Intense Circularly Polarized Laser Pulses Research Articles

Effect of Ion Motion on Generation and Lifetime of Magnetic Fields in a Cluster Plasma Irradiated with Intense Circularly Polarized Laser Pulses

Effect of Ion Motion on Generation and Lifetime of Magnetic Fields in a Cluster Plasma Irradiated with Intense Circularly Polarized Laser Pulses

Two-dimensional electron returning trajectory in nonsequential double ionization by strong-field circularly polarized laser pulses

We investigate the features of electron returning trajectories in recollision-induced nonsequential double ionization by intense circularly polarized laser pulses within a classical ensemble model. We find typical two-dimensional returning trajectories of the first ionized electron that can be decomposed into local circular motion driven by the laser field and overall elliptical motion due to the Coulombic attraction of the parent ion. Such trajectories burst within a narrow time window of less than two laser cycles and end periodically with kinetic energies around the instantaneous ponderomotive energy after many laser cycles, which is very different from the well-known simple one-dimensional returning trajectories in the case of linear laser polarization.

Frustrated double ionization of atoms in circularly polarized laser fields

We theoretically study frustrated double ionization (FDI) of atoms subjected to intense circularly polarized laser pulses using a three-dimensional classical model. We find a ‘knee’ structure of FDI probability as a function of intensity, which is similar to the intensity dependence of nonsequential double ionization probability. The observation of FDI is more favourable when using targets with low ionization potentials and short driving laser wavelengths. This is attributed to the crucial role of recollision therein, which can be experimentally inferred from the photoelectron momentum distribution generated by FDI. This work provides novel physical insights into FDI dynamics with circular polarization.

A strong-field approach with realistic wave functions to the above-threshold ionization of Ba+

We study the above-threshold ionization of atoms in intense circularly polarized laser pulses. In order to compute photoelectron energy spectra, we apply the strong-field approximation with different models for the initial state wave function. Specifically, we compare the spectra for singly ionized Barium (Ba+) using hydrogenic wave functions and realistic one-particle wave functions obtained by multiconfiguration Dirac–Hartree–Fock computations, respectively. As a particular example, we discuss the dependence of the photoelectron spectra on the magnetic quantum number m of the initial state and we reproduce the well known m-selectivity in strong-field ionization. Here, we show that the photoelectron spectra exhibit noticeable differences for the two models of the initial state and that the m-selectivity is enhanced when realistic wave functions are used. We conclude that the description of strong-field processes within the strong-field approximation will benefit from a realistic description of the initial atomic state.

Effect of circularly polarized laser pulse beam waist radius on the dynamic and radiation characteristics of collided electrons

In this paper, the trajectory as well as the temporal and spatial characteristics of the electromagnetic radiation of high-energy electrons collided by fields of relativistically circularly polarized laser pulses are studied theoretically and calculated numerically, based on Lorentz equation, energy equation of electrons, and the basic equations of electron radiation. Results shows that the head-on collision of a single counter-streaming electron and an intense circularly polarized laser pulse can produce an ultra-intense pulse with zeptosecond duration in the direction where the azimuth angle and the polar angle . The amplitude of the electron trajectory and the peak power of its radiation are positively related to the beam waist radius. However, as the radius of the laser beam increases, its influence on the dynamic and radiation characteristics of the electron weaken gradually and become negligible when the beam waist radius is greater than 4 times the laser wavelength.

Molecular electron recollision dynamics in intense circularly polarized laser pulses

Extreme UV and x-ray table top light sources based on high-order harmonic generation (HHG) are focused now on circular polarization for the generation of circularly polarized attosecond pulses as new tools for controlling electron dynamics, such as charge transfer and migration and the generation of attosecond quantum electron currents for ultrafast magneto-optics. A fundamental electron dynamical process in HHG is laser induced electron recollision with the parent ion, well established theoretically and experimentally for linear polarization. We discuss molecular electron recollision dynamics in circular polarization by theoretical analysis and numerical simulation. The control of the polarization of HHG with circularly polarized ionizing pulses is examined and it is shown that bichromatic circularly polarized pulses enhance recollision dynamics, rendering HHG more efficient, especially in molecules because of their nonspherical symmetry. The polarization of the harmonics is found to be dependent on the compatibility of the rotational symmetry of the net electric field created by combinations of bichromatic circularly polarized pulses with the dynamical symmetry of molecules. We show how the field and molecule symmetry influences the electron recollision trajectories by a time–frequency analysis of harmonics. The results, in principle, offer new unique controllable tools in the study of attosecond molecular electron dynamics.

The effects of helical magnetostatic wiggler on the modulation instability of a laser pulse propagating through a hot magnetoplasma

In this paper, the effects of helical magnetostatic wiggler on the modulation instability of a laser pulse propagating through a hot magnetoplasma have been investigated. Using the relativistic fluid model, the propagation of intense circularly polarized laser pulse along the wiggler field has been considered. A non-linear equation which describes the interaction of laser pulse with hot magnetoplasma in the quasi-neutral approximation for both of the right- and left-hand polarizations has been derived. The effect of the wiggler magnetic strength on the growth rate of the laser pulse has been discussed. The results indicate that for the right-hand polarization, the increase of wiggler frequency leads to increasing of growth rate. Inversely, in left-hand polarization with increasing of wiggler frequency, growth rate decreases. Moreover, it is found that in hot magnetoplasma employing the wiggler field improves declines the growth of modulation instability in right-and left-hand polarization. In addition, it was found that growth rate in the high plasma temperatures saturates and reaches to a constant norm.

Nonlinear interaction of intense left- and right-hand polarized laser pulse with hot magnetized plasma

In this article, self-focusing of an intense circularly polarized laser pulse in the presence of an external oblique magnetic field in hot magnetized plasma, using Maxwell’s equations and the relativistic fluid momentum equation, is studied. An envelope equation governing the spot size of the laser beam for both of left- and right-hand polarizations has been derived and the effects of the plasma temperature and oblique magnetic field on the electron density distribution of hot plasma with respect to variation of the normalized laser spot size has been investigated. Numerical results depict that in right-hand polarization, self-focusing of the laser pulse along the propagation direction in hot magnetized plasma becomes better and more compressed with increasing $\unicode[STIX]{x1D703}$. Inversely, in left-hand polarization, increase of $\unicode[STIX]{x1D703}$ in an oblique magnetic field leads to enhancement of the spot size and reduction self-focusing. Besides, in the plasma density profile, self-focusing of the laser pulse improves in comparison with no oblique magnetic field. Also it is shown that plasma temperature has a key role in the laser spot size, normalized laser output power and the variation of plasma density.

Self-focusing and de-focusing of intense left and right-hand polarized laser pulse in hot magnetized plasma: Laser out-put power and laser spot-size

In this work, self-focusing of an intense circularly polarized laser pulse in hot magnetized plasma using Maxwell’s equations and relativistic fluid momentum equation is investigated. An envelope equation governing the spot-size of laser beam for both of left- and right-hand polarizations has been derived and the effects of the plasma temperature and magnetic field on the spatial electron density distribution of hot plasma with respect to variation of normalized laser spot-size has been studied. Numerical results indicate that with increasing the magnetic field strength self-focusing of the right-hand polarization laser pulse increases, while for the left-hand polarization, laser de-focusing enhances. Furthermore, it is found that for right-hand polarization of the incident laser, when the external magnetic field strength enhances the laser spot-size reduces and laser pulse becomes focused. In fact, effect magnetic field on the density profile in right-hand polarization is more prominent. Also it is shown that plasma temperature has a significant role in the, laser spot-size, laser out-put power and the variation of plasma density.

Improving the quality of proton beams via double targets driven by an intense circularly polarized laser pulse

A new scheme is proposed to improve the quality of proton beams via ultra-intense laser pulse interacting with double plasma targets, which consist of a pre-target with relatively low density and a main target with high density. Both one- and two-dimensional Particle-in-Cell simulations show that, the using of an appropriate pre-target can help to obtain a much stronger longitudinal charge separation field in contrast to using only the main target. And proton beam with lower momentum divergence, better monochromaticity and collimation, as well as higher current density is generated. Moreover, due to the strengthened coupling between the laser pulse and targets, the energy conversion from laser pulse to protons is also increased.

High-order-harmonic generation in molecular sequential double ionization by intense circularly polarized laser pulses

We present effects of electron energy transfer by electron collisions on high-order-harmonic generation (HHG) in molecular sequential double ionization by intense circularly polarized laser pulses. Results from numerical solutions of time-dependent Schr\"odinger equations for extended (large internuclear distance) ${\mathrm{H}}_{2}$ where electrons are entangled and hence delocalized by exchange show that HHG with cutoff energy up to ${I}_{p}+24{U}_{p}$ can be obtained, where ${I}_{p}$ is the molecule ionization potential and ${U}_{p}={I}_{0}/4{\ensuremath{\omega}}_{0}^{2}$ (in atomic units) is the ponderomotive energy for pulse intensity ${I}_{0}$ and frequency ${\ensuremath{\omega}}_{0}$. A time-frequency analysis is employed to identify electron collisions for the generation of harmonics. Extended HHG arises from electron energy exchange, which agrees well with the prediction of a classical two electron collision model. Results for nonsymmetric ${\mathrm{HHe}}^{+}$ where initially electrons are localized on He are also compared and confirm the role of initial electron delocalization via entanglement for obtaining extended HHG plateaus.

Propagation of intense laser pulses in strongly magnetized plasmas

Propagation of intense circularly polarized laser pulses in strongly magnetized inhomogeneous plasmas is investigated. It is shown that a left-hand circularly polarized laser pulse propagating up the density gradient of the plasma along the magnetic field is reflected at the left-cutoff density. However, a right-hand circularly polarized laser can penetrate up the density gradient deep into the plasma without cutoff or resonance and turbulently heat the electrons trapped in its wake. Results from particle-in-cell simulations are in good agreement with that from the theory.

Self-focusing of circularly polarized laser pulse propagating through a magnetized non-Maxwellian plasma

Self-focusing of an intense circularly polarized laser pulse propagating through a magnetized non-Maxwellian plasma is investigated. Based on a relativistic two-fluid model, nonlinear equation describing dynamics of the slowly varying amplitude is obtained. The evolution of laser spot size is studied and effect of non-Maxwellian distribution of charge density on the spot size is considered. It is shown that the existence of super-thermal particles leads to the enhancement of the self-focusing quality of plasma.

Molecular photoelectron holography with circularly polarized laser pulses

We investigate the photoelectron momentum distribution of molecular-ion H2+driven by ultrashort intense circularly polarized laser pulses. Both numerical solutions of the time-dependent Schrödinger equation (TDSE) and a quasiclassical model indicate that the photoelectron holography (PH) with circularly polarized pulses can occur in molecule. It is demonstrated that the interference between the direct electron wave and rescattered electron wave from one core to its neighboring core induces the PH. Moreover, the results of the TDSE predict that there is a tilt angle between the interference pattern of the PH and the direction perpendicular to the molecular axis. Furthermore, the tilt angle is sensitively dependent on the wavelength of the driven circularly polarized pulse, which is confirmed by the quasiclassical calculations. The PH induced by circularly polarized laser pulses provides a tool to resolve the electron dynamics and explore the spatial information of molecular structures.

Efficient proton beam generation from a foam-carbon foil target using an intense circularly polarized laser

Energetic proton acceleration from interaction of intense circularly polarized laser pulse with a foam-carbon foil target is investigated using two-dimensional particle-in-cell simulation. It is found that protons from this target are accelerated to much higher energy in comparison with double-layer targets and simple flat targets. This result can be attributed to an enhancement of energy conversion efficiency from laser to electrons inside foam region. When a large number of energetic electrons are generated from the nearcritical plasma, they transport through the foil and form a strong backside sheath field which accelerates protons more efficiently.

Ultrashort x-ray pulse generation by nonlinear Thomson scattering of a relativistic electron with an intense circularly polarized laser pulse

The nonlinear Thomson scattering of a relativistic electron with an intense laser pulse is calculated numerically. The results show that an ultrashort x-ray pulse can be generated by an electron with an initial energy of 5 MeV propagating across a circularly polarized laser pulse with a duration of 8 femtosecond and an intensity of about $1.1\ifmmode\times\else\texttimes\fi{}{10}^{21}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$, when the detection direction is perpendicular to the propagation directions of both the electron and the laser beam. The optimal values of the carrier-envelop phase and the intensity of the laser pulse for the generation of a single ultrashort x-ray pulse are obtained and verified by our calculations of the radiation characteristics.

Circularly polarized molecular high-order harmonic generation inH2+with intense laser pulses and static fields

Molecular high-order harmonic generation (MHOHG) by a combined intense circularly polarized laser pulse and static electric field has been studied from the appropriate time-dependent Schr\"odinger equation (TDSE) for the ${{\mathrm{H}}_{2}}^{+}$ molecular ion. It is found that for a particular static field strength derived from a classical model, efficient MHOHG spectra are obtained with maximum energy ${I}_{p}$ $+$ 9.05${U}_{p}$, where ${I}_{p}$ is the ionization potential and ${U}_{p}={E}_{0}^{2}/4{m}_{e}{\ensuremath{\omega}}_{0}^{2}$ is the ponderomotive energy at amplitude ${E}_{0}$ and frequency ${\ensuremath{\omega}}_{0}$ of the circularly polarized laser pulse. The static field controls recollision of the electron with parent ions and is confirmed by numerical solutions of the ${{\mathrm{H}}_{2}}^{+}$ TDSE at equilibrium. To produce circularly polarized MHOHG spectra, a combination of an elliptically polarized pulse and a static electric field is found to be most efficient. A time-frequency analysis obtained via Gabor transforms is employed to identify electron recollision times responsible for the generation of these high-order harmonics. It is found that only single recollision trajectories contribute to the circularly polarized harmonics, thus generating new sources for high-frequency circularly polarized attosecond pulses.

Ionization of oriented carbonyl sulfide molecules by intense circularly polarized laser pulses

We present combined experimental and theoretical results on strong-field ionization of oriented carbonyl sulfide molecules by circularly polarized laser pulses. The obtained molecular frame photoelectron angular distributions show pronounced asymmetries perpendicular to the direction of the molecular electric dipole moment. These findings are explained by a tunneling model invoking the laser-induced Stark shifts associated with the dipoles and polarizabilities of the molecule and its unrelaxed cation. The focus of the present article is to understand the strong-field ionization of one-dimensionally-oriented polar molecules, in particular asymmetries in the emission direction of the photoelectrons. In the following article [Phys. Rev. A 83, 023406 (2011)] the focus is to understand strong-field ionization from three-dimensionally-oriented asymmetric top molecules, in particular the suppression of electron emission in nodal planes of molecular orbitals.

Axial magnetic field generation by intense circularly polarized laser pulses in underdense plasmas

Axial magnetic field generation by intense circularly polarized laser beams in underdense plasmas has been studied with three-dimensional particle-in-cell simulations and by means of theoretical analysis. Comparisons between analytical models and simulation results have identified an inverse Faraday effect as the main mechanism of the magnetic field generation in inhomogeneous plasmas. The source of azimuthal nonlinear currents and of the axial magnetic field depends on the transverse inhomogeneities of the electron density and laser intensity. The fields reach a maximum strength of several tens of megagauss for laser pulses undergoing relativistic self-focusing and channeling in moderately relativistic regime. Ultrarelativistic laser conditions inhibit magnetic field generation by directly reducing a source term and by generating fully evacuated plasma channels.

Ionization of oriented targets by intense circularly polarized laser pulses: Imprints of orbital angular nodes in the two-dimensional momentum distribution

We solve the three-dimensional time-dependent Schr\"{o}dinger equation for a few-cycle circularly polarized femtosecond laser pulse interacting with an oriented target exemplified by an Argon atom, initially in a $3\text{p}_{x}$ or $3\text{p}_{y}$ state. The photoelectron momentum distributions show distinct signatures of the orbital structure of the initial state as well as the carrier-envelope phase of the applied pulse. Our \textit{ab initio} results are compared with results obtained using the length-gauge strong-field approximation, which allows for a clear interpretation of the results in terms of classical physics. Furthermore, we show that ionization by a circularly polarized pulse completely maps out the angular nodal structure of the initial state, thus providing a potential tool for studying orbital symmetry in individual systems or during chemical reactions.