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We present a theoretical study of transient absorption and reshaping ofextreme ultraviolet (XUV) pulses by helium atoms dressed with a moderatelystrong infrared (IR) laser field. We formulate the atomic response using boththe frequency-dependent absorption cross section and a time-frequency approachbased on the time-dependent dipole induced by the light fields. The latterapproach can be used in cases when an ultrafast dressing pulse inducestransient effects, and/or when the atom exchanges energy with multiplefrequency components of the XUV field. We first characterize the dressed atomresponse by calculating the frequency-dependent absorption cross section forXUV energies between 20 and 24 eV for several dressing wavelengths between 400and 2000 nm and intensities up to 10^12 W/cm^2. We find that for dressingwavelengths near 1600 nm, there is an Autler-Townes splitting of the 1s ---> 2ptransition that can potentially lead to transparency for absorption of XUVlight tuned to this transition. We study the effect of this XUV transparency ina macroscopic helium gas by incorporating the time-frequency approach into asolution of the coupled Maxwell-Schr\"odinger equations. We find rich temporalreshaping dynamics when a 61 fs XUV pulse resonant with the 1s ---> 2ptransition propagates through a helium gas dressed by an 11 fs, 1600 nm laserpulse.

Transient absorption and reshaping of ultrafast XUV light by laser-dressed helium