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

TL;DR

This paper theoretically investigates how helium atoms dressed with an IR laser can transiently absorb and reshape ultrafast XUV pulses, revealing effects like Autler-Townes splitting and temporal pulse reshaping.

Contribution

It introduces a combined frequency-dependent and time-frequency approach to analyze transient XUV absorption in laser-dressed helium, including macroscopic propagation effects.

Findings

Autler-Townes splitting near 1600 nm dressing wavelength

Potential XUV transparency at specific transitions

Rich temporal reshaping of XUV pulses in helium gas

Abstract

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 10^12 W/cm^2. 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-Schr\"odinger 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.