Generation of an extreme ultraviolet supercontinuum and isolated sub-50 as pulse in a two-colour laser field

Abstract

We theoretically study high-order harmonic generation when a helium ion is exposed to a two-colour laser field, which is synthesized by a 5 fs/800 nm laser pulse and a 64 fs/2400 nm laser pulse. Our numerical results show that the harmonic spectrum exhibits an ultrabroad extreme ultraviolet supercontinuum when the initial state is prepared as a coherent superposition of the ground state and the first excited state. By superposing a series of properly selected harmonics, an isolated attosecond pulse with a duration of 47 as is obtained. Compared with the case of the ground state in a one-colour field, the intensity of this isolated attosecond pulse is six orders of magnitude higher. We also demonstrate these results in terms of the time–frequency analysis and the semiclassical three-step model.