Quantum interference in attosecond transient absorption of laser-dressed helium atoms

TL;DR

This paper explores how attosecond transient absorption spectra of helium atoms, influenced by a delayed IR laser, reveal sub-cycle atomic dynamics through interference effects and bound state modifications.

Contribution

It demonstrates that the delay-dependent absorption spectrum encodes attosecond-scale atomic dynamics, highlighting interference between excitation pathways and IR-induced bound state modifications.

Findings

Absorption spectrum modulations occur at sub-IR-cycle timescales.

Interference between multi-photon excitation pathways affects absorption.

IR field modifies bound state lines, revealing ultrafast dynamics.

Abstract

We calculate the transient absorption of an isolated attosecond pulse by helium atoms subject to a delayed infrared (\ir) laser pulse. With the central frequency of the broad attosecond spectrum near the ionization threshold, the absorption spectrum is strongly modulated at the sub-\ir-cycle level. Given that the absorption spectrum results from a time-integrated measurement, we investigate the extent to which the delay-dependence of the absorption yields information about the attosecond dynamics of the atom-field energy exchange. We find two configurations in which this is possible. The first involves multi photon transitions between bound states that result in interference between different excitation pathways. The other involves the modification of the bound state absorption lines by the IR field, which we find can result in a sub-cycle time dependence only when ionization limits the duration of the strong field interaction.