Direct Observation of Depolarization Shift of the Intersubband Resonance

We have studied the intersubband resonance of $\mathrm{GaAs}/\mathrm{AlGaAs}$ multi-quantum-well systems by comparing photon drag and absorption spectra obtained by in-plane photocurrent and photoconduction measurements. The peak absorption at room temperature is found to be blueshifted from the photon drag resonance by as much as $33{\mathrm{cm}}^{-1\mathrm{}}$. We argue that this difference gives directly the depolarization shift, since the resonant photon drag current is driven by the Doppler effect, which is a $k$-vector dependent single particle process.