  We investigate the evolution of extreme ultraviolet (XUV) spectral lineshapes
in an optically-thick helium gas under near-infrared (IR) perturbation. In our
experimental and theoretical work, we systematically vary the IR intensity,
time-delay, gas density and IR polarization parameters to study lineshape
modifications induced by collective interactions, in a regime beyond the single
atom response of a thin, dilute gas. In both experiment and theory, we find
that specific features in the frequency-domain absorption profile, and their
evolution with propagation distance, can be attributed to the interplay between
resonant attosecond pulse propagation and IR induced phase shifts. Our
calculations show that this interplay also manifests itself in the time domain,
with the IR pulse influencing the reshaping of the XUV pulse propagating in the
resonant medium.
