  We investigate the absorption line shapes of laser-dressed atoms beyond the
single-atom response, by using extreme ultraviolet (XUV) attosecond pulse
trains to probe an optically thick helium target under the influence of a
strong infrared (IR) field. We study the interplay between the IR-induced phase
shift of the microscopic time-dependent dipole moment and the
resonant-propagation-induced reshaping of the macroscopic XUV pulse. Our
experimental and theoretical results show that as the optical depth increases,
this interplay leads initially to a broadening of the IR-modified line shape,
and subsequently to the appearance of new, narrow features in the absorption
line.
