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TECHNICAL PAPERS

# Advances in Laser Cooling of Solids

[+] Author and Article Information
X. L. Ruan

Department of Mechanical Engineering,  University of Michigan, Ann Arbor, MI 48109

M. Kaviany

Department of Mechanical Engineering,  University of Michigan, Ann Arbor, MI 48109kaviany@umich.edu

J. Heat Transfer 129(1), 3-10 (Jun 18, 2006) (8 pages) doi:10.1115/1.2360596 History: Received January 26, 2006; Revised June 18, 2006

## Abstract

We review the progress on laser cooling of solids. Laser cooling of ion-doped solids and semiconductors is based on the anti-Stokes fluorescence, where the emitted photons have a mean energy higher than that of the absorbed photons. The thermodynamic analysis shows that this cooling process does not violate the second law, and that the achieved efficiency is much lower than the theoretical limit. Laser cooling has experienced rapid progress in rare-earth-ion doped solids in the last decade, with the temperature difference increasing from $0.3to92K$. Further improvements can be explored from the perspectives of materials and structures. Also, theories need to be developed, to provide guidance for searching enhanced cooling performance. Theoretical predictions show that semiconductors may be cooled more than ion-doped solids, but no success in bulk cooling has been achieved yet after a few attempts (due to the fluorescence trapping and nonradiative recombination). Possible solutions are discussed, and net cooling is expected to be realized in the near future.

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## Figures

Figure 3

Process for laser cooling of a semiconductor in which a laser photon with frequency ωph,i is absorbed followed by emission of an up-converted fluorescence photon with frequency ωph,e

Figure 2

Energy spectra of all three carriers in irradiated Yb3+:Y2O3. Yb3+ has only two main electronic levels: F7∕24 and F5∕24. Carriers may interact with each other as energy and momentum conservations are met (26).

Figure 4

The energy diagram for laser cooling of a solid, where radiation is the only external thermal load

Figure 5

A control volume showing various energies in and out in the laser cooling of a solid (37)

Figure 6

(a) The absorption and fluorescence spectra of Yb3+:ZBLANP, with the mean fluorescence wavelength marked (11). (b) The normalized temperature difference with respect to the pumping wavelength. Cooling is detected as the pumping wavelength is tuned longer than the mean fluorescence wavelength (11).

Figure 7

Experimental apparatus used for observing laser cooling of Yb:ZBLANP (15). Pump radiation from a cw Ti:sapphire laser undergoes mode scrambling within an external multimode silica fiber before injection into the sample fiber, which is positioned upon a sample mount (inset), imposing an extremely low conductive thermal load. Unabsorbed pump radiation is collected from the output end of the sample fiber and reinjected into the sample with the help of an external high reflector. Finally, emitted fluorescence is collected with a third internal optic and is spectrally resolved to determine the temperature.

Figure 8

Modified structures for enhanced laser cooling performance: (a) Cavity arrangement for multiple passes (42), (b) A micrograph of Yb3+:Y2O3 nanopowder (51).

Figure 9

Predicted cooling efficiency as a function of temperature, for the 2 and 8wt%Yb3+:Y2O3 in the linear regime for two optical pathlengths (45).

Figure 1

(a) Three fundamental energy carriers in rare-earth ion doped solids irradiated by laser light: Photons from the pumping fields, phonons from the host crystal, and electrons of the doped ions. (b) Principles of laser cooling in rare-earth ion doped crystal. The electron is excited by absorbing a photon and one or more lattice phonons, and then decays by emitting a higher energy photon.

Figure 10

Possible structures for extracting the fluorescence from the high refractive-index semiconductors: (a) An index-matching dome lens attached on the cooling element (48). (b) A nanogap structure to couple the evanescent waves out of the cooling element (50).

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