Abstract

Energy transfer from strong pump beams to weak signal beams by self-diffraction at dynamic- free carrier gratings in crystalline silicon has been investigated. The gratings are due to the intersecting pump and signal beams generating a periodic electron-hole pair distribution by interband absorption. Single pulses of a mode-locked Nd:YAG laser were used. Because of the short pulse width of 50 ps, carrier relaxation and diffusion can be neglected for intersection angles up to 30°. At small intersection angles θ, the interaction is described as a three-wave mixing process. The third beam is produced by diffraction of the pump beam into a direction opposite the signal beam. At large angles θ, the third beam does not appear because of interference quenching so that two-wave mixing takes place. Maximum signal amplification up to 20 was observed for small and large angles. A sharp gain minimum was measured at angles of θ = 2.5-3°, the transition region between two- and three-wave mixing. Theoretical calculations of the gain depending on the energy density of the pump pulse are in qualitative agreement with measurements (Fig. 1). Above 80-100 mJ/cm² a decrease of signal amplification takes place due to free carrier absorption.

© 1987 Optical Society of America