Coherent exciton dynamics as a function of quantum-well number

We have investigated the coherent dynamics of excitons in a single, a double, and a quintuple quantum-well structure under resonant excitation. In pump and probe experiments on the quintuple quantum well we observe a fast decay of the excitonic population within the initial coherent regime. This fast initial population decay is less pronounced for the double quantum well and is almost absent for the single quantum well. The energy linked to the initially lost coherent population is found in the coherent electromagnetic emission induced by the first-order polarization. More specifically, the intensity of this directional light emission increases quadratically with the number of quantum wells. Interference effects and electromagnetic coupling effects between polarizations located in distinct quantum wells probably play a crucial role in the superlinear coherent emission process.