Three-photon spectroscopy of excitons and polaritons in alkali halides

Three-photon spectra of nine alkali halides are presented. The nonlinear-optical method of three-photon spectroscopy allows one to measure resonances on the upper polariton branch and the longitudinal exciton at different values of the total k vector. The analysis of the polariton dispersion yields reliable values of the longitudinal-transverse splitting. In Cs halides new exciton lines of ${\Gamma }_5^{\mathrm{-}}$ symmetry are resolved which are forbidden for one-photon optical transitions. In high magnetic fields transitions to paraexcitons of ${\Gamma }_5^{\mathrm{-}}$ and ${\Gamma }_3^{\mathrm{-}}$ symmetry are allowed. The g values for the uppermost valence band and the lowest conduction band are derived from the Zeeman splitting of the paraexcitons and the transverse polariton. In addition, the exchange energy for KI and RbI is gained from the three-photon spectra in high magnetic fields.