The role of reflections in the generation of a time delay in strong field ionization

TL;DR

This paper investigates the origin of time delay in strong field tunneling ionization, revealing that reflections within the barrier cause a measurable delay at the tunnel exit, which can be eliminated by accounting for these reflections.

Contribution

It demonstrates that the tunneling time delay arises from sub-barrier reflections and can be nullified when these reflections are properly considered within the wavefunction.

Findings

Time delay exists due to sub-barrier interference effects.

Removing reflections from the wavefunction eliminates the observed delay.

Reflections at the barrier surface are responsible for the delay.

Abstract

The problem of time delay in tunneling ionization is revisited. The origin of time delay at the tunnel exit is analysed, underlining the two faces of the concept of the tunnelling time delay: the time delay around the tunnel exit and the asymptotic time delay at a detector. We show that the former time delay, in the sense of a delay in the peak of the wavefunction, exists as a matter of principle and arises due to the sub-barrier interference of the reflected and transmitted components of the tunneling electronic wavepacket. We exemplify this by describing the tunnelling ionization of an electron bound by a short-range potential within the strong field approximation in a "deep tunnelling" regime. If sub-barrier reflections are extracted from this wavefunction, then the time delay of the peak is shown to vanish. Thus, we assert that the disturbance of the tunnelling wavepacket by the reflection from the surface of the barrier causes a time delay in the neighbourhood of the tunnel exit.