Interplay between Coulomb-focusing and non-dipole effects in strong-field ionization with elliptical polarization

J Daněk,L Gallmann,M Klaiber,K Z Hatsagortsyan,B Willenberg,B W Mayer,C H Keitel,C R Phillips,U Keller,J Maurer

doi:10.1088/1361-6455/aaba42

Abstract

We study strong-field ionization and rescattering beyond the long-wavelength limit of the dipole approximation with elliptically polarized mid-IR laser pulses. Full three-dimensional photoelectron momentum distributions (PMDs) measured with velocity map imaging and tomographic reconstruction revealed an unexpected sharp ridge structure in the polarization plane (2018 Phys. Rev. A 97 013404). This thin line-shaped ridge structure for low-energy photoelectrons is correlated with the ellipticity-dependent asymmetry of the PMD along the beam propagation direction. The peak of the projection of the PMD onto the beam propagation axis is shifted from negative to positive values when the sharp ridge fades away with increasing ellipticity. With classical trajectory Monte Carlo simulations and analytical analysis, we study the underlying physics of this feature. The underlying physics is based on the interplay between the lateral drift of the ionized electron, the laser magnetic field induced drift in the laser propagation direction, and Coulomb focusing. To apply our observations to emerging techniques relying on strong-field ionization processes, including time-resolved holography and molecular imaging, we present a detailed classical trajectory-based analysis of our observations. The analysis leads to the explanation of the fine structure of the ridge and its non-dipole behavior upon rescattering while introducing restrictions on the ellipticity. These restrictions as well as the ionization and recollision phases provide additional observables to gain information on the timing of the ionization and recollision process and non-dipole properties of the ionization process.

Highlights

Strong-field ionization in mid-infrared laser fields has gained a lot of attention for the generation of coherent soft x-rays with high harmonic generation (HHG) [1] and for the discovery of a variety of strong field characteristica, like the observation of holographic electron interferences [2] and low-energy structures [3,4,5]
With classical trajectory Monte Carlo (CTMC) simulations and analytical methods we identified the associated ionization dynamics for this sharp ridge to be due to Coulomb focusing of slow recollisions of electrons with a momentum approaching zero
We identified a subspace in the momentum representation of the tunnel ionized electron wave packet, which in an elliptically polarized laser field shows recollision dynamics similar to the linear polarization case

Summary

INTRODUCTION

Strong-field ionization in mid-infrared (mid-IR) laser fields has gained a lot of attention for the generation of coherent soft x-rays with high harmonic generation (HHG) [1] and for the discovery of a variety of strong field characteristica, like the observation of holographic electron interferences [2] and low-energy structures [3,4,5] These processes are typically described through the recollision of the electron wave packet with the residual ion (or parent ion). Discoveries like low energy structures [3,4,5] and holographic interference patterns [2] were first observed at mid-IR wavelengths Another characteristic phenomenon of strong-field ionization at mid-IR wavelengths is the onset of non-dipole effects.

EXPERIMENTAL DETAILS

COULOMB FOCUSING AT ELLIPTICAL POLARIZATION

NON-DIPOLE EFFECTS

CONCLUSIONS