Internal collision double ionization of molecules driven by co-rotating two-color circularly polarized laser pulses

The double ionization process of molecules driven by co-rotating two-color circularly polarized fields is investigated with a three-dimensional classical ensemble model. Numerical results indicate that a considerable part of the sequential double ionization(DI) events of molecules occur through internal collision double ionization(ICD), and the ICD recollision mechanism is significantly different from that in non-sequential double ionization(NSDI). By analyzing the results of internuclear distances R = 5 a.u. and 2 a.u., these two recollision mechanisms are studied in depth. It is found that the dynamic behaviors of the recollision mechanisms of NSDI and ICD are similar. For NSDI, the motion range of electrons after the ionization is relatively large, and the electrons will return to the core after a period of time. In the ICD process,electrons will rotate around the parent ion before ionization, and the distance of the electron motion is relatively small. After a period of time, the electrons will come back to the core and collide with another electron. Furthermore, the molecular internuclear distance has a significant effect on the electron dynamic behavior of the two ionization mechanisms. This study will help to understand the multi-electron ionization process of complex systems.

Electron correlations in nonsequential double ionization of argon atoms by elliptically polarized laser pulses

Using a classical ensemble model, we investigate the correlation behaviour of electrons originating from nonsequential double ionization （NSDI） of argon atoms by the elliptically polarized laser pulses. Because of the ellipticity, not only the first electron to return but also the later return of tunneled electrons contribute significantly to NSDI. We mainly discuss two kinds of events of NSDI originating from the first and the second return separately. For the NSDI resulting from the recollision of the first return, the correlated electron momentum spectrum along the long axis of the laser polarization plane reveals an obvious V-like shape, located at the first and third quadrant. However, for the NSDI resulting from the recollision of the second return, the momenta of two electrons are distributed in the four quadrants uniformly. By analysing the trajectories of these two kinds, we find that the recollision energy and the laser phase at recollision are different for the first and second returning trajectories, which are responsible for the difference in the correlated behavior of the final electron momentum.

Effect of elliptical polarizations on nonsequential double ionization in two-color elliptically polarized laser fields

Using the classical ensemble model, we investigate the nonsequential double ionization（NSDI） of Ar and Mg in the two-color elliptically polarized laser pulse for different elliptical polarizations. Numerical results show that for Ar atoms the NSDI yield increases as the ellipticity increases, which is different from the case of Mg atoms. Moreover, the correlated behavior in the correlated electron momentum along the x direction and ion momentum distributions of Ar atoms are influenced by the ellipticity. By statistical analysis of different times, we can conclude that the ellipticity may be responsible for the NSDI processes. The correlated momenta distributions along the x direction at the recollision time are demonstrated and the results show that the travelling time and ellipticity can affect the emitted directions of both electrons.

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 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.

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.

Dynamic study of compressed electron layer driven by linearly polarized laser

The dynamics of the compressed electron layer（CEL） are investigated when a linearly polarized（LP） laser pulse irradiates a plasma target. The turbulent motion of the CEL is investigated by a simple model, which is verified by particlein-cell（PIC） simulations. It is found that the compressed layer disperses in a few cycles of the laser duration, because the CEL comes back with a large velocity in the opposite direction of the laser incident. A larger wavelength laser can be used to tailor the proton beam by reducing the turbulence of the CEL in the region of the LP laser acceleration.

Generation of elliptical isolated attosecond pulse from oriented H_(2)^(+)in a linearly polarized laser field

We investigate the ellipticity of the high-order harmonic generation from the oriented H_(2)^(+)exposed to a linearly polarized laser field by numerically solving the two-dimensional time-dependent Schrodinger equation(2 D TDSE).Numerical simulations show that the harmonic ellipticity is remarkably sensitive to the alignment angle.The harmonic spectrum is highly elliptically polarized at a specific alignment angleθ=30°,which is insensitive to the variation of the laser parameters.The position of the harmonic intensity minima indicates the high ellipticity,which can be attributed to the two-center interference effect.The high ellipticity can be explained by the phase difference of the harmonics.This result facilitates the synthesis of a highly elliptical isolated attosecond pulse with duration down to 65 as,which can be served as a powerful tool to explore the ultrafast dynamics of molecules and study chiral light-matter interaction.

Semiclassical investigation of Coulomb focusing effects in atomic above-threshold ionization with elliptically polarized laser fields

We investigate atomic above-threshold ionization in elliptically polarized strong laser fields with a semiclassical approach.With increasing laser intensity,the Coulomb focusing（CF） effects are found to become stronger in both parallel and perpendicular directions with respect to the polarization plane.The dependence of CF effects on tunnel exit,initial transverse momentum distribution and laser electric field is analyzed.It was revealed that the effects of tunnel exit are most prominent with variation of the laser intensity,and the other two factors both play non-negligible roles.Our results provide a deeper insight to the recent experiments of Coulomb asymmetry[Shafir D,et al.,2013 Phys.Rev.Lett.111 023005 and Li M,et al,2013 Phys.Rev.Lett.111 023006].

Decoding the electron dynamics in high-order harmonic generation from asymmetric molecular ions in elliptically polarized laser fields

The high-order harmonic generation from an asymmetric molecular ion is theoretically investigated based on the Born-Oppenheimer model with two-dimensional electron dynamics.It is shown that the harmonic intensity changes periodically in elliptically polarized laser fields.The periodical character is ellipticity-dependent.By establishing the physical image,the periodicity of the harmonic intensity can be ascribed to the contributions of the ground state and the excited state.Furthermore,the electron dynamics from different electronic states can be selected via combining the elliptically polarized laser field with a static electric field.The harmonics dominated either by ground state or excited state are emitted once in an optical cycle in the combined laser field.

Semi-integer harmonic generation from an argon atom by bichromatic counter-rotating circularly polarized laser field

We theoretically investigated the properties of the high-order harmonic generation from an argon atom by bichromatic counter-rotating circularly polarized(BCCP)laser field.The harmonic emission processes have been illustrated by numerically solving the two-dimensional time-dependent Schr¨odinger equation of an atom in intense laser fields.It is found that with the decrease of the right-circularly polarized laser wavelength,the harmonic spectra are gradually splitting and the harmonic orders move towards the higher frequency.Meanwhile,the integer and semi-integer harmonic emission will be generated when the frequency ratios of right-and left-circularly polarized lasers are semi-integer.The emission mechanism of the semi-integer-order harmonics has been investigated by using the rules of photon absorption and emission.

Role of potential on high-order harmonic generation from atoms irradiated by bichromatic counter-rotating circularly polarized laser fields

We investigate high-order harmonic generation from atoms irradiated by bichromatic counter-rotating circularly polarized laser pulses by numerically solving the time-dependent Schrödinger equation.It is found that the minimum energy position of the harmonic spectrum and the non-integer order optical radiation are greatly discrepant for different atomic potentials.By analyzing the quantum trajectory of the harmonic emission,discrepancies among the harmonic spectra from different potentials can be attributed to the action of the potential on the ionized electrons.In addition,based on the influence of the driving light intensity on the overall intensity and ellipticity of higher order harmonics,the physical conditions for generating a high-intensity circularly polarized harmonic can be obtained.

Correlation between the magic wavelengths and the polarization direction of the linearly polarized laser in the Ca^+ optical clock

The magic wavelengths for different Zeeman components are measured based on the ^40Ca^＋ optical clock. The dynamic dipole polarizability of a non-zero angular moment level has correlation with the polarization direction of the linearly polarized laser beam, and we show that the four hyperfine structure levels of 4S1/2,m=±1/2 and 3d5/2,m=±1/2 for ^40Ca^＋ have the same dynamic dipole polarizability at the magic wavelength and a certain polarization direction. In addition, the existence of a specific direction of polarization may provide a new idea for improving the precision of magic wavelength measurement in experiment.

Relative phase effect of nonsequential double ionization in Ar by two-color elliptically polarized laser field

We investigated the nonsequential double ionization(NSDI)in Ar by two-color elliptically polarized laser field with a three-dimensional(3D)classical ensemble method.We study the relative phase effect of NSDI and distinguish two particular recollision channels in NSDI,which are recollision–impact ionization(RII)and recollision-induced excitation with subsequent ionization(RESI),according to the delay-time between the recollision and the final double ionization.The numerical results indicate that the ion momentum distribution is changed and the triangle structure is more obvious with the decrease of the relative phase.We also demonstrate that the RESI process always dominates in the whole double ionization process and the ratio of RESI and RII channels can be influenced by the relative phase.

Exploration of High-Harmonic Generation from the CS2 Molecule by the Lewenstein Method in Two-Color Circularly Polarized Laser Field

The high harmonic generation (HHG) from the CS2 molecule in intense laser fields is investigated using the extended Lewenstein method. The initial state is the highest-occupied molecular orbital of the CS2 molecule, which can be well described by Gaussian wave packet using GAMESS-UK package. Compared with the case of the elliptical laser, the HHG can be extended in two-color circularly polarized laser field. The time-frequency analysis and classical electron trajectory as well as the ionization yield curve are also presented to further explain the underlying mechanism. After adding a static electric field on the z-direction, the single quantum path control is realized and the supercontinuum spectra are obtained. Moreover, an isolated 110 as pulse can be obtained by superposing the harmonics from 130th to 180th order.

High-Order-Harmonic Generation from a Relativistic Circularly Polarized Laser Interacting with Over-Dense Plasma Grating

The simple surface current model is extended to study the generation of high-order harmonics for a relativistic circularly polarized laser pulse interacting with a plasma grating surface. Both exact relativistic electron dynamics and optical interference of surface periodic structure are considered. It is found that high order harmonics in the specular direction are obviously suppressed whereas the harmonics of the grating periodicity are strongly enhanced and folded into small solid angles with respect to the surface direction. The conversion efficiency of certain harmonics is five orders of magnitude higher than that of the planar target cases. It provides an effective approach to generate a coherent radiation within the so-called ＇water window＇ while maintaining high conversion efficiency and narrow angle spread.

Atomic high-order harmonic generation from a circularly polarized laser pulse

We demonstrate theoretically that the high-order harmonic of an atom can be generated by a circularly polarized laser pulse.The harmonic spectrum shows a clear cutoff with an energy Ip ＋ 2Up.In particular,the high-order harmonic generation comes from the multiple recombination of the ionized electron with non-zero initial velocity.These results are verified by the classical model theory and the time-frequency analysis of a harmonic spectrum.

Elliptically polarized high-order harmonic generation of Ar atom in an intense laser field

High-order harmonic generation(HHG) of Ar atom in an elliptically polarized intense laser field is experimentally investigated in this work.Interestingly,the anomalous ellipticity dependence on the laser ellipticity(ε) in the lower-order harmonics is observed,specifically in the 13rd-order,which displays a maximal harmonic intensity at ε ≈ 0.1,rather than at ε = 0 as expected.This contradicts the general trend of harmonic yield,which typically decreases with the increase of laser ellipticity.In this study,we attribute this phenomenon to the disruption of the symmetry of the wave function by the Coulomb effect,leading to the generation of a harmonic with high ellipticity.This finding provides valuable insights into the behavior of elliptically polarized harmonics and opens up a potential way for exploring new applications in ultrafast spectroscopy and light–matter interactions.

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.

Dynamical analysis of the effect of elliptically polarized laser pulses on molecular alignment and orientation

In this Letter, we study the molecular alignment and orientation driven by two elliptically polarized laser pulses.It is shown that the field-free molecular alignment can be achieved in a three-dimensional（3D） case, while the field-free molecular orientation is only along the x and y directions, and that the field-free alignment and orientation along different axes are related to the populations of the rotational states. It is demonstrated that changing the elliptic parameter is efficient for controlling both in-pulse and post-pulse molecular alignment and orientation. The delay time also has an influence on the field-free molecular alignment and orientation.

Ultrafast photoionization of ions and molecules by orthogonally polarized intense laser pulses: Effects of the time delay

We present the photoelectron momentum distributions(PMDs) and the photoelectron angular distributions(PADs) of He+ ions, aligned H2+ molecules and N2 molecules by intense orthogonally polarized laser pulses. Simulations are performed by numerically solving the corresponding two-dimensional time-dependent Schr?dinger equations(TDSEs) within the single-electron approximation frame. Photoelectron momentum distributions and photoelectron angular distributions present different patterns with the time delays Td, illustrating the dependences of the PMDs and PADs on the time delays by orthogonally polarized laser pulses. The evolution of the electron wavepackets can be employed to describe the intensity of the PADs from the TDSE simulations for N2 molecules.