Intensity enhancement of the attosecond pulse generation from the high-order harmonic spectra has been theoretically investigated through solving the three-dimensional time-dependent Schrödinger equation. It is found that with the introduction of the down-chirped pulse, the temporal frequency of the down-chirp region is decreased. As a result, the ionized electrons can receive much more energy during its acceleration in this region, showing the extension of the harmonic cutoff. Moreover, due to the multi-harmonic emission events contribute to the higher harmonics, the intensity of the harmonic cutoff from the down-chirped pulse is 1.5 orders of magnitude higher than those from the chirp-free pulse. Further, by properly introducing the asymmetric inhomogeneous effect, the plasmonic enhancement of the laser intensity in the negative direction is larger than that in the positive direction. As a consequence, the ionized electron from the down-chirp region with the negative amplitude can obtain the additional acceleration, thus leading to the further extension of the harmonic cutoff. Especially when the spatial position of the inhomogeneous field is chosen to be the negative value, due to the improved enhancement of the laser intensity, not only the harmonic cutoff is extended but also the harmonic yield is near-stable, showing a 175 eV supercontinuum with the single short quantum path contribution. Finally, by directly superposing the selected harmonics, three attosecond XUV pulses with the full widths at half maximum of 38, 35 and 36 as can be obtained, which are nearly 1.5 orders of magnitude enhancement compared with the chirp-free homogeneous field case.
Relativistic plasma aperture for laser intensity enhancement
