Ytterbium(3+) and thulium(3+) doped fluoroaluminate glasses for anti-Stokes cooling.

摘要

Materials that exhibit anti-Stokes fluorescent cooling are receiving renewed interest as a viable means of refrigeration. While anti-Stokes cooling has been shown in a number of solids, progress has been limited in part by the properties of the refrigeration media.;This study demonstrates that alternate refrigeration media can overcome a number of problems encountered with fluorozirconate glasses, and tungstate crystals. In particular, fluoroaluminate glasses offer improved corrosion resistance and environmental stability, while still providing the benefits inherent to fluoride glasses such as good rare earth solubility, near quantum-efficient fluorescence, low Stokes shifts, and net shape processing.;Fluoroaluminate glasses capable of exhibiting anti-Stokes cooling were prepared and their thermal and optical properties were characterized. The glass compositions examined were 40AlF3·12BaF2·22CaF 2·16YF3·10SrF2 (mol%) doped with either ytterbium or thulium. Ytterbium and thulium levels ranged from 0.5 x 1020/cm3 to 4.0 x 1020 /cm3. Additions of ytterbium and thulium were seen to stabilize the glass matrix until high dopant concentrations. The effect of dopant concentration on the optical and thermal properties is clearly distinguishable, and indicates that the glass network is stabilized until microcrystallite formation occurs at a higher loading.;For the Yb3+ specimens, optical pumping at 980 nm showed an intense emission peak at 980 nm. Increasing the Yb3+ concentration from 0.5 x 1020/cm3 to 3.0 x 10 20/cm3 produced a broad and intense peak centered at 1010 nm. Additionally, optical pumping at 930 nm produced an intense peak at 1060 nm for samples with less than 1.0 x 1020/cm 3 Yb3+. As doping levels increased, the emission intensity of the 1060 nm peak decreased and contributed to extensive broadening of the 1010 nm peak.;For the Tm3+ samples, optical pumping at 800 nm resulted in emission at 1460 nm, 1800 nm, 2300 nm, and 2900 nm. The 1800 nm emission, which is due to the 3F4-to-3H 6 transition and is of interest to anti-Stokes cooling, increased in intensity with increasing Tm3+ concentration up to 1.0 x 1020/cm3.;Examination of the optical absorption, emission, and excited state lifetime data indicates these glasses should demonstrate the anti-Stokes effect, and recent anti-Stokes experiments confirm optical cooling in these glasses.