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Above Threshold Ionization in Tightly Focused, Strongly Relativistic Laser Fields

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Abstract

At the ultra-high intensities now accessible with the latest generation of petawatt class lasers, v×B forces play a dramatic role on the dynamics of tunnel-ionized electrons. At these intensities electrons are driven nearly parallel to the laser propagation direction, gaining high energies as they ride in phase with the laser field over a long distance [1]. At sub- and near relativistic intensity ATI electrons can be well described by models, which assume that the laser electric and magnetic fields are purely transverse to the laser propagation axis. This assumption leads to a well defined relationship between the electrons’ final energy and ejection angle [2]. It is known however, that the actual electric and magnetic fields near a focus have components that are parallel to the laser propagation axis [3]. The magnitude of these longitudinal fields is smaller than the transverse fields by a factor of ~ 1/kw0 (where k is the laser wave number and w0 is the focal radius). Since these fields are typically at least an order of magnitude smaller than the transverse fields, even for tightly focused beams, they are usually unimportant. However our theoretical and numerical investigations on strong field ATI in the relativistic regime have shown that these longitudinal fields are very important. We find that, while the electron energies produced can be quite high, approaching 1 GeV at intensity of 5 × 1021 W/cm2, they are strongly influenced by the presence of longitudinal fields near the focus. These longitudinal electric fields serve to decelerate the electrons, pushing them out of phase with the field [4]. This decreases significantly the energy the electrons can acquire in the field and leads to large differences in the ATI electron angular distribution (this phenomenon is illustrated in Fig. 1). In this presentation we demonstrate a theoretical basis for the great effect of longitudinal electric fields on electron dynamics in case of ultrarelativistic ATI and support it by numerical simulations.

© 2003 Optical Society of America

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