User:Melgaard/sandbox/Optical Refrigeration

Optical Refrigeration

Optical refrigeration (a.k.a. laser cooling of solids) is a form of laser cooling in which vibrational energy, and hence the temperature, of a macroscopic solid is reduced through anti-Stokes fluorescence. The name "optical refrigeration" helps distinguish laser cooling of solids from the more commonly referenced doppler cooling (see also laser cooling). Even though the mechanisms are different for laser cooling of different phases of matter, the same general principle applies: heat removal through the energy transfer from low energy to high energy light. Laser cooling of gases is a well developed field achieving temperatures <1 nK by reducing the translational motion of atoms, for instance via doppler cooling, culminating in a Nobel Prize [1]. The cooling mechanism for solids is very different. The translational motion cannot be reduced because the atom locations in space are effectively fixed by the surrounding atomic structure. Without translational motion, doppler cooling does not apply to a solid. There is instead vibrational motion. After absorption of incident laser light, it is the annihilation of phonons, the quantized vibrational modes of a solid, that removes energy from the solid by transferring energy into the fluorescence.

History
During the 19th century, it was assumed fluorescence energy must always be Stokes shifted from the excitation energy. In other words, the fluorescence energy must always be less than the excitation energy. This argument follows from an interpretation of the second law of thermodynamics, and matches intuition. In general, observations followed this principle, however, a significant number of examples were being presented that provided evidence for anti-Stokes shifted fluorescence. While difficult to explain, by the early 20th century enough evidence had been presented to convince scientists of the validity of the anti-Stokes fluorescence measurements.

The first suggestion to use anti-Stokes fluorescence as a cooling mechanism was made by Peter Pringsheim in 1929, with a suggestion of using Sodium vapor. At the time, a controversy brewed as to whether cooling could be achieved since it was assumed to violate the second law of thermodynamics, since the material itself would lose entropy as it cooled. This argument falls short of considering the full system, namely the inclusion of the entropy of light. This controversy was quashed by Landau in 1946 when he assigned entropy to fluorescence. The basic argument can be understood when looking at what occurs between the incoming and outgoing light. Initially, monochromatic coherent light (and hence low entropy) from a laser is absorbed, followed by broadband isotropic (higher entropy) fluorescence. Even though the energy of the fluorescence is slightly higher (~kT), entropy of the full system increases, agreeing with the second law of thermodynamics. Shortly after, the suggestion to use anti-Stokes fluorescence cooling in a rare-earth doped solid was made in 1950 by Kastler.

Demonstration of anti-Stokes cooling was still missing two crucial ingredients. The first was the lack of a narrow band, intense light source, and the second was high purity (low absorption) host materials. Thanks to the invention of the laser in 1964, and the advancements in the material of long haul fiber optics such as ZBLAN, the first demonstration of anti-Stokes fluorescence cooling was demonstrated at Los Alamos National Laboratory, where researchers achieved 0.3 K cooling in a Yb3+:ZBLAN.

Since then, several advancements have been made utilizing different rare-earth ions in a wide variety of hosts.

Thus far, the best proffering material has been LiYF4:Yb3+ (also written Yb3+:YLF, Yb3+:LiYF4).