Zhao Song-Feng

Study of Microwave Multiphoton Transition of Rydberg Potassium Atom by Using B-Spline

The B-spline expansion technique and time-dependent two-level approach are applied to study the interaction between the microwave field and potassium atoms in a static electric field. We obtain theoretical multiphoton resonance spectra that can be compared with the experimental data. We also obtain the time evolution of the final state in different microwave fields.

Retrieval of Electron Return Time from High-order Harmonics Generated in a Mixture of He and Ne Gases

In terms of single-atom induced dipole moment by Lewenstein model, we present the macroscopic high-order harmonic generation from mixed He and Ne gases with different mixture ratios by solving three-dimensional Maxwells equation of harmonic held. And then we show the validity of mixture formulation by Wagner et al. [Phys. Rev. A 76 (2007) 061403(R)] in macroscopic response level. Finally, using least squares fitting we retrieve the electron return time of short trajectory by formulation in Kanai et al. [Phys. Rev. Lett. 98 (2007) 153904] when the gas jet is put after the laser focus.

Synthesis of Multi-Color Long Laser Pulses for Strong Attosecond Pulse Generation

We theoretically investigate the high-order harmonics and attosecond pulses generated in a single mid-infrared and synthesized two- and three-color fields. We demonstrate that one can obtain a sub-cycle field by combining two or more long pulses at incommensurate frequencies, if the phases and amplitudes of each field are properly optimized. Compared with the one-color field, the harmonic yields in synthesized fields can be enhanced more than two orders with the same total laser power. The technique of waveform synthesizing shows the possibility of generating intense single attosecond pulses by using longer driving laser pulses without the need of gating techniques with experiment.

Investigation of Angular Dependence of Strong-Field Tunneling Ionization for Asymmetric Diatomic Molecule HeH2+

We determine the structure parameters for the asymmetric heteronuclear diatomic molecule HeH2+ at several internuclear distances with the molecular wavefunctions obtained by solving the time-independent Schrodinger equation with B-spline basis. Then the angular dependence of strong-field ionization rates of HeH2+ are investigated with the molecular tunneling ionization theory. We show that the shape of several lowly excited states (i.e. 2pσ, 2pπ, 3dδ) for HeH2+ are reflected in the orientation dependent ionization rates very well, however, the angle-dependent ionization rate fails to follow the angular distribution of the asymptotic electron density for the ground state 1sσ. We also show that the internuclear distance dependent ionization probabilities are in a good agreement with the more accurate result obtained from the numerical solution of the time-dependent Schrodinger equation.

Destructive and Constructive Interference of High-Order Harmonic Generation in Mixture of He and Ne Gases

Using the single-atom induced dipole moment under strong field approximation as a source, we suggest a model to simulate the macroscopic high-order harmonic generation (HHG) from the mixed gases (He and Ne) interacting with intense infrared laser by solving the three-dimensional Maxwells equation of the harmonic field. Regular destructive interference (DI) and constructive interference (CI) are observed in the macroscopic HHG spectra when the gas jet is put at a good phase-matching position. A semiclassical model of short and long electron trajectories is applied to interpret the DI and CI of HHG qualitatively.

Two-dimensional photoelectron momentum distribution of hydrogen in intense laser field

Two-dimensional (2D) electron momentum distributions and energy spectra of a hydrogen in an intense laser field are calculated by solving the time-dependent Schrodinger equation combined with the window-operator technique. Compared with the standard projection technique, the window-operator technique has the advantage that the continuum states of atoms can be avoided in the calculation. We show that the 2D electron momentum distributions and the energy spectra from those two techniques accord quite well with each other if an appropriate energy width is used in the window operator.

Positions and Widths of Anticrossings for Potassium Rydberg Stark States

The B-spline expansion technique is applied to study the anticrossings for potassium Rydberg states in a static electric field. The results of our calculation indicate that the anticrossings are caused mainly by the core interaction or by the fine structure interaction. Our results for the positions and the widths of the anticrossings are in good agreement with experimental data.

The high-harmonic conversion efficienciesin optimized two-color laser pulses

High-order harmonic generation (HHG) is a remarkable frequency up-conversion process in which an ultrafast femtosecond laser pulse at high intensity (>10 14 W/cm 2 ) interacts with a gaseous target to generate coherent extreme ultraviolet radiation. High-order harmonics, due to their high repetition rate operation, tunable wavelength and high coherence degree, have been widely used in many fields. HHG is currently a cornerstone of attosecond pulse, and promises to be a valuable tabletop light source. Their main drawback, however, is the low conversion efficiency, which limits many applications. In this work we present a promising way to increase harmonic yield by using the synthesized two-color field, whose validity is demonstrated both on single-atom and macroscopic levels. The single-atom harmonic spectrum is calculated by the strong field approximation method. The macroscopic propagation of the fundamental and harmonic fields in an ionizing medium is obtained by solving Maxwell equations, which involves the dispersion, absorption, Kerr and plasma effects. The waveform of two-color laser field is determined by using a genetic algorithm to optimize the maximum single-atom Ne harmonic yield, for the lasers with fundamental wavelengths of 800 nm to 3000 nm. Optimizations are performed on condition that the cutoff and total laser power for the single-color field and the optimized wave are the same. The optimizations show that the optimal wavelengths and intensities meet the conditions of λ 1 / λ 2 ≈ 3 , I 1 / I 2 ≈ 5 for two component fields, and carrier-envelop phases are f 1 =0, f 2 =1.6 p . Our analysis show that the spreading of the electron wavepacket is almost the same in the optimized and single-color field. But the ionization level and yields of harmonics emitted from short-trajectory returning electrons are enhanced in the optimized field. Thus, the harmonic conversion efficiency generated by the optimized laser field can be increased by at least one order without increasing total laser power even after considering macroscopic propagation effects. We also compare the wavelength scaling laws of harmonic yield generated by the single- and optimized two-color fields. For the single atom, the present wavelength scaling in the single-color field agrees with the previous results, which scales as l - 5 ~ l - 8 . In the two-color field, the scaling law is slightly worse than that in the homochromous field mainly due to different contributions of short and long trajectories. While for the macroscopic high harmonic generation, its efficiency falls more dramatically with increasing wavelength because of the unfavourable phase-matching and dominant contribution of harmonics from short-trajectory electrons, with a very unfavorable scaling law of l - 8 ~ l - 13 . Although we can not obtain a much better wavelength scaling of harmonic yield in the two-color fields, the conversion efficiency can be enhanced more than one order. Further analysis shows that the stability of optimized waveform is maintained in consideration of relative intensity, wavelength and phase fluctuations of ±10%, ±40 nm and ±0.05 p , respectively. Due to the great advent of the ultrafast lasers technology, such as waveform synthesizing, optical parametric amplification and optical parametric chirped-pulse amplification, this work gives helpful guidance for intense harmonic generation.

THz wave emission from argon in two-color laser field

Terahertz (THz) wave emission from argon atom in a two-color laser pulses is studied numerically by solving the one-dimensional (1D) time-dependent Schr?dinger equation. The THz spectra we obtained include both discontinuous and continuum ones. By using the special basis functions that we previously proposed, our analysis points out that the discontinuous and continuum parts are contributed by bound?bound and continuum?continuum transition of atomic energy levels. Although the atomic wave function is strongly dressed during the interaction with laser fields, our identification for the discontinuous part of the THz wave shows that the transition between highly excited bound states can still be well described by the field-free basis function in the tunneling ionization regime.

Interference Effect of Direct Photodetachment for H− Ions in a Short Laser Pulse

The electron detachment of negative hydrogen ions exposed to a few-cycle linearly polarized laser pulse is investigated in the context of the strong field approximation evaluated by the saddle-point method. The results show that the angular distributions of the laser-induced photoelectron obtained by the saddle-point method are in good agreement with both the experimental results and the numerical-integration results. More importantly, we show that the unusual maximum in the angular distributions of photoelectrons for negative hydrogen ions can be explained by the quantum interference effect based on the saddle-point method.