Third-order nonlinear optical spectroscopy in CuCl

The production of light at the combination frequency $2{\omega }_1$-${\omega }_2$ by two incident-tunable coherent beams at the frequencies ${\omega }_1$ and ${\omega }_2$, respectively, has been studied in CuCl, as $2{\omega }_1$ is varied across the $Z_3$-exciton-polariton dispersion curve, and/or ${\omega }_1-{\omega }_2$ is varied across the longitudinal phonon-polariton dispersion curve. The interference of the excitonic and Raman resonances with the nonresonant electronic nonlinear susceptibility gives new information about the nonlinear properties of the $Z_3$ exciton and its damping as a function of temperature. A detailed theoretical and experimental comparison with earlier work on two-photon absorption and second-harmonic generation in CuCl is presented.