Ultrafast time-dependent absorption in a macroscopic three-level helium gas
We present a theoretical study of the transient absorption of an ultrafast, extreme ultraviolet (XUV) pulse in the presence of an infrared (IR) control pulse in a macroscopic three-level helium gas. We concentrate on cases in which the IR pulse is shorter than the XUV pulse and induces transient transparency and amplification of the XUV light via coherent control. We give a time-dependent picture of the absorption process through the time dependence of the populations and relative coherences of the three states. We show that the XUV absorption can be dynamically changed into emission via phase control of the XUV-resonant 2p state through IR-induced coupling between the 2p and the 2s states. This can lead to compression and amplification of the XUV pulse when propagating through a laser-dressed macroscopic medium. We discuss the influence of atomic decoherence times on the XUV reshaping and show that fast decoherence means that the laser dressing acts more like a passive transmission window and less like a dynamical redistribution of energy. © 2013 American Physical Society.