Transient grating investigation of exciton diffusion and fluorescence quenching inNdxTa1−xP5O14crystals

Abstract

The use of degenerate four-wave-mixing techniques to investigate exciton migration in highly concentrated laser materials is discussed. A theoretical derivation is presented of the signal beam intensity for this technique using the geometric arrangement common for energy-transfer studies. The results demonstrate effects that occur when the pump beams are not exactly phase matched and the instabilities encountered for very small pump-beam-crossing angles. Applying this technique to crystals of ${\mathrm{Nd}}_{x}{\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{P}}_{5}{\mathrm{O}}_{14}$ shows that exciton diffusion takes place in a given direction with diffusion lengths between 0.18 and 0.36 \ensuremath{\mu}m for samples with $x$ ranging from 0.2 to 1.0, respectively. Fluorescence quenching is shown to vary linearly with concentration at high values of $x$ and quadratically at low values of $x$. This is consistent with an exciton migration and trapping mechanism at high concentrations and cross relaxation at low concentrations.