  A thin and dense plasma layer is created when a sufficiently strong laser
pulse impinges on a solid target. The nonlinearity introduced by the
time-dependent electron density leads to the generation of harmonics. The pulse
duration of the harmonic radiation is related to the risetime of the electron
density and thus can be affected by the shape of the incident pulse and its
peak field strength. Results are presented from numerical
particle-in-cell-simulations of an intense laser pulse interacting with a thin
foil target. An analytical model which shows how the harmonics are created is
introduced. The proposed scheme might be a promising way towards the generation
of attosecond pulses.
  PACS number(s): 52.40.Nk, 52.50.Jm, 52.65.Rr
