Pinpointing the macroscopic signatures of attosecond transient absorption in helium: Reshaped spectral splitting and persistent quantum beating

Abstract

Although attosecond transient absorption spectroscopy (ATAS) has been widely used in the study of ultrafast dynamics of atoms, molecules, and solids, there is a need for further theoretical exploration of macroscopic effects beyond the response of single atoms. Here, the extreme ultraviolet (XUV) absorbance of He atoms in the presence of a delayed near-infrared (NIR) pulse is studied theoretically, from the limit of dilute-gas medium to high densities. By numerically solving the time-dependent Schr\"odinger equation coupled with Maxwell's equations, we elucidate the macroscopic signatures in the absorption spectrum of an optically dense medium, including previously recognized XUV spectral pulse shaping, a complex spectral splitting, and a persistent quantum beating, based on the energies of the states involved. An analytical expression for optical density is deduced directly to pinpoint the physical origins of the various absorption features in a quantitative way, which helps one grasp the intuitive picture of the macroscopic absorption spectrum and provides guidance to explore applications such as optical pulse shaping and XUV molecular quantum beat spectroscopy.