Circularly Polarized Laser Field Research Articles

Recollision induced excitation-ionization with counter-rotating two-color circularly polarized laser field

Nonsequential double ionization of Ar by a counter-rotating two-color circularly polarized laser field is theoretically investigated. At the combined intensity in the “knee” structure range, the double ionization occurs mainly through recollision induced excitation followed by subsequent ionization of Ar+∗. By tracing the history of the recollision trajectories, we explain how the relative intensity ratio of the two colors controls the correlated electron dynamics and optimizes the ionization yields. The major channels contributing to enhancing the double ionization are through the elliptical trajectories with smaller travel time but not through the triangle shape or the other long cycle trajectories. Furthermore, the correlated electron dynamics could be limited to the attosecond time scale by adjusting the relative intensity ratio. Finally, the double ionization from doubly excited complex at low laser intensity is qualitatively discussed.

Brilliant petawatt gamma-ray pulse generation in quantum electrodynamic laser-plasma interaction

We show a new resonance acceleration scheme for generating ultradense relativistic electron bunches in helical motions and hence emitting brilliant vortical γ-ray pulses in the quantum electrodynamic (QED) regime of circularly-polarized (CP) laser-plasma interactions. Here the combined effects of the radiation reaction recoil force and the self-generated magnetic fields result in not only trapping of a great amount of electrons in laser-produced plasma channel, but also significant broadening of the resonance bandwidth between laser frequency and that of electron betatron oscillation in the channel, which eventually leads to formation of the ultradense electron bunch under resonant helical motion in CP laser fields. Three-dimensional PIC simulations show that a brilliant γ-ray pulse with unprecedented power of 6.7 PW and peak brightness of 1025 photons/s/mm2 /mrad2 /0.1% BW (at 15 MeV) is emitted at laser intensity of 1.9 × 1023 W/cm2.

Knee structure in double ionization of noble atoms in circularly polarized laser fields

Nonsequential double ionization is characterized by a knee structure in the plot of double-ionization probability versus laser intensity. In circularly polarized (CP) laser fields, this structure has only been observed for Mg atoms. By choosing laser fields according to a scaling law, we exhibit the knee structure in CP laser fields for Ar and He atoms. The collision of the ionized electron with the core enhances the ionization of the second electron and forms the knee structure. The electron recollision is universal in CP laser fields, but the ionization probability in the knee region decreases as the wavelength of the driven field increases. For experimental observations, it is beneficial to use target atoms with small ionization potentials and laser fields with short wavelengths.

Exploration of High-Harmonic Generation from the CS2 Molecule by the Lewenstein Method in Two-Color Circularly Polarized Laser Field**Supported by the National Natural Science Foundation of China under Grant Nos 11574117, 11271158, 61575077 and 11575071.

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.

Spin-polarized photoelectrons produced by strong-field ionization of randomly aligned nitric oxide

We investigate the effect of the molecular alignment of nitric oxide (NO) on nonadiabatic tunnel ionization of degenerate valence orbitals in strong circularly polarized laser fields and on spin polarization of photoelectrons. Our numerical study shows that not only for the alignment parallel to the laser propagation axis [Liu, K.; Barth, I. Phys. Rev. A 2016, 94, 043402] but also for arbitrary alignment angles except for perpendicular alignment, the counter-rotating molecular orbital with respect to the circular polarization of the laser field projected on the plane perpendicular to the molecular axis is ionized more easily. Due to the nonadiabatic effect and the nodal structure of the valence orbitals of NO, the ionization maxima for the and orbitals in right circularly polarized laser fields are obtained for the molecular orientations at the polar angles of around and , respectively. Considering the spin-orbital entanglement in the doubly degenerate electronic ground state of NO, the spin-polarization of photoelectrons is high for parallel alignment, decreases upon increasing the polar angle and vanishes for perpendicular alignment. Averaging over all alignment angles, non-zero spin-polarization of photoelectrons for randomly aligned NO is preserved.

Nonadiabatic tunnel ionization of current-carrying orbitals of prealigned linear molecules in strong circularly polarized laser fields

Nonadiabatic tunnel ionization of current-carrying orbitals of prealigned linear molecules in strong circularly polarized laser fields

Enhancement of terahertz radiation by using circularly polarized two-color laser fields

Terahertz radiation from tunneling ionization of gaseous atoms and molecules in the two-color laser fields with various polarizations has been investigated. We experimentally demonstrate that the efficiency of terahertz emission in the circularly polarized laser fields with the same helicity is 5 times higher than that with linearly polarized two-color femtosecond pulses in high laser intensity. By solving time-dependent Schrödinger equation, this enhancement is well explained based on the analysis of electron tunneling ionization and subsequent dynamics.

Possibility of introducing spin into attoscience with spin-polarized electrons produced by a bichromatic circularly polarized laser field

We show that the electrons, produced in strong-bicircular-field-induced above-threshold ionization of inert-gas atoms, have a large spin asymmetry if the ions exhibit fine-structure splitting. For a bicircular field, which consists of two coplanar counterrotating circularly polarized fields of frequencies $\ensuremath{\omega}$ and $2\ensuremath{\omega}$, the spin-asymmetry parameter changes rapidly with the electron energy. Since the electron--parent-ion rescattering in a counterrotating bicircular field is characterized on the attosecond time scale, this spin asymmetry may introduce the spin degree of freedom into attoscience. We show that the high-energy backward and low-energy forward scattered electrons, which are produced on the scale of a fraction of the laser cycle, exhibit spin asymmetry.

Laser-induced polarization of a quantum spin system in the steady-state regime

The effect of the circularly polarized laser field on quantum spin systems in the steady-state regime, in which relaxation plays the central role, has been studied. The dynamical mean-field-like theory predicts several general results for the behavior of the time-average magnetization caused by the laser field. The induced magnetization oscillates with the frequency of the laser field (while Rabi-like oscillations, which modulate the latter in the dynamical regime, are damped by the relaxation in the steady-state regime). At high frequencies, that magnetization is determined by the value to which the relaxation process is directed. At low frequencies the slope of that magnetization as a function of the frequency is determined by the strength of the laser field. The anisotropy determines the resonance behavior of the time-averaged magnetization in both the ferromagnetic and antiferromagnetic cases with nonzero magnetic anisotropy. Nonlinear effects (in the magnitude of the laser field) have been considered. The effect of the laser field on quantum spin systems is maximal in resonance, where the time-averaged magnetization, caused by the laser field, is changed essentially. Out of resonance the changes in the magnetization are relatively small. The resonance effect is caused by the nonzero magnetic anisotropy. The resonance frequency is small (proportional to the anisotropy value) for spin systems with ferromagnetic interactions and enhanced by exchange interactions in the spin systems with antiferromagnetic couplings. We show that it is worthwhile to study the laser-field-induced magnetization of quantum spin systems caused by the high-frequency laser field in the steady-state regime in ``easy-axis'' antiferromagnetic spin systems (e.g., in Ising-like antiferromagnetic spin-chain materials). The effects of the Dzyaloshinskii-Moriya interaction and the spin-frustration couplings (in the case of the zigzag spin chain) have been analyzed.

High-order harmonic generation of N2 molecule in two-color circularly polarized laser fields**Project supported by the National Natural Science Foundation of China (Grant Nos. 61575077, 11271158, and 11574117).

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.

Generating Isolated Elliptically Polarized Attosecond Pulses Using Bichromatic Counterrotating Circularly Polarized Laser Fields.

We theoretically demonstrate the possibility to generate both trains and isolated attosecond pulses with high ellipticity in a practical experimental setup. The scheme uses circularly polarized, counterrotating two-color driving pulses carried at the fundamental and its second harmonic. Using a model Ne atom, we numerically show that highly elliptic attosecond pulses are generated already at the single-atom level. Isolated pulses are produced by using few-cycle drivers with controlled time delay between them.

Momentum partition between constituents of exotic atoms during laser-induced tunneling ionization

The tunneling ionization of exotic atoms such as muonic hydrogen, muonium and positronium in a strong laser field of circular polarization is investigated taking into account the impact of the motion of the center of mass on the the tunneling ionization dynamics. The momentum partition between the ionization products is deduced. The effect of the center of mass motion for the momentum distribution of the ionization components is determined. The effect scales with the ratio of the electron (muon) to the atomic core masses and is nonnegligible for exotic atoms, while being insignificant for common atoms. It is shown that the electron (muon) momentum shift during the under-the-barrier motion due to the magnetically induced Lorentz force has a significant impact on the momentum distribution of the atomic core and depends on the ratio of the electron to the atomic core masses.

Strong-field ionization with two-color circularly polarized laser fields

Strong-field ionization provides fundamental insight into light-matter interactions, encoding the structure of atoms and molecules on the sub\aa{}ngstr\om and subfemtosecond scales. In this Rapid Communication, we explore an important regime: strong-field ionization by two-color circularly polarized laser fields. In contrast to past work using linearly polarized drivers, we probe electron trajectories that are driven in a two-dimensional plane, thus separating the tunneling angle from the rescattering angle. This allows us to make several findings. First, we observe a single-lobed electron distribution for co-rotating fields, and a three-lobed distribution for counter-rotating fields, providing experimental validation of the theoretical model explaining the generation of circularly polarized high harmonic light. Second, we discover that there is significant electron-ion rescattering using counter-rotating fields, but not with co-rotating fields. Finally, we show that the rescattered electrons are well separated from the directly ionized electrons, in striking contrast to similar low-energy structures seen with linearly polarized fields. These findings help overcome the long-standing problem of how to decouple the tunneling and rescattering steps in strong-field ionization, which will enable new dynamic probes of atomic and molecular structure.

Influence of pulse duration on above-threshold ionization in intensive circularly polarized laser field

The above-threshold ionization (ATI) process of argon in intensive circularly polarized laser field has been investigated by solving the time-dependent Schrödinger equation in length gauge with emphasis on pulse duration effect. It is shown that the influence of pulse duration is obvious on the transition between multiple-photon and tunneling ionization regimes when the Keldysh parameter is no more than 3.25. That is, photoelectron ionization by shorter pulse occurs in the tunneling regime mainly, and yet the multiple-photon absorption appears more easily with longer pulse. However, such influence disappears when the Keldysh parameter is sufficiently large and the ATI always appears in the multiple-photon ionization regime.

Ionization and excitation of the excited hydrogen atom in strong circularly polarized laser fields

In the recent work of Herath et al. [T. Herath, L. Yan, S. K. Lee, and W. Li, Phys. Rev. Lett. 109, 043004 (2012)] the first experimental observation of a dependence of strong-field ionization rate on the sign of the magnetic quantum number $m$ [of the initial bound state $(n,l,m)]$ was reported. The experiment with nearly circularly polarized light could not distinguish which sign of $m$ favors faster ionization. We perform ab initio calculations for the hydrogen atom initially in one of the four bound substates with the principal quantum number $n=2$, and irradiated by a short circularly polarized laser pulse of $800\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$. In the intensity range of ${10}^{12}\ensuremath{-}{10}^{13}\phantom{\rule{0.28em}{0ex}}\mathrm{W}/\mathrm{c}{\mathrm{m}}^{2}$ excited bound states play a very important role, but also up to some ${10}^{15}\phantom{\rule{0.28em}{0ex}}\mathrm{W}/\mathrm{c}{\mathrm{m}}^{2}$ they cannot be neglected in a full description of the laser-atom interaction. We explore the region that with increasing intensity switches from multiphoton to over-the-barrier ionization and we find, unlike in tunneling-type theories, that the ratio of ionization rates for electrons initially counter-rotating and corotating (with respect to the laser field) may be higher or lower than 1.

Hole dynamics and spin currents after ionization in strong circularly polarized laser fields

We apply the time-dependent analytical R-matrix theory to develop a movie of hole motion in a Kr atom upon ionization by strong circularly polarized field. We find rich hole dynamics, ranging from rotation to swinging motion. The motion of the hole depends on the final energy and the spin of the photoelectron and can be controlled by the laser frequency and intensity. Crucially, hole rotation is a purely non-adiabatic effect, completely missing in the framework of quasistatic (adiabatic) tunneling theories. We explore the possibility to use hole rotation as a clock for measuring ionization time. Analyzing the relationship between the relative phases in different ionization channels we show that in the case of short-range electron-core interaction the hole is always initially aligned along the instantaneous direction of the laser field, signifying zero delays in ionization. Finally, we show that strong-field ionization in circular fields creates spin currents (i.e. different flow of spin-up and spin-down density in space) in the ions. This phenomenon is intimately related to the production of spin-polarized electrons in strong laser fields Barth and Smirnova (2013 Phys. Rev. A 88 013401). We demonstrate that rich spin dynamics of electrons and holes produced during strong field ionization can occur in typical experimental conditions and does not require relativistic intensities or strong magnetic fields.

Numerical verification of the theory of nonadiabatic tunnel ionization in strong circularly polarized laser fields

We verify the theory of nonadiabatic ionization of degenerate valence orbitals in strong circularly polarized laser fields by numerically solving the two-dimensional time-dependent Schrödinger equation for an effective one-electron potential of neon. The numerically calculated ionization ratios of the and orbitals agree well with the theoretical results (i.e., the counter-rotating electron tunnels more easily). However, for strong laser pulses and low laser frequencies, the adiabatic laser-dressed orbitals play an important role. In a Floquet treatment of a three-level model, we find that in this regime the ionization ratio of initial and orbitals depends strongly on the orbital energy order of valence s and orbitals. We also show that the emission angles of valence and electrons are different and should be observable in attoclock experiments.

Analytical solutions for diatomic Rydberg quasimolecules in a laser field

In our previous works we studied analytically helical Rydberg states and circular Rydberg states of two-Coulomb-center systems consisting of two nuclei of charges Z and Z′, separated by a distance R, and one electron. We obtained energy terms of these Rydberg quasimolecules for a field-free case, as well as under a static electric field or under a static magnetic field. In the present paper we study such systems under a laser field. For the situation where the laser field is linearly-polarized along the internuclear axis, we found an analytical solution for the stable helical motion of the electron valid for wide ranges of the laser field strength and frequency. We also found resonances, corresponding to a laser-induced unstable motion of the electron, that result in the destruction of the helical states. For the situation where such Rydberg quasimolecules are under a circularly-polarized field, polarization plane being perpendicular to the internuclear axis, we found an analytical solution for circular Rydberg states valid for wide ranges of the laser field strength and frequency. We showed that both under the linearly-polarized laser field and under the circularly-polarized laser field, in the electron radiation spectrum in the addition to the primary spectral component at (or near) the unperturbed revolution frequency of the electron, there appear satellites. We found that for the case of the linearly-polarized laser field, the intensities of the satellites are proportional to the squares of the Bessel functions J 2 (s), (q = 1, 2, 3, ...), where s is proportional to the laser field strength. As for the case of the circularly-polarized field, we demonstrated that there is a red shift of the primary spectral component — the shift linearly proportional to the laser field strength. Under a laser field of a known strength, in the case of the linear polarization the observation of the satellites would be the confirmation of the helical electronic motion in the Rydberg quasimolecule, while in the case of the circular polarization the observation of the red shift of the primary spectral component would be the confirmation of the specific type of the phase modulation of the electronic motion. Conversely, if the laser field strength is unknown, both the relative intensities of the satellites and the red shift of the primary spectral component could be used for measuring the laser field strength.

Positronium Decay in a Circular Polarized Laser Field

We calculate the lifetime of both the o-Ps and the p-Ps positronium annihilation decay Ps → γγ in the strong circular polarized laser field. We take a strategy of the factorization to separate the effects caused by the Coulomb interaction and the strong laser field interaction. It is factorized in the time direction but not in the space direction. Our results show that in the laser with long wavelength and high intensity, the lifetimes of those Ps states are dramatically increased. For CO2 laser with 10 μm wavelength and 1013 W/cm2 intensity, lifetime of the spin-single positronium is increased by 108 times. Our result is consistent with those obtained by solving the Schödinger equation. This effect may be useful for the high harmonic generation (HHG) effects provided with the Ps [Phys. Rev. Lett. 70 (1993) 774; Phys. Rev. Lett. 93 (2004) 013601].

Relativistic elastic scattering of hydrogen atom by positron impact in a circularly polarized laser field

In the framework of the first Born approximation and using the Dirac–Volkov formalism, we investigate the relativistic model describing the laser assisted elastic differential cross sections (DCSs) for positron scattering by the hydrogen atom. Both the DCSs for electron and positron are notably modified by the circularly polarized laser field, particularly in the regime of high energies and medium intensities. Comparing the two numerical DCSs, we recover the well-known result that the elastic scattering differential cross sections with and without laser field for a hydrogen atom by electron and positron impact are the same (Greiner and Reinhardt. Quantum Electrodynamics, 2nd ed., Springer Verlag, New York, Berlin, Heidelberg. 1994). The numerical results reveal that the differential cross section for positron – hydrogen atom scattering is significantly reduced with the increase of the electric field strength.