By solving the time-dependent Schrödinger equation (TDSE) accurately with
time-dependent generalized pseudospectral (TDGPS) method, we theoretically investigated
the high-order-harmonic generation (HHG) from three dimensional (3D) Hydrogen
atom in ultraviolet (UV)-assisted chirped fields. When a 128 nm UV pulse is
added on a chirped fundamental field, the HHG spectra is greatly broadened and enhanced,
which is quite similar as the HHG from H atom initially prepared in the first
excited state in the chirped field only. Besides, the HHG of H atom in the combination
of a chirped fundamental field and a 256 nm UV pulse case is also investigated.
The HHG process is illustrated by the semi-classical three-step model and the time-frequency
analysis. The ionization probability and electron wavepacket as functions
of time are also calculated to further illustrate this phenomenon. Furthermore, we also
discuss the influence of time delay between the chirped fundamental field and the 128
nm UV pulse on HHG process. Finally, by superposing the harmonics in the range of
200th-260th order, an isolated attosecond pulse with a duration of about 64 as can be
generated.
