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Journal of the Physical Society of Japan

Possibility of Optical Cooling of Ruby

Ikuji Tsujikawa, Tsuyoshi Murao

Published Apr 1, 1963• 18 citations
Publisher

Abstract

The possibility of cooling by an optical process of ruby is pointed out. When Cr 3+ ions in ruby are optically pumped by photons with a given energy and emit photons with higher energies, the lattice energy can be extracted as light, so that a cooling effect appears. If some possible heating effects did not overcome this cooling effect, the temperature would be actually reduced. Cooling rates and cooling capacities are given. If we could have light input powers 10 -1 Watt or 10 -2 Watt for optical pumping, it might be possible to cool from 80°K or 20°K to temperatures below 1°K, respectively. Technical problems and possible heating effects are discussed.

DOI
10.1143/jpsj.18.503

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Possibility of Optical Cooling of Ruby

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The possibility of cooling by an optical process of ruby is pointed out. When C^<3+> ions in ruby are optically pumped by photons with a given energy and emit photons with higher energies, the lattice energy can be extracted as light, so that a cooling effect appears. If some possible heating effects did not overcome this cooling effect, the temperature would be actually reduced. Cooling rates and cooling capacities are given. If we could have light input powers 10^<-1> Watt or 10^<-2> Watt for optical pumping, it might be possible to cool from 80°K or 20°K to temperatures below 1°K, respectively. Technical problems and possible heating effects are discussed.

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Physical Review B•Jul 6, 2007•8 citations
The possibility of cooling by an optical process of ruby is pointed out. When Cr 3+ ions in ruby are optically pumped by photons with a given energy and emit photons with higher energies, the lattice energy can be extracted as light, so that a cooling effect appears. If some possible heating effects did not overcome this cooling effect, the temperature would be actually reduced. Cooling rates and cooling capacities are given. If we could have light input powers 10 -1 Watt or 10 -2 Watt for optical pumping, it might be possible to cool from 80°K or 20°K to temperatures below 1°K, respectively. Technical problems and possible heating effects are discussed.
The possibility of cooling by an optical process of ruby is pointed out. When C^<3+> ions in ruby are optically pumped by photons with a given energy and emit photons with higher energies, the lattice energy can be extracted as light, so that a cooling effect appears. If some possible heating effects did not overcome this cooling effect, the temperature would be actually reduced. Cooling rates and cooling capacities are given. If we could have light input powers 10^<-1> Watt or 10^<-2> Watt for optical pumping, it might be possible to cool from 80°K or 20°K to temperatures below 1°K, respectively. Technical problems and possible heating effects are discussed.
A near-minimal instance of optical cooling is experimentally presented, wherein the internal-state entropy of a single atom is reduced more than twofold by illuminating it with broadband, incoherent light. Since the rate of optical pumping by a thermal state increases monotonically with its temperature, the cooling power in this scenario increases with higher thermal occupation, an example of a phenomenon known as cooling by heating. In contrast to optical pumping using coherent, narrow-band laser light, here, we perform the same task with fiber-coupled, broadband sunlight, the brightest laboratory-accessible source of continuous blackbody radiation.
Absorption transitions between two excited optical states in ruby have been studied by comparing the absorption coefficients measured under optically pumped and unpumped conditions. The spectrum has been determined in the π-and σ-polarizations in the photon energy range from 5,500 to 45,000 cm -1 . Several absorption bands have been found which arise from the transitions from the excited states t 2g 3 2 E g and t 2g 3 2 T 1g of the Cr 3+ ion. From the locations and intensities of these bands, the terminal states of the observed transitions are assigned to the charge transfer state, the t 2g 3 2 T 2g state, and the doublet states belonging to the electron configuration t 2g 3 e g . Calculated energies between these states and the metastable states, t 2g 3 2 E g and t 2g 3 2 T 1g , are in good agreement with the peak energies of the observed absorption bands. The present experiment gives new information inaccessible by the ordinary ground state absorption measurement, such as the locations of several doublet states with the configuration t 2g 2 e g and the intensities of doublet-doublet transitions in ruby.
When an atom or a molecule interacts with a light beam, the light emitted or absorbed carries valuable information about the atomic or molecular structure. This phenomenon underlies the whole field of spectroscopy. But the interaction of a photon with an atom can be used to manipulate the atom as well as to probe its structure. For example, in an approach called optical pumping, invented by Alfred Kastler, one can use the resonant exchange of angular momentum between atoms and polarized photons to align or orient the spins of atoms or to put them in nonequilibrium situations. In his original 1950 paper Kastler also proposed using optical pumping to cool and to heat the internal degrees of freedom, calling the phenomena the “effet luminofrigorique” and the “effet luminocalorique.” Another famous example of the use of photon-atom interaction to control atoms is laser cooling. This technique relies on resonant exchange of linear momentum between photons and atoms to control their external degrees of freedom and thus to reduce their kinetic energy. Laser cooling was suggested independently by Theodor Hänsch and Arthur Schawlow for neutral atoms and by David Wineland and Hans Dehmelt for trapped ions. In an article written three years ago for PHYSICS TODAY (June 1987, page 34), Wineland and Wayne Itano presented the principle of laser cooling and the potential applications of cold atoms to fields of physics such as ultrahigh resolution spectroscopy, atomic clocks, collisions, surface physics and collective quantum effects. At that time laser cooling had brought temperatures down to a few hundred microkelvin, but unexpected improvements during the last three years have dramatically lowered those temperatures to only a few microkelvin. We now feel we understand the new physical mechanisms responsible for these very low temperatures.
The optical spectrum due to the exchange coupled Cr 3+ ions in ruby has been studied theoretically. The effect of the lowering of the Cr 3+ site symmetry due to the formation of a pair was taken into account. For the 2nd neighbor pair, a good fitting of the excited state energies between theory and experiment was obtained. The effect of the higher order exchange is found to be very important for this pair. It is also found that the deviation of the Cr 3+ site symmetry from C 3 is rather large for this pair. For the 4th neighbor pair, the level diagram of the excited state was obtained and the absorption intensities were calculated. Most of the observed lines in this pair are explained by this theory. The lines so called N 1 and N 2 are found to be the transitions due to the exchange induced electric dipole.
Under appropriate conditions, absorption of light by a solid can initiate a process by which it is cooled. In particular, energy is extracted from a material when its absorption of a photon is followed by emission of a photon of higher energy. This up-conversion requires some of the solid's electrons to garner energy from atomic vibrations. Here, two schemes for laser cooling via localized electronic states are addressed. The first scheme utilizes the ground state and an excited state of a localized center. In this two-level scheme, the cooling process is initiated with photon absorption in the extreme low-energy tail of a localized state's vibrationally broadened absorption spectrum. The subsequent atomic relaxation transfers energy of especially large vibratory atomic strains into electrical energy that is then extracted via photon emission. The second scheme involves the ground state and two excited states of a localized center. Cooling is facilitated when (i) the photoexcitation of an electron from its ground state to the lower excited level is followed by (ii) electron-phonon-induced promotion to the uppermost level and the subsequent (iii) return of the electron to its ground state with emission of a photon of higher energy than that of the absorbed photon. However, competing relaxation processes contribute to heating. The net cooling power per unit volume is maximized for both schemes, thereby determining characteristics of localized electronic systems that foster optical cooling. The cooling power per unit volume is greatest at high temperatures and falls rapidly as the thermal energy is reduced below each system's luminescence Stokes shift. Moreover, cooling via the three-level scheme is most effective when (i) the energy separation between excited states is smaller than the thermal energy and (ii) the degeneracy of the highest-lying excited state is much larger than that of the center's middle level. These restrictive conditions appear to be satisfied for the three-level systems that to date exhibit laser cooling at or somewhat below room temperature: severely localized crystal-field-split fff-electron states of rare-earth ions.