Optical dephasing of rare earth ions in mixed crystalline and size-restricted systems

Abstract

High resolution optical hole burning spectroscopy of rare earth ions is used to study the dynamics of disordered and size-restricted crystalline solids. Materials described include mixed crystals of Ca1−xLaxF2+x and Ca1−xYxF2+x with Eu3+ and Pr3+ and sol–gel prepared nanometer scale γ-Al2O3:Eu3+. All of these materials exhibit, at low temperatures, a temperature dependence of their spectral hole linewidth which is similar to the dynamical properties observed in amorphous materials such as glasses. This suggests an enhanced density of states at low frequencies. The results on the mixed alkaline earth fluorides are discussed in relation to far infrared and heat capacity measurements, which indicate the presence of low frequency excitations. It appears that while the density of defect states responsible for the dynamics increases with Y or La content, the density saturates at low concentrations. Both persistent and transient hole burning mechanisms occur for nanoscale sol–gel γ-Al2O3:Eu3+. The nearly exponential increase in the linewidth above 7 K is explained by Raman scattering by phonons associated with the size-restricted nature of the phonon density of states.