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

Recent advances in laser cooling of solids

Galina A. Nemova, Raman Kashyap

Published Oct 11, 2013• 3 citations
Publisher

Abstract

The recent achievements devoted to cooling of solids with a laser are presented in this paper. We discuss the latest results of traditional laser cooling of solids based on rare earth ions and new techniques based on colloidal lead-salt quantum dots doped in a glass host, laser cooling in Tm3+-doped oxy-fluoride glass ceramic. Relatively short (microsecond) lifetime of the excited level of the PbSe QDs compared to the millisecond lifetime of the excited level of RE ions allows an acceleration of the cooling process and provides an opportunity to use new materials with higher phonon energy as hosts, which are normally considered unsuitable for cooling with RE ions. Another new approach to the laser cooling problem based on super-radiance has been considered in this paper. The advantages of optical refrigeration with rare earth doped semiconductors, in which not only optically active electrons of the 4f shell but the valence and conduction bands of the host material are involved in cooling cycle is discussed. It is shown that involving the valence and conduction bands of the host in the cooling cycle allows the pump wavelength to be shorter than mean fluorescence wavelength. Raman laser cooling of solids as well as observation of spontaneous Brillouin cooling have been presented.

DOI
10.1117/12.2037972

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Journal of the Optical Society of America B•Oct 11, 2012•25 citations
The recent achievements devoted to cooling of solids with a laser are presented in this paper. We discuss the latest results of traditional laser cooling of solids based on rare earth ions and new techniques based on colloidal lead-salt quantum dots doped in a glass host, laser cooling in Tm3+-doped oxy-fluoride glass ceramic. Relatively short (microsecond) lifetime of the excited level of the PbSe QDs compared to the millisecond lifetime of the excited level of RE ions allows an acceleration of the cooling process and provides an opportunity to use new materials with higher phonon energy as hosts, which are normally considered unsuitable for cooling with RE ions. Another new approach to the laser cooling problem based on super-radiance has been considered in this paper. The advantages of optical refrigeration with rare earth doped semiconductors, in which not only optically active electrons of the 4f shell but the valence and conduction bands of the host material are involved in cooling cycle is discussed. It is shown that involving the valence and conduction bands of the host in the cooling cycle allows the pump wavelength to be shorter than mean fluorescence wavelength. Raman laser cooling of solids as well as observation of spontaneous Brillouin cooling have been presented.
Theoretical schemes for laser cooling with nanoparticles have been presented and comprehensively investigated. It is shown that specially designed samples based on nanoparticles can be used to improve the process of laser cooling of solids. One of the proposed schemes is based on lead salt colloidal quantum dots (QDs) doped in a glass host. The second one is based on Tm3+ doped oxy fluoride glass ceramic. It has been shown that lead salt colloidal QDs doped in a glass host can operate as artificial atoms. Very short (microsecond range) radiative lifetimes of the excited 1Sh level of PbSe QDs in comparison with the relatively long (millisecond) radiative lifetime of rare-earth (RE) ions allows the cooling process to be accelerated and to use new hosts with relatively high maximum phonon energy, which have so far been considered not suitable for cooling with RE ions. It has been shown that the second sample, which is based on Tm3+ doped oxy fluoride glass ceramic provides the unique combination of high chemical and mechanical stability of the oxide glass, which is important for a number of applications, and the low phonon energy of the fluoride nano-crystals, which trap a majority of Tm3+ ions participating in the cooling process. This is highly beneficial for laser cooling of solids, since the effective embedding of rare-earth ions in the crystalline phase with low phonon energy provides a high quantum efficiency for the 3F4 → 3H6 transition involved in the cooling cycle in the Tm3+ ions, which is a key parameter for laser cooling of solids.
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.
This review discusses the progress and ongoing efforts in optical refrigeration. Optical refrigeration is a process in which phonons are removed from a solid by anti-Stokes fluorescence. The review first summarizes the history of optical refrigeration, noting the success in cooling rare-earth-doped solids to cryogenic temperatures. It then examines in detail a four-level model of rare-earth-based optical refrigeration. This model elucidates the essential roles that the various material parameters, such as the spacing of the energy levels and the radiative quantum efficiency, play in the process of optical refrigeration. The review then describes the experimental techniques for cryogenic optical refrigeration of rare-earth-doped solids employing non-resonant and resonant optical cavities. It then examines the work on laser cooling of semiconductors, emphasizing the differences between optical refrigeration of semiconductors and rare-earth-doped solids and the new challenges and advantages of semiconductors. It then describes the significant experimental results including the observed optical refrigeration of CdS nanostructures. The review concludes by discussing the engineering challenges to the development of practical optical refrigerators, and the potential advantages and uses of these refrigerators.
A new approach to cool rare earth doped solids with optical super-radiance (SR) is presented. SR is the coherent, sharply directed spontaneous emission of photons by a system excited with a pulsed laser. We consider an Yb3+ doped ZBLAN sample pumped at the wavelength 1015nm with a rectangular pulsed source with a power of ~700W and duration of 20ns. The intensity of the SR is proportional to the square of the number of excited ions. This unique feature of SR permits an increase in the rate of the cooling process in comparison with the traditional laser cooling of the rare earth doped solids with anti-Stokes spontaneous incoherent radiation (fluorescence). This scheme overcomes the limitation of using only low phonon energy glasses for laser cooling.
We present a theoretical scheme for laser cooling of rare-earth-doped solids with optical superradiance (SR), which is the coherent, sharply directed spontaneous emission of photons by a system of laser-excited rare-earth ions in the solid-state host (glass or crystal). We consider an Yb+{}^{+}+-doped ZnF4{}_{4}4​-BaF2{}_{2}2​-LaF3{}_{3}3​-AlF3{}_{3}3​-NaF (ZBLAN) sample pumped at a wavelength 1015 nm, with a rectangular pulsed source with a power of \ensuremath{\sim}433 W and a duration of 10 ns. The intensity of the SR is proportional to the square of the number of excited ions. This unique feature of SR permits an increase in the rate of the cooling process in comparison with the traditional laser cooling of the rare-earth-doped solids with anti-Stokes spontaneous incoherent radiation (fluorescence). This scheme overcomes the limitation of using only low phonon energy glasses for laser cooling.
We present a theoretical scheme for laser cooling in Tm3+-doped oxy-fluoride glass ceramic (GC). Tm3+-doped GC has a unique combination of high chemical and mechanical stability related to the oxide glass and low phonon energy of the fluoride nanocrystals, which trap a majority of Tm3+ ions. This unique property of GC makes it attractive for a number of applications. The effective embedding of rare-earth ions in the crystalline phase with low phonon energy provides high quantum efficiency for the F43→H63 transition in the Tm3+ ions participating in the cooling cycle.