Optical refrigeration (also called laser refrigeration or anti-Stokes fluorescent cooling) is a technique for cooling a macroscopic crystal (or a piece of glass) with a laser beam. The crystal must be doped, e.g. with ytterbium or thulium ions, which are excited by the laser beam. The laser wavelength is chosen such that it is longer than the average wavelength of the resulting fluorescence. This means that the energy of the absorbed photons is lower than the average energy of the emitted photons, so that energy is removed from the crystal. Of course, it is essential that the quantum efficiency of the fluorescence is high, and that nearly all fluorescence light can leave the crystal without being absorbed, e.g. by impurities: a single absorbed photon would offset the cooling effect of many other photons.
Cooling a piece of ZBLAN glass in a “laser fridge” from room temperature down to 208 K has been demonstrated , and 110 K have been achieved with Yb:LiYF4 (Yb:YLF) . In theory, even temperatures of the order of 77 K (liquid nitrogen) should be reachable. Certain ytterbium-doped crystal materials, particularly tungstates such as Yb:KGW = Yb:KGd(WO4)2, appear to be suitable for this purpose.
Possible applications of laser refrigeration are the replacement of Stirling coolers and the like (avoiding moving parts, vibrations, etc.), but also radiation-balanced lasers, where the internal heat generation is essentially compensated by optical refrigeration.
It is instructive to consider entropy changes associated with laser refrigeration. The reduction in thermal entropy of the cooled device is more than compensated by the increase in entropy which arises from the conversion of narrow-band focused laser light into fluorescence light, which has a much higher entropy due to the many spatial modes and different frequencies involved in the emission.
See also the article on laser cooling, which deals with the cooling of microscopic particles, rather than macroscopic samples. The physical principles behind such cooling methods are rather different from those of optical refrigeration.
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