Extension of the high-order harmonics and an isolated sub-100 as pulse generation in a two-colour laser field

We theoretically investigate the high-order harmonic generation (HHG) when a helium ion model is irradiated by a two-colour laser field, which is synthesized by a 5 fs pulse at 800 nm and a 10 fs pulse at 394 nm. The harmonic spectrum reveals a double-plateau structure and an extreme ultraviolet supercontinuum spectrum in the lower second plateau can be superposed to produce attosecond pulses. By increasing the intensity of 10 fs pulse at 394 nm, the cutoff of the second plateau is extended effectively and the harmonics in the second plateau become in phase. Consequently, an isolated 63 as pulse is observed. These results can be explained in terms of the time?frequency characteristics of attosecond pulses and a semiclassic three-step model.