We present a theoretical study of transient absorption and reshaping of extreme ultraviolet (XUV) pulses by helium atoms dressed with a moderately strong infrared (IR) laser field. We formulate the atomic response using both the frequency-dependent absorption cross section and a time-frequency approach based on the time-dependent dipole induced by the light fields. The latter approach can be used in cases when an ultrafast dressing pulse induces transient effects, and/or when the atom exchanges energy with multiple frequency components of the XUV field. We first characterize the dressed atom response by calculating the frequency-dependent absorption cross section for XUV energies between 20 and 24 eV for several dressing wavelengths between 400 and 2000 nm and intensities up to 1012 W/cm2. We find that for dressing wavelengths near 1600 nm, there is an Autler-Townes splitting of the 1s→2p transition that can potentially lead to transparency for absorption of XUV light tuned to this transition. We study the effect of this XUV transparency in a macroscopic helium gas by incorporating the time-frequency approach into a solution of the coupled Maxwell-Schrödinger equations. We find rich temporal reshaping dynamics when a 61-fs XUV pulse resonant with the 1s→2p transition propagates through a helium gas dressed by an 11-fs, 1600-nm laser pulse. © 2011 American Physical Society.
Publication Source (Journal or Book title)
Physical Review A - Atomic, Molecular, and Optical Physics
Gaarde, M., Buth, C., Tate, J., & Schafer, K. (2011). Transient absorption and reshaping of ultrafast XUV light by laser-dressed helium. Physical Review A - Atomic, Molecular, and Optical Physics, 83 (1) https://doi.org/10.1103/PhysRevA.83.013419