The zero vector potential mechanism of attosecond absorption

A new mechanism for the absorption of energy during the interaction between an ultra-intense laser pulse and a sharp-edged overdense plasma, which we term the zero vector potential (ZVP) mechanism, is presented. The ZVP-mechanism is a nonponderomotive absorption mechanism that should dominate in the interaction of very strong short laser pulses (a0>>1) with overdense plasmas in the case of sharp density gradients. In the ZVP-mechanism the existence of moving zeroes in the vector potential of the relativistic skin layer is crucial to the generation of both fast electron bunches and coherent x-rays. We demonstrate that the laser energy is absorbed from the plasma on the attosecond timescale in the form of electron bunches with unprecedentedly short duration. The numerical simulations are able to validate all qualitative and quantitative aspects of the ZVP-mechanism.