An effective method to enhance the harmonic efficiency by using the He[Formula: see text] ions chain has been proposed. It is found that (i) with the increasing number (2[Formula: see text]) of He[Formula: see text] ions, the intensities of the harmonics are enhanced and the cutoffs of the harmonics are extended. (ii) With the increasing nuclear distance ([Formula: see text] of different He[Formula: see text] ions, the intensities of the harmonics are reduced and the cutoffs of the harmonics are decreased. (iii) By using the up-chirp or the down-chirp laser fields, the cutoffs of the harmonics and the intensities of the harmonics in the cutoff region can be further extended and enhanced, respectively. The quantum analyses are shown to explain the generated process of the harmonics. (iv) By selecting the harmonics from an example case of He[Formula: see text] ions chain ([Formula: see text], [Formula: see text][Formula: see text]a.u.), 33[Formula: see text]as and 45[Formula: see text]as pulses with the intensity enhancements of four and five orders of magnitude can be produced.

An effect method to obtain the single attosecond pulses (SAPs) by using the inhomogeneous polarization gating (PG) technology in the crossed metal nanostructure has been proposed and investigated. It is found that by properly injecting the gas into the negative position of the nanostructure, the emitted harmonics at the very highest orders used for the SAPs only occurs at one side of the region inside the nanostructure. As a result, not only the extension of the harmonic cutoff can be achieved, but also the modulations of the harmonics can be decreased. Further, with the introduction of a weak near-infrared field, the harmonic yield can be enhanced by 3.5 orders of magnitudes in comparison with that from the single PG technology. Finally, by superposing the selected harmonics from the inhomogeneous PG scheme, a 32 as SAP with the high signal intensity can be obtained.

A potential scheme to obtain the high-intensity KeV isolated attosecond pulses (IAPs) from the high-order harmonic generation (HHG) has been proposed and studied via the superposition of the multi-color inhomogeneous laser beams. The scheme can be separated into three steps. Firstly, with the superposition of two-color mid-infrared field and a few-cycle infrared field at the proper delay time and the carrier envelope phase, the optimal HHG spectrum can be found. Secondly, with the help of the positive inhomogeneous effect in space, the harmonic emission peaks (HEPs) from the positive half-cycle can be well selected. Secondly, by properly adding a unipolar pulse with the positive peak, the harmonic cutoff can be extended to the KeV region and the harmonic plateau is only contributed by the single HEP. Thirdly, with the introduction of the UV pulse, the signal of the harmonic plateau can be enhanced by at least 500 dB due to the UV-resonance-enhancement-ionization. Finally, through the Fourier transformation of some selected harmonics, the sub-35 as IAPs in the KeV region can be obtained.

An effective method to obtain the single attosecond pulses (SAPs) by using the multi-cycle plasmon-driven double optical gating (DOG) technology in the specifically designed metal nanostructures has been proposed and investigated. It is found that with the introduction of the crossed metal nanostructures along the driven and the gating polarization directions, not only the harmonic cutoff can be extended, but also the efficient high-order harmonic generation (HHG) at the very highest orders occurs only at one side of the region inside the nanostructure. As a result, a 93 eV supercontinuum with the near stable phase can be found. Further, by properly introducing an ultraviolet (UV) pulse into the driven laser polarization direction (which is defined as the DOG), the harmonic yield can be enhanced by two orders of magnitude in comparison with the singe polarization gating (PG) technology. However, as the polarized angle or the ellipticity of the UV pulse increase, the enhancement of the harmonic yield is slightly reduced. Finally, by superposing the selected harmonics from the DOG scheme, a 30 as SAP with intensity enhancement of two orders of magnitude can be obtained.