Trajectory analysis of few-cycle strong field ionization in two-color circularly polarized fields

Bichromatic circularly polarized fields provide a useful tool to probe the ionization dynamics.In this work, we compare the photoelectron momentum distribution in few-cycle bichromatic field of different helicities.The spectral features are analyzed with semiclassical trajectories derived from the strong field approximation.In particular, the interference fringes in momentum distribution are investigated by tracking the ionization time and tunneling exits of released photoelectrons.Different types of trajectories that contribute to the interference fringes are elucidated.     

Isolated attosecond pulse generation with few-cycle two-color counter-rotating circularly polarized laser pulses

Most of the schemes for generating isolated attosecond pulses(IAP) are sensitive to the carrier-envelope phase(CEP)of the driving lasers. We propose a scheme for generating IAP using two-color counter-rotating circularly polarized(TCCRCP) laser pulses. The results demonstrate that the dependence of the IAP generation on CEP stability is largely reduced in this scheme. IAP can be generated at most of CEPs. Therefore, the experiment requirements become lower.     

Helicity-dependent time delays in multiphoton ionization by two-color circularly polarized laser fields

By numerically solving the three-dimensional time-dependent Schrödinger equation, we have invest-tigated multiphoton ionization of hydrogen atom in the two-color circularly polarized (TCCP) laser fields consisting of a strong 400 nm and a much weaker 800 nm pulses. Due to the presence of perturb-bative 800 nm laser pulse, sideband peaks emerge between the above-threshold ionization rings in the photoelectron momentum distributions. Our numerical results show that the sideband peaks exhibit one-lobe structure in the co-rotating TCCP laser fields, while it displays the three-lobe structure in the counter-rotating TCCP laser fields. Moreover, the photoelectron yield of sidebands in the co-rotating TCCP fields is much higher than those of the counter-rotating TCCP fields. These phenomena could be well explained from the perspective of the photon-absorption channels via the selection rules. In-terestingly, an obvious phase shift between the sidebands of different orders from the co-rotating and counter-rotating TCCP fields is observed. This shift indicates the helicity-dependent time delay in the one-photon continuum-continuum transition process.     

Evolution of photoelectron angular distributions in a train of identical,circularly polarized few-cycle laser pulses

Photoelectron angular distribution (PAD) of atoms irradiated by a train of identical, circularly polarized few-cycle laser pulses is studied in the frame of a nonperturbative scattering theory. Our study shows that the PADs vary with the kinetic energy of photoelectron, the carrier-envelope phase, and the pulse duration. We find that along with increasing of the kinetic energy of photoelectron or with decreasing of the pulse duration or the both, the original one maximum of PAD splits into two maxima; the newly produced two maxima evolve to the opposite pole of the symmetric axis, and finally incorporate as a new maximum located in the symmetric axis.Received: 26 February 2004, Published online: 10 August 2004PACS: 32.80.Rm Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states) - 42.50.Hz Strong-field excitation of optical transitions in quantum systems; multiphoton processes; dynamic Stark shift - 42.65.Re Ultrafast processes; optical pulse generation and pulse compression     

双色反向旋转圆偏振激光场下氩原子高次谐波的产生

通过二阶分裂算符法数值求解了二维含时薛定谔方程,研究了双色反向旋转圆偏振激光场作用下氩原子的高次谐波的产生.研究表明,双色反向旋转圆偏振激光场的幅值比对高次谐波发射的效率有较大影响,当幅值比为1.4时高次谐波的发射效率最高,并应用时频分析方法、经典三步模型以及利萨茹图形等解释了高次谐波发射的物理机制.研究了不同幅值比情况下谐波场的旋转偏振特性,发现对不同频段的谐波谱的电场进行叠加,谐波场的偏振特性随幅值比的不同而发生变化,但谐波场的旋向性与入射激光脉冲的旋向性相同,不受幅值比变化的影响.     

Spatial, temporal and spectral emission characteristics from electron oscillation driven by an intense circularly polarized few-cycle laser pulse

The spatial, temporal and spectral emission characteristics of radiation generated from electron oscillations driven by an intense circularly polarized few-cycle laser pulse have been investigated theoretically and numerically using a single electron model. For a femtosecond driving laser pulse with duration of one optical cycle, the maximal radiation emitted by the electron comprises only one electromagnetic pulse having durations much shorter than the optical cycle and belonging to the attosecond range. It is discovered that the influence of the initial phase on the process of full spatial characteristics of the radiation is apparent for intense few-cycle laser pulse. The characteristics can be used to measure the initial phase of intense circularly polarized few-cycle laser pulse in experiments.     

Harmonic generation by atoms in circularly polarized laser fields: far-off and near resonance regimes

The generation of harmonics by atoms interacting with two laser fields having coplanar circular polarizations and an integral frequency ratio is addressed through ab initio numerical simulations. A detailed characterization of a few specific harmonics is given. In particular, the two different cases where the total energy absorbed through photons is far off or close to the energy gap between different atomic states are investigated. It is found that the conversion efficiency in the harmonic generation is strongly dependent on the inner atomic structure and in certain specific cases it can be significantly enhanced within a small frequency range. PACS 42.50.Hz; 42.65.Ky     

High-order harmonic generation of N2molecule in two-color circularly polarized laser fields

The generation of high-order harmonics and the attosecond pulse of the N_2 molecule in two-color circularly polarized laser fields are investigated by the strong-field Lewenstein model. We show that the plateau of spectra is dramatically extended and a continuous harmonic spectrum with the bandwidth of 113 e V is obtained. When a static field is added to the x direction, the quantum path control is realized and a supercontinuum spectrum can be obtained, which is beneficial to obtain a shorter attosecond pulse. The underlying physical mechanism is well explained by the time–frequency analysis and the semi-classical three-step model with a finite initial transverse velocity. By superposing several orders of harmonics in the combination of two-color circularly polarized laser fields and a static field, an isolated attosecond pulse with a duration of 30 as can be generated.     

Helicity of harmonic generation and attosecond polarization with bichromatic circularly polarized laser fields

We theoretically investigate the high-order harmonic generation(HHG) of helium atom driven by bichromatic counterrotating circularly polarized laser fields. By changing the intensity ratio of the two driving laser fields, the spectral chirality of the HHG can be controlled. As the intensity ratio increases, the spectral chirality will change from positive-to negativevalue around a large intensity ratio of the two driving fields when the total laser intensity keeps unchanged. However, the sign of the spectral chirality can be changed from positive to negative around a small intensity ratio of the two driving fields when the total laser intensity changes. At this time, we can effectively control the helicity of the harmonic spectrum and the polarization of the resulting attosecond pulses by adjusting the intensity ratio of the two driving laser fields. As the intensity ratio and the total intensity of the driving laser fields increase, the relative intensity of either the left-circularly or right-circularly polarized harmonic can be enhanced. The attosecond pulses can evolve from being elliptical to near linear correspondingly.     

Electron acceleration by tightly focused radially polarized few-cycle laser pulses

Within the framework of plane-wave angular spectrum analysis of the electromagnetic field structure, a solution valid for tightly focused radially polarized few-cycle laser pulses propagating in vacuum is presented. The resulting field distribution is significantly different from that based on the paraxial approximation for pulses with either small or large beam diameters. We compare the electron accelerations obtained with the two solutions and find that the energy gain obtained with our new solution is usually much larger than that with the paraxial approximation solution.     

The above-threshold detachment of negative ions in half-cycle laser field

We study partial detachment rate and photodetachment asymmetry of F⁻ detached by half-cycle linearly polarized laser field using numerical simulation. Similar to photodetachment for negative ions in few-cycle laser fields, we find that partial detachment rates of a couple opposite directions in the above-threshold detachment of F⁻ are not equal, the detachment is asymmetric. Furthermore, the photodetachment asymmetry degree increases with carrier-envelop phase (CEP) as the peak laser intensity becoming stronger or the pulse width becoming shorter. The maximal asymmetry degree is stronger with higher laser intensity. We confirm the effect of the CEP, laser intensity and pulse width on the above-threshold detachment of F⁻ in half-cycle laser fields. It provides a possible way to determine the CEP of half-cycle laser fields by measuring detached photoelectrons.     

Generating monoenergetic proton beam by using circularly polarized laser

The interaction of ultrashort intense circularly polarized laser with ultra thin overdense foil is studied by particle-in-cell simulation and analytic model. It is found that with the balance between pondermotive force and electrostatic force, highly quasi-monoenergetic proton beam can be generated by Phase Stable     

Photoelectron momentum distributions of Ne and Xe dimers in counter-rotating circularly polarized laser fields

The strong-field ionization of dimers is investigated theoretically in counter-rotating circularly polarized laser fields. By numerically solving the two-dimensional (2D) time-dependent Schrödinger equation (TDSE) with the single-electron approximation (SEA) frame, we present the photoelectron momentum distributions (PMDs) and photoelectron angular distribution (PADs) of aligned Ne and Xe dimers. It is found that the PMDs and PADs strongly depend on the time delays by counter-rotating circularly polarized laser pulses. The results can be explained by the ultrafast photoionization model and the evolution of electron wave packets for Ne and Xe dimers. Besides, We make a comparison of PMDs between Ne atom and Ne dimer.     

High-order harmonic generation of the N2 molecule in two-color circularly polarized laser fields

The generation of high-order harmonics and the attosecond pulse of the N₂ molecule in two-color circularly polarized laser fields are investigated by the strong-field Lewenstein model. We show that the plateau of spectra is dramatically extended and a continuous harmonic spectrum with the bandwidth of 113 eV is obtained. When a static field is added to the x direction, the quantum path control is realized and a supercontinuum spectrum can be obtained, which is beneficial to obtain a shorter attosecond pulse. The underlying physical mechanism is well explained by the time-frequency analysis and the semi-classical three-step model with a finite initial transverse velocity. By superposing several orders of harmonics in the combination of two-color circularly polarized laser fields and a static field, an isolated attosecond pulse with a duration of 30 as can be generated.     

Electron detachment from negative hydrogen and lithium ions by an ultrashort laser pulse

A model of electron detachment from negative ions is based on the approximation of instantaneous perturbation. A simple analytical representation for the ground state of the weakly bound electron is chosen to reproduce the radial distribution of electron density at any distance from the center of the ion. The probabilities of electron detachment from negative hydrogen and lithium ions are derived as functions of the transferred momentum of the laser field. The contribution of the inner shells of ions to the electron detachment is estimated. The momentum distribution of electrons in the final state is obtained. The effect of a pulse train on the total probability of detachment is considered for different pulse polarities. The conditions for the partial electron return to the initial state are revealed.     

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 Schr o¨dinger 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.     

Recollision dynamics and phase diagram for nonsequential double ionization with circularly polarized laser fields

A semiclassical quasistatic model is used to investigate the recollision dynamics in circularly polarized laser fields. A velocity window for recollision to occur is found. Only when the return electron's orbits are irregular does significant double ionization take place. The model reproduces the experimental results for magnesium and explains the apparently conflicting experimental results in terms of an analytical formula that demarcates the phase diagram for the nonsequential double ionization in circularly polarized laser fields.