Intersubband absorption in highly photoexcited semiconductor quantum wells

Transient mid infrared (MIR) absorption spectroscopy is used to investigate transitions between higher electronic subbands in semiconductor quantum well (QW) structures after interband photoexcitation with intense picosecond pulses in the visible spectral range. Our investigation focuses on the e2–e3 intersubband transition in an asymmetric undoped GaAs/AlGaAs QW structure. At an injected nonequilibrium carrier density of View the MathML source, an e2–e3 absorption band at View the MathML source with a spectral width of View the MathML source is found. For a higher density studied, View the MathML source, the band is broadened and blueshifted by View the MathML source. Intersubband absorption signals are distinguished from free-carrier absorption signals in the MIR by their characteristic time behavior.