Laser Pulse Propagation Direction Research Articles

Nondipole signatures in ionization and high-order harmonic generation

We analyze nondipole effects arising in ionization and high-order harmonic generation for a two-dimensional hydrogen atom irradiated with either low- or high-frequency laser pulses. In the low-frequency case, the electron wave packet dynamics is dominated by rescattering processes within the laser pulse. Here both odd- and even-order harmonics are generated in the direction of the laser field propagation and polarization, respectively. For high-frequency pulses, such rescattering processes can be neglected. We demonstrate that a significant portion of photoelectrons is detected opposite to the laser pulse propagation direction as a consequence of their postpulse wave packet spreading and interaction with the parent ion. This is accompanied by rich interference structures formed in the momentum distributions of photoelectrons. Our results follow from the numerical solution of the time-dependent Schr\"odinger equation, which is based on the Suzuki-Trotter scheme with the split-step Fourier approach. The method relies on a Hamiltonian decomposition, where except for the components depending exclusively on the momentum or on the position operators, there are also terms depending on both momentum and position operators in particular configurations. We demonstrate that, as long as the latter does not depend on noncommuting coordinates of the momentum and position operators, nondipole effects in laser-matter interactions can be studied without applying extra approximations and unitary operations.

Nonlinear spectral features of the relativistic interaction between electron and ultrashort laser pulse

Features of the radiation spectra are investigated with the quantum electrodynamic theory for the nonlinear scattering interaction between relativistic electron and few-cycle linearly polarized laser pulse. The angle between the moving direction of the electron and that of the laser pulse affects the scattered radiation characteristics. When the electron has a head-on collision with the laser pulse having a carrier envelope phase as zero, the angular distribution of the scattered radiation spectrum is symmetrical with respect to the laser pulse propagation direction. Such a symmetry disappears when the electron collides with the laser obliquely and the direction of most of the radiation energy shifts at an acute angle towards the electron direction. The CEP has significant effects on the spectral angular distribution. The CEP influence is determined by the exertion of the electromagnetic field on the motion of the electron, whose trajectory overlap in phase space leads to the interference between scattered radiation in different intervals of the interaction process. The supercontinuum radiation could be produced under certain CEPs. An external electrostatic field applied as the environment for the laser-electron interaction is another important factor affecting the scattered radiation features by modulating the harmonic components and their angular distribution. This study may help the analysis of the spectral data from relativistic laser-solid target experiments.

Influence of transverse magnetic field on the properties of laser ablation produced nickel oxide nanoparticles

Influence of transverse magnetic field on the nickel produced plasma plume and structural and optical properties of nickel oxide nanoparticles produced by pulsed laser ablation (PLA) method have been investigated experimentally. Ablation container was placed between the poles of permanent magnets. Strength of external magnetic field was controlled by the distance between magnets. The direction of the magnetic field was perpendicular to the direction of laser pulse propagation. 5 samples were synthesized in the presence of magnetic fields with different strengths in distilled water. Ablation was carried out by 1064 nm wavelength beam of pulsed Nd:YAG laser of 7 ns pulse width. Effects of external magnetic field on the properties of nickel oxide nanoparticles were characterized by X-ray diffraction patterns, field emission scanning electron microscope images, transmission electron microscope microimages, UV–Vis-NIR absorption spectra, dynamic light scattering patterns, FTIR and photoluminescence spectra. Furthermore, magnetic properties of synthesized nanoparticles were studied using their hysteresis curve which were recorded by vibrating sample magnetometer. Results show that with increasing the strength of external magnetic field, the intensity of XRD peaks of synthesized nanoparticles was increased while their size was decreased. Applying the external magnetic field caused the cyclotron motion of the charged particles in the plasma plume on the surface of target which increased their energy, and decreased their agglomeration.

Sub-cycle time resolution of multi-photon momentum transfer in strong-field ionization

During multi-photon ionization of an atom it is well understood how the involved photons transfer their energy to the ion and the photoelectron. However, the transfer of the photon linear momentum is still not fully understood. Here, we present a time-resolved measurement of linear momentum transfer along the laser pulse propagation direction. We can show that the linear momentum transfer to the photoelectron depends on the ionization time within the laser cycle using the attoclock technique. We can mostly explain the measured linear momentum transfer within a classical model for a free electron in a laser field. However, corrections are required due to the parent-ion interaction and due to the initial momentum when the electron enters the continuum. The parent-ion interaction induces a negative attosecond time delay between the appearance in the continuum of the electron with minimal linear momentum transfer and the point in time with maximum ionization rate.

Effect of external magnetic field on wakefield generation in underdense plasma slab using backward semi-Lagrangian Vlasov code

Laser wakefield excitation in the interaction of femtosecond laser pulse with an underdense thin magnetized plasma slab is studied by one dimensional relativistic Vlasov-Maxwell system of equations. The interaction is modeled by relativistic Vlasov equation in propagation direction of laser pulse and fluid equations in transverse direction. Backward semi-Lagrangian method (BSL) is used for numerical solution of Vlasov equation. Generation of wakefields by a right and left handed circularly polarized (RCP) and (LCP) laser pulses in the interaction with nonmagnetized and magnetized plasma slab are examined and compared. Two directions are considered for external magnetic field, parallel and antiparallel to direction of laser pulse propagation. The results show that applying magnetic field along (opposite to) the propagation direction of RCP (LCP) laser pulse increases amplitude of density steepening and consequently wakefields amplitude, and mean kinetic energy of electrons compared with nonmagnetized plasma. The results are in complete agreement with previous analytical and numerical reports.

Investigation the influence of injection angle on electron acceleration in laser wake-field accelerator with the presence of external magnetic field

In this paper, the influence of initial momentum and the injection angle of electron in to the magnetized plasma in a laser wake-field accelerator have been studied.It has been seen that the use of external magnetic field in the opposite direction of laser pulse propagation leads to an increase in the wake-field amplitude and a decrease in laser pulse group velocity in comparison to the use of external magnetic field in same direction of laser pulse propagation or unmagnetized plasma case. The results show, the effect of initial momentum variation on the final energy gained by electron is lower in comparison with the effect of injection angle variation. It is observed when the injection angle is increased, the electron can stay more in acceleration phase and reach to more final energy. The maximum final energy of accelerated electron is about 2.5 GeV when the magnetic field is applied along the reverse direction. Also, the electron energy is increased with the increase of injection angle and lowest energy is related to the case of magnetic field applied along the reverse direction.

Plasma waves excitation by a short pulse of focused laser radiation

The excitation of plasma waves by a short pulse of focused laser radiation is studied. Since the excitation of waves depends strongly on the pulse form, we described in detail its form and the conditions when the expression for the pulse field is applicable. Limitations on the pulse duration and the degree of laser radiation focusing are given. The basis for studying the excitation of plasma waves is the equation for potential electric fields. This equation describes the dispersion, weak damping due to collisions of electrons with ions, and wave excitation due to the ponderomotive effect of a short pulse of laser radiation. The dispersion of waves is described by a small integral term that takes into account the thermal motion of electrons. The effect of electron collisions on the damping of waves is described by the Fokker-Planck collision integral. The expression for the ponderomotive force is written taking into account the fact that the laser pulse propagates with a group velocity close to the speed of light. From the equation for waves, we find the Fourier transform of the electric field, which makes it possible to analyze the spectral composition of the excited waves and their radiation patterns. When radiation is weakly focused, waves are excited along the direction of laser pulse propagation. In the case of strong focusing, plasma waves are excited at an angle to this direction, and the greater the angle magnitude, the greater the difference of wave frequency from the electron plasma frequency.

The effect of external magnetic field on the density distributions and electromagnetic fields in the interaction of high-intensity short laser pulse with collisionless underdense plasma

Laser absorption in the interaction between ultra-intense femtosecond laser and solid density plasma is studied theoretically here in the intensity range \( I{\lambda^2} \simeq 10^{14}{-}10^{16}\;{\text{W}}\;{{\text{cm}}^{-2}} \;\upmu{{\text{m}}^{2}} \). The collisionless effect is found to be significant when the incident laser intensity is less than \( 10^{16}\;{\text{W}}\;{{\text{cm}}^{-2}}\;\upmu{{\text{m}}^{2}} \). In the current work, the propagation of a high-frequency electromagnetic wave, for underdense collisionless plasma in the presence of an external magnetic field is investigated. When a constant magnetic field parallel to the laser pulse propagation direction is applied, the electrons rotate along the magnetic field lines and generate the electromagnetic part in the wake with a nonzero group velocity. Here, by considering the ponderomotive force in attendance of the external magnetic field and assuming the isothermal collisionless plasma, the nonlinear permittivity of the plasma medium is obtained and the equation of electromagnetic wave propagation in plasma is solved. Here, by considering the effect of the ponderomotive force in isothermal collisionless magnetized plasma, it is shown that by increasing the laser pulse intensity, the electrons density profile leads to steepening and the electron bunches of plasma become narrower. Moreover, it is found that the wavelength of electric and magnetic field oscillations increases by increasing the external magnetic field and the density distribution of electrons also grows in comparison to the unmagnetized collisionless plasma.

Effect of obliqueness of external magnetic field on the characteristics of magnetized plasma wakefield

A direct three-dimensional model to study the wakefield in underdense magnetized plasma is introduced. The model is based on an analytic procedure by Laplace transformation for calculating the magnetized plasma wake equations. Wakefield is excited using a high-intensity ultrashort laser beam. In the presence of external magnetic field perpendicular to the direction of the laser pulse propagation direction, plasma electrons rotate around the magnetic field lines, leading to the generation of an electromagnetic component of the plasma wakes at plasma frequency. This component is polarized perpendicularly to the direct current magnetic field lines and propagates in the forward direction and normal direction with respect to the laser pulse propagation direction, both perpendiculars to the direction of the applied magnetic field. Intensity of the radiation in different plasma densities and different magnetic field strengths has been observed.

Nonlinear interaction of charged particles with strong laser pulses in a magnetic undulator

Centre of Strong Fields Physics, Yerevan State University, 1 A. Manukian, Yerevan 0025, Armenia(Received 17 June 2010; published 4 August 2010)Laser acceleration due to the nonlinear-threshold phenomena of charged particle ‘‘reflection’’ andcapture by slowed wave in a magnetic undulator is considered. The obtained numerical results prove theparticle reflection and capture phenomena in the field of actual laser pulses with temporal and spaceprofiles which lead to the particles acceleration. In contrast to the reflection regime where particleacceleration takes place already at the constant undulator step, in the capture regime it is necessary toincrease adiabatically the undulator step along the laser pulse propagation direction by the certain self-consistent variation law corresponding to acceleration rate.

Accelerating protons to therapeutic energies with ultraintense, ultraclean, and ultrashort laser pulses

Proton acceleration by high-intensity laser pulses from ultrathin foils for hadron therapy is discussed. With the improvement of the laser intensity contrast ratio to 10(-1) achieved on the Hercules laser at the University of Michigan, it became possible to attain laser-solid interactions at intensities up to 10(22) W/cm2 that allows an efficient regime of laser-driven ion acceleration from submicron foils. Particle-in-cell (PIC) computer simulations of proton acceleration in the directed Coulomb explosion regime from ultrathin double-layer (heavy ions/light ions) foils of different thicknesses were performed under the anticipated experimental conditions for the Hercules laser with pulse energies from 3 to 15 J, pulse duration of 30 fs at full width half maximum (FWHM), focused to a spot size of 0.8 microm (FWHM). In this regime heavy ions expand predominantly in the direction of laser pulse propagation enhancing the longitudinal charge separation electric field that accelerates light ions. The dependence of the maximum proton energy on the foil thickness has been found and the laser pulse characteristics have been matched with the thickness of the target to ensure the most efficient acceleration. Moreover, the proton spectrum demonstrates a peaked structure at high energies, which is required for radiation therapy. Two-dimensional PIC simulations show that a 150-500 TW laser pulse is able to accelerate protons up to 100-220 MeV energies.

Optimum small-scale management of random beam perturbations in a femtosecond laser pulse

We have performed statistical simulations of stochastic filament control in a femtosecond laser pulse. Suppression of random multiple filaments is performed by means of a mesh introduced into the transverse beam section at the laser system output. The optimum size of a single square mesh unit contains (3.1-3.2)Pcr for self-focusing in the medium. Controlled filaments are formed in several groups along the laser pulse propagation direction. Almost simultaneous formation of multiple filaments can be attained by means of spatial modulation of the mesh transmission coefficient.

Monte Carlo analysis of electron-positron pair creation by powerful laser-ion impact

We consider electron-positron pair creation by the impact of very powerful laser pulses with highly charged ions. In contrast to our foregoing work with rather limited angular configurations of pair creation, we extend these calculations to even higher laser intensities, and we use the Monte Carlo method to numerically analyze the rates of pair creation for arbitrary angular distributions. We also evaluate the intensity dependence of the total rates of pair creation. Thus we demonstrate that our laser-induced process shows stabilization, because beyond a specific laser power the total rates of pair creation decreases. Our analysis of the angular distributions of the created electron-positron pairs leads to the conclusion that pairs are predominantly emitted in the direction of laser pulse propagation.

Generation of short pulse radiation from magnetized wake in gas-jet plasma and laser interaction

New features of a high power tunable radiation from the magnetized plasma wakes are studied. This Letter covers some aspects of the problem, which was previously discussed in details in [Phys. Rev. E 68 (2003) 026409]. A gas-jet flow is used to generate the sharp boundary plasma. Wakefield is excited by a mode locked Ti:sapphire laser beam operating at 800 nm wavelength with the pulse width of 100 fs (FWHM) and maximum energy of 100 mJ per pulse with 10 Hz repetition rate. The neutral density of gas-jet flow is measured with a Mach–Zehnder interferometer. Strength of the applied external dc magnetic field normal to the direction of laser pulse propagation varies from 0 to 8 kG in the interaction region. Radiation is observed, in the forward direction due to the axial component of the magnetized wakefield and in the normal direction due to the radial component of the magnetized wakefield, both perpendicular to the direction of the applied magnetic field. The frequency of the emitted radiation with the pulse width of 200 ps (detection limit), measured by the method of time-of-flight, is in the millimeter wave range. Radiations are polarized perpendicularly to the laser pulse propagation direction and dc magnetic field lines as is expected from the theory. Electrodynamic properties of the radiation have been studied at different plasma densities and magnetic field strengths.

Interaction of powerful laser pulse with magnetized plasma

In this paper the generation of electrostatic wake field and electron acceleration in this field by intense laser pulse propagating in plasma in the presence of external magnetic field is investigated. It is shown that if the change in the laser pulse wave amplitude (approximation of the constant laser pulse wave field) is not taken into account, it is possible by switching to Lagrangian variables to reduce the system of equations describing excitation of the wake field and electron acceleration to a simplified system of ordinary differential equations (ODE), which can be easily integrated. This makes it possible to find the energy, transferred to electron oscillations in the wake field for different angles between the direction of laser pulse propagation and magnetic field. Stability of the laser pulse propagating with arbitrary angle to external magnetic field towards excitation of parametric and decay instabilities with involvement of upper hybrid and lower hybrid waves is investigated, as well. Equations describing these instabilities are derived and growth rates are found analytically for a small amplitude wake field and with the help of numerical calculations in the case of the finite wake field amplitude.

Harmonic generation in ring-shaped molecules

We study numerically the interaction between an intense circularly polarized laser field and an electron moving in a potential which has a discrete cylindrical symmetry with respect to the laser pulse propagation direction. This setup serves as a simple model, e.g., for benzene and other aromatic compounds. From general symmetry considerations, within a Floquet approach, selection rules for the harmonic generation [O. Alon Phys. Rev. Lett. 80 3743 (1998)] have been derived recently. Instead, the results we present in this paper have been obtained solving the time-dependent Schroedinger equation ab initio for realistic pulse shapes. We find a rich structure which is not always dominated by the laser harmonics.

Atoms interacting with intense, high-frequency laser pulses: Effect of the magnetic-field component on atomic stabilization

At sufficiently high laser intensities, the magnetic-field component of the laser field can strongly influence the stabilization of atoms in the high-frequency regime by inducing motion along the laser pulse propagation direction. Using a two-dimensional model atom, we investigate how the duration of the laser pulse affects stabilization in this nondipole regime. Results obtained using a fully spatially dependent vector potential are compared to a long-wavelength approximation. We also discuss nondipole effects when the atom interacts with two counterpropagating pulses.