title: Breakdown of the dipole approximation in strong-field ionization.
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 10¹³ W/cm². 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 semiclassical simulations, we identify the combined action of the magnetic field of the laser pulse and the Coulomb potential as the origin of our observations.
authors.affiliation: Department of Physics, ETH Zurich, 8093 Zurich, Switzerland.
authors.forename: A
authors.initials: A
authors.lastname: Ludwig
authors.affiliation: Department of Physics, ETH Zurich, 8093 Zurich, Switzerland.
authors.forename: J
authors.initials: J
authors.lastname: Maurer
authors.affiliation: Department of Physics, ETH Zurich, 8093 Zurich, Switzerland.
authors.forename: B W
authors.initials: BW
authors.lastname: Mayer
authors.affiliation: Department of Physics, ETH Zurich, 8093 Zurich, Switzerland.
authors.forename: C R
authors.initials: CR
authors.lastname: Phillips
authors.affiliation: Department of Physics, ETH Zurich, 8093 Zurich, Switzerland and Institute of Applied Physics, University of Bern, 3012 Bern, Switzerland.
authors.forename: L
authors.initials: L
authors.lastname: Gallmann
authors.affiliation: Department of Physics, ETH Zurich, 8093 Zurich, Switzerland.
authors.forename: U
authors.initials: U
authors.lastname: Keller