Effects of inhomogeneous laser field in time and space on the generation of broad spectral continuum and ultrashort attosecond pulse

The inhomogeneous effects of laser field in time and space on the generations of high-order harmonic spectra and attosecond pulses from He atom have been investigated with the assistance of the frequency-chirping technique and the metallic nanostructure. We find that in different combinations of chirps and spatial inhomogeneous effects, not only can the harmonic cutoff be extended, but the single harmonic emission peak (HEP) can also be selected to contribute to the spectral continuum. In detail, with the combinations of (i) symmetric chirp in frequency and positive inhomogeneous effect in space or (ii) asymmetric down chirp in frequency and negative inhomogeneous effect in space, two broad spectral continua with bandwidths of 496 eV and 480 eV can be obtained. Further, by properly adding IR or UV controlling pulse, the harmonic emission efficiency can be enhanced by several orders of magnitude. The enhancement of harmonic yield from adding UV pulse is higher than that from adding IR pulse. Moreover, the difference of harmonic enhancement between UV combined field and IR combined field is much more obvious at lower controlling laser intensity. Through analyzing the ionization probability, we find that the higher enhancement of harmonic yield from UV combined field is because of UV-resonance-enhancement-ionization between the ground state and the excited state of He atom. Finally, by properly superposing some harmonics, a number of attosecond pulses with pulse durations of 45 as can be obtained.