Dynamics of chromium ion valence transformations in Cr,Ca:YAG crystals used as laser gain and passive Q-switching media

Abstract

Thermal treatment of YAG crystals co-doped with Cr4+ and Ca2+ exhibits reversible oxidation–reduction reactions of the chromium ions, which depend on both temperature and ambient atmosphere. Dynamics of these reactions were studied by the spectral absorption and fluorescence characteristics of Cr,Ca:YAG crystals following different thermal treatment procedures. The absorption spectra were resolved into different peaks, of modified Lorentzian line-shape. Full reduction of Cr4+ into Cr3+ is obtained by prolonged heating under vacuum, or in an ambient atmosphere of Ar:5 vol.% H2 mixture at ∼1000 °C. Saturated oxidation is obtained by prolonged heating at elevated temperatures in free air. The oxygen diffusion coefficient is estimated as (1.85 ± 0.2) × 10−6 cm2/s at 850 °C, with an activation-energy of (1.7 ± 0.07) eV. Only a small fraction of Cr3+ ion concentration transforms into Cr4+. The latter may reside in either octahedral or tetrahedral sites, each exhibiting a different absorption spectrum. Transition from octahedral sites to tetrahedral ones is thermally activated, although no unique thermal activation energy may be assigned to the process. Knowing the process parameters now allows one to optimize oxidization procedures administered to Cr,Ca:YAG crystal devices that are used as laser gain media or as saturable absorbers.