Isolated sub-30-attosecond pulse generation using a multicycle two-color chirped laser and a static electric field

We present a theoretical investigation of high-order harmonic generation in a chirped two-color laser field, which is synthesized by a 10-fs/800-nm fundamental chirped pulse and a 10-fs/1760-nm subharmonic pulse. It is shown that a supercontinuum can be produced using the multicycle two-color chirped field. However, the supercontinuum reveals a strong modulation structure, which is not good for the generation of an isolated attosecond pulse. By adding a static electric field to the multicycle two-color chirped field, not only the harmonic cutoff is extended remarkably, but also the quantum paths of the high-order harmonic generation （HHG） are modified significantly. As a result, both the extension of the supercontinuum and the selection of a single quantum path are achieved, producing an isolated 23-as pulse with a bandwidth of about 170.6 eV. Furthermore, the influences of the laser intensities on the supercontinuum and isolated attosecond pulse generation are investigated.

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.

Single Ultrashort Attosecond Pulse Generation via Combination of Chirped Fundamental Laser and an Ultraviolet Controlling Pulse

We theoretically study the high-order harmonic generation （HHG） from a hydrogen atom in an intense few-cycle chirped fundamental laser in combination with an ultraviolet （uv） controlling pulse. The high-order harmonic spectrum is calculated by solving the time-dependent Schr6dinger equation using the split-operator method. In our calculation, we present the difference of the high-order harmonic spectrum from one-dimensional （1D） model hydrogen atom and three-dimensional （3D） real hydrogen atom. We found that the plateau of the high-order harmonic generation from the 1D ease and 3D case are all extended effectively to Iv -k 35Up due to the presence of the chirped laser pulse and the HHG supercontinuum spectrum is generated by adding an ultraviolet controlling pulse at a proper time, but the efficiency of the HHC for 3D case is more higher at the near cut-off region than the 1D case. Therefore, the generation of the attosecond pulse by synthesizing the harmonics near cut-off region have some slight differences between 1D and 3D simulations. As a real 3D case study, we show that an isolated 18 as pulse with a bandwidth of 232.5 eV is generated directly by optmizing the combination laser fields.

Generation of Isolated Attosecond Pulse from Asymmetric Molecular Ions by Introducing Half-Cycle-Like Laser Fields

We propose an efficient method for the generation of an isolated attosecond pulse from the asymmetric molecular ions HeH^2＋ by adding a half-cycle-like field （HCLF） to the fundamental driving laser field. The high-order harmonic generation （HHG） is investigated by numerically sowing the time-dependent Schrodinger equation. By performing the time-frequency distributions and the electronic wave packet probability densities, we find that the optimizing combined field is not only useful for extending the HHG cutoff, but also for simplifying the recombination channels through controlling the electron localization. In addition, by adjusting the intensity of the HCLF, a dominant short quantum path is selected to contribute the HHG spectrum. As a result, a 75-as isolated attosecond pulse is obtained by superposing a proper range of the harmonics.

Generation of Linear Isolated Sub-60 Attosecond Pulses by Combining a Circularly Polarized Pulse with an Elliptically Polarized Pulse

The two-color circularly polarized pulses scheme was proposed to generate isolated attosecond pulses in our previous work [Phys. Rev. A 87 （2013） 043406], while the polarization of the attosecond pulse was not investigated. We show a supplementary explanation of this scheme and present another scheme to generate linear isolated attosecond pulses by combining a circularly polarized pulse with an elliptically polarized pulse. High-order harmonic generation and quantum path control are investigated to compare these two schemes. Both schemes can obtain supercontinuum spectra plateau from about 200eV to 550eV, which belong to the water window region. It is found that the latter scheme can clearly eliminate the short quantum path and extend the harmonic plateau. A linear isolated attosecond pulse with a duration of sub-6Oas can be generated by superposing a bandwidth of 70eV.

Generation of an extreme ultraviolet supercontinuum with a multicycle chirped laser and a static electric field

We theoretically present a method for generating an ultrabroad extreme ultraviolet （XUV） supercontinuum by using the combination of a multicycle chirped laser and a static electric field. At a low laser intensity, the spectral cutoff is extended to the 495th order harmonic, and the bandwidth of the supercontinuum spectrum is broadened to 535 eV. At a high laser intensity, the harmonic cutoff is enlarged to the 667th order, and a supercontinuum covering a bandwidth of 1035 eV is generated. In these two cases, the long quantum path is removed, and the short quantum path is selected. Especially for the relatively high laser intensity, an isolated 23-attosecond pulse with a bandwidth of about 170.6 eV is directly obtained. Finally, we also analyze the influences of the chirp parameter and the duration of the chirped pulse as well as the static field strength on the supereontinuum.

Controlling the contributions to high-order harmonic generation from different nuclei of N_2 with an orthogonally polarized two-color laser field

The generation of high-order harmonic and the attosecond pulse of the N2 molecule with an orthogonally polarized two-color laser field are investigated by the strong-field Lewenstein model.We show that the control of contributions to high-order harmonic generation（HHG） from different nuclei is realized by properly selecting the relative phase.When the relative phase is chosen to be φ= 0.4π,the contribution to HHG from one nucleus is much more than that from another.Interference between two nuclei can be suppressed greatly; a supercontinuum spectrum of HHG appears from 40 e V to125 e V.The underlying physical mechanism is well explained by the time–frequency analysis and the semi-classical threestep model with a finite initial transverse velocity.By superposing several orders of harmonics,an isolated attosecond pulse with a duration of 80 as can be generated.

Extension of Harmonic Cutoff and Generation of Isolated Sub-30 as Pulse in a Two-Color Chirped Laser Field

We theoretically investigate high-order harmonic and isolated attosecond pulse generation in a two-color chirped laser field,which is synthesized by a 9 fs/800 nm fundamental chirped pulse and a 9 fs/1600 nm controlling chirped pulse.Our numerical results show that,by using this method,not only is the harmonic cutoff significantly extended to the 948th order harmonic,but also the bandwidth of the supercontinuum spectrum is effectively broadened to about 1342 eV.In addition,due to the introduction of the chirp,the long quantum path is suppressed and only the short one is selected,and then an isolated 28 as pulse with a bandwidth of 155 eV is obtained directly.

Multiphoton quantum dynamics of many-electron atomic and molecular systems in intense laser fields

We present the recent new developments of time-dependent Schrödinger equation and time-dependent density-functional theory for accurate and efficient treatment of the electronic structure and time-dependent quantum dynamics of many-electron atomic and molecular systems in intense laser fields.We extend time-dependent generalized pseudospectral(TDGPS)numerical method developed for time-dependent wave equations in multielectron systems.The TDGPS method allows us to obtain highly accurate time-dependent wave functions with the use of only a modest number of spatial grid point for complex quantum dynamical calculations.The usefulness of these procedures is illustrated by a few case studies of atomic and molecular processes of current interests in intense laser fields,including multiphoton ionization,above-threshold ionization,high-order harmonic generation,attosecond pulse generation,and quantum dynamical processes related to multielectron effects.We conclude this paper with some open questions and perspectives of multiphoton quantum dynamics of many-electron atomic and molecular systems in intense laser fields.

The simplest route to generating a train of attosecond pulses

We report on two novel routes to generating a train of attosecond pulses from a broad discrete spectrum in the near-infrared–visible–ultraviolet range.One extends an integer-temporal-Talbot(ITT)concept to include high-order spectral dispersions and generates a pulse train that completely satisfies the transform-limited condition.The other numerically explores the optimum conditions under which we can obtain an attosecond pulse train that approximately satisfies the transform-limited condition.The second method is more practical than the first.Either of these methods is extremely simple and robust;we need only to place a few thin dispersive materials in the optical path and to adjust their thicknesses without spatially dispersing the frequency components.We numerically demonstrate the generation of a train of attosecond pulses with a transform-limited pulse duration of 728 as and a repetition period of 8.03 fs in gaseous parahydrogen.