Vacuum heating versus skin layer absorption of intense femtosecond laser pulses

The crossing of the narrow skin layer in solid targets by electrons in a time shorter than a laser cycle represents one of the numerous collisionless absorption mechanisms of intense laser-matter interaction. This kinetic effect is studied at normal and oblique laser beam incidence and particle injection by a test particle approach in an energy interval extending into the relativistic domain. Three main results obtained are the strong dependence of the energy gain by the single particle on the instant of injection relative to the phase of the light wave, the reflection of the particles primarily contributing to absorption well in front of the target rather than in the Debye layer, and the low degree of absorption hardly exceeding the 10% limit. The simulation results offer a more unambiguous interpretation of the absorption mechanism often referred to as “vacuum heating.” In particular, it is clearly revealed that the absorption in the vacuum region prevails on that originating from the skin layer. Relativistic ponderomotive effects are also tested, however their contribution to absorption is not significant.