Beyond the single-atom response in absorption lineshapes: Probing a dense, laser-dressed helium gas with attosecond pulse trains

TL;DR

This study explores how dense, laser-dressed helium gas affects absorption line shapes when probed with attosecond pulse trains, revealing complex interplay between microscopic phase shifts and macroscopic pulse reshaping.

Contribution

It demonstrates the combined effects of IR-induced phase shifts and resonant propagation on absorption lines in dense helium, extending understanding beyond single-atom responses.

Findings

Line broadening with increased optical depth

Emergence of narrow absorption features

Interplay between microscopic and macroscopic effects

Abstract

We investigate the absorption line shapes of laser-dressed atoms beyond the single-atom response, by using extreme ultraviolet (XUV) attosecond pulse trains to probe an optically thick helium target under the influence of a strong infrared (IR) field. We study the interplay between the IR-induced phase shift of the microscopic time-dependent dipole moment and the resonant-propagation-induced reshaping of the macroscopic XUV pulse. Our experimental and theoretical results show that as the optical depth increases, this interplay leads initially to a broadening of the IR-modified line shape, and subsequently to the appearance of new, narrow features in the absorption line.