SURFACE SELECTIVITY IN 4-WAVE-MIXING - TRANSIENT GRATINGS AS A THEORETICAL AND EXPERIMENTAL EXAMPLE

A theoretical treatment of transient grating diffraction is derived for gratings that are spatially nonuniform in the direction perpendicular to the sample surface. This treatment is readily generalized to any four-wave mixing experiment. Both reflection and transmission geometries of diffraction are examined for the standard transient grating case, in which both grating excitation beams are incident upon the same side of the sample. For samples in which the grating amplitude perpendicular to the sample surface varies slowly relative to the optical wavelength, the reflection geometry is shown to probe only the surface or the interface, while the transmission geometry probes the bulk of the sample. An experimental example using four transient grating geometries (two reflection, two transmission) is shown to yield significantly different temporal responses, illustrating the nature of the theoretical predictions. The sample is a thin molecular crystal upon a substrate. Both electronic excitations (excitons) and wave-guided acoustic modes are generated and probed. Distinct signals are obtained from the bulk, the crystal-substrate interface, and the free-crystal face. Model calculations are presented that illuminate the behavior of the experimental example.