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André Ludwig: Breakdown of the Dipole Approximation in Strong-Field Ionization

December 15, 2014

How electron dynamics is altered by the magnetic field component of the light as well as the ion’s Coulomb force onto the escaping electron 

The electric dipole approximation is a concept widely used in atomic, molecular and optical science. Here, Ursula Keller and co-workers report the experimental observation of the breakdown of the dipole approximation in its long-wavelength-limit, which is less well known than its short-wave counterpart.
The results concern the important case of linearly polarized light and identify the relevant signature in photoelectron momentum distributions obtained with strong-field ionization of various atomic species. This breakdown is manifested in asymmetries of the photoelectron momentum distributions in beam propagation. The maximum of the photoelectron distribution is shifted opposite to the beam propagation direction, what appears counterintuitive in the frame of the radiation pressure picture brought forward in earlier work.

The authors could pinpoint the combined action of the magnetic field and the Coulomb potential as the cause for their observations. To the best of their knowledge, the results represent the first observation of non-dipole effects in strong field ionization with mid-infrared pulses. The results have implications for all strong field processes that are based on rescattering. This includes the generation of high harmonics, in particular in the hard x-ray regime, photoelectron holography and photoelectron diffraction, which are relevant for a wide audience in the general physics community, specifically for the growing field seeking to exploit the advantages of transferring these general methods to long-wavelength lasers.

A. Ludwig, J. Maurer, B.W. Mayer, C.R. Phillips, L. Gallmann, and U. Keller (2014) Breakdown of the Dipole Approximation in Strong-Field Ionization. Phys. Rev. Lett. 113, 243001 (10.1103/PhysRevLett.113.243001)

Fig. 1. Illustration of the wavelength-intensity parameter space in strong-field ionization, taking the magnetic field component into account. The area where the dipole approximation is considered as valid (dipole oasis) is depicted as green
dotted region. The well-known short-wavelength dipole limit arises for wavelengths on the order of the atomic scale, i.e. for
λ =1 a.u.. The long-wavelength limit arises due to the laser magnetic field component, and is characterized by the ratio Up/2c =1 a.u..

Fig. 2. (a) Typical projected photoelectron momentum distribution (PMD) of xenon recorded at an intensity of
6×1013W/cm2 with linear polarization using a VMIS at a center wavelength of 3.4 μm. We show the plane spanned by
the laser polarization (labeled px) and propagation (labeled pz) direction. The orange arrow depicts the center spot resulting
from field-ionization of highly-excited Rydberg states used as reference for pz=0 a.u., and the dashed boxes indicate the areas taken for the momentum-offset analysis.
(b) Projections of the PMD onto the beam propagation direction together with Lorentzian fits. The orange curve (squares) is used to set the pz=0 a.u. reference and the offset of the maximum of the photoelectron distribution is extracted from the fit on the green markers (circles).

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