Undressing a Collective Intersubband Excitation in a Quantum Well

We have experimentally measured the 1--2 intersubband absorption in a single 40 nm wide modulation-doped Al0.3Ga0.7As/GaAs square quantum well as a function of frequency, intensity, and charge density. The low-intensity depolarization-shifted absorption occurs near 80cm^-1 (10 meV or 2.4 THz), nearly 30% higher than the intersubband spacing. At higher intensities, the absorption peak shifts to lower frequencies. Our data are in good agreement with a theory proposed by Za{}u ż \.{z}{}ny, which attributes the redshift to a reduction in the depolarization shift as the excited subband becomes populated.