Analytical approach to high harmonics spectrum in the nanobunching   regime
  With the high-order harmonic generation (HHG) from plasma sur- faces it is possible to turn a laser pulse into a train of attosecond or even zeptosecond pulses in the backward radiation. These attosecond pulses may have amplitude several orders of magnitude higher 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 an analytical expression that describes the electron density pro- file and obtain a good agreement with particle-in-cell simulation results. We investigate the most efficient case of HHG at moderate laser intensity (I = 2*10^20 W/cm^2 ) on the over-dense plasma slab with an exponential profile pre-plasma. Subsequently we calculate the spectra of a single at- tosecond pulse from the backward radiation using our expression for the density shape in combination with the equation for the spectrum of the nanobunch radiation. 
Mykyta Cherednychek and Alexander Pukhov
physics.plasm-ph