Collective radiative decay of light- and heavy-hole exciton polaritons in multiple-quantum-well structures

Time- and spectrally resolved reflectivity measurements on multiple-quantum -well structures excited by ultrashort laser pulses are presented. In Bragg structures, where the interwell distance amounts to half the optical wavel ength at the exciton resonance, superradiant coupling of the polarization d ynamics within different quantum wells leads to an enhancement of the refle ctivity strength and linewidth. We show that the superradiance is not limit ed to the heavy-hole exciton but can he observed even on excited and light- hole exciton states as well as on correlated electron-hole pair states. The se coupling effects grow with the number of quantum wells. For our samples containing 60 and 30 quantum wells, pronounced superradiant enhanced reflec tivity is present even at high excitation intensities and high temperatures , showing the insensitivity of radiative coupling with respect to incoheren t scattering processes. Our findings are in good agreement with solutions o f the semiconductor Maxwell-Bloch equations.