The generation of the combination frequency $2{\omega }_1-{\omega }_2$ in CuCl by two incident waves, at frequencies ${\omega }_1$ and ${\omega }_2$, respectively, is measured as a function of ${\omega }_1$. When $2{\omega }_1$ equals the frequency of the first longitudinal-exciton resonance, a resonant dispersive behavior in the production of the intensity at $2{\omega }_1-{\omega }_2$ is observed.

• Received 27 September 1973

DOI:https://doi.org/10.1103/PhysRevB.9.1853