Low-level photomodulation of exciton absorption in CdTe single quantum wells

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Abstract
An optical nonlinearity at very low light levels in a CdTe/CdZnTe single quantum well at 4 K is reported. Excitation above the band gap produces a significant decrease in the heavy‐hole excitonabsorption at intensities down to μW/cm2. The dependence of the change in absorption on the pump intensity is sample dependent and is strongly sublinear. At low pump intensity the recovery of the absorption after switching off the pump is exponential with a time constant of about 150 ms, which is nearly independent of pump intensity up to ∼3 mW/cm2. Above 30 μW/cm2 an additional process on a time scale ∼100 μs is observed. The rise time after turning on the pump varies inversely as pump intensity. It is shown that the nonlinearity arises from the presence of excess electrons in the CdTequantum well, which reduce the excitonic absorption by phase‐space filling. These electrons are charge compensated by holes trapped in the barrier. The time and intensity dependence of the optical nonlinearity can be fitted by a kinetic model of the trapping, in which a range of traps with different recovery times participates.