  We investigated the giant resonance in Xenon by high-order harmonic
generation spectroscopy driven by a two-color field. The addition of a
non-perturbative second harmonic component parallel to the driving field breaks
the symmetry between neighboring sub-cycles resulting in the appearance of
spectral caustics at two distinct cut-off energies. By controlling the phase
delay between the two color components it is possible to tailor the harmonic
emission in order to amplify and isolate the spectral feature of interest. In
this paper we demonstrate how this control scheme can be used to investigate
the role of electron correlations that give birth to the giant resonance in
Xenon. The collective excitations of the giant dipole resonance in Xenon
combined with the spectral manipulation associated with the two color driving
field allow to see features that are normally not accessible and to obtain a
quantitative good agreement between the experimental results and the
theoretical predictions.
