Above-threshold ionization of hydrogen atom in chirped laser fields

Above-threshold ionization （ATI） of a hydrogen atom exposed to chirped laser fields is investigated theoretically by solving the time-dependent Schrodinger equation. By comparing the energy spectra, the two-dimensional momentum spectra, and the angular distributions of photoelectron for the laser pulses with different chirp rates, we show a very clear chirp dependence both in the multiphoton and tunneling ionization processes but no chirp dependence in the single-photon ionization. We find that the chirp dependence in the multiphoton ionization based ATI can be attributed to the excited bound states. In the single-photon and tunneling ionization regimes, the electron can be removed directly from the ground state and thus the excited states may not be very important. It indicates that the chirp dependence in the tunneling ionization based ATI processes is mainly due to the laser pulses with different chirp rates,

Broadband Terahertz Wave Generation from Monolayer Graphene Driven by Few-Cycle Laser Pulse

We theoretically investigate the characteristics of terahertz(THz) radiation from monolayer graphene exposed to normal incident few-cycle laser pulses, by numerically solving the extended semiconductor Bloch equations. Our simulations show that the THz spectra in low frequency regions are highly dependent on the carrier envelope phase(CEP) of driving laser pulses. Using an optimal CEP of few-cycle laser pulses, we can obtain broadband strong THz waves, due to the symmetry breaking of the laser-graphene system. Our results also show that the strength of the THz spectra depend on both the intensity and central wavelength of the laser pulses. The intensity dependence of the THz wave can be described by the excitation rate of graphene, while wavelength dependence can be traced back to the band velocity and the population of graphene. We find that a near single-cycle THz pulse can be obtained from graphene driven by a mid-infrared laser pulse.

An improved method for the investigation of high-order harmonic generation from graphene

High-order harmonic generation (HHG) of bulk crystals in strong laser field is typically investigated with semiconductor Bloch equations (SBEs). However, in the length gauge, it suffers from the divergence for the crystals with a zero band gap, such as graphene, using both Bloch- and Houston-states expansion methods. Here, we present a method of solving the SBEs based on time-dependent Bloch basis, which is equivalent to semiconductor Bloch equations in the velocity gauge. Using this method, we investigate the HHG of a single-layer graphene. It is found that our results for population are in good agreement with the other results. For a initial condition py = 0, we find the electrons just move in single valence band or conduction band, which are in accord with classical results. Our simulations on the HHG dependence of polarization of driving laser pulse confirm that 5th, 7th, and 9th harmonic yields increase to the maximal value when laser ellipticity ε ≈ 0.3. What is more, similar to the case of atoms in the laser field, the total strength of 3rd harmonic decrease monotonically with the increase of ε. In addition, we simulate the dependence of HHG on crystallographic orientation with respect to the polarization direction of linear mid-infrared laser pulse, and the results reveal that for higher harmonics, their radiation along with the change of rotation angle θ reflects exactly the sixfold symmetry of graphene. Our method can be further used to investigate the behaviors of other materials having Dirac points (i.e., surface states of topological insulators) in the strong laser fields.

High-order harmonic generation of Li^+ with combined infrared and extreme ultraviolet fields

We investigate high-order harmonic generation （HHG） ofLi＋ ion driven by an intense infrared （IR） laser field in corn-bination with a weak XUV pulse. To achieve this, we first construct an accurate single-active electron angular-momentumdependent model potential of Li＋ ion, by which the accurate singlet energy levels of Li＋ for the ground state and excited states with higher quantum numbers can be obtained. Then, we solve numerically the three dimensional time-dependent Schr/Sdinger equation of Li＋ ion by means of the generalized pseudospectral method to obtain HHG. Our results show that the strength of assisted XUV is not amplified during the harmonic generation process, but the yield of HHG power spectrum in the whole plateau has a significant enhancement. Furthermore, the optimal phase delay between the IR and XUV pulses allows the production of ultrabroadband supercontinuum spectra. By superposing some harmonics, a strong new single 27-attosecond ultrashort pulse can be obtained.

Dynamic stabilization of Na atom in an intense pulsed laser field

We investigate theoretically the ionization properties of the valence electron for the alkali metal atom Na in an intense pulsed laser field by solving numerically the time-dependent Schrodinger equation with an accurate l-dependent model potential.By calculating the variations of the ionization probabilities with laser peak intensity for wavelengths ranging from 200 nm to 600 nm,our results present a dynamic stabilization trend for the Na atom initially in its ground state（3 s） and the excited states（3 p and 4 s） exposed to an intense pulsed laser field.Especially a clear ＂window＂ of dynamic stabilization at lower laser intensities and longer wavelengths for the initial state 4 s（the second excited state） is found.By analyzing the time-dependent population distributions of the valence electron in the bound states with the different values of principal quantum number n and orbital quantum number l,we can attribute the dynamic stabilization to the periodic population in the low-excited states since the valence electron oscillates rapidly between the lowly excited states and the continuum states.

Retrieval of Angle-Dependent Strong-Field Ionization by Using High Harmonics Generated from Aligned N_(2) Molecules

We propose a method to retrieve the angle-dependent strong-field ionization of highest occupied molecular orbital(HOMO)from high-order harmonic generation(HHG)of aligned molecules.This method is based on the single-molecule quantitative rescattering model with known alignment distribution and photo-recombination cross sections of fixed-in-space molecules.With the macroscopic HHG of aligned N2 molecules,we show that angledependent ionization of HOMO can be successfully retrieved at both low and high degrees of alignment.We then show that the error in the retrieved angular dependence of ionization becomes larger if the uncertainty in the alignment distribution is introduced in the retrieval procedure.We also examine that the retrieved ionization of HOMO is much deviated from the accurate one if the intensity of probe laser becomes higher such that inner HOMO-1 can contribute to HHG.

Determination of structure parameters in strong-field ionization models of atoms

The Ammosov–Delone–Krainov(ADK) and Perelomov–Popov–Terent'ev(PPT) ionization models were widely used in strong-field physics and attosecond science due to their many attractive advantages such as simpler analytical formula, less computational demands, and satisfied accuracy of ionization rate. Based on the density-functional theory, we systematically determine accurate structure parameters of 25 atoms, 24 positive ions and 13 negative ions and tabulate for future applications. The wave function with correct asymptotic behavior is obtained by solving the time-independent Schr?dinger equation with B-spline basis sets and the accurate structure parameters are extracted from this wave function in the asymptotic region. The accuracies of structure parameters are carefully examined by comparing the ionization probabilities(or yields) calculated by PPT and ADK models with those of solving the threedimensional time-dependent Schr?dinger equation and the experimental data.