Nonlinear reflection properties of germanium associated with thermal effects

Thermal excitations on a germanium surface under simultaneous irradiation by two monochromatic optical beams, one strong and one weak, are predicted as functions of the angular separation and frequency difference between the beams, their relative polarization, their intensities, and pulse durations. Nonlinear optical reflection for Q-switched ruby laser pulses is then described. Weak reflected and diffracted beam intensities show tendencies in which the former is preferentially enhanced for a downshifted weak beam frequency, while the latter depends only on the shift magnitude. Both are suppressed for large shifts or large angular separations between input beams.