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Time-resolved measurements of transmission in resonance with the 1 S quadrupole polariton in Cu,O reveal a long-lasting signal which is modulated by pronounced oscillations with time-dependent period. These oscillations are interpreted as "propagation beats", which are caused by superposition of excited states of both polariton branches which propagate coherently through the crystal. At higher excitation intensities the decay becomes faster and the beats disappear. Magneto-quantum beats are also influenced significantly by propagation effects in transmission measurements.

The three-level optical Stark effect is due to a dynamical coupling of two excited levels. The coupling is achieved by a high intensity CO, laser. Results of experiments on multiple quantum wells (MQW) and excitons in Cu,O are presented. In the MQW the coupling between the two lowest conduction band sublevels leads to characteristic changes in the one-photon spectra of the lowest exciton transitions. In Cu,O the CO, laser pulse couples 1 S and 2 P excitons. In this case the one-photon spectrum of the 2 P exciton (dipole allowed) and also of the 1 S spectrum (quadrupole allowed) show drastic changes. Most of the experimental results can be explained quantitatively in terms of a nonlinear susceptibility.Der optische Starkeffekt im Dreizustandssystem beruht auf der dynamischen Kopplung zweier angeregter Zustande. Die Kopplung wird mit einem hochintensiven C0,-Laser erreicht. Es werden experimentelle Ergebnisse an Quantentrogstrukturen (MQW) und Exzitonen in Cu,O vorgestellt. Im MQW fuhrt die Kopplung der beiden tiefsten Leitungsbandzustande zu charakteristischen Anderungen der Einphotonenspektren der tiefsten Exzitonubergange. In Cu,O koppelt der C0,-Laserpuls 1 Sund 2 P-Exzitonen. In diesem Fall zeigt das Einphotonenspektrum des 2 P-Exzitons (dipolerlaubt) wie auch das des 1 S-Exzitons (quadrupolerlaubt) drastische Anderungen. Der groDte Teil der experimentellen Ergebnisse kann quantitativ mittels einer nichtlinearen Suszeptibilitat erklart werden.') PF 500500, D-4600 Dortmund 50, FRG.

PACS. 71.35 -Excitons and related phenomena. PACS. 42.50 -Quantum optics. PACS. 78.47 -Time-resolved optical spectroscopies and other ultrafast optical measurements in condensed matter.

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