POLARITON MOTIONAL NARROWING IN SEMICONDUCTOR MULTIPLE-QUANTUM WELLS

We demonstrate a new fast-scan ultrafast coherent reflectivity techniq ue that tracks the amplitude and phase of excitons in GaAs heterostruc tures at densities down to 4 x 10(6) cm(-2). Observed growth in the ex citon coherence over the first 1.5 ps is direct evidence for the prese nce of interfering polariton modes. Polariton theory provides a good a ccount of the data as the number of quantum wells is increased. Polari ton motional narrowing of spectral lines from multiple quantum wells i s demonstrated both experimentally and theoretically, annealing out th e inhomogeneous broadening of the exciton energies.