Abstract

We present a detailed theory of multiwave mixing via a laser-induced thermal grating in a thin film. The two incident lasers consist of one strong pump and a relatively weaker probe beam. These lasers are coherent with respect to each other but may be frequency shifted. This produces a stationary (frequency shift is zero) or a moving (non-zero frequency shift) intensity grating on the film. This in turn creates a spatially coincident or a nonlocal refractive-index grating. We solved the 3-D heat diffusion problem as well as the coupled Maxwell wave equations for the pump, probe, and various diffracted beams in a self-consistent manner. Under easily realized geometries and using high thermal index materials such as liquid crystals, very large amplification of the probe beam (more than 20) can be easily achieved with a cw pump beam of a few W/cm². Experimentally, we have verified the theoretical predictions using cw and pulsed CO₂ lasers. The wave mixing effects are studied as a function of pump beam intensities, grating constants, temperature, pump-to-pump ratio, and sample thickness.

© 1987 Optical Society of America