Abstract
A study of the structure of the electric and magnetic fields of ultraintense laser pulses focused by an off-axis parabolic mirror is reported. At first, a theoretical model is laid out, whose final equations integration allows the space and time structure of the fields to be retrieved. The model is then employed to investigate the field patterns at different times within the optical cycle, for off-axis parabola parameters normally employed in the context of ultraintense laser–plasma interaction experiments. The results show that nontrivial, complex electromagnetic field patterns are observed at the time at which the electric and magnetic fields are supposed to vanish. The importance of this effect is then studied for different laser polarizations, $f$ numbers and off-axis angles.
Highlights
Off-axis parabolic (OAP) mirrors have become an essential tool to focus ultrashort laser pulses down to micrometre size spots, allowing relativistic intensities ( 1018 W · cm−2) to be reached
The model is employed to investigate the field patterns at different times within the optical cycle, for off-axis parabola parameters normally employed in the context of ultraintense laser–plasma interaction experiments
The usage of OAP is envisaged as essential to get tight focusing of the generation >10 PW scale lasers in order to reach an intensity on target in the 1022–1024 W · cm−2 range, allowing strong field quantum electrodynamics (QED) phenomena such as radiation reaction, vacuum polarization and pair production to be investigated[2, 3]
Summary
Off-axis parabolic (OAP) mirrors have become an essential tool to focus ultrashort laser pulses down to micrometre size spots, allowing relativistic intensities ( 1018 W · cm−2) to be reached. [31], where a theoretical model is presented predicting that the polarization of a beam focused by an off-axis ellipsoidal mirror exhibits a (smooth) spatial dependence in the focal plane This observation only concerned the time averaged pattern of the polarization direction and did not describe any change occurring at a sub-cycle level. Our framework is based on a full vector diffraction treatment and retains a time dependence of the fields as provided by their initial phases, allowing the electromagnetic field pattern to be retrieved at any given time within the optical cycle Using this model and solving the resulting integrals by numerical calculations, we study in Section 3.1, the field maps at different times, showing that, as a result of the off-axis focusing, electric (magnetic) fields are generated, during the optical cycle, along directions different from the original polarization direction (or original magnetic field direction, respectively).
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