Optical Cooling of Solids

Optical cooling of solids is, in principle, more difficult than for gases, because the usual result of illumination is the generation of heat (phonons) and light. However, in 1995 R. 1. Epstein and co-workers observed for the first time laser-induced cooling 

The cooling power will be equal to the emitted power minus the absorbed power, Pgooi = — Pabs, and it can easily be written in terms of the pump wavelength, X. 

In the ideal case of a quantum efficiency equal to one (the number of emitted photons being equal to the number of absorbed photons), we can easily obtain that 

EXAMPLE 6.7 Estimate the cooling efficiency, defined by ) cooi =, of a 

Ytf -ZBLANP sample when it is illuminated at room temperature by a laser tuned at 1000 nm. 

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In spite of the usually low cooling efficiencies (see the exercise above), recent experiments have demonstrated an anti-Stokes cooling from room temperature to 77 K within a certain internal volume of Yb + doped fluorochloride and fluoride glasses under high photon irradiances (Fernandez et ai, 2000). Future practical applications of optical cooling of solids include cooling systems for spacecraft electronics and detectors, as well as for superconductive circuits. 

Finally, in the last section of this chapter (Section 6.6), we will treat two aspects that are of great relevance in the optical spectroscopy of solids. First, we will introduce a semi-empirical method (due to Judd, 1962 Ofelt,1962) that analyzes the absorption spectra of trivalent rare earth ions in crystals to search for new efficient phosphors and solid state lasers. Secondly, we will treat a relatively new topic related to optical centers in solids the optically induced cooling of trivalent ytterbium doped solids. 

Hehlen, M.P., 2009. Design and fabrication of rare-earth-doped laser cooling materials. In Epstein, R.I., Sheik-Bahae, M. (Eds.), Optical Refrigeratitm Science and Applications of Laser Cooling of Solids. Wiley, Weinheim, Germany, pp. 37-74. 

Melgaard, S.D., 2013. Cryogenic optical refrigeration laser cooling of solids below 123 K. Ph.D. University of New Mexico, Albuquerque, New Mexico. 

The isomerized rosin, 245 g. (0.72 mole) (Note 1), is placed in a 1-1. Erlenmeyer flask and dissolved in 375 ml. of acetone by heating the mixture on a steam bath. To this solution, at incipient boiling, is added slowly and with vigorous agitation (Note 5) 127 g. (0.81 mole) of diamylamine 2 (Note 6). Upon cooling to room temperature, crystals appear in the form of rosettes. The mass is agitated, cooled well in an ice bath, and filtered by suction. The crystalline salt is washed on a Buchner funnel with 150 ml. of acetone and dried in a vacuum oven at 50° for 1 hour. The optical rotation of this material is [< ]d —18° (Note 4). The solid is recrystallized four times from acetone. Each time a sufficient quantity (20 ml. per g.) of acetone is used to obtain complete solution, and the solvent is evaporated until incipient precipitation of the salt occurs. The yield of product is 118 g. [ ]d —60° (Note 4). An additional 29 g. of product, having the same rotation, can be recovered from filtrates of the previous crystallizations. 

The vibrational spectrum of benzene around 1000 cnf has also been measured. IQ. Benzene was physisorbed on a cooled copper substrate in the vacuum chamber. Figure 19 shows the transmission for several thicknesses of benzene and a prism separation of 3 cm. The thickness was determined from the measured transmission in transparent regions using Eg. (7). The solid curves were calculated from Eqs. (5) and (6) using optical constants for benzene obtained from an ordinary transmission experiment.il The benzene film was assumed to be isotropic. Of the two absorption lines seen, one belongs to an in-plane vibrational mode, and one to an out-of-plane vibration. Since the electric field of the SEW is primarily perpendicular to the surface, the benzene molecules are clearly not all parallel or all perpendicular to the copper surface. Also it should be noted that the frequencies are the same (within the experimental resolution) as those of solid benzene22 and of nearly the same width. These features indicate that the benzene interacts only weakly with the copper surface, as would be expected for physisorbed molecules. 

There is no special crystallization technique required to prepare chiral crystals from achiral organic compounds. Ordinary crystallization techniques can be employed i.e., a hot saturated solution of a compound in a suitable solvent is slowly cooled, or a saturated solution is slowly evaporated to obtain crystals. Whether the molecule is chiral can be easily differentiated by the measurement of optical rotation in solution using a polarimeter. However, there are great difficulties in measuring the optical rotation of crystalline substances due to the large birefringence [33]. Solid-state circular dichroism (CD) spectral measurements of... 

Our first steps toward the single-molecule regime arose from work at IBM Research in the early 1980s on persistent spectral hole-burning effects in the optical transitions of impurities in solids (for a review, see [20]). Briefly, if a molecule with a strong zero-phonon transition and minimal Franck-Condon distortion is doped into a solid and cooled to liquid helium temperatures, the optical absorption becomes inhomogeneously broadened (Fig. 2.2A). The width of the lowest electronic transition for any one molecule (homogeneous width, Yjj) becomes very small because few phonons are present, while at the... 

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