Sub-Optical-Cycle Dynamics of Electron Nano-Bunches in Relativistic Laser-Plasma Interactions: Insights From Numerical Simulations*

The reflection of relativistically-intense few-cycle laser pulses from solid targets offers an efficient route toward the production of isolated attosecond pulses. A combination of theory and particle-in-cell (PIC) simulations has previously suggested that the origin of this radiation is the sub-cycle acceleration of high-density, nanometer-scale electron bunches (nanobunches). During this acceleration, the electrons follow synchrotron-like trajectories, emitting harmonics of the laser fundamental in the reflection and transmission directions. The formation and structure of the electron nanobunches are closely related to the emitted radiation. Here, multidimensional PIC simulations taking into account transverse laser forces are run and the results compared with those obtained from 1D PIC. On the short time scales considered here, we show that these nanobunches also form in 2D PIC simulations with similar structure to those found in 1D.