Sub-barrier recollisions and the three classes of tunneling time delays in strong-field ionization

TL;DR

This paper explores how sub-barrier recollisions influence tunneling time delays in strong-field ionization, revealing that interference effects modify the observed delays and proposing a new characteristic time for tunneling initiation.

Contribution

It introduces a theoretical analysis of sub-barrier recollisions' impact on tunneling time delays and suggests a new time parameter for tunneling initiation, supported by experimental data.

Findings

Interference reduces the tunneling time delay at the exit but retains a positive value.

Sub-barrier recollisions affect the asymptotic and exit time delays differently.

A new tunneling time characteristic for wave packet initiation is proposed.

Abstract

Tunneling ionization is characterized by a negative time delay, observed asymptotically as a specific shift of the photoelectron momentum distribution, which is caused by the interference of the sub-barrier recolliding and direct ionization paths. In contrast, a \textit{Gedankenexperiment} following the peak of the wavefunction shows a positive tunneling time delay at the tunnel exit, considering only the direct ionization path. In this paper, we investigate the effects of sub-barrier recollisions on the time delay pattern at the tunnel exit. We conclude that the interference of the direct and recolliding trajectories decreases the tunneling time delay at the exit by the value equal to the asymptotic time delay maintaining, however, its sizeable positive value. Finally, we discuss the recent experiment [Light: Science \& Applications 11, 1 (2022)] addressing the tunneling time in a modified two-color attoclock setup. The analysis of the experimental findings with our theoretical model indicates the physical necessity to introduce a new time characteristic for tunneling ionization -- the time delay describing the initiation of the tunneling wave packet.