Origin of the blueshift in the intersubband infrared absorption in GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As multiple quantum wells

The intersubband transition (IT) in GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As multiple-quantum-well samples measured by the infrared-absorption technique at 5 K is studied as a function of the two-dimensional electron-gas density (σ). A blueshift in the peak-position energy of the IT is observed as σ increases. Single-particle calculations of the optical-absorption spectra, which are obtained by using the nonparabolic-anisotropic envelope-function approximation (EFA), indicate that the peak-position energy should show a redshift as σ is increased. We found that it is necessary to incorporate many-body corrections (in particular electron-electron intrasubband exchange and direct Coulomb interchange energies), depolarization, and excitonlike shifts in the EFA calculations in order to account for the experimental peak-position energy and blueshift as σ is increased.