Very intense laser pulses focused on a solid target release high-energy electrons. These are accelerated by the electromagnetic field of the laser, which is inherently oscillatory. The conditions of the interaction must therefore be carefully optimized, so that the acceleration is efficient and can last beyond one half of an optical cycle.

Recent modelling and experiments have shown that it is possible to increase the coupling between the laser and the plasma electrons by using a target with a nanostructured surface in the manner of a diffraction grating. A surface "plasma wave" can then produce relativistic electron packets of energy with a high current.

The researchers are currently adding another condition: a carefully chosen rotation of the laser wave front at the focal point, which allows the surface plasma wave to be reinforced. This wave then contributes more effectively to the acceleration of the electron packets, which leads to a significant gain (65%) in electron energy.

Particle-in-cell simulations reveal that the optimum is obtained when the sliding velocity of the wave front center with respect to the laser beam focus point is merged with the propagation velocity of the surface plasma wave. In this case, it would be possible to create – using laser pulses of 25 fs (1 femtosecond = 10^-15 s) and 4.7 x 10¹⁹ W/cm² and a rotating wave front of up to 67 mrad – electron packets of 8 fs in duration, with an energy of 70 MeV and a total charge of 50 pC (1 picocoulomb = 10^-12 C).

This study paves the way for measurements that can be performed with ultra-high intensity (UHI) laser facilities currently available in France, and in particular at Iramis.

This work by the Laboratoire des Solides Irradiés (École Polytechnique, CNRS, CEA) was carried out in collaboration with the Laboratory for the Use of Intense Lasers (École Polytechnique, CNRS, CEA, Sorbonne University) and the University of Pisa (Italy).