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 e(2)-e(3) intersubband transition in an asymmetric undoped GaAs/AlGaAs QW structure. At an injected nonequilibrium carrier density of I X 10(13) cm(-2)/QW, an e(2)-e(3) absorption band at 99 meV with a spectral width of 5 meV is found. For a higher density studied, 3 X 10(13) cm(-2)/QW, the band is broadened and blueshifted by 30 meV. Intersubband absorption signals are distinguished from free-carrier absorption signals in the MIR by their characteristic time behavior. (C) 2003 Elsevier B.V. All rights reserved.