Investigation of the laser-induced lineshape change in attosecond transient absorption spectra by employing a time-dependent generalized Floquet approach

TL;DR

This paper introduces a time-dependent generalized Floquet approach to analyze laser-induced lineshape changes in attosecond transient absorption spectra, revealing the roles of adiabatic and non-adiabatic phases in spectral modifications.

Contribution

It develops a novel TDGF method for understanding lineshape changes, integrating adiabatic and non-adiabatic phase effects, and compares it with existing models like LIP and RWA.

Findings

The TDGF approach accurately describes lineshape changes.

The LIP model is valid under the adiabatic theorem.

The RWA is applicable when higher-order IR effects are negligible.

Abstract

We introduce a time-dependent generalized Floquet (TDGF) approach to calculate attosecond transient absorption spectra of helium atoms subjected to the combination of an attosecond extreme ultraviolet (XUV) pulse and a delayed few-cycle infrared (IR) laser pulse. This TDGF approach provides a Floquet understanding of the laser-induced change of resonant absorption lineshape. It is analytically demonstrated that, the phase shift of the time-dependent dipole moment that results in the lineshape changes consists of the \emph{adiabatic} laser-induced phase (LIP) due to the IR-induced stark shifts of adiabatic Floquet states and the \emph{non-adiabatic} phase correction due to the non-adiabatic IR-induced coupling between adiabatic Floquet states. Comparisons of the spectral lineshape calculated based on the TDGF approach with the results obtained with the LIP model [S. Chen \emph{et al.}, Phys. Rev. A \textbf{88}, 033409(2013)] and the rotating-wave approximation (RWA) are made in several typical cases. It is suggested in the picture of adiabatic Floquet states that, the LIP model works as long as the generalized adiabatic theorem [A. Dodin \emph{et al.}, Phys. Rev. X Quantum \textbf{2}, 030302(2021)] fulfils, and the RWA works when the higher-order IR-coupling effect in the formation of adiabatic Floquet states is neglectable.