant extra-cavity scheme with an enhancement factor of 12.8. Cooling efficiency of 1.27% and a temperature drop of 14.3 K/W are obtained with 79% of the incident laser power being absorbed. Based on our results, a sample with background absorption of α=4.2×10(-4) cm(-1) can be potentially cooled down to ∼145 K. Our investigation shows that Yb3+-doped LuLiF4 crystal is potentially a promising candidate for solid-state refrigeration.
A diode laser with a power of 178 mW is used to pump a 2.2% (wt.) Yb 3 -doped YLiF 4 crystal in an optical extracavity, and a resonant cavity-enhanced laser cooling for Yb 3 -doped fluoride crystal is proposed and demonstrated. The pump laser enhancement factor so far obtained is up to 8.6 with the resonant cavity. Given that 82% of incident laser power is absorbed, the cooling efficiency is calculated as 1.08% and the temperature drop is 12.3 K/W. Accordingly, the combination of the diode laser-featuring low cost, long life, small weight, compact volume, and low power consumption-and the simple resonant cavity for laser cooling of solids is promising in some important applications in optical refrigeration of space sensors and detectors, etc.
A theoretical study of intra-cavity laser cooling by anti-Stokes luminescence in a rare-earth doped glass is per- formed. Compared with cooling in an external cavity by multipassing the radiation, intra-cavity cooling has the advantage of high pumping power and high-absorbed power. However, one must ensure that the cavity can still form a laser by locating the material in the cavity. A model is developed to evaluate the enhancement factor and the absorbed power. The results show that for a low optical density, especially when the sample length is less than 2 mm, the intracavity configuration is a very efficient method for laser cooling. The diode laser, which may become the best candidate for our model, is briefly discussed.
A simple model is developed to study the laser cooling of solids. The condition of laser cooling of a solid is developed. By using some parameters of the Yb 3+ ion, which is most widely used in laser cooling, we then calculate the cooling power and the cooling efficiency. In order to make a more precise analysis, the effect of fluorescent reabsorption, which is unavoidable in the cooling process, is discussed using the random walk model. Taking Tm 3+ ion as an example, we derive the average number of absorption events and determine the change in quantum efficiency due to reabsorption. Finally, we obtain the red-shift of the fluorescent wavelength and the requirement of sample dimension.OCIS codes: 140.3320, 160.2540, 300.2530. doi: 10.3788/COL201210.031401.As early as 1929, Pringsheim proposed an idea to cool a solid material by using anti-Stokes fluorescence. In this process, the material absorbs a pumping photon with a longer wavelength and soon afterwards emits a fluorescence photon with a shorter wavelength; the energy difference between the two photons, which results from the internal energy of the material, is removed by the fluorescence radiation, and the material is cooled. Many researchers have initially opposed this idea as they cannot relate laser to refrigeration. However, in 1995, the first demonstration of net laser cooling of a solid was reported [1] , and a Yb 3+ -doped fluorozirconate glass ZBLANP was cooled to 0.3 K below room temperature. Thereafter, people have carried out a series of theoretical and experimental studies for laser cooling of solids and obtained great progress [2−12] . Since anti-Stokes fluorescence cooling has been successfully achieved, explanations about the experimental result have been continually explored. Lamouche et al. found a theoretical model [12] and derived the cooling efficiency in arbitrary temperature by evaluating the experimental spectroscopy. The relationship between the mean fluorescent wavelength and the temperature was also determined. As the model of Lamouche is very complex, we propose a simple two-level system to analyze the micro course of laser cooling and then calculate the cooling power and cooling efficiency. We then discuss several main parameters that influence cooling power and determine the relationship between temperature and time.In order to obtain efficient laser cooling of solids, the key is to choose the proper fluorescent center and its level structure, as well as the appropriate energy gap. Taking Tm 3+ ions as example, their energy manifolds are shown in Fig.
We predict enhanced laser cooling performance of rare-earth-ions-doped glasses containing nanometre-sized ultrafine particles, which can be achieved by the enhancement of local field around rare earth ions, owing to the surface plasma resonance of small metallic particles. The influence of energy transfer between ions and the particle is theoretically discussed. Depending on the particle size and the ion emission quantum efficiency, the enhancement of the absorption is predicted. It is concluded that the absorption are greatly enhanced in these composite materials, the cooling power is increased as compared to the bulk material.compact, all-solid state cryocooler, these materials are good candidates for the cooling element.
Laser cooling of solid material has become a new developing research area in recent years. Tm3+ doped ZrF4-BaF2-LaF3-AlF3-NaF-PbF2 glass is one of the hot materials in this field. Compared with Yb3+, Tm3+ has better cooling potential. Up to date, one of the main factors restricting the cooling effect is fluorescent reabsorption. In this paper, firstly, using several spectral parameters of Tm3+, the reabsorption effect is calculated by stochastic model which is a semianalytical approach to this problem. The average number of absorption events is obtained. Afterwards, the effect of fluorescence trapping due to total internal reflection is analyzed. The results show that the quantum efficiency will be lowed by 0.5%1% due to reabsorption, that the redshift of the mean fluorescence wavelength is in the range of 210 nm, and that the cooling efficiency and the cooling power decrease. Finally, after discussion, we find that the use of a small size and a long thin geometry will benefit to the fluorescence emission and cooling effect.
We propose a new method to cool the Yb 3+ -doped ZBLANP glass in a standing-wave cavity. There are two advantages of this cavity-enhanced technique: the pumping power is greatly enhanced and the absorption of the cooling material is greatly increased. We introduce the basic principle of the cavity-enhanced laser cooling and discuss the cooling effect of a solid-state material in a cavity. From the theoretical study, it is found that the laser cooling effect is strongly dependent on the reflectivity of the cavity mirrors, the length of the solid material, the surface scattering of the material, and so on. Some optimal parameters for efficient laser cooling are obtained.
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.
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