Breakdown of the Dipole Approximation in Strong-Field Ionization

TL;DR

This paper demonstrates the breakdown of the electric dipole approximation in strong-field ionization at mid-infrared wavelengths, showing how magnetic and Coulomb effects influence photoelectron distributions at high intensities.

Contribution

It reveals the limits of the dipole approximation in strong-field ionization and identifies magnetic and Coulomb effects as key factors in this breakdown.

Findings

Shift in photoelectron momentum opposite to beam propagation with increasing intensity

Magnetic field and Coulomb potential jointly cause observed effects

Breakdown occurs in the long-wavelength limit at high intensities

Abstract

We report the breakdown of the electric dipole approximation in the long-wavelength limit in strong-field ionization with linearly polarized few-cycle mid-infrared laser pulses at intensities on the order of 10$^{13}$ W/cm$^2$. Photoelectron momentum distributions were recorded by velocity map imaging and projected onto the beam propagation axis. We observe an increasing shift of the peak of this projection opposite to the beam propagation direction with increasing laser intensities. From a comparison with semi-classical simulations, we identify the combined action of the magnetic field of the laser pulse and the Coulomb potential as origin of our observations.