Abstract
In the present paper, the electronic intersubband transitions in semiconductor quantum well structures are investigated using transient mid infrared absorption spectroscopy after interband photoexcitation with intense picosecond pulses in the visible spectral range. The focus of our investigations is on the e2–e3 intersubband transition in an asymmetric undoped GaAs / AlGaAs quantum well (QW) structure at room temperature. At injected nonequilibrium carrier densities of 1×1013 cm -2 per QW, an e2–e3 absorption band at 99 meV is found which is blue-shifted with increasing carrier density. Intersubband absorption signals are distinguished from free-carrier absorption signals in the mid infrared (MIR) by their characteristic time behavior.
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