Liu Xue-Shen

Selection of quantum path in high-order harmonics and isolated sub-100 attosecond generation in few-cycle spatially inhomogeneous laser fields*

We theoretically study the selection of the quantum path in high-order harmonics (HHG) and isolated attosecond pulse generation from a one-dimensional (1D) model of a molecule in few-cycle inhomogeneous laser fields. We show that the inhomogeneity of the laser fields play an important role in the HHG process. The cutoff of the harmonics can be extended remarkably, and the harmonic spectrum becomes smooth and has fewer modulations. We investigate the time-frequency profile of the time-dependent dipole, which shows that the short quantum path is enhanced and the long quantum path disappears in spatially inhomogeneous fields. The semi-classical three-step model is also applied to illustrate the physical mechanism of HHG. The influence of driving field carrier-envelop phase (CEP) on HHG is also discussed. By superposing a series of properly selected harmonics, an isolated attosecond pulse (IAP) with duration 53 as can be obtained by a 15-fs, 1600-nm laser pulse with the parameter e = 0.0013 (e is the parameter that determines the order of inhomogeneity of the laser field).

Dynamic properties of the cubic nonlinear Schrödinger equation by symplectic method

The dynamic properties of a cubic nonlinear Schrodinger equation are investigated numerically by using the symplectic method with different space approximations. The behaviours of the cubic nonlinear Schrodinger equation are discussed with different cubic nonlinear parameters in the harmonically modulated initial condition. We show that the conserved quantities will be preserved for long-time computation but the system will exhibit different dynamic behaviours in space difference approximation for the strong cubic nonlinearity.

Symplectic Schemes and the Shooting Method for Eigenvalues of the Schr dinger Equation

The one-dimensional time-independent Schrodinger equation is transformed into a Hamiltonian canonical equation by means of the Legendre transformation, then the symplectic schemes and a new shooting method extended to the eigenvalues of the Schrodinger equation. The method is applied to the calculations of one-dimensional harmonic oscillator, an anharmonic oscillator and the hydrogen atom. The numerical results are in good agreement with the exact ones.

Quantum path control and isolated attosecond pulse generation in the combination of near-infrared and terahertz pulses

We present an efficient and realizable scheme for the generation of an ultrashort single attosecond (as) pulse from H atom with a 800-nm fundamental laser field combined with a terahertz (THz) field. The high-order harmonic generation (HHG) can be obtained by solving the time-dependent Schrodinger equation accurately and efficiently with time-dependent generalized pseudo-spectral (TDGPS) method. The result shows that the plateau of high-order harmonics is extended and the broadband spectra can be produced by the combined laser pulse, which can be explained by the corresponding ionization probability. The time–frequency analysis and semi-classical three-step model are also presented to further investigate this mechanism. Besides, by the superposition of the harmonics near the cutoff region, an isolated 133-as pulse can be obtained.

Soliton breathers in spin-1 Bose—Einstein condensates

We consider a spin-1 Bose—Einstein condensate trapped in a harmonic potential with different nonlinearity coefficients. We illustrate the dynamics of soliton breathers in two-component and three-component states by numerically solving the one-dimensional time-dependent coupled Gross—Pitaecskii equations (GPEs). We present that two condensates with repulsive interspecies interactions make elastic collision and novel soliton breathers are created in two-component state. We also demonstrate novel soliton breathers in three-component state with attractive coupling constants. Furthermore, possible reasons for creating soliton breathers are discussed.

Relative Phase Dependence of Double Ionization in a Synthesized Laser Pulse

We investigate the double ionization process of a three-dimensional model atom interacting with a synthesized laser pulse and explore the mechanism of non-sequential double ionization varying with the value of relative phase. The result shows that the recollision probability decreases when the value of the relative phase increases. The momentum spectra of electrons in the sequential ionization region are also illustrated.

Double ionization of helium interacting with elliptically polarized laser pulse by classical ensemble simulations

This paper uses the classical ensemble method to study the double ionization of a 2-dimensional (2D) model helium atom interacting with an elliptically polarized laser pulse. The classical ensemble calculation demonstrates that the ratio of double to single ionization decreases with the increasing ellipticity of the driving field. The classical scenario shows that there are hardly any e–e recollisions with the circularly polarized laser pulse. The double ionization probability is studied for linearly and circularly polarized laser pulses. The classical numerical results are consistent with the semiclassical rescattering mechanism and in agreement with the experimental results and the quantum calculations qualitatively.

High-Order Harmonic Generation from a Model of Ar+ Ionized Clusters

We theoretically study high-order harmonic generation (HHG) due to the interaction of a model of Ar+ ionized clusters with intense laser pulses. The plateau of HHG can be extended, which results from the application of a multi-well potential. The classical theory is used to explain the phenomenon.

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.

Behaviour of Cubic Nonlinear Schr?dinger Equation by Using the Symplectic Method

The dynamic properties of cubic nonlinear Schrodinger equation are investigated numerically by using the symplectic method. We show that the trajectories in phase space will exhibit different behaviour (elliptic orbit or homoclinic orbit) with the increase of nonlinear perturbation. We illustrate this phenomenon by mean of linearized stability analysis. The theoretical analysis is consistent with the numerical results.