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Accelerating electrons without diverting them using a radially polarized laser

​Researchers from the CEA-Iramis and the Laboratoire d'Optique Appliquée (ENSTA/X/CNRS) have succeeded for the first time in efficiently accelerating electrons to relativistic energies using a radially polarized laser.

Published on 27 April 2021

Femtosecond laser beams are now so intense (> 1018 W/cm²) that they can accelerate electrons to relativistic speeds. But it is not so simple, since the electric field of a laser tends to make the electrons oscillate if their speed is too low. To take full advantage of the acceleration by the laser's electric field (~ 1013 V/m), it is necessary to inject electrons already having relativistic energy into the beam of light so that they remain long enough in the acceleration field.

The solution proposed in 2015 by a collaboration between the LOA and Iramis led to the first experimental demonstration of vacuum laser acceleration. This achievement was based on the use of a "plasma mirror" that fulfills a triple function: produce a plasma, reflect the laser, and inject the fast electrons into the laser field in the desired optimal configuration.

However, there was still a fundamental limitation: the electric field associated with the laser light was perpendicular to the propagation direction of the beam, and the electrons accelerated by the laser were diverted from the optical axis!

To overcome this angular scattering, the researchers had the idea of using a radially polarized laser. This kind of polarization is produced by a spatial shaping of the laser beam using an eight-quadrant phase plate. Thus, at each point of the beam, the (transverse) electric field points towards the center of the beam and the laser intensity profile becomes circular. When this beam is strongly focused, the resulting electric field becomes longitudinal at the focus point. With this type of polarization, the electrons can be accelerated along the optical axis without diverging.

This is what was demonstrated in an experimental campaign conducted with the CEA's UHI100 laser. Electrons were accelerated to relativistic speeds with radially polarized ultra-high intensity laser pulses, and their angular divergence was reduced by half compared to the 2015 experiment (which used a conventional linear polarization).

Precise observations of the accelerated electrons and the resulting laser harmonics, combined with three-dimensional simulations of the experimental results, enabled the researchers to understand the complexity of the interactions involved and to identify possible paths forward. The radial polarization of the laser pulses must be of the highest possible quality and the laser incidence perpendicular to the plasma mirror. To be continued!

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