The release of the fast electrons accompanied by the emission of ultra-short EM pulses, in particular, has not yet been experimentally provided. The duration is on the nanosecond timescale and corresponds to well-known pulses in the GHz domain whose features have been widely investigated 26, 27, 28.Ī complete picture of the several processes involved in laser-target interactions is experimentally complicated to obtain since different diagnostic techniques are usually needed. The second component is due to the neutralization currents that flows through into the target to balance the net positive charge left on it. Their propagation, measured through coil-like structures 23 or wires 24, 25, occurs on tens-picosecond time scale. The first one consists in ultra-short EM pulses in the THz domain 21, 22 and is mostly related to the current associated with fast electrons. Such a recirculation generates huge electromagnetic pulses 18, 19, 20, up to some teravolt per meter (TV/m) depending on the laser intensity, whose spectral content is constituted by two main components. The hot electrons bounce back and forth and continue to ionize the matter creating a plasma. The unbalanced positive charge left on target leads thus to the formation of a strong electric potential that locks the majority of hot electrons close to the target 16. Some of them (fast electrons) are energetic enough to completely escape the target charging it rapidly. These hot electrons propagate through target and then are ejected 17. When a laser pulse operating at relativistic intensities ( \(\) W/cm 2) irradiates a solid target, a force driven by the laser field is produced and accelerates electrons up to relativistic velocities 12, 13, 14, 15, 16. It is therefore fundamental to have a clear and precise understanding of the interaction process in the transient regime, where all the customary models that assume thermal equilibrium are stretched to their proper end of justification and beyond. More specifically, the transfer of energy from the laser field to the particles in the bulk of the target lies in the heart of all the processes and acts as a complex initial condition. Multi-Terawatt laser pulses with femtosecond duration have opened new horizons in research of nonlinear transient phenomena like astrophysics in laboratory 1, 2, particle acceleration 3, 4, 5, material science 6, 7, surface phenomena 8 (breakdown and surface manipulation), nuclear 9 and medical physics 10, 11. Our results provide a snapshot of huge pulses, up to 0.6 teravolt per meter, emitted with multi-megaelectronvolt electron bunches with sub-picosecond duration and are able to explore the processes involved in laser-matter interactions at the femtosecond timescale. The temporal evolution of the interaction is probed with a novel femtosecond resolution diagnostics that enables the differentiation of the contribution by the high-energy forerunner electrons and the radiated electromagnetic pulses generated by the currents of the remaining charges on the target surface. Here we present experimental results related to the fields and charges generated by the interaction of an ultra-short high-intensity laser with metallic targets. The entire process, moreover, is difficult to probe since it develops close to target on the sub-picosecond timescale and ends after some picoseconds. Deciphering its evolution is a complicated task that strongly depends on the details of the early phase of the interaction, which acts as complex initial conditions. The interaction of high-power ultra-short lasers with materials offers fascinating wealth of transient phenomena which are in the core of novel scientific research.
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