Role of contrast of a relativistic femtosecond laser pulse interacting with solid and structured targets

We consider the effect of a pre-plasma layer inevitably present in experiments on the acceleration of electrons and ions during interaction of a relativistic femtosecond laser pulse with a dense plasma. The interaction regimes are identified in which the presence of such a layer can significantly increase the average and maximum energies of electrons. The regimes are discussed in which an artificial nanosecond prepulse makes it possible to produce a collimated electron beam with a high charge and an average energy of up to 10 ponderomotive energies in the direction of the reflected or incident laser beam. It is shown that the acceleration of ions, as a rule, requires an ultrahigh contrast of the laser pulse, since the parameters of the accelerated ion beams deteriorate significantly in the presence of preplasma or due to the evaporation of a thin-film target. The regimes of interaction of laser pulses with thick targets, in which heavy multiply charged ions can be accelerated by cleaning the surface with a prepulse, are also discussed. An essential part of the review is devoted to the interaction of radiation with micro- and nanostructured targets. Both the methods of their fabrication and the issues related to the interaction of a femtosecond laser pulse and its contrast with such structures are considered.