Optical cooling

Figure Cl.4.9. Usual cooling (carrier) and repumping (sideband) transitions when optically cooling Na atoms. The repumper frequency is nonnally derived from the cooling transition frequency with electro-optic modulation.

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... 

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. 

Fig. 3. Exploded view of the thin film spectroscopy apparatus, showing the relative positions of the optical cooling surface, the interferometer, the optical pathway for measurement and the material source for the film. W, window (polished quartz or sapphire), O, ground surface onto which windows are glued, LED, light emitting diode, PD, photodiode...

Neuhauser, W. Hohenstatt M. Toschek, P.E. Dehmelt H.G. Visual observation and optical cooling of electrodynamically contained ions. Appl. Phys. 1978,17,123-129. 

An interesting aspect of collision-aided radiative excitation is its potential for optical cooling of vapors. Since the change in kinetic energy of the collision partners per absorbed photon can be much larger than that transferred by photon recoil (Sect. 9.1), only a few collisions are necessary for cooling to low temperatures compared with a few thousand for recoil cooling [1093]. 

In this section we discuss the new technique of optical cooling, which decreases the velocity of atoms to a small interval around v = 0. Optical cooling down to temperatures of a few micro Kelvin has been achieved by combining optical and evaporative cooling even the nanoKelvin range was reached. This brought the discovery of quite new phenomena, such as Bose-Einstein condensation or atom-lasers, and atomic fountains [1109-1 111]. 

Although the recoil effect is very small when a single photon is absorbed, it can be used effectively for optical cooling of atoms by the cumulative effect of many absorbed photons. This can be seen as follows When atom A stays for the time T in a laser field that is in resonance with the transition j) ), the atom may absorb and emit a photon tko many times, provided the optical pumping cycle is short compared to T and the atom behaves like a true two-level system. This means that the emission of fluorescence photons fuo by the excited atom in level k) brings the atom back only to the initial state i), but never to other levels. With the saturation parameter S = Bikp (Oik)/Aik, the fraction of excited atoms becomes... 

For the experimental realization of optical cooling, which uses a collimated beam of atoms and a counterpropagating cw laser (dye laser or diode laser. Fig. 9.6) the following difficulties have to be overcome during the deceleration time the Doppler-shifted absorption frequency o) t) = k v(t) changes with the decreasing velocity V, and the atoms would come out of resonance with the monochromatic laser. 

Most optical cooling experiments have been performed up to now on alkali atoms, such as Na or Rb, using a single-mode cw dye laser. The velocity decrease of the atoms is monitored with the tunable probe laser L2, which is sufficiently weak... 

A very interesting alternative laser for optical cooling of atoms in a collimated beam is the modeless laser [1131], which has a broad spectral emission (without mode stmcture, when averaged over a time of T > 10 ns, with an adjustable bandwidth and a tunable center frequency). Such a laser can cool all atoms regardless of their velocity if its spectral width Ao>l is larger than the Doppler shift Acod = vok [1132]. 

If the atoms have been optically cooled before they pass the standing laser wave, they experience a large collimation in the maximum of the standing wave, but are not affected in the nodes. The laser wave acts like a transmission grating that channels the transmitted atoms and focuses the different channels (Fig. 9.12) [1139]. 

A schematic diagram of an apparatus for optical cooling of atoms, deflection of the slowed-down atoms, and focusing is depicted in Fig. 9.13. 

The optical cooling techniques discussed so far are restricted to true two-level systems because the cooling cycle of induced absorption and spontaneous emission has to be performed many times before the atoms come to rest. In molecules the fluorescence from the upper excited level generally ends in many rotational-vibrational levels in the electronic ground state that differ Irom the initial level. Therefore most of the molecules cannot be excited again with the same laser. They are lost for further cooling cycles. 

A very interesting optical cooling technique starts with the selective excitation of a collision pair of cold atoms into a bound level in an upper electronic state (Fig. 9.17). While this excitation occurs at the outer turning point of the upper-state potential, a second laser dumps the excited molecule down into a low vibrational level of the electronic ground state by stimulated emission pumping (photo-induced association). In favorable cases the level u = 0 can be reached. If the colliding atoms... 

A promising nonoptical technique relies on cooling of molecules by collisions with cold atoms. If the gas mixture of atoms and molecules can be trapped in a sufficiently small volume long enough to achieve thermal equilibrium between atoms and molecules, the optically cooled atoms act as a heat sink for the molecules, which will approach the same temperature as the atoms (sympathetic cooling) [1145]. 

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