Anisotropic Interference of Three-Wave and Double Two-Wave Frequency Mixing in GaAs

It is shown that two-step, two-wave mixing makes a substantial contribution to the total three-wave mixing process in a noncentrosymmetric crystal. The efficiency of generating $2{\ensuremath{\omega}}_{1}\ensuremath{-}{\ensuremath{\omega}}_{2}$ was observed as a function of the difference frequency ${\ensuremath{\omega}}_{1}\ensuremath{-}{\ensuremath{\omega}}_{2}$, the electric polarization vector, and the propagation direction. This method allowed the determination of the true third-order susceptibility ${\ensuremath{\chi}}^{(3)}$, in both sign and magnitude, in terms of ${[{\ensuremath{\chi}}^{(2)}]}^{2}$.