With the high-order harmonic generation (HHG) from plasma surfaces 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 profile 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 attosecond pulse from the backward radiation using our expression for the density shape in combination with the equation for the spectrum of the nanobunch radiation.

We present a new mechanism of attosecond extreme-ultraviolet (XUV) pulses generation from a relativistic laser-driven overdense plasma surfaces in the wavebreaking regime. Through particle-incell simulations and analysis, we demonstrate that the observed ultrashort XUV emission for the parameters we considered is predominantly due to a strong plasma-density oscillation subsequent to wavebreaking. The coupling of the strong density variation and the transverse fields in the front surface layer gives rise to the transmitted emission with frequencies mainly around the local plasma frequency. This mechanism provides new insights into the scenarios of XUV generation from solid surfaces and the dynamics of laser-plasma interactions.

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