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Improving laser cooling efficiencies of Yb-doped yttrium aluminum garnet by utilizing non-resonant anti-Stokes emission at high temperatures

Yuta Nakayama, Yukihiro Harada, Takashi Kita

Published Nov 15, 2019• 8 citations
Publisher

Abstract

Laser cooling in solids has been demonstrated, and cooling from room temperature to cryogenic temperatures has so far been the main focus of research. However, operation at high temperatures can be advantageous. We propose that laser cooling of Yb-doped yttrium aluminum garnet ceramic discs is enhanced above 300 K. Photoluminescence (PL) spectra is evidence that the phonon-assisted energy transfer from resonant states, which exhibit a narrow PL peak, to inhomogeneously distributed energy states becomes stronger at higher temperatures. This results in an enhanced non-resonant anti-Stokes PL at high temperatures. The ideal cooling efficiency determined from the PL spectrum at 470 K is 1.7 times higher than that at 300 K.

DOI
10.1364/oe.27.034961

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Applied Physics Letters•Jan 6, 1997•31 citations
Laser cooling in solids has been demonstrated, and cooling from room temperature to cryogenic temperatures has so far been the main focus of research. However, operation at high temperatures can be advantageous. We propose that laser cooling of Yb-doped yttrium aluminum garnet ceramic discs is enhanced above 300 K. Photoluminescence (PL) spectra is evidence that the phonon-assisted energy transfer from resonant states, which exhibit a narrow PL peak, to inhomogeneously distributed energy states becomes stronger at higher temperatures. This results in an enhanced non-resonant anti-Stokes PL at high temperatures. The ideal cooling efficiency determined from the PL spectrum at 470 K is 1.7 times higher than that at 300 K.
We demonstrated an enhancement of the ideal laser-cooling efficiency in Yb-doped yttrium aluminum garnet, (Yb:Y)AG, and Er-doped yttrium aluminum garnet, (Er:Y)AG, ceramic discs above 300 K. The temperature-dependent photoluminescence (PL) spectrum indicates that the phonon-assisted energy transfer from the narrowband, resonant state into the inhomogeneously distributed energy states is enhanced with increasing the temperature. The enhanced ideal cooling efficiency of the (Yb:Y)AG ceramic disc and the (Er:Y)AG ceramic disc at 470 K is 1.7 times higher than that at 300 K. The enhanced ideal cooling efficiency of (Er:Y)AG at 470 K marked 4.0% which is 1.8 times higher than that of (Yb:Y)AG.
Cooling via efficient emission of anti-Stokes photoluminescence can be utilized to realize novel cooling devices. It has been shown that rare earth-doped materials are good candidates for solid-state laser cooling because of their narrow emission spectra and discrete energy levels. We systematically study laser cooling efficiency of rare earth-doped oxide crystals. The photoluminescence spectrum evidences that the phonon-assisted energy transfer from resonant states to inhomogeneously distributed energy states sensitively depends on temperature. We discuss detailed properties of the cooling efficiency of Yb-doped yttrium aluminum oxide crystals and, furthermore, touch emission properties of their crystal thin films.
Yttrium aluminum perovskite (YAP) is a host material that can provide a strong emission from a rare-earth dopant and it has a lower phonon energy than yttrium aluminum garnet (YAG). Therefore, YAP is a promising material for optical refrigeration and radiation balanced laser. We measured the photoluminescence (PL) spectra of ytterbium (Yb)-doped YAP and compared them with those of Yb-doped YAG. The estimated ideal laser cooling efficiencies of Yb-doped YAP and Yb-doped YAG were comparable at 300 K. Based on the temperature-dependent anti-Stokes PL, we found that the laser cooling power of Yb-doped YAP at 470 K is 14.3 times higher than that at 200 K. This enhancement at higher temperatures is 3.2 times larger than that observed for Yb-doped YAG. We attributed the higher laser cooling power of Yb-doped YAP to a lower multi-phonon relaxation rate (and/or a higher energy transfer rate) and an antenna effect caused by the energy transfer from Yb ions that are located at the Y-site to Yb ions that are surrounded by an inhomogeneous alloy structure. The calculated small signal gain of (Yb:Y)AP is 3.5 times larger than that of (Yb:Y)AG. The larger small signal gain of (Yb:Y)AP arises from its strong absorbance and small Stark splitting width.
Temperature dependence of an enhanced excited state absorption was investigated in erbium-doped garnet crystals using 787.3 and 787.8 nm laser diodes. The enhanced absorption was obtained at temperature ranges of 15–570 K, 15–420 K, and 15–1200 K in a yttrium aluminum garnet, a yttrium scandium gallium garnet, and a lutetium aluminum garnet, respectively. Absorption peaks corresponding to both the I1615/2−I49/2 and the I1613/2−I211/2 transitions of Er3+ in garnets were shifted from those for 300 K at high temperature. The enhanced absorption weakened at high temperature because an absorption cross section from the I413/2 level to the H211/2 level decreased due to the shift of the absorption peaks. It has been confirmed that a mechanism of the enhanced absorption can be explained by calculating the rate equations based on an analytical model for a five-level system of the Er3+ ion. It is suggested that the enhanced absorption weakened at temperatures lower than 12 K because the number of phonons decreased due to low temperature.
Laser cooling of solids with anti-Stokes fluorescence is currently attracting widespread attention because of the wide range of its applications, including all-optical cryocooling for airborne and space-based applications, heat suppression in high-power lasers, and cooling of nanoparticles for biological and mesoscopic physics. This laser cooling process has typically very low (only a few%) efficiency and its enhancement is therefore highly desirable. In this work, we propose to leverage Mie resonance to enhance anti-Stokes fluorescence cooling in RE-doped nanoparticles. As an example, we consider an Yb3+:YAG nanosphere pumped at the long wavelength tail of the Yb3+ absorption spectrum, at 1030 nm. We show that if the radius of the nanosphere is adjusted to the pump wavelength in such a manner that the pump excites some of the Mie resonant modes of the sample, the cooling power density generated in the sample is considerably enhanced and the temperature of the sample is consequently appreciably decreased.
Using a laser diode modulated at 250 and 500 MHz, dependence of enhanced absorption on both sample length and temperature was investigated in an erbium doped yttrium aluminum garnet crystal. The transmitted laser intensity decreased with a decay time of 300 ps for sample lengths greater than 3.0 mm and temperatures higher than 50 K when the incident laser intensity increased. In addition, an emission peak was observed near 835 nm corresponding to the transition from the S3/24 level to the I13/24 in Er3+. It was suggested that the enhanced absorption was caused by stimulated emission in three excited states.