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Pan Stanford Publishing eBooks

Chapter 8 Bulk Cooling Efficiency Measurements of Yb-Doped Fluoride Single Crystals and Energy Transfer–Assisted Anti-Stokes Cooling in Co-Doped Fluorides

Azzurra Volpi, Alberto Di Lieto, M. Tonelli

Published Aug 31, 2016• 0 citations
Publisher

Abstract

To overcome the ETU-induced heating and realize efficient and enhanced laser cooling in Ho3+-doped solids, in Section 4.3, we will provide a modified excitation scheme based on ground-state absorption (GSA) of 5I8 Æ 5I7 at around 2.07 μm and ESA of 5I5 Æ 5F5 at around 2.28 μm. Compared with the conventional pumping scheme, the co-pumping scheme ameliorates the cooling performance significantly through alleviating the upconversion-quenching-induced heat [12], that is, the heat-producing 5I5 Ho3+ions are pumped to the 5F5 state by the laser at 2.28 μm. Fortunately, the 5F5 Ho3+ ions barely transit to the lower nearby state and tend to decay radiatively. Therefore, major heat produced in the conventional laser cooling of Ho3+-doped solids is avoided, and both cooling efficiency and cooling power density can be improved. In Section 4.4, we broaden the research domain of optical refrigeration to rare earth ion co-doped systems. Specifically, when Ho3+–Tm3+ co-doped YLiF4 crystals are pumped at around 2.07 μm, Ho3+ ions will populate at the 5I7 manifold. Meanwhile, Tm3+ ions will also be sensitized by absorbing energy transferred from 5I7 Ho3+ ions. Since the average energy of fluorescence photons generated by Tm3+3F4 Æ 3H6 radiative transitions is higher than that generated by Ho3+5I7 Æ 5I8 radiative transitions, energy transfer (ET) from Ho3+ to Tm3+ ions will induce annihilation of host phonons, and bring enhancement in both the cooling efficiency and the cooling power. Therefore, this cooling mechanism is referred next as energy transfer-enhanced laser cooling (ETLC) of solids [11]. 4.2 Conventional Laser Cooling of Holmium- The main upconversion process and the anti-Stokes fluorescence cooling (ASFC) processes of Ho3+-doped fluoride crystals are illustrated in Fig. 4.1. The ground-state (5I8) Ho3+ ions are pumped to the first excited state (5I7) with laser excitations of energy hν (solid red arrow on the left) and then relax thermally among the Stark levels in a timescale of picoseconds with host phonon absorption. Part of the Ho3+5I7 ions de-excite by emitting fluorescence photons of mean energy hν10 (solid blue arrow, ν10 > ν), which accounts for the ASFC process. The rest of the 5I7 Ho3+ ions de-excited and transfer their energy to the neighboring 5I7 Ho3+ ions, causing population accumulation at the third excited state (5I5) with host phonon absorption (solid green arrows). Comparison between the energy mismatch and the host phonon energy (for YLiF4 crystal host, ħω ≈ 450 cm-1) indicates that about two host phonons are annihilated for each ETU process (see Table 4.1).

DOI
10.1201/9781315364728-9

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Energy transfer enhanced laser cooling in Ho^3+ and Tm^3+-codoped lithium yttrium fluoride

Guang-Zong Dong, Xinlu Zhang, Li Li

We report a theoretical scheme for laser cooling of solids based on energy transfer usually found in rare-earth codoped materials. The cooling scheme enables a large enhancement in the cooling efficiency with regard to the standard anti-Stokes fluorescence cooling. A Ho3+ and Tm3+-codoped low-phonon crystal (LiYF4) sample is investigated to find that the cooling efficiency increases, and then decreases with the increasing of the resonant absorption. The optimal cooling efficiency is predicted to exceed 5%. The maximum cooling power density could be promoted greatly by applying the codoped cooling scheme. The cooling scheme is also valid for other rare-earth (for example, Tm3+ and Er3+, or Er3+ and Yb3+) codoped materials.

Journal of the Optical Society of America B•Mar 13, 2013•18 citations

Efficient and enhanced optical refrigeration beyond 2 μm in Ho^3+-doped solids via copumping scheme

Guang-Zong Dong, Xinlu Zhang

We report a theoretical scheme for realizing efficient and enhanced laser cooling in Ho3+-doped solids, based on ground-state absorption (GSA) of I58→I57 at around 2.07 μm and excited-state absorption (ESA) of I55→F55 at around 2.28 μm. The upconversion-quenching-induced heat that usually is found in Ho3+ optical refrigeration can be significantly alleviated by using the copumping scheme, so the cooling power density and the cooling efficiency are greatly improved. The scheme works at temperatures above 133 K, and can shorten the time needed to reach the minimum attainable temperature of the cooling system. Besides, the dopant concentration is allowed to be promoted to around 1%. The copumping scheme can also be applied to other multilevel (such as Tm3+ and Er3+) optical refrigeration systems to improve the cooling performance.

Journal of the Optical Society of America B•Jun 26, 2014•3 citations

Enhanced laser cooling of rare-earth-ion-doped nanocrystalline powders

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The enhanced laser cooling performance of rare-earth-ion-doped nanocrystalline powders is predicted, compared to the bulk material, using ${\mathrm{Yb}}^{3+}:{\mathrm{Y}}_{2}{\mathrm{O}}_{3}$ as the model material. This is achieved by enhancing the off-resonance, phonon-assisted absorption, which is proportional to the three factors considered in this paper: the dopant concentration, the pumping field energy, and the excitation coefficient. Using the energy transfer theory for concentration quenching, the optimum concentration corresponding to the maximum cooling power is found to be considerably larger than the currently used value, suggesting noticeable enhancement effects for laser cooling. The pumping field energy is enhanced in random nanopowders compared with bulk crystals under the same irradiation, due to the multiple scattering of photons. Photons are thus localized in the medium and do not propagate through, increasing the photon absorption of the pumping beam (and it is shown that the reabsorption of the fluorescence is negligible). Using molecular dynamics simulations, the phonon density of states (DOS) of the nanopowder is calculated, and found to have broadened modes, and extended small tails at low and high frequencies. The second-order electronic transition rate for the anti-Stokes luminescence is calculated using the Fermi golden rule, which includes the influence of this phonon DOS, and is shown to have enhancement effects on the laser cooling efficiency using nanopowders. It is finally concluded that these three enhancement mechanisms are essentially to increase the population of the three participating carriers (electron, photon, and phonon) in the interacting volume, and this also points out directions for enhancing laser cooling performance in bulk materials.

Physical Review B•Apr 21, 2006•72 citations

Chapter 7 Deep Laser Cooling of Rare Earth–Doped Crystals by Stimulated Raman Adiabatic Passage

Andrei V. Ivanov, Yu. Rozhdestvensky, E. Yu. Perlin

Expectedly there are challenging issues to be tackled. For example, the purity level of the (undoped) fiber needs to be high to avoid excessive parasitic absorption of photons by impurities. When the fiber cooler and the object to be cooled are macroscopic (centimeter scale or larger), managing the (conductive and convective) heat and photon flows is also a pertinent issue. These and related issues will be discussed below. In Section 3.2, we describe our theoretical developments in this investigation. We discuss the calculation of the cooling powers of the Tm atoms and of a model isolated fiber and the simulation of heat flow in the cooling of the heat load. In Section 3.3, we discuss experimental progress in fabricating high-purity fiber and temperature measurements under optical pumping conditions. 3.2 Theoretical Developments 3.2.1 Modeling the Cooling of Thulium AtomsThe basic cooling agents in our setup are the thulium (Tm) atoms doping the fiber. We describe in this subsection a photoexcitation model of a Tm atom in a glass host, which we use to estimate its laser cooling efficiency. More detail can be found in [7]. The model, given in Ref. [8], is shown in Fig. 3.2. It consists of four energy levels, each with a degree of degeneracy gi, as designated in the figure. The lower two levels represent the manifold of states created by the splitting of the 13-fold degenerate ground state (3H6) of Tm by interactions with the glass host, and the upper two levels represent the split ninefold excited-state (3F4) manifold. The pump laser energy hn is tuned to the energy between |1> and |2>, which is the lowest excitation energy between the two manifolds. The pumped Tm atoms de-excite (incoherently) to a level in the ground-state manifold via luminescence, at rate Wrad, and nonradiative transitions, at a rate Wnr. If the de-excitation happens sufficiently slowly, the excited atoms have time to thermalize in the excited-state manifold. The luminescent photons are thus at higher energies than that of the pump photons. In each cycle of operation in which a Tm atom absorbs a pump photon and emits a luminescent photon, which then escapes, heat is removed from the system. The population dynamics of the four-level model obeys the following rate equations [8]: ( ) g B/inc1 1 112 1 2 2 3 1 1 02 02 E k TIdN g gRN N N N w N N edt g h g ds n -Ê ˆÊ ˆ= - - + + - -Á ˜Á ˜Ë ¯ Ë ¯ (3.1) u B/inc 32 1 dN dt RN w N g 2 2= - - - Ê ËÁ ˆ ¯˜ -d u B/ (3.3) N N N N Ntotal = + + +0 1 32 (3.4)where Ni and gi are the population density and degree of degeneracy, respectively, of level |i>, and Ntotal is the total density of Tm atoms. The pump laser has intensity Iinc and frequency n. R = 2Wrad + 2Wnr, and s12 denotes the photoabsorption cross section of the Tm atoms. dEg and w1 are the energy difference between the two levels and the intramanifold relaxation rate in the ground-state manifold, and dEu and w2 are the corresponding quantities for the excited-state manifold. T is the sample temperature, and kB is the Boltzmann constant. In Eq. 3.1, the first term on the right side represents stimulated absorption from and emission into the pump laser mode. The second term models the decay of the excited manifold population and the third term the thermalization within a manifold. The terms in Eqs. 3.2 and 3.3 have the same interpretation.

Pan Stanford Publishing eBooks•Aug 31, 2016•0 citations

Coherent pumping for fast laser cooling of doped crystals

Andrei V. Ivanov, Yu. Rozhdestvensky, E. Yu. Perlin

A mechanism for laser cooling of crystals doped with rare-earth ions is proposed. The mechanism involves two-photon Raman scattering through the dipole-allowed 5d ion level, electron–phonon transitions between the Stark-split sublevels, and fluorescence from the excited ion level. It is shown that, under the conditions of coherent pumping, the total population of the ground level is transferred to the excited level of the rare-earth ion, thus increasing the cooling power and accelerating the cooling process. The involvement of dipole-allowed transitions yields an increase in the cooling efficiency by 14% compared to the efficiency of direct pumping at transitions between the 4f levels. Estimations show that the temperature of the sample cooled by the proposed mechanism can reach 82.9 K for current materials with a background absorption coefficient of 4.0×10−4 cm−1.

Journal of the Optical Society of America B•Apr 14, 2015•11 citations

Chapter 10 Fluoride Glasses and Fibers

Mohammed Saad

Figure 6.1 Anti-Stokes fluorescence emission in a two-level system. Additionally, only a fraction of the absorbed photons result in excitation of coolant atoms. The rest are absorbed by background impurities. With the absorption coefficient of the coolant atoms designated by αc and that of the background by αb, the proportion of absorbed photons that contribute to cooling is h a c b( ) = + (6.3) If the assumption is made that phonon-mediated absorp-tion is proportional to the occupation probability of the phonon mode, namely n k T= (exp[ / ]-1)B -1W , the efficiency becomes h a a aabs c c b/( )= +n n . Then at high temperatures (kBT >> ħΩ)), ηabs varies little with temperature when the background absorp-tion is low (αb/αc << 1). On the other hand at low temperatures, as n Æ 0, ηabs decreases in proportion to n . Hence the absorption efficiency drops dramatically through the important cryogenic re-gime. The overall cooling is determined by the product of the various efficiency factors. h h h n n ËÁ ˆ ¯˜ (6.4) From Eqs. 6.1 and 6.4 it is easy to show that the cooling power density per unit frequency is Pc/V = ηabsηextαtotIv(vfi – v)/v. Here Iv= I0g(v) is the intensity per unit frequency and g(v) is a normalized spectral line shape function. V is the interaction volume. If the bandwidth of the light source equals the full linewidth 2Γ of the optical transition, and the density of atoms in the interaction volume is N/V, the energy loss rate per atom is R = 2ΓηcαtotIv(V/N) = ηcαtotI0(V/N) (6.5) The condition for net cooling can be expressed in a simple way based on Eq. 6.4, since ηc must exceed zero. By assuming that the laser detuning is optimized at the phonon frequency Ω, we can rewrite the numerator using hvft – hv = ħΩ. Furthermore the average phonon energy is ħΩ ≈ kBT near the boundary of the classical regime [15]. Then the condition for laser cooling (ηc > 0) becomes h h next abs B> -1 k T h (6.6) An important limitation of refrigeration based on anti-Stokes fluorescence emerges from this condition by determining the temperature at which the cooling efficiency drops to zero (ηc = 0). By solving Eq. 6.6 as an equality, it is found that a lower bound exists for the attainable temperature. The minimum temperature Tmin that can be reached with anti-Stokes cooling found from Eq. 6.6 is T h ( / ) ( ) ( / ) = + - + Ê ËÁ ˆ ¯˜ n a a h a a 1 (6.7) According to Eq. 6.7, if the quantum efficiency is high (next ≈ 1), the temperature limit for laser cooling by anti-Stokes emission is primarily determined by the frequency of light and the background-to-resonant-absorption ratio. The dependence on these two factors arises because heat input to the system is proportional to incident photon energy and absorptive heating efficiency. Only when the background absorption coefficient vanishes could arbitrarily low temperatures be reached. For the moderate levels of background absorption encountered in practice in solids, this places a severe constraint on temperatures that can be reached by laser cooling. As a simple example of this, the anti-Stokes cooling limit predicted by Eq. 6.7 at a wavelength of 1 micron is Tmin = 145.3 K when the external quantum efficiency is ηext = 0.90 and background absorption coefficient is αb = αc/1000. The lower bound on attainable temperatures can be interpreted in another way. It arises because the cooling rate depends on phonon occupation, a factor that diminishes as temperature decreases. Eventually the rate of anti-Stokes cooling drops below the rate of heating from background impurity absorption and cooling is no longer possible [16]. For 3D refrigeration of solids to be as effective as laser cooling of gases, techniques capable of maintaining high rates of cooling through the cryogenic temperature range are needed. This is discussed further in what follows. In the remainder of this chapter momentum is incorporated into the discussion to investigate whether the fundamental temperature limitation of anti-Stokes fluorescence cooling can be evaded when the dynamics are viewed comprehensively. Judging from the successes of laser cooling of gases, cooling schemes that conserve momentum and energy simultaneously during optical interactions are the most efficient. Yet momentum is disregarded in the anti-Stokes fluorescence method. Other techniques may offer the capability of balancing the momentum and energy requirements more consistently to accommodate phonon dispersion (Fig. 6.2).

Pan Stanford Publishing eBooks•Aug 31, 2016•0 citations
To overcome the ETU-induced heating and realize efficient and enhanced laser cooling in Ho3+-doped solids, in Section 4.3, we will provide a modified excitation scheme based on ground-state absorption (GSA) of 5I8 Æ 5I7 at around 2.07 μm and ESA of 5I5 Æ 5F5 at around 2.28 μm. Compared with the conventional pumping scheme, the co-pumping scheme ameliorates the cooling performance significantly through alleviating the upconversion-quenching-induced heat [12], that is, the heat-producing 5I5 Ho3+ions are pumped to the 5F5 state by the laser at 2.28 μm. Fortunately, the 5F5 Ho3+ ions barely transit to the lower nearby state and tend to decay radiatively. Therefore, major heat produced in the conventional laser cooling of Ho3+-doped solids is avoided, and both cooling efficiency and cooling power density can be improved. In Section 4.4, we broaden the research domain of optical refrigeration to rare earth ion co-doped systems. Specifically, when Ho3+–Tm3+ co-doped YLiF4 crystals are pumped at around 2.07 μm, Ho3+ ions will populate at the 5I7 manifold. Meanwhile, Tm3+ ions will also be sensitized by absorbing energy transferred from 5I7 Ho3+ ions. Since the average energy of fluorescence photons generated by Tm3+3F4 Æ 3H6 radiative transitions is higher than that generated by Ho3+5I7 Æ 5I8 radiative transitions, energy transfer (ET) from Ho3+ to Tm3+ ions will induce annihilation of host phonons, and bring enhancement in both the cooling efficiency and the cooling power. Therefore, this cooling mechanism is referred next as energy transfer-enhanced laser cooling (ETLC) of solids [11]. 4.2 Conventional Laser Cooling of Holmium- The main upconversion process and the anti-Stokes fluorescence cooling (ASFC) processes of Ho3+-doped fluoride crystals are illustrated in Fig. 4.1. The ground-state (5I8) Ho3+ ions are pumped to the first excited state (5I7) with laser excitations of energy hν (solid red arrow on the left) and then relax thermally among the Stark levels in a timescale of picoseconds with host phonon absorption. Part of the Ho3+5I7 ions de-excite by emitting fluorescence photons of mean energy hν10 (solid blue arrow, ν10 > ν), which accounts for the ASFC process. The rest of the 5I7 Ho3+ ions de-excited and transfer their energy to the neighboring 5I7 Ho3+ ions, causing population accumulation at the third excited state (5I5) with host phonon absorption (solid green arrows). Comparison between the energy mismatch and the host phonon energy (for YLiF4 crystal host, ħω ≈ 450 cm-1) indicates that about two host phonons are annihilated for each ETU process (see Table 4.1).
We report a theoretical scheme for laser cooling of solids based on energy transfer usually found in rare-earth codoped materials. The cooling scheme enables a large enhancement in the cooling efficiency with regard to the standard anti-Stokes fluorescence cooling. A Ho3+ and Tm3+-codoped low-phonon crystal (LiYF4) sample is investigated to find that the cooling efficiency increases, and then decreases with the increasing of the resonant absorption. The optimal cooling efficiency is predicted to exceed 5%. The maximum cooling power density could be promoted greatly by applying the codoped cooling scheme. The cooling scheme is also valid for other rare-earth (for example, Tm3+ and Er3+, or Er3+ and Yb3+) codoped materials.
We report a theoretical scheme for realizing efficient and enhanced laser cooling in Ho3+-doped solids, based on ground-state absorption (GSA) of I58→I57 at around 2.07 μm and excited-state absorption (ESA) of I55→F55 at around 2.28 μm. The upconversion-quenching-induced heat that usually is found in Ho3+ optical refrigeration can be significantly alleviated by using the copumping scheme, so the cooling power density and the cooling efficiency are greatly improved. The scheme works at temperatures above 133 K, and can shorten the time needed to reach the minimum attainable temperature of the cooling system. Besides, the dopant concentration is allowed to be promoted to around 1%. The copumping scheme can also be applied to other multilevel (such as Tm3+ and Er3+) optical refrigeration systems to improve the cooling performance.
The enhanced laser cooling performance of rare-earth-ion-doped nanocrystalline powders is predicted, compared to the bulk material, using Yb3+:Y2O3{\mathrm{Yb}}^{3+}:{\mathrm{Y}}_{2}{\mathrm{O}}_{3}Yb3+:Y2​O3​ as the model material. This is achieved by enhancing the off-resonance, phonon-assisted absorption, which is proportional to the three factors considered in this paper: the dopant concentration, the pumping field energy, and the excitation coefficient. Using the energy transfer theory for concentration quenching, the optimum concentration corresponding to the maximum cooling power is found to be considerably larger than the currently used value, suggesting noticeable enhancement effects for laser cooling. The pumping field energy is enhanced in random nanopowders compared with bulk crystals under the same irradiation, due to the multiple scattering of photons. Photons are thus localized in the medium and do not propagate through, increasing the photon absorption of the pumping beam (and it is shown that the reabsorption of the fluorescence is negligible). Using molecular dynamics simulations, the phonon density of states (DOS) of the nanopowder is calculated, and found to have broadened modes, and extended small tails at low and high frequencies. The second-order electronic transition rate for the anti-Stokes luminescence is calculated using the Fermi golden rule, which includes the influence of this phonon DOS, and is shown to have enhancement effects on the laser cooling efficiency using nanopowders. It is finally concluded that these three enhancement mechanisms are essentially to increase the population of the three participating carriers (electron, photon, and phonon) in the interacting volume, and this also points out directions for enhancing laser cooling performance in bulk materials.
Expectedly there are challenging issues to be tackled. For example, the purity level of the (undoped) fiber needs to be high to avoid excessive parasitic absorption of photons by impurities. When the fiber cooler and the object to be cooled are macroscopic (centimeter scale or larger), managing the (conductive and convective) heat and photon flows is also a pertinent issue. These and related issues will be discussed below. In Section 3.2, we describe our theoretical developments in this investigation. We discuss the calculation of the cooling powers of the Tm atoms and of a model isolated fiber and the simulation of heat flow in the cooling of the heat load. In Section 3.3, we discuss experimental progress in fabricating high-purity fiber and temperature measurements under optical pumping conditions. 3.2 Theoretical Developments 3.2.1 Modeling the Cooling of Thulium AtomsThe basic cooling agents in our setup are the thulium (Tm) atoms doping the fiber. We describe in this subsection a photoexcitation model of a Tm atom in a glass host, which we use to estimate its laser cooling efficiency. More detail can be found in [7]. The model, given in Ref. [8], is shown in Fig. 3.2. It consists of four energy levels, each with a degree of degeneracy gi, as designated in the figure. The lower two levels represent the manifold of states created by the splitting of the 13-fold degenerate ground state (3H6) of Tm by interactions with the glass host, and the upper two levels represent the split ninefold excited-state (3F4) manifold. The pump laser energy hn is tuned to the energy between |1> and |2>, which is the lowest excitation energy between the two manifolds. The pumped Tm atoms de-excite (incoherently) to a level in the ground-state manifold via luminescence, at rate Wrad, and nonradiative transitions, at a rate Wnr. If the de-excitation happens sufficiently slowly, the excited atoms have time to thermalize in the excited-state manifold. The luminescent photons are thus at higher energies than that of the pump photons. In each cycle of operation in which a Tm atom absorbs a pump photon and emits a luminescent photon, which then escapes, heat is removed from the system. The population dynamics of the four-level model obeys the following rate equations [8]: ( ) g B/inc1 1 112 1 2 2 3 1 1 02 02 E k TIdN g gRN N N N w N N edt g h g ds n -Ê ˆÊ ˆ= - - + + - -Á ˜Á ˜Ë ¯ Ë ¯ (3.1) u B/inc 32 1 dN dt RN w N g 2 2= - - - Ê ËÁ ˆ ¯˜ -d u B/ (3.3) N N N N Ntotal = + + +0 1 32 (3.4)where Ni and gi are the population density and degree of degeneracy, respectively, of level |i>, and Ntotal is the total density of Tm atoms. The pump laser has intensity Iinc and frequency n. R = 2Wrad + 2Wnr, and s12 denotes the photoabsorption cross section of the Tm atoms. dEg and w1 are the energy difference between the two levels and the intramanifold relaxation rate in the ground-state manifold, and dEu and w2 are the corresponding quantities for the excited-state manifold. T is the sample temperature, and kB is the Boltzmann constant. In Eq. 3.1, the first term on the right side represents stimulated absorption from and emission into the pump laser mode. The second term models the decay of the excited manifold population and the third term the thermalization within a manifold. The terms in Eqs. 3.2 and 3.3 have the same interpretation.
A mechanism for laser cooling of crystals doped with rare-earth ions is proposed. The mechanism involves two-photon Raman scattering through the dipole-allowed 5d ion level, electron–phonon transitions between the Stark-split sublevels, and fluorescence from the excited ion level. It is shown that, under the conditions of coherent pumping, the total population of the ground level is transferred to the excited level of the rare-earth ion, thus increasing the cooling power and accelerating the cooling process. The involvement of dipole-allowed transitions yields an increase in the cooling efficiency by 14% compared to the efficiency of direct pumping at transitions between the 4f levels. Estimations show that the temperature of the sample cooled by the proposed mechanism can reach 82.9 K for current materials with a background absorption coefficient of 4.0×10−4 cm−1.
Figure 6.1 Anti-Stokes fluorescence emission in a two-level system. Additionally, only a fraction of the absorbed photons result in excitation of coolant atoms. The rest are absorbed by background impurities. With the absorption coefficient of the coolant atoms designated by αc and that of the background by αb, the proportion of absorbed photons that contribute to cooling is h a c b( ) = + (6.3) If the assumption is made that phonon-mediated absorp-tion is proportional to the occupation probability of the phonon mode, namely n k T= (exp[ / ]-1)B -1W , the efficiency becomes h a a aabs c c b/( )= +n n . Then at high temperatures (kBT >> ħΩ)), ηabs varies little with temperature when the background absorp-tion is low (αb/αc << 1). On the other hand at low temperatures, as n Æ 0, ηabs decreases in proportion to n . Hence the absorption efficiency drops dramatically through the important cryogenic re-gime. The overall cooling is determined by the product of the various efficiency factors. h h h n n ËÁ ˆ ¯˜ (6.4) From Eqs. 6.1 and 6.4 it is easy to show that the cooling power density per unit frequency is Pc/V = ηabsηextαtotIv(vfi – v)/v. Here Iv= I0g(v) is the intensity per unit frequency and g(v) is a normalized spectral line shape function. V is the interaction volume. If the bandwidth of the light source equals the full linewidth 2Γ of the optical transition, and the density of atoms in the interaction volume is N/V, the energy loss rate per atom is R = 2ΓηcαtotIv(V/N) = ηcαtotI0(V/N) (6.5) The condition for net cooling can be expressed in a simple way based on Eq. 6.4, since ηc must exceed zero. By assuming that the laser detuning is optimized at the phonon frequency Ω, we can rewrite the numerator using hvft – hv = ħΩ. Furthermore the average phonon energy is ħΩ ≈ kBT near the boundary of the classical regime [15]. Then the condition for laser cooling (ηc > 0) becomes h h next abs B> -1 k T h (6.6) An important limitation of refrigeration based on anti-Stokes fluorescence emerges from this condition by determining the temperature at which the cooling efficiency drops to zero (ηc = 0). By solving Eq. 6.6 as an equality, it is found that a lower bound exists for the attainable temperature. The minimum temperature Tmin that can be reached with anti-Stokes cooling found from Eq. 6.6 is T h ( / ) ( ) ( / ) = + - + Ê ËÁ ˆ ¯˜ n a a h a a 1 (6.7) According to Eq. 6.7, if the quantum efficiency is high (next ≈ 1), the temperature limit for laser cooling by anti-Stokes emission is primarily determined by the frequency of light and the background-to-resonant-absorption ratio. The dependence on these two factors arises because heat input to the system is proportional to incident photon energy and absorptive heating efficiency. Only when the background absorption coefficient vanishes could arbitrarily low temperatures be reached. For the moderate levels of background absorption encountered in practice in solids, this places a severe constraint on temperatures that can be reached by laser cooling. As a simple example of this, the anti-Stokes cooling limit predicted by Eq. 6.7 at a wavelength of 1 micron is Tmin = 145.3 K when the external quantum efficiency is ηext = 0.90 and background absorption coefficient is αb = αc/1000. The lower bound on attainable temperatures can be interpreted in another way. It arises because the cooling rate depends on phonon occupation, a factor that diminishes as temperature decreases. Eventually the rate of anti-Stokes cooling drops below the rate of heating from background impurity absorption and cooling is no longer possible [16]. For 3D refrigeration of solids to be as effective as laser cooling of gases, techniques capable of maintaining high rates of cooling through the cryogenic temperature range are needed. This is discussed further in what follows. In the remainder of this chapter momentum is incorporated into the discussion to investigate whether the fundamental temperature limitation of anti-Stokes fluorescence cooling can be evaded when the dynamics are viewed comprehensively. Judging from the successes of laser cooling of gases, cooling schemes that conserve momentum and energy simultaneously during optical interactions are the most efficient. Yet momentum is disregarded in the anti-Stokes fluorescence method. Other techniques may offer the capability of balancing the momentum and energy requirements more consistently to accommodate phonon dispersion (Fig. 6.2).