Spatiotemporal structure of the combined field of high-order harmonics and generation of attosecond pulses

An investigation is made of the feasibility of separating a train of attosecond short-wavelength pulses from a combined field of high-order harmonics. Semiclassical and quantum-mechanical theories of the generation of high-order harmonics are used to consider the problem of minimum duration of a pulse which can be generated by summing the fields of several adjacent harmonics near a harmonic-generating gas layer. A study is made of the spatial structure of the field of harmonics generated in a thin layer subjected by a Gaussian laser beam. It is shown that harmonics of different orders have different divergences and that they focus at different distances when generated in a converging beam. This effect may be utilised to separate harmonics of specific orders, particularly in the generation of attosecond pulses. High-intensity fundamental-frequency and third-harmonic radiations should be suppressed by other methods. Calculations predict that pulses of duration down to 90 as and of intensity comparable with that of the third harmonic can be generated.