Breakdown of dipole approximation
The electric dipole approximation is widely used in atomic, molecular and optical physics and is typically related to a regime for which the wavelength is much larger than the atomic structure. However, studies have shown that in strong laser fields another regime exists where the dipole approximation breaks down [1]. During the ionization process in intense laser fields and at long wavelengths the photoelectrons can reach higher velocities such that the magnetic field component of the laser field becomes significant. The ionization dynamics and the final momentum of the electron is therefore modified by the entire Lorentz force. In contrast the magnetic field interaction is neglected in the dipole approximation.
Rapid developments in laser technology and advancements in the accuracy of the measurements techniques have enabled the observation of the influence of such non-dipole e ects on the final angular photoelectron momentum distributions (PMD). More recently the number of studies on ionization beyond the dipole approximation has increased significantly, providing more important insight into fundamental properties of ionization processes, as summarised in an invited review article in 2021 [2].
For example we have shown that the final three dimensional photoelectron momentum spectra [1] and the holographic interferences [3] are significantly affectedby the non-dipole drift with the parent-ion interaction. In addition the linear multiphoton momentum transfer on a sub-cycle time scale and the sharing of the transferred linear photon momenta between the electron and the ion [4].
For example with the attoclock technique we measured how much momentum is transferred to the photoelectron in the direction of laser propagation with sub-femtosecond resolution at high laser intensities, where multiple photons are involved in the ionization process. This has been an unresolved question [4,5].
[1] A. Ludwig, J. Maurer, B. W. Mayer, C. R. Phillips, L. Gallmann, U. Keller
Download"Breakdown of the dipole approximation in strong-field ionization" (PDF, 613 KB)vertical_align_bottom
Phys. Rev. Lett., vol. 113, 243001, 2014
[2] J. Maurer, U. Keller, Invited Topical Review
Download"Ionization in intense laser fields beyond the electric dipole approximation: concepts, methods, achievements and future directions" (PDF, 2.7 MB)vertical_align_bottom
Journal of Physics B: Atomic, Molecular and Optical Physics, vol. 54, 094001 (18pp), 2021
[3] B. Willenberg, J. Maurer, U. Keller, J. Danek, M. Klaiber, N. Teeny, K. Z. Hatsagortsyan, C. H. Keitel
Download"Holographic interferences in strong-field ionization beyond the dipole approximation: The influence of the peak and focal-volume-averaged laser intensities" (PDF, 1.2 MB)vertical_align_bottom
Phys. Rev. A, vol. 100, 033417, 2019
[4] B. Willenberg, J. Maurer, B. W. Mayer, U. Keller
Download"Sub-cycle time resolution of multi-photon momentum transfer in strong-field ionization" (PDF, 1.3 MB)vertical_align_bottom
Nature Communication, vol. 10, 5548, 2019
[5] News item Link: https://ulp.ethz.ch/news/ulp-news/2019/12/sub-femtosecond-timing-of-the-linear-momentum-transfer-to-the-photoelectron-in-multi-photon-ionization.html