PROPAGATION OF CONFINED EXCITONIC POLARITONS AT HIGH-DENSITIES

The propagation of excitonic. polaritons is investigated in the nonlin ear optical regime by real-time-resolved pump-and-probe measurements o n a femtosecond time scale. High-quality ZnSxSe1-x/ZnSe/ZnSxSe1-x hete rostructures are used which. in linear absorption, show clearly resolv ed Fabry-Perot resonances of polaritons confined in the ZnSe layer. Wi thout pumping, the transmitted probe pulses exhibit a complex temporal beating behavior caused by quantum beating between heavy-hole and lig ht-hole polaritons as well as by propagation beats. For increasing flu x of a preceding pump pulse the transmitted power is temporally redist ributed from later times toward the leading maximum of the pulse. Addi tionally, the amplitude of the beats continuously decreases. Almost qu antitative agreement is found between the experimental data and calcul ations based on semiclassical polariton theory, clearly demonstrating the strong influence of spatial dispersion also on nonlinear polariton propagation. This allows determination of the density dependence of t he oscillator strength, damping, and energy shift of the heavy-hole po lariton.