  Intersubband absorption in modulation-doped quantum wells is usually
appropriately described as a collective excitation of the confined
two-dimensional electron gas. At sufficiently low electron density and low
temperatures, however, the in-plane disorder potential is able to damp the
collective modes by mixing the intersubband charge-density excitation with
single-particle localized modes. Here we show experimental evidence of this
transition. The results are analyzed within the framework of the density
functional theory and highlight the impact of the interplay between disorder
and the collective response of the two-dimensional electron gas in
semiconductor heterostructures.
