Ideal Laser Cooling Efficiency Utilizing Anti-Stokes Luminescence in Yb-Doped Yttrium Aluminum Garnet Powder Crystals

Laser cooling in rare-earth doped material using anti-Stokes photoluminescence (PL) caused by phonon annihilation realizes novel cooling devices without generating heat and vibration. Yb-doped yttrium aluminum garnet powder crystals, (Y:Yb)AG with the Yb concentration from 2 to 13 mol%, were fabricated by a solid state reaction method. PL of (Y:Yb)AG excited at 659 nm shows the maximum intensity at the Yb concentration of 6 mol% because of concentration quenching of the PL. When we resonantly excite at 1030 nm corresponding to the energy distance between the E5 and E3 levels of the f-f transition of Yb3+, obvious anti-Stoke PL signal has been observed at 968 nm. This result indicates that phonons are absorbed, and, then, an up-converted photon with the energy between the E5 and E1 levels is emitted. The relative cooling efficiency, defined by a product of the ideal cooling efficiency and the integrated PL intensity, becomes maximum at the 1030-nm excitation. The ideal cooling efficiency was estimated to be 1.9% at the Yb concentration of 6 mol% and the 1030-nm excitation at room temperature.