UNDRESSING A COLLECTIVE INTERSUBBAND EXCITATION IN A QUANTUM-WELL

We have experimentally measured the 1-2 intersubband absorption in a s ingle 40 nm wide modulation-doped Al0.3Ga0.7As/GaAs square quantum wel l as a function of frequency, intensity, and charge density. The low-i ntensity depolarization-shifted absorption occurs near 80 cm(-1) (10 m eV or 2.4 THz), nearly 30% higher than the intersubband spacing. At hi gher intensities, the absorption peak shifts to lower frequencies. Our data are in good agreement with a theory proposed by Zaluzny, which a ttributes the redshift to a reduction in the depolarization shift as t he excited subband becomes populated.