Abstract
Light-matter interaction based on strong laser fields enables probing the structure and dynamics of atomic and molecular systems with unprecedented resolutions, through high-order harmonic spectroscopy, laser-induced electron diffraction, and holography. All strong-field processes rely on a primary ionization mechanism where electrons tunnel through the target potential barrier lowered by the laser field. Tunnel ionization is, thus, of paramount importance in strong-field physics and attoscience. However, the tunneling dynamics and properties of the outgoing electronic wave packets often remain hidden beneath the influence of the subsequent scattering of the released electron onto the ionic potential. Here, we present a joint experimental-theoretical endeavor to characterize the influence of sub-barrier dynamics on the amplitude and phase of the wave packets emerging from the tunnel. We use chiral molecules, whose photoionization by circularly polarized light produces forward-backward asymmetric electron distributions with respect to the light propagation direction. These asymmetric patterns provide a background-free signature of the chiral potential in the ionization process. We first implement the attoclock technique, using bicircular two-color fields. We find that, in the tunnel-ionization process, molecular chirality induces a strong forward-backward asymmetry in the electron yield, while the subsequent scattering of the freed electron onto the chiral potential leads to an asymmetric angular streaking of the electron momentum distribution. In order to access the phase of the tunneling wave packets, we introduce subcycle gated chiral interferometry. We employ an orthogonally polarized two-color laser field whose optical chirality is manipulated on a sub-laser-cycle timescale. Numerical simulations are used to interpret the electron interference patterns inherent to this interaction scheme. They show that the combined action of the chiral potential and rotating laser field not only imprints asymmetric ionization amplitudes during the tunneling process, but also induces a forward-backward asymmetric phase profile onto the outgoing electron wave packets. Chiral light-matter interaction thus induces subtle angular-dependent shaping of both the amplitude and the phase of tunneling wave packets.
- Received 19 May 2021
- Accepted 13 October 2021
DOI:https://doi.org/10.1103/PhysRevX.11.041056
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
The tunneling of a particle through a barrier is one of the most fascinating quantum phenomena. The motion taking place under the barrier, in a region forbidden by classical mechanics, is the subject of intense debate. Many experiments aim at measuring the time taken by the particle to go through the tunnel. Here, we take a completely different direction, revealing the influence of the dynamics under the barrier on the motion of the outgoing particle.
Our approach combines two key elements: the barrier is chiral—its structure cannot be superimposed on its mirror image—and it rotates in time. Specifically, our barrier holds the electrons inside a chiral molecule, set spinning by a photoionizing laser field whose polarization rotates. The electrons must pass through the spinning barrier of the molecule to escape. The clockwise or counterclockwise rotation of the molecule kicks the now-liberated electrons toward opposite directions. This chiral tunneling is like the rotation of a nut on a bolt, which translates the nut forward or backward depending on the handedness of the thread. By appropriately shaping the laser field, we produce quantum interference patterns that show that the electrons exiting a left- or right-handed barrier are phase shifted. The chiral motion within the barrier thus leaves its signature on both the amplitude and phase of the electron wave packets emerging from the tunnel.
Beyond an interest in chiral imaging, the imprint of the sub-barrier dynamics on the tunneling electrons will impact all forms of attosecond strong-field spectroscopies, notably photoelectron holography and high-harmonic generation.
