Efficient production of fast electrons from femtosecond laser interaction with solid targets

The interactions of intense, p-polarized, femtosecond laser pulses with solid-density plasma targets is studied with particle-in-cell simulation. At large angles of incidence the density profile is rapidly modified by a ‘‘push-pull’’ combination of ponderomotive force and the space-charge effect of electrons held outside the solid, eventually leading to efficient absorption over a wide range of laser irradiance. For I${\lambda }^2$>${10}^{17}$ W ${\mathrm{cm}}^{\mathrm{-}2}$ μ${\mathrm{m}}^2$, the final absorption rate peaks at 45°–50° and is largely independent of intensity; the average absorption fraction and the shape of the fast electron distribution both depend on the ratio of the pulse length to the ion shelf expansion time ${\mathit{t}}_{\mathit{s}}$∝${\mathrm{\omega }}_0^{\mathrm{-}1}$(${\mathit{m}}_{\mathit{i}}$/${\mathit{Zm}}_{\mathit{e}}$ ${)}^{1/2}$.