Linewidths and Thermal Shifts of Spectral Lines in Neodymium-Doped Yttrium Aluminum Garnet and Calcium Fluorophosphate

Measurements in the 4.2-500°K temperature range are reported of the widths, shapes, and thermal shifts of the 1.06- and 0.9-μ lines of the ${\mathrm{Nd}}^{3+}$ ions in yttrium aluminum garnet (YAlG) and calcium fluorophosphate. The results are interpreted in terms of phonon-impurity interactions and static crystal strains. The obtained effective Debye temperature of YAlG is compared with the phonon density spectrum deduced from vibronic sidebands of the $R$ lines in YAlG: ${\mathrm{Cr}}^{3+}$. The $R\to Z_5$ and $R\to Y_6$ transitions of YAlG: Nd are observed to shift exceptionally to the higher energies with an increase of temperature. This is tentatively ascribed to the pushing effect of nearby higher-lying manifolds. Comparison of widths of several lines in YAlG: Nd at low temperatures reveals that the fluctuation of the second-degree crystal-field parameters contributes to the inhomogeneous linebroadenings. Discussions are also presented on the suitability of hard crystals with high phonon cutoff frequencies for the laser host materials.