The method of three-photon difference-frequency generation is used to measure resonances on the lower polariton branch in CuC1. Due to the small linewidth of the resonances (smallest width 80 FeVj it is possible to determine the splitting of the lower branch in a magnetic field at different k-values for the first time. Resonances on the upper polariton branches and on the longitudinal exciton branches can also be excited with the use of two-and three-photon sum-frequency generation and two-photon absorption. The rather peculiar wavevector-dependent splitting of the lower polariton branch is explained quantitatively by taking into account the field induced mixing of the Z,-and Zl, ,-excitons.Mit Hilfe der Dreiphotonen-Differenzfrequenzerzeugung konnen Resonanzen auf dem unteren Polaritonast in CuCl angeregt werden. Aufgrund der geringen Linienbreite der Resonanzen (kleinste Breite 80 peV) kann erstmals die k-abhingige Magnetfeldaufspaltung des unteren Astes gemessen werden. Mittels der Zwei-und Dreiphotonen-Summenfrequenzerzeugung und der Zweiphotonenabsorption konnen aul3erdem sowohl Resonanzen auf den oberen Polaritonasten als auch die longitudinalen Exzitonen angeregt werden. Die wellenzahlabhangige Aufspaltung des unteren Polaritonastes wird quantitativ durch die Beriicksichtigung der feldinduzierten Mischung der Z,-und Z,, ,-Exzitonen erkliirt.

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Resonances on the lower polariton branch of KI and RbI x e measured by the use of three-photon difference-frequency generation (TP-DFG). From these data the energy of the lowest transverse exciton state is derived. Due to the small linewidth of the resonances it is possible to measure the k-dependent magnetic field splitting of the lower branch for the first time. The results allow to determine the exchange energy, the longitudinal-transverse splitting and the g-values of the uppermost valence band (r, -symmetry) and the lowest conduction band (TZ-symmetry) with much higher accuracy than before.

Three-photon diA'erence-frequency generation (TP-DFG) is used to excite resonances on the lower exciton-polariton branch in alkali halides. The line shape of the resonances is investigated in KI and Rbl for temperatures between 1.5 and 800 K. A drastic variation of the linewidth ranging from 0.2 to about 50 meV is observed as a function of temperature and polariton energy. The experimental results are interpreted in the framework of polariton response theory, which links the shape of the TP-DFG resonance to the complex dielectric function. From the measured positions and widths of the resonance peaks, accurate data on dispersion and absorption are obtained for the region of the exponential tail of the excitonic absorption edge. The method allows us to determine the parameters of the Urbach-Martienssen rule by measurements on large single crystals.