Spatial-diffusion measurements in impurity-doped solids by degenerate four-wave mixing.

A method is introduced that measures the spatial-migration rate of electronic excitation in condensed media over distances of the order of 0.1 microm. Two volume holographic gratings of widely differing modulation periods are simultaneously produced, using a phase-conjugate wave geometry of degenerate four-wave mixing. Spatial migration results in a reduced scattering efficiency of one grating and is observed as a polarization rotation of the backward-going output wave. Upper limits are placed on the diffusion constants in pink ruby and Nd(3+)-doped silicate glass.