The generation of high-intensity single attosecond x-ray pulses from high-order harmonic generation (HHG) has been investigated via superposition of multi-laser beams. The results can be separated into three parts: (i) single-color chirp modulation; (ii) ultraviolet (UV)-chirp two-color modulation; and (iii) multi-laser beam modulation. Firstly, for the single chirp modulation, with the introduction of the up-chirp, although the harmonic cutoff can be slightly extended, the efficiency of the HHG is reduced. With the introduction of the down-chirp, both the cutoff and the efficiency of the HHG can be improved compared with the up-chirp. Moreover, as the pulse duration increases, a larger harmonic cutoff can be found. Secondly, by appropriate addition of a UV pump pulse, the efficiency of HHG can be further enhanced due to UV-resonance-enhancement-ionization. Thirdly, with the assistance of the third controlling pulse or the inhomogeneous effect of the laser field, the harmonic cutoff can be further extended and keV-HHG can be produced. Finally, through Fourier transformation of some selected harmonics on the spectral continuum, high-intensity x-ray pulses (including the keV region) with durations of sub-90 as can be obtained.

High-order harmonic spectra and attosecond source generation from Ar atom driven by the inhomogeneous laser¯eld in frequency and space have been theoretically investigated. The results showed that (i) for the case of the frequency modulation of the laser, the harmonic cuto® can be remarkably extended with the introduction of the chirp of the laser¯eld, (ii) for the case of the space modulation of the laser, the harmonic cuto® can be further extended by using the positive spatial inhomogeneous¯eld, (iii) by properly adding a terahertz controlling¯eld, the intensity of the harmonic yield can be enhanced by 2 orders of magnitude, showing a 775 eV supercontinuum. As a result,¯ve isolated attosecond pulses (IAPs) from 60 as to 22 as can be obtained.

Spatial distribution and quantum trajectory control of the molecular harmonic spectra from H[Formula: see text] are theoretically investigated through the combination of a few-cycle 800[Formula: see text]nm pulse and a half-cycle pulse. Results show that by properly adding the half-cycle pulse, the harmonic cutoffs from the recombination process are extended, and the harmonic intensities from the positive-[Formula: see text] direction are suppressed. Moreover, as the internuclear distance increased, the modulations on the harmonic spectra are reduced. The time-dependent wave function and the time-frequency analyses of the harmonic spectra are shown to explain the above physical mechanism. Further, by adding the laser pulse to the bowtie-shaped gold nanostructure separated by an air gap [Formula: see text][Formula: see text]nm, much higher harmonic cutoff order can be obtained in the positive-[Formula: see text] region. Finally, two isolated attosecond pulses with the full width at half maximum of 34as and 31as are obtained.

No abstract

No abstract

High-order harmonic spectra and attosecond pulse generation from Rydberg atom (He[Formula: see text]) driven by the spatially inhomogeneous field have been theoretically investigated. (i) Firstly, with an electron initially in a single excited Rydberg state (nth), the harmonic yield can be enhanced due to the decreased ionization potential, and a maximum enhancement can be obtained when the initial state is prepared as the third excited state (n = 3). However, the low cutoff energy from the excited state is unbeneficial to the generation of the higher photon pulse. Thus, with the further introduction of the laser chirp, not only the harmonic cutoff is extended, but also the harmonic modulation is reduced. As a result, five super-bandwidths from 63 eV to 267 eV can be found. (ii) Secondly, by preparing the initial state as a coherent superposition of excited state, the harmonic yield can be further enhanced, especially for the coherent superposition of the first and the third (n = 1 + 3) and the second and the fourth (n = 2 + 4) excited states, the harmonic yield is enhanced by 4–8 orders of magnitude compared with the case of the single ground initial state. Furthermore, by properly adding the laser pulse into the spatially inhomogeneous region (gap center [Formula: see text] a.u.) from left [Formula: see text] to right [Formula: see text], much higher cutoff energies can be obtained in the left region. As a consequence, two super-bandwidths of 248 eV and 496 eV can be obtained. Finally, by properly superposing the harmonics, a series of sub-25-as pulses with intensity enhancement of 5–8 orders of magnitude can be produced.

The generation of the high-order harmonic and the attosecond pulse from He atom driven by the near-infrared (NIR) field combined with the XUV pulse has been theoretically investigated by solving the three-dimensional time-dependent Schrödinger equation. The results show that by properly adding the XUV pulse into the NIR field, (i) not only the harmonic yield is enhanced, caused by the laser-induced excited state effect; but also the multi-cutoff extension of the harmonics can be found, caused by the absorption of the extra XUV photons during its recombination process. (ii) With the introduction of the XUV pulse, the frequency modulation of the high-order harmonic generation (HHG) can be found and controlled. (iii) By directly superposing the harmonics in the cutoff region, two isolated XUV pulses with the durations of 150 as can be obtained.

No abstract