Three-photon difference-frequency spectroscopy of polaritons in alkali halides

Abstract
Three-photon difference-frequency generation is used to study resonances on the lower polariton branch of alkali halides. The resonances show a spectacularly small linewidth of about 0.12 meV. This small linewidth allows us for the first time to resolve the splitting of the lower polariton in a magnetic field. A pronounced dependence of this splitting on the total k vector is observed.