Four-wave-mixing in radiatively coupled multiple-quantum-well structures: A model study

The influence of radiative coupling on the four-wave-mixing signals of multiple-quantum-well structures is investigated theoretically for excitation at the excitonic resonance. It is shown that amplitude and shape of the FWM signals depend strongly on the number of quantum wells N in the sample and on the periodicity of the structure. For a fixed delay time τ₁₂, the signal amplitude of a Bragg structure increases quadratically with N due to constructive interference as long as the radiative decay rate N_γrad is small compared to the inverse delay time τ₁₂^-1. For large N, if the enhanced radiative decay rate exceeds the inverse delay time, superradiant emission leads to a decrease of the observed signals with increasing N. The signals of anti-Bragg structures with an odd number of quantum wells decrease with increasing N due to destructive interference effects whereas anti-Bragg structures with an even number of quantum wells behave similarly to Bragg structures.