High-intensity attosecond X-ray source generation by using low-intensity spatial inhomogeneous mid-infrared pulse combined with an ultraviolet pulse

Abstract

We theoretically investigate the high-order harmonic spectra and the attosecond pulse generation from He[Formula: see text] ion driven by the spatial inhomogeneous combined field, consisting of a two-color low-intensity mid-infrared pulse ([Formula: see text] W/cm2) and an ultraviolet (UV) pulse ([Formula: see text] W/cm2). The numerical results show that due to the spatial inhomogeneous effect, the harmonic cutoffs are remarkably extended, and a 986 eV super-continuum can be obtained. Further with the proper introduction of an UV pulse (from 60 nm to 20 nm) to the two-color field, the harmonic yields are enhanced, especially for adding the 30 nm pulse case, a 1113 eV super-continuum (corresponding to a 34.1 dB enhanced field) with less harmonic modulation and six orders of magnitude enhancement can be obtained. Similar calculations have been performed for He atom and an 896 eV super-continuum with four orders of magnitude enhancement can be produced. Time–frequency harmonic distribution and time-dependent electron populations show that the single short quantum path contribution is responsible for the less harmonic modulation and the laser-induced electron excitation is responsible for the harmonic enhancement. Finally, by properly superposing the harmonics, a series of sub-50-as pulses including the whole X-ray region with 3–6 orders of magnitude enhancement can be obtained.