  Absorption is usually expected to be detrimental to quantum coherence
effects. However, the situation for complex absorption spectra has been little
studied yet. We consider the resonance fluorescence of excitons in a
semiconductor quantum well. The creation of excitons requires absorption of the
incoming pump-laser light. Thus, the absorption spectrum of the medium acts as
a spectral filter for the emitted light. Surprisingly, absorption can even
improve quantum effects, as is demonstrated for the squeezing of the resonance
fluorescence of the quantum-well system. This effect can be explained by an
improved phase matching due to absorption.
