• High Power Laser Science and Engineering
  • Vol. 8, Issue 4, 04000e44 (2020)
I. Makos1、2, I. Orfanos1、2, E. Skantzakis1, I. Liontos1, P. Tzallas1、3, A. Forembski4, L. A. A. Nikolopoulos4, and D. Charalambidis1、3、*
Author Affiliations
  • 1Foundation for Research and Technology - Hellas, Institute of Electronic Structure & Laser, 70013Heraklion (Crete), Greece
  • 2Department of Physics, University of Crete, 70013Heraklion (Crete), Greece
  • 3ELI-ALPS, ELI-Hu Non-Profit Ltd., H-6720Szeged, Hungary
  • 4School of Physical Sciences, Dublin City University, Dublin 9, Ireland
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    Motivated by the achieved high intensities of novel extreme ultraviolet (XUV) radiation sources, such as free electron lasers and laser-driven high harmonic generation beamlines, we elaborate on their perspective in inducing observable strong field effects. The feasibility of extending such effects from the infrared and visible spectral regimes in the XUV domain is supported through numerically calculated models of near-future experiments. We highlight the advancement of performing studies in the time domain, using ultra-short XUV pulses, which allows for the temporal evolution of such effects to be followed. Experimental and theoretical obstacles and limitations are further discussed.

    1 Introduction

    The interaction of intense femtosecond (fs) laser radiation with atoms/molecules may lead to a substantial distortion of the atomic/molecular potentials and intrinsic dynamics. A well-known result of such a distortion is the so-called tunneling ionization that underlies processes such as high harmonic generation (HHG)[1] and above-threshold ionization (ATI)[2]. Tunneling ionization occurs when an infrared (IR) pulse is distorting the Coulomb potential of the system, forming potential barriers that oscillate with the laser period, through which an electron can tunnel out into the continuum. Whether tunneling or multiphoton is the main mechanism of photoionization depends on the interplay between the radiation’s field strength, frequency, pulse duration as well as the ionization energy of the atom/molecule. The key parameter that defines the mechanism underlying a photoionization process is the Keldysh or adiabatic parameter[3]

    with ${E_{\rm ion}}$ being the ionization energy and ${U}_p$ the ponderomotive potential[4], i.e., the mean kinetic energy of the oscillation of a free electron interacting with the radiation field
    where $m$ and $e$ are the mass and charge of the electron, respectively, $x$ is its position, and ${E}_0$ and $\omega$ are the electric field amplitude and the angular frequency of the radiation, respectively. A practical form of Equation (2) is