Thulium cross-relaxation in a low phonon energy crystalline host

We report on the cross-relaxation of Tm³⁺ in a low phonon energy chloride host. The temperature and concentration dependence for Tm³⁺ cross-relaxation in the host YCl₃ was studied between 300 and 500 K, in crystals of YCl₃ with concentrations ranging from 0.7 to 7×10²⁰ions/cm³. Crystals were grown from melts of anhydrous YCl₃ and TmCl₃ powders under Cl₂ atmosphere using a self-seeded vertical Bridgeman technique. Fluorescence spectra and lifetimes resulting from quasi-cw pumping with a 0.5-W, 811-nm diode were collected and analyzed for the spectral region 1100--2000 nm versus temperature for three crystals with varying Tm³⁺ densities. Each of the fluorescence spectra contain three broad features centered at 1200, 1490, and 1800 nm that reflect the populations of the first three excited levels of Tm³⁺. Rates for multiphonon relaxation and cross-relaxation were determined from the fluorescence lifetime data. An increase in 1200-nm fluorescence as the temperature rises is evidence of an endothermic cross-relaxation process. This suggests a fundamentally new mechanism for optical cooling. A macroscopic rate equation model with its temperature and concentration dependence given by theories for phonon-assisted energy transfer is fit to the spectral data.