Three-Photon Spectroscopy of Exciton Polaritons in Alkali Halides

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Abstract
Three-photon spectroscopy is introduced as a useful tool for the investigation of electronic transitions in solids. This technique is used to study resonances on the upper polariton branch, and even on the longitudinal exciton. This is the first such application for a system with inversion symmetry. The polariton structure of the lowest exciton in KI is analyzed with use of different wave-vector configurations. The fit of the experimental data yields a longitudinal-transverse splitting of 110 meV. For the more complicated exciton structure of CsI, a correspondingly complicated polariton structure is expected. For CsI, results are presented only for one wave-vector configuration.