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"Laser Cooling of Solids" icon

Laser Cooling of Solids

Enhanced laser cooling of ion doped nanocrystalline powders (e.g., Yb3+: Y2O3) can be achieved by enhancing the anti-Stokes, off-resonance absorption, which is proportional to the three design-controlled factors, namely, dopant concentration, pumping field energy, and anti-Stokes transition rate. The optimum dopant concentration for cooling shows that higher dopant concentration increases absorption, while decreases quantum efficiency. Using the energy transfer theory for concentration quenching, the optimum concentration corresponding to the maximum cooling power is found. 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, increasing the photon absorption of the pumping beam. 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. These three enhancement mechanisms increase the number of the three participating carriers (electron, photon, and phonon) in the interacting volume.

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