Relativistic interaction of ultrashort laser pulses with micro- or nano-structured targets has recently raised a huge interest for its potential application in many fields, including laser-driven particle acceleration, warm dense matter creation or even Inertial Confinement Fusion. The main reason of such an interest lies in the more efficient absorption of the laser energy, if compared to a flat target, which is mainly due to the extended penetration depth of laser light into the nanostructured target compared to the collisionless skin depth of a flat target and to the enhancement of the local electric field around the structures. The enhanced energy transfer from the laser pulse to the solid, that in the more favourable cases can exceed the 90%, yields matter in extreme conditions, with multi-keV temperatures and pressures in the Gigabar range, which can be hardly reached in laboratory.
The interaction conditions, including both the target geometry and the laser pulse temporal profile, strongly affect the energy transfer and their effect should be investigated for achieving a better control of the process. Picosecond pedestal typical of ultrashort pulses, for example, can damage the more tiny structures and fill with a preplasma the nano-scale gaps before the arrival of the main peak. This can strongly degrade the coupling of the laser pulse with this class of targets.
We here report on the comparative study of relativistic interaction mechanisms of a high-contrast laser pulse with wire-structures of different size, revealing a transition from a coherent particle heating to a stochastic plasma heating regime, which occurs when migrating from micro-scale to nano-scale structures. Experiments and kinetic simulations suggest that the coherent heating regime, occurring for large gaps between the wires, favour the generation of high-energy electrons via direct laser acceleration into the channels. Differently, the stochastic heating regime, obtained for gaps between the wires which are smaller than the amplitude of electron quivering in the laser field, results in less energetic electrons and in the generation of a multi-keV high-pressure plasma.
The two regimes appear potentially suitable for laser-driven ion/electron acceleration schemes and warm dense matter investigation and strong shock generation, respectively.