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The results of the transient grating technique applied to single crystals of silicon have been analyzed, taking into account free carrier absorption and nonlinear recombination. Using different configurations of this technique, the exposure and decay characteristics of gratings in the volume or surface of silicon of different properties (pure, doped with deep or shallow traps, ion implanted, or amorphous) have been investigated. The presence of impurities does not change the dominant mechanism of refractive index modulation by the photogenerated nonequilibrium carriers. Increased damage of Si leads to a decrease in carrier diffusion (implanted Si) with, in the case of amorphous Si, domination of grating decay by carrier recombination. The properties of gratings in high external dc or ac (microwave) electric fields enables one to evaluate hot carrier diffusion coefficients.