Li Suyu

High-intensity molecular harmonic generation without ionization

We theoretically investigate high-order harmonic generation from H2+ in an infrared laser field. Our numerical simulations show that a highly efficient plateau structure exists in the molecular harmonic spectrum. Under the action of the infrared laser pulse, the bound electronic wave packet in a potential well has enough time to tunnel through the effective potential barrier, which is formed by the molecular potential and the infrared laser field, and then recombine with the neighboring nucleus emitting a harmonic photon. During the entire dynamic process, because the wave packet is mainly located in the effective potential, the diffusion effect is of no significance, and thus a highly efficient harmonic plateau can be achieved. Specifically, the cut-off frequency of the plateau is linearly scaled with the peak amplitude of the infrared laser electric field, which may open another route to examine the internuclear distance of the molecule. Furthermore, one may detect the molecular bond lengths using the harmonic plateau.

Isolated attosecond pulse generation from atom radiated by a three-color laser pulse

We theoretically investigate high-order harmonic and attosecond pulse generation from helium atom in a three-color laser field, which is synthesized by 10 fs/800 nm Ti-sapphire laser and a two-color field consisting of 30 fs/532 nm and 30 fs/1330 nm pulses. Compared with harmonic spectrum generated by a monochromatic field, the harmonics generated from the synthesized three-color field show a supercontinuum spectrum with a bandwidth of 235 eV, ranging from the 154th to the 306th order harmonic. This phenomenon can be attributed to the fact that the ionization of atoms as well as motion of ionized electron can be effectively controlled in the three-color field. Therefore, an isolated 46-as pulse can be generated by superposing supercontinuum from the 160th to the 210th order harmonics.

Atomic high-order harmonic generation from a circularly polarized laser pulse

We demonstrate theoretically that the high-order harmonic of an atom can be generated by a circularly polarized laser pulse. The harmonic spectrum shows a clear cutoff with an energy Ip + 2Up. In particular, the high-order harmonic generation comes from the multiple recombination of the ionized electron with non-zero initial velocity. These results are verified by the classical model theory and the time-frequency analysis of a harmonic spectrum.

Re-Heating Effect on the Enhancement of Plasma Emission Generated from Fe Under Femtosecond Double-Pulse Laser Irradiation*

In this paper, we present a study on the effect of inter-pulse delay using femtosecond double-pulse laser-induced breakdown spectroscopy in a collinear geometry. The temporal evolution of spectral intensity is performed for the lines of Fe I 423.60 nm, Fe I 425.08 nm and Fe I 427.18 nm. It is found that, by selecting appropriate inter-pulse delay, the signal enhancement can be significantly increased compared with the single-pulse case. A three-fold enhancement in the current experiment is obtained. The plasma temperature and electron density are also investigated based on the theory of Boltzmann plot and Stark broadening. We attribute the main mechanism for emission enhancement to the plasma re-heating effect.

Field-free orientation of diatomic molecule via the linearly polarized resonant pulses

We propose a scheme to coherently control the field-free orientation of NO molecule whose rotational temperature is above 0 K. It is found that the maximum molecular orientation is affected by two factors: one is the sum of the population of M = 0 rotational states and the other is their distribution, however, their distribution plays a much more significant role in molecular orientation than the sum of their population. By adopting a series of linearly polarized pulses resonant with the rotational states, the distribution of M = 0 rotational states is well rearranged. Though the number of pulses used is small, a relatively high orientation degree can be obtained. This scheme provides a promising approach to the achievement of a good orientation effect.

Defocusing role in femtosecond filamentation: Higher-order Kerr effect or plasma effect?*

The femtosecond filamentation in the classical and high-order Kerr (HOK) models is numerically investigated by adopting multi-photon ionization (MPI) cross section with different values. It is found that in the case that the MPI cross section is relatively small, there exists a big difference between the electron density as well as clamped intensity calculated in the classical model and those calculated in the HOK one, while in the case that the MPI cross section is relatively large, the electron density and clamped intensity calculated in the two models are nearly in agreement with each other, and under this circumstance, even if the higher-order nonlinear terms do exist, the free-charge generation and the associated defocusing in a filament are enough to mask their effects. The different behaviors of the maximum intensity and on-axis electron density at the collapse position with the pulse duration provides an approach to determine which effect plays the dominant defocusing role. These results demonstrate that it is ionization that results in the difference between the two models.

Investigation on the influence of atomic potentials on the above threshold ionization

We systematically investigate the influence of atomic potentials on the above-threshold ionization (ATI) spectra in one-dimensional (1D) cases and compare them with the three-dimensional (3D) case by numerically solving the time-dependent Schrodinger equation. It is found that the direct ionization plateau and the rescattering plateau of the ATI spectrum in the 3D case can be well reproduced by the 1D ATI spectra calculated from the supersolid-core potential and the soft-core potential, respectively. By analyzing the factors that affect the yield of the ATI spectrum, we propose a modified-potential with which we can reproduce the overall 3D ATI spectrum. In addition, the influence of the incident laser intensities and frequencies on the ATI spectra calculated from the proposed modified potential is studied.

Observation of molecular rotation during femtosecond laser filamentation in air by a pump-probe longitudinal diffraction method

Utilizing a pump-probe longitudinal diffraction method, we have experimentally observed dynamics of air molecular rotation during femtosecond filamentation. By adjusting only the pump pulse energy, a recurrence of phase shift variation of the probe throughout a range of delay of at least 100 ps was achieved.

Calculation of the Duration of the Atomic Tunneling Ionization in a Strong Electrostatic Field by Using Bohmian Trajectories Approach

The duration of a bound electron tunneling through the barrier formed by atomic potential and electrostatic field is calculated by the Bohmian trajectories scheme. The time of the tunneling ionization decreases with the increase of the amplitude of the electrostatic field. By using the information about the position, velocity and force of the Bohmian trajectories, the dynamical process of tunneling through the barrier is investigated.

Influence of multi-photon excitation on the atomic above-threshold ionization*

Using the time-dependent pseudo-spectral scheme, we solve the time-dependent Schrodinger equation of a hydrogen-like atom in a strong laser field in momentum space. The intensity-resolved photoelectron energy spectrum in above-threshold ionization is obtained and further analyzed. We find that with the increase of the laser intensity, the above-threshold ionization emission spectrum exhibits periodic resonance structure. By analyzing the population of atomic bound states, we find that it is the multi-photon excitation of bound state that leads to the occurrence of this phenomenon, which is in fairly good agreement with the experimental results.