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 $2{\omega }_1-{\omega }_2$ was observed as a function of the difference frequency ${\omega }_1-{\omega }_2$, the electric polarization vector, and the propagation direction. This method allowed the determination of the true third-order susceptibility ${\chi }^{\left(3\right)}$, in both sign and magnitude, in terms of ${\left[{\chi }^{\left(2\right)}\right]}^2$.

• Received 5 July 1972

DOI:https://doi.org/10.1103/PhysRevLett.29.865