Relativistic plasma control for single attosecond pulse generation

To describe the high harmonic generation at plasma surfaces in the relativistic regime, we introduce the concept of {\it apparent reflection point} (ARP). It appears to an external observer that the radiation electric field is zero at the ARP. The relativistic dynamics of the ARP completely defines the generation of high harmonics and attosecond pulses. The ARP velocity is a smooth function of time. The corresponding $\gamma$-factor, however, has sharp spikes at the times when the tangential vector potential vanishes and the surface velocity becomes close to the speed of light. We show that managing the laser polarization, one can efficiently control the ARP dynamics, e.g., to gate a single (sub-)attosecond pulse out of the short pulse train generated by a multi-cycle driver. This relativistic control is demonstrated numerically by particle-in-cell simulations.