Picosecond nonradiative processes in neodymium-doped crystals and glasses: Mechanism for the energy gap law

We present measurements of the (4)G(7/2) emission lifetime for 26 Nd-doped crystals and glasses, including phosphates, silicates, oxides, garnets, apatites, and fluorides. The fluorescent decays were found to range from 70 ps to 41 ns, depending on the host medium. We show that the (4)G(7/2) emission lifetime is correlated to the I-4(11/2) relaxation time to within a factor of two, by comparing the present results with previous directly-determined I-4(11/2) lifetimes. This (4)G(7/2) - I-4(11/2) correlation is useful because the I-4(11/2) state is the terminal level for the common 1.06 mu m Nd-laser transition, while the (4)G(7/2) emission time can be measured in a relatively straightforward manner. A model of nonradiative decay based on dipole-dipole energy transfer is developed and found to be supported by our data. It allows independent calculation of the parameters of the energy gap law, in addition to fitting the nonradiative lifetime data. (C) 1998 Elsevier Science B.V. All rights reserved.