Sub-picosecond snapshots of fast electrons from high intensity laser-matter interactions

Abstract

The interaction of a high-intensity short-pulse laser with thin solid targets produces electron jets that escape the target and positively charge it, leading to the formation of the electrostatic potential that in turn governs the ion acceleration. The typical timescale of such phenomena is on the sub-picosecond level. Here we show, for the first time, temporally-resolved measurements of the first released electrons that escaped from the target, so-called fast electrons. Their total charge, energy and temporal profile are provided by means of a diagnostics based on Electro-Optical Sampling with temporal resolution below 100 fs.

Novel Single-Shot Diagnostics for Electrons from Laser-Plasma Interaction at SPARC_LAB

“…The resulting shape is as the one expected in Figure 2b. In order to evaluate the experimental results, we developed a numerical simulation tool [20]. By delaying the probe laser, the position of the EOS signal changes accordingly: the signals shift down because, in the meantime, the induced local birefringence moves far from the path of the travelling electrons (cf.…”

“…The FLAME laser is focused onto a metallic target. The EOS diagnostics, based on a ZnTe crystal placed 1 mm downstream from the target, allows for measuring the temporal profile of the emitted electron bunch by means of an ancillary laser beam (probe), directly split from the main laser, probing the local birefringence induced by the electric field [20].…”

“…By measuring the relative delay with respect to the reference time, it is possible to estimate the energy of the emitted electrons [20]. Indeed, the EOS being a single-shot device, it can be used as a time of arrival monitor allowing for measuring its time of flight [42,43].…”

“…Therefore, the fast electron charge can be estimated from the signal intensity. The emitted bunch travels normally to the crystal surface and moves below it while the probe laser crosses the crystal with a non-zero incidence angle; (Bottom) Spatial encoding process: (a) the bunch Coulomb field makes the crystal birefringent; (b) while the electric field penetrates into the crystal, the local birefringence shifts downwards; (c) the probe laser crosses the crystal and its polarization is rotated: the resulting signal comes from the blue region, i.e., where the local birefringence and the probe laser temporally overlapped [20]. …”

“…Those electrons, called fast electrons, are responsible for exit surface ionization and ion/proton acceleration and they have been studied in the past with different approaches [14][15][16][17][18][19]. With this technique, we succeed with measuring for the first time the temporal profile of the electric field carried by fast electrons generated during the interaction with an unprecedented resolution below 100 fs [20,21]. Furthermore, we have investigated the role of the target shape in the fast electron emission in order to optimize the ion and proton acceleration process.…”