  With the generation of high order harmonics (HHG) on the plasma surface it is
possible to turn the laser pulse into a train of attosecond or even zeptosecond
pulses in the back radiation. These attosecond pulses may have amplitude
several orders of magnitude larger than that of the laser pulse under
appropriate conditions. We study this process in detail, especially the
nanobunching of the plasma electron density. We derive the analytical
expression that describes the electron density profile and obtain a good
agreement with particle-in-cell simulations. We investigate the most efficient
case of HHG at moderate laser intensity (a0 = 10) on the over dense plasma slab
with an exponential profile per-plasma. Subsequently we calculate the spectra
of single attosecond pulses from back radiation using our expression for
density shape in combination with the equation for spectrum of nanobunch
radiation.
