Femtosecond induced transparency and absorption in the extreme ultraviolet by coherent coupling of the He 2s2p (1P0) and 2p2 (1Se) double excitation states with 800 nm light

TL;DR

This study demonstrates femtosecond laser-induced transparency and absorption phenomena in the extreme ultraviolet by coherently coupling helium double excitation states, revealing Autler-Townes splitting and the importance of ionization effects.

Contribution

It introduces a novel observation of EIT-like behavior in the EUV using femtosecond spectroscopy and models the effects including optical field ionization of excited states.

Findings

Observation of Autler-Townes doublet in EUV spectra

Both transparency and absorption are induced by laser dressing

Qualitative agreement achieved when ionization effects are included

Abstract

Femtosecond high-order harmonic transient absorption spectroscopy is used to observe electromagnetically induced transparency-like behavior as well as induced absorption in the extreme ultraviolet by laser dressing of the He 2s2p (1Po) and 2p2 (1Se) double excitation states with an intense 800 nm field. Probing in the vicinity of the 1s2 \to 2s2p transition at 60.15 eV reveals the formation of an Autler-Townes doublet due to coherent coupling of the double excitation states. Qualitative agreement with the experimental spectra is obtained only when optical field ionization of both double excitation states into the N = 2 continuum is included in the theoretical model. Because the Fano q-parameter of the unperturbed probe transition is finite, the laser-dressed He atom exhibits both enhanced transparency and absorption at negative and positive probe energy detunings, respectively.