Thulium cross-relaxation in a low phonon energy crystalline host

Abstract

We report on the cross-relaxation of ${\mathrm{Tm}}^{3+}$ in a low phonon energy chloride host. The temperature and concentration dependence for ${\mathrm{Tm}}^{3+}$ cross-relaxation in the host ${\mathrm{YCl}}_{3}$ was studied between 300 and 500 K, in crystals of ${\mathrm{YCl}}_{3}$ with concentrations ranging from 0.7 to $7\ifmmode\times\else\texttimes\fi{}{10}^{20}{\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{s}/\mathrm{c}\mathrm{m}}^{3}.$ Crystals were grown from melts of anhydrous ${\mathrm{YCl}}_{3}$ and ${\mathrm{TmCl}}_{3}$ powders under ${\mathrm{Cl}}_{2}$ 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 ${\mathrm{Tm}}^{3+}$ 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 ${\mathrm{Tm}}^{3+}.$ 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.