Liqiang Feng

Theoretical investigation of the asymmetric molecular harmonic emission and the attosecond pulse generation

The harmonic emission of the asymmetric charged molecule HeH2+ ion is theoretically investigated by solving the one-dimensional-time-dependent Schrödinger equation. Results show that the laser-induced electron transfer process between the vibrational state and the ground electronic state is responsible for the resonant peak on the harmonic spectrum, and that the two ionization pathways are behind the two identified cutoff energies. In addition, by optimizing the asymmetric harmonic emission, the generations of the ultrashort attosecond pulses are also discussed.

High-intensity attosecond pulse generation by using inhomogeneous laser field in frequency and space

High-order harmonic spectra and attosecond source generation from Ar atom driven by the inhomogeneous laser field 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 cutoff can be remarkably extended with the introduction of the chirp of the laser field, (ii) for the case of the space modulation of the laser, the harmonic cutoff can be further extended by using the positive spatial inhomogeneous field, (iii) by properly adding a terahertz controlling field, the intensity of the harmonic yield can be enhanced by 2 orders of magnitude, showing a 775eV supercontinuum. As a result, five isolated attosecond pulses (IAPs) from 60 as to 22 as can be obtained.

Spatial distribution and quantum trajectory control of the molecular harmonic spectra

Spatial distribution and quantum trajectory control of the molecular harmonic spectra from H2+ are theoretically investigated through the combination of a few-cycle 800nm 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-x 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 g = 18nm, much higher harmonic cutoff order can be obtained in the positive-x0 region. Finally, two isolated attosecond pulses with the full width at half maximum of 34as and 31as are obtained.

Intensity enhancement of attosecond XUV pulse by using asymmetric inhomogeneous mid-infrared down-chirped field

Intensity enhancement of the attosecond pulse generation from the high-order harmonic spectra has been theoretically investigated through solving the three-dimensional time-dependent Schrodinger 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.