OPTICAL DEPHASING OF RARE-EARTH IONS IN MIXED CRYSTALLINE AND SIZE-RESTRICTED SYSTEMS

High resolution optical hole burning spectroscopy of rare earth ions i s used to study the dynamics of disordered and size-restricted crystal line solids. Materials described include mixed crystals of Ca1-xLax2+x and Ca1-xYxF2+x with Eu3+ and Pr3+ and sol-gel prepared nanometer sca le gamma-Al2O3:Eu3+. All of these materials exhibit, at low temperatur es, a temperature dependence of their spectral hole linewidth which is similar to the dynamical properties observed in amorphous materials s uch as glasses. This suggests an enhanced density of states at low fre quencies. The results on the mixed alkaline earth fluorides are discus sed in relation to far infrared and heat capacity measurements, which indicate the presence of low frequency excitations. It appears that wh ile the density of defect states responsible for the dynamics increase s with Y or La content, the density saturates at low concentrations. B oth persistent and transient hole burning mechanisms occur for nanosca le sol-gel gamma-Al2O3:Eu3+. The nearly exponential increase in the li newidth above 7 K is explained by Raman scattering by phonons associat ed with the size-restricted nature of the phonon density of states.