Generation of Ultra-short Light Pulses by a Rapidly Ionizing Thin Foil

A thin and dense plasma layer is created when an intense 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 Typeset using REVTEX 1 In recent years several mechanisms generating harmonics of electromagnetic radiation have been discovered. Among these high-order harmonics from gases [1] and harmonics production from the plasma-vacuum boundary when a laser pulse impinges on a solid target [2] are the most prominent ones. Especially the high-order gas-harmonics, exhibiting a “plateau” instead of a rapid decrease with the harmonic order, seem to be a promising source for xuv “water-window”-radiation. Apart from the effort to make progress towards shorter wavelengths, another goal is to achieve shorter pulse durations because the temporal resolution in pump-probe experiments clearly depends on the pulse length. One scheme proposed to generate attosecond pulses is based on phase-matching pulse trains which are produced by a laser pulse focused into a jet of rare gases [3]. Another method makes use of the fact that the efficiency of gas-harmonics generation is sensitive to the ellipticity of the incident laser light [4]. The method to generate an ultra-short low order harmonic laser pulse as proposed in this Letter is based on a completely different mechanism. We would like to stress in advance that the method is not appropriate for generating particularly high order harmonics efficiently. We assume a linearly polarized laser pulse impinging perpendicularly on a thin foil target. In the following analytical and numerical calculations the whole setup is treated one dimensionally in space, i.e., the laser pulse propagates along x and the electric field is in ydirection. The foil will be ionized by the pulse. To calculate the pulse propagation through a medium with varying free electron density one has to solve the inhomogeneous wave equation ∂ ∂x E(x, t)− 1 c ∂ ∂t E(x, t) = 1 ε0c ∂ ∂t j(x, t). (1) The Green’s function of this equation is G(x, x, t, t) = −cΘ[c(t − t) − |x − x|]/2 where Θ(y) is the step-function, i.e., Θ(y) = 1 for y > 0 and 0 otherwise. The solution of (1) can be written as the sum of the incident field E0(x, t) and the radiation field produced by the current j(x, t), i.e., E(x, t) = E0(x, t) + Er(x, t), with Er(x, t) = − 1 2cε0 ∫