Gaussian Laser Pulse Research Articles

Comment on “Even-order harmonic generation from nonlinear Thomson backscatter in a tightly focused Gaussian laser pulse” [Phys. Plasmas 29, 043102 (2022)

Hong et al. [Phys. Plasmas 29, 043102 (2022)] researched the nonlinear Thomson backscatter in a highly focused Gaussian linear laser pulse. They studied the law of backward harmonic spectrum and electron motion trajectory and put forward the view that “the electron will be pushed to the −z-axis by the responsible force of the falling edge of the laser pulse when the interaction time is long enough.” In this Comment, we study the electron motion law under the precise laser pulse expression. We find and explain that the initial position of the electron is the reason why the electron is pushed to the −z-axis. Only when the initial position of the electron is in the −z half-axis, the electron will be pushed away at the end of the pulse.

Ripple formation with intense Gaussian femtosecond laser pulses close to the damage threshold

The formation of laser-induced periodic surface structures (LIPSS or ripples) is a topic that has been investigated for almost 60 years. More recently with the advent of ultrashort laser pulses this subject has regained interest, in particular, due to interaction regimes that have not been present so far. Consequently a lot of work has been done in that field, especially with comprehensive experimental and theoretical investigations of the scaling of ripple parameters on laser pulse duration, wavelength, applied fluence, shot number and so on. However, there are still a lot of questions. The present work addresses an important issue on that subject. In particular, ripple formation is investigated at high laser intensity, namely at an intensity sufficiently large to generate a femtosecond-laser induced plasma. Thus ripple formation occurs close to damage threshold. Experimental results and theoretical discussion of ripple formation and the interrelation to laser pulse energy deposition, energy transport and sample damage originating from the optical interaction and additional thermal effects, respectively, are discussed. Most important, a reduction of ripple formation threshold with laser intensity and fluence, respectively, has been observed which is associated by a super-linear increase of the ripple area. The scaling of this reduction with laser fluence obtained from theoretical estimates is in good agreement with the experimental data.

Response to “Comment on ‘Even-order harmonic generation from nonlinear Thomson backscatter in a tightly focused Gaussian laser pulse’” [Phys. Plasmas 29, 114703 (2022)

In a recent Comment, Chang et al. [Phys. Plasmas 29, 114703 (2022)] have studied the electron dynamics for an electron accelerating in a tightly focused Gaussian laser pulse with the higher order correction terms, and it is found that the initial position of the electron is the reason why the electron is pushed to the −z axis at the end. In this Reply, the electron dynamics and its nonlinear Thomson backscattering in a tightly focused Gaussian laser pulse with the higher order correction terms are presented, and it is found that the result is consistent with the lowest order approximation case in our paper [Hong et al., Phys. Plasmas 29, 043102 (2022)]. Meanwhile, it is also found that when the longitudinal deceleration effect of the ponderomotive force on the electron introduced by the falling part of the tightly focused laser pulse is greater than the longitudinal acceleration effect of the rising part, the electron that is initially stationary will be slightly pushed to the −z axis at the end, which can well explain the “return” phenomenon of the electron in the longitudinal direction both in our paper and in the Comment.

Above-100 MeV proton beam generation from near-critical-density plasmas irradiated by moderate Laguerre–Gaussian laser pulses

High-quality protons have many potential applications in tumor therapy, proton radiography, fast ignition and nuclear physics. Over recent decades, significant progress has been made in generating such proton beams with lasers. However, it is still challenging to achieve a compact proton beam with an energy of hundreds of MeV under the laser intensities currently available. Here, we propose a novel scheme to produce above-100 MeV protons in nonuniform near-critical-density (NCD) plasmas driven by a Laguerre–Gaussian (LG) laser pulse. When a linearly-polarized LG laser pulse with the intensity of ∼1020 W cm−2 enters the NCD plasmas with a trapezoidal density profile, a donut-like double-channel structure with an electron column on the axis is produced behind the laser pulse, together with a unique, strong magnetic field. At the end of the density plateau, the magnetic field begins to expand in both longitudinal and transverse directions. The magnetic pressure force displaces the electrons with regard to the protons, resulting in a quasi-static electric field . This electrostatic field can remain robust and be sustaintained for a long time since the magnetic field decreases very slowly, i.e. . Thus, the protons can be accelerated by the longitudinal electrostatic field and confined by the generated transverse focusing field. Three dimensional particle-in-cell simulations confirmed that a well-defined above-100 MeV proton beam with a cut-off energy at least 3.5 times larger than that of the normal Gaussian laser could be obtained, making the proton beams a potential source for tumor therapy in the future.

Coupled effect of spatio-temporal variation of Laguerre–Gaussian laser pulse on electron acceleration in magneto-plasma

In this work, the acceleration of electrons via incidence of a Laguerre–Gaussian laser pulse in a plasma medium with relativistic non-linearity in the presence of a uniform external magnetic field along the direction of propagation has been presented. The moment theory approach has been used to obtain the two non-linear second-order differential equations, which have been further solved numerically to determine the variation in the radial and temporal width of the laser pulse in the plasma medium. The propagation of the laser pulse in the plasma medium excites the electron plasma wave, which acts as a wakefield for electron acceleration. The effect of radial and temporal variation on electron acceleration has also been investigated. Furthermore, the spatio-temporal variation and energy gain have been studied for different Laguerre modes of lasers, their intensities, and plasma densities.

Cosh-Gaussian laser pulse influenced electron acceleration in an ion channel

The electron acceleration in a prepared ion channel is studied theoretically by using a radially polarized (RP) cosh-Gaussian (ChG) laser pulse. The peculiar propagation properties of ChG laser cause it to focus early and over a shorter time than a Gaussian laser pulse, making it suitable for accelerating electrons to extremely high energies over a small duration. The electrostatic field formed by an ion channel prevents electrons from escaping the interaction zone due to their transverse oscillations, whereas the decentering parameter of the ChG laser pulse influences electron energy gain. This combined role of RP-ChG laser and the effect of an ion channel causes an enhancement in the electron energy gain significantly to the order of GeV with laser intensity (∼) in an ion density channel (∼) with decentered parameter (∼2.15) for an incident laser pulse at initial phase ().

Ultra-intense vortex laser generation from a seed laser illuminated axial line-focused spiral zone plate.

Relativistic vortex laser has drawn increasing attention in the laser-plasma community owing to its potential applications in various domains, e.g., generation of energetic charged particles with orbital angular momentum (OAM), high OAM X/γ-ray emission, high harmonics generation, and strong axial magnetic-field production. However, the generation of such relativistic vortex laser is still a challenge to the current laser technology. Using micro-structure targets named axial line-focused spiral zone plate (ALFSZP), we propose a novel scheme for ultra-intense vortex laser generation. In the scheme, a relativistic Gaussian laser pulse irradiates an ALFSZP, and diffracts as it passes through the ALFSZP. Due to the focusing and radial Hilbert transform capabilities of the ALFSZP, the seed laser is converted efficiently to a vortex one which is then well focused in a tunable focal volume. Three-dimensional particle-in-cell simulations indicate that using a seed laser pulse with intensity of 1.3 × 1020 W/cm2, the vortex laser intensity achieved is as high as 1.3 × 1021 W/cm2 with the averaged angular momentum per photon up to 0.73ℏ, promising diverse applications in various fields aforementioned.

Bright γ-ray source with large orbital angular momentum from the laser near-critical-plasma interaction

We propose a new laser-plasma-based method to generate bright γ-rays carrying large orbital angular momentum by interacting a circularly polarized Laguerre–Gaussian laser pulse with a near-critical hydrogen plasma confined in an over-dense solid tube. In the first stage of the interaction, it is found via fully relativistic three-dimensional particle-in-cell simulations that high-energy helical electron beams with large orbital angular momentum are generated. In the second stage, this electron beam interacts with the laser pulse reflected from the plasma disc behind the solid tube, and helical γ beams are generated with the same topological structure as the electron beams. The results show that the electrons receive angular momentum from the drive laser, which can be further transferred to the γ photons during the interaction. The γ beam orbital angular momentum is strongly dependent on the laser topological charge l and laser intensity a 0, which scales as . A short (duration of 5 fs) isolated helical γ beam with an angular momentum of −3.3 × 10−14 kg m2 s−1 is generated using the Laguerre–Gaussian laser pulse with l = 2. The peak brightness of the helical γ beam reaches 1.22 × 1024 photons s−1 mm−2 mrad−2 per 0.1% BW (at 10 MeV), and the laser-to-γ-ray angular momentum conversion rate is approximately 2.1%.

Spatiotemporal dynamics of circularly polarized Gaussian laser pulse in a magnetized plasma

AbstractSpatiotemporal evolution of intense circularly polarized Gaussian laser pulse propagating in plasma is investigated. Self‐focusing and self‐compression properties of the laser pulse are studied, taking into account the ponderomotive nonlinearity, magnetic field, and state of wave polarization effects. The coupled differential equations governing the beam width (in space) and pulse length (in time) parameters are obtained via paraxial ray and Wentze−Kramers−Brillouin approximations and solved numerically. Numerical simulation showed that the pulse is compressed (in time and space) to a significant extent at a different normalized distance due to the effect of the nonlinearity of the medium. It is shown that for the right‐hand circularly polarized laser, the strength of both self‐focusing and self‐compression of the laser pulse along the propagation direction is increased by increasing the value of the magnetic field, and consequently, the normalized intensity of the pulse is enhanced as it propagates through the plasma. In the case of a left‐hand circularly polarized laser, an increase in the magnetic field causes a decrease in the strength of both self‐focusing and self‐compression of the laser pulse, especially in the higher values. To analyse the evolution of the laser spot size, a three‐dimensional view of the normalized laser intensity at different points of the distance of propagation has also been plotted. Moreover, the results indicate a specific laser intensity range with a “saturation point” where the compression process reaches its maximum value, and outside of this range, it vanishes.

Enhanced radiation of nonlinear Thomson backscattering by a tightly focused Gaussian laser pulse and an external magnetic field

The electron dynamics and the Thomson backscattering of an electron moving in a combined field of a tightly focused Gaussian laser pulse and an external uniform magnetic field are investigated in detail. It is found that by considering the tightly focused Gaussian laser pulse, the electron can be pushed out from the laser pulse by the ponderomotive force, resulting in the symmetry breaking of the electron dynamics from the Gaussian envelope, which can dramatically enhance the radiation intensity. It is also found that by introducing an external magnetic field, the emergence of the cyclotron motion of the electron under the external magnetic field also breaks the symmetry of the electron dynamics and enhances the radiation. Especially in the resonance case, i.e., the cyclotron frequency of the electron is close to the laser frequency, the emission spectrum is further enhanced due to the great extension of the interaction time and the symmetry breaking by the beat wave between the helix motion and the cyclotron motion of the electron in the combined field, and a platform with high radiation intensity containing the THz band has also appeared.

Improving the electrostatic design of the Jefferson Lab 300 kV DC photogun.

The 300kV DC high voltage photogun at Jefferson Lab was redesigned to deliver electron beams with a much higher bunch charge and improved beam properties. The original design provided only a modest longitudinal electric field (Ez) at the photocathode, which limited the achievable extracted bunch charge. To reach the bunch charge goal of approximately few nC with 75ps full-width at half-maximum Gaussian laser pulse width, the existing DC high voltage photogun electrodes and anode-cathode gap were modified to increase Ez at the photocathode. In addition, the anode aperture was spatially shifted with respect to the beamline longitudinal axis to minimize the beam deflection introduced by the non-symmetric nature of the inverted insulator photogun design. We present the electrostatic design of the original photogun and the modified photogun and beam dynamics simulations that predict vastly improved performance. We also quantify the impact of the photocathode recess on beam quality, where recess describes the actual location of the photocathode inside the photogun cathode electrode relative to the intended location. A photocathode unintentionally recessed/misplaced by sub-millimeter distance can significantly impact the downstream beam size.

Observation of transverse injection and enhanced beam quality in laser wakefield acceleration of isolated electron bunches using an optimized plasma waveguide.

The laser wakefield acceleration of monoenergetic multi-GeV electron beams in the bubble regime is investigated via particle-in-cell simulations considering laser guiding of sub-petawatt pulses by an optimized plasma waveguide. The density profile of the plasma has a transverse transition from a low value for the laser guiding central channel to an optimal higher value for the surrounding plasma. Multidimensional particle-in-cell simulations in the nonlinear bubble regime show that when the spot size of the Gaussian laser pulse is matched to the diameter of the low-density laser-guiding plasma channel, electron self-injection can be transversely provided from the surrounding high-density plasma mitigating the need for a minimum electron density of the low-density channel to trigger the self-injection. Accordingly, the pump depletion and electron dephasing lengths can be increased by reducing the electron density of the axial channel, and the electron bunch can be accelerated to considerably longer distances. As a result, the energy gain of the trapped electrons, injected from the surrounding high-density region, can be efficiently enhanced. Under such conditions, a completely localized electron bunch with considerably decreased energy spread (<2%) and enhanced peak energy (∼2.5GeV) is accelerated over a length of ∼6mm by a sub-petawatt laser pulse (∼86TW).

The generation and control of serpentine femtosecond laser filament array

Some potential applications require controllable curved femtosecond laser filaments. We show how a gas lattice medium with periodic refractive index profiles can generate a serpentine filament array of a femtosecond laser Gaussian pulse. The simulation findings show that the curved characteristics (bending times and degree), spatial evolution (onset, termination and interval) and energy fluence profiles of the filaments, can be flexibly controlled by adjusting the various parameters of the gas lattice. When collapsing Gaussian laser pulse is coupled into the asymmetric gas lattice, the optical waveguide effect in a medium with a refractive index gradient plays a vital role in forming the continuously curved trajectory. By adjusting the gas lattice parameters, serpentine filaments with customised characteristics can also be obtained. Our research provides a new method for manipulating the strong nonlinear propagation trajectories, which is for the first time to tailor serpentine filaments by utilising a modulated dielectric medium and offers a unique solution for many potential applications.

Guiding of Laguerre–Gaussian pulses in high-order plasma channels

In laser wakefield accelerators, guiding of drive laser pulses in preformed plasma channels plays a key role to overcome laser diffraction for effective acceleration. Different from guiding schemes studied previously, where a Gaussian laser pulse and a parabolic plasma channel were investigated, here we investigate the guiding of Laguerre–Gaussian (LG) pulses in plasma channels. Analytical studies and three-dimensional particle-in-cell simulations show that the matched conditions still exist for high order laser pulses and high order plasma channels. For usual Gaussian and high order LG pulses, the second order parabolic channel gives the best guiding. Although the laser pulse can also be guided in even higher order channels, its envelope deforms during propagation. For laser pulses with combined multi-LG modes, determined by their azimuthal orbit angular momenta, there is axisymmetric or non-axisymmetric evolution for the transverse laser intensity profile. The preformed plasma channel can guide the combined pulses but the transverse intensity profile of the laser pulses always evolves.

Combined Effect of Spatio-Temporal Dynamics of Laser Pulse on Electron Acceleration in Relativistic Plasma

This article presents the spatial and temporal dynamics of high-power Gaussian laser pulse in relativistic plasma. At higher intensities, the self-focusing and self-compression of laser pulse take place due to the variation in the mass of plasma electrons moving with speed comparable to the speed of light. The two non-linear coupled differential equations for spatial and temporal pulsewidth parameters have been obtained using the moment theory approach. These equations have been solved numerically using the Runga-Kutta method. The variation in the spatial and temporal pulsewidth parameters with the distance of propagation is then used to calculate the energy gained by the electrons, which are trapped by the excited electron plasma wave during laser-plasma interaction. It is observed that spatio-temporal dynamics play an important role in electron acceleration in relativistic plasma.

Numerical investigation of guiding of Gaussian laser pulse in plasma channel with radial variation in presence of Kerr effect by Runge–Kutta and extrapolation methods

Numerical investigation of guiding of Gaussian laser pulse in plasma channel with radial variation in presence of Kerr effect by Runge–Kutta and extrapolation methods

Prompt acceleration of a μ + beam in a toroidal wakefield driven by a shaped steep-rising-front Laguerre–Gaussian laser pulse

Recent experimental data for anomalous magnetic moments strongly indicates the existence of new physics beyond the Standard Model. Energetic μ + bunches are relevant to μ + rare decay, spin rotation, resonance and relaxation (μSR) technology, future muon colliders, and neutrino factories. In this paper, we propose prompt μ + acceleration in a nonlinear toroidal wakefield driven by a shaped steep-rising-front Laguerre–Gaussian (LG) laser pulse. An analytical model is described, which shows that a μ + beam can be focused by an electron cylinder at the centerline of a toroidal bubble and accelerated by the front part of the longitudinal wakefield. A shaped LG laser with a short rise time can push plasma electrons, generating a higher-density electron sheath at the front of the bubble, which can enhance the acceleration field. The acceleration field driven by the shaped steep-rising-front LG laser pulse is about four times greater than that driven by a normal LG laser pulse. Our simulation results show that a 300 MeV μ + bunch can be accelerated to 2 GeV and its transverse size is focused from an initial value of w 0 = 5 μm to w = 2 μm in the toroidal bubble driven by the shaped steep-rising-front LG laser pulse with a normalized amplitude of a = 22.

Light-Induced Magnetization at the Nanoscale.

Triggering and switching magnetic moments is of key importance for applications ranging from spintronics to quantum information. A noninvasive ultrafast control at the nanoscale is, however, an open challenge. Here, we propose a novel laser-based scheme for generating atomic-scale charge current loops within femtoseconds. The associated orbital magnetic moments remain ferromagnetically aligned after the laser pulses have ceased and are localized within an area that is tunable via laser parameters and can be chosen to be well below the diffraction limit of the driving laser field. The scheme relies on tuning the phase, polarization, and intensities of two copropagating Gaussian and vortex laser pulses, allowing us to control the spatial extent, direction, and strength of the atomic-scale charge current loops induced in the irradiated sample upon photon absorption. In the experiment we used He atoms driven by an ultraviolet and infrared vortex-beam laser pulses to generate current-carrying Rydberg states and test for the generated magnetic moments via dichroic effects in photoemission. Abinitio quantum dynamic simulations and analysis confirm the proposed scenario and provide a quantitative estimate of the generated local moments.

Direct laser acceleration of electrons from a plasma mirror by an intense few-cycle Laguerre–Gaussian laser and its dependence on the carrier-envelope phase

A direct acceleration scheme to generate high-energy, high-charge electron beams with an intense few-cycle Laguerre–Gaussian (LG) laser pulse was investigated using three-dimensional particle-in-cell simulations. In this scheme, an intense LG laser pulse was irradiated onto a solid density plasma slab. When the laser pulse is reflected, electrons on the target front surface are injected into the longitudinal electric field of the laser and accelerated further. We found that the carrier-envelope phase (CEP) of the few-cycle laser pulse plays a key role in the electron injection and acceleration process. Using a three-cycle LG laser pulse with and an appropriate CEP, an about 60 pC electron beam could be obtained at a maximum energy of 16 MeV. In comparison, when a laser pulse with mismatched CEP was used, a total of 4 pC electron beam with a maximum energy of 3.5 MeV was obtained. Linear scaling of electron energy to the laser strength was shown up to at which a quasi-monoenergetic electron beam of 850 MeV energy with a charge equal to 600 pC could be obtained. These results demonstrate that high-energy electron beams can be stably generated through direct laser acceleration using a CEP-controlled intense few-cycle LG laser pulse.

Even-order harmonic generation from nonlinear Thomson backscatter in a tightly focused Gaussian laser pulse

The electron dynamics and the Thomson backscattering spectra for an electron accelerating in a tightly focused Gaussian laser pulse are first investigated in detail. It is found that for a tightly focused Gaussian laser pulse, the ponderomotive force introduced due to the non-uniform intensity distribution of the laser pulse has the tendency to push out the electron from the laser pulse, which leads to the trajectory symmetry-breaking of the electron and then the generation of the even-order harmonics at the same time. Further, for the tightly focused Gaussian laser pulse, changes in several laser parameters, such as the increase of the laser peak amplitude, lengthening of the pulse width, and decrease of the beam waist, lead earlier to the relative ejected position of the electron to the laser pulse, which causes the more obvious trajectory symmetry-breaking of the electron, and then the more intensive peak intensity of the even-order harmonics. It is different from the well-known results of the plane waves and the Gaussian laser pulse with uniform transverse intensity distribution and provides a possible way for the generation of the even-order harmonics in nonlinear Thomson backscattering.