Direct observation of depolarization shift of the intersubband resonance

We have studied the intersubband resonance of GaAs/AlGaAs multi-quantum-wel l systems by comparing photon drag and absorption spectra obtained by in-pl ane photocurrent and photoconduction measurements. The peak absorption at r oom temperature is found to be blueshifted from the photon drag resonance b y as much as 33 cm(-1) We argue that this difference gives directly the dep olarization shift, since the resonant photon drag current is driven by the Doppler effect, which is a k-vector dependent single particle process.