This Letter presents for the first time a scheme to generate intense high-order optical vortices that carry orbital angular momentum in the extreme ultraviolet region based on relativistic harmonics from the surface of a solid target. In the three-dimensional particle-in-cell simulation, the high-order harmonics of the high-order vortex mode is generated in both reflected and transmitted light beams when a linearly polarized Laguerre-Gaussian laser pulse impinges on a solid foil. The azimuthal mode of the harmonics scales with its order. The intensity of the high-order vortex harmonics is close to the relativistic region, with the pulse duration down to attosecond scale. The obtained intense vortex beam possesses the combined properties of fine transversal structure due to the high-order mode and the fine longitudinal structure due to the short wavelength of the high-order harmonics. In addition to the application in high-resolution detection in both spatial and temporal scales, it also presents new opportunities in the intense vortex required fields, such as the inner shell ionization process and high energy twisted photons generation by Thomson scattering of such an intense vortex beam off relativistic electrons. Light beams can exhibit helical wave fronts: the light phase "winds up" around the spatial beam center and forms an optical vortex. The phase wind imprints an orbital angular momentum (OAM) to the beam [1,2]. The characteristic helical phase profiles of optical vortices are described by expðilϕÞ multipliers, where ϕ is the azimuthal coordinate and the integer number l is their topological charge, corresponding to the order of the mode. The total phase accumulated in one full annular loop is 2πl, and an OAM of lℏ is carried by per photon for an l-order linearly polarized optical vortex beam. Based on this, the high-order optical vortex beam provides a powerful tool in optical information to investigate the entanglement state [3] and for studies of cold atoms and enhancing atomic transition [4][5][6][7].In order to provide more quantum information and for other potential applications, high-order vortex beams are required. However, limited by the etching resolution, the common method using forked diffraction grating [8] or the spiral phase plates [1] to generate the optical vortex beams is difficult to be used to obtain them. Many studies have attempted to generate light beams with OAM. For example, a relativistic electron beam can act as a mode converter that interacts with a laser in a helical undulator [9-11] and high-energy photons in MeV-GeV with OAM can be obtained by Compton backscattering of twisted laser photons off relativistic electrons [12], where the mode of the Laguerre-Gaussian (LG) pulse remains unchanged. In addition, in view of the gas high-order harmonics generation (HHG) scheme [13][14][15], because of the confluence of OAM and HHG, this scheme has an extraordinarily promising perspective. The observed harmonics possess a helical wave front in both experimental [16,17] and theoretical...

When a relativistic laser pulse with a high photon density interacts with a specially tailored thin foil target, a strong torque is exerted on the resulting spiral-shaped foil plasma, or "light fan." Because of its structure, the latter can gain significant orbital angular momentum (OAM), and the opposite OAM is imparted to the reflected light, creating a twisted relativistic light pulse. Such an interaction scenario is demonstrated by particle-in-cell simulation as well as analytical modeling, and should be easily verifiable in the laboratory. As an important characteristic, the twisted relativistic light pulse has a strong torque and ultrahigh OAM density. DOI: 10.1103/PhysRevLett.112.235001 PACS numbers: 52.38.-r, 03.50.De, 42.50.Tx, 52.59.-f Prompted by the fast development of laser techniques [1], light-matter interaction has entered the regime of a relativistic laser-plasma interaction. Over the past few decades, a number of novel mechanisms and schemes have been proposed. Among these mechanisms and schemes, the most promising application is for use in laser-driven plasma accelerator science, such as laser wakefield acceleration of electrons [2] and a laser driving foil to accelerate protons [3]. Laser-plasma interaction can also be an efficient source of high-order harmonic generation (HHG) [4], x rays [5], and even gamma rays [6,7]. One of the key issues in the above mechanisms is how to make use of the laser ponderomotive force efficiently to pump a strong charge separation field in plasma, which is the origin of particle acceleration. Hence, it is the force (the accelerating force, the confining force, etc.) that people care about most in relativistic laser plasma physics. The effect of another important dynamical quantity, the torque, although as important as force, has not been revealed for a relativistic laser pulse. How to observe the orbital angular momentum (OAM) in laser-plasma interaction and how the appearance of OAM would essentially affect the process are of special interest. Circularly polarized light carries a spin angular momentum of AEℏ per photon; however, the total OAM of a normal Gaussian pulse, commonly found in the current chirped pulse amplification technology, is zero. Therefore, observation of the torque and OAM in relativistic laserplasma interaction is rare.OAM has been discussed extensively for weak light [8-13] and extreme ultraviolet light [14][15][16]. Since Allen et al. first showed that a Laguerre-Gaussian (LG) laser pulse has finite OAM [8], many applications using twisted light have been found [9][10][11]. The OAM of a twisted light can be transferred to matter. More interestingly, several phenomena observed in astrophysics, like pulsars, are related to the OAM of light and plasma [17,18]. Thus, simulating and investigating such an immense process in a laboratory on the Earth would be of great convenience. Recently, Mendonca et al. have derived the solutions of plasma wave with OAM [19,20]. They also created a donut plasma wakefield using an intense laser with OAM f...

Extreme-ultravoilet (XUV) attosecond pulses with durations of a few tens of attosecond have been successfully applied for exploring ultrafast electron dynamics at the atomic scale. But their weak intensities limit the further application in demonstrating nonlinear responses of inner-shell electrons. Optical attosecond pulses will provide sufficient photon flux to initiate strong-field processes. Here we proposed a novel method to generate an ultra-intense isolated optical attosecond pulse through relativistic multi-cycle laser pulse interacting with a designed gas-foil target. The underdense gas target sharpens the multi-cycle laser pulse, producing a dense layer of relativistic electrons with a thickness of a few hundred nanometers. When the dense electron layer passes through an oblique foil, it emits single ultra-intense half-cycle attosecond pulse in the visible and ultraviolet spectral range. The emitted pulse has a peak intensity exceeding 1018 W/cm2 and full-width-half-maximum duration of 200 as. The peak power of this attosecond light source reaches 2 terawatt. The proposed method relaxes the single-cycle requirement on the driving pulse for isolated attosecond pulse generation and significantly boosts the peak power, thus it may open up the route to new experiments tracking the nonlinear response of inner-shell electrons as well as nonlinear attosecond phenomena investigation.

We present a new magnetic field generation mechanism in underdense plasmas driven by the beating of two, co-propagating, Laguerre-Gaussian (LG) orbital angular momentum (OAM) laser pulses with different frequencies and also different twist indices. The resulting twisted ponderomotive force drives up an electron plasma wave with a helical rotating structure. To second order, there is a nonlinear rotating current leading to the onset of an intense, static axial magnetic field, which persists over a long time in the plasma (ps scale) after the laser pulses have passed by. The results are confirmed in three-dimensional particle-in-cell simulations and also theoretical analysis. For the case of 300 fs duration, 3.8×10 17 W/cm 2 peak laser intensity we observe magnetic field of up to 0.4 MG. This new method of magnetic field creation may find applications in charged beam collimation and microscale pinch.

Nowadays, human's understanding of the fundamental physics is somehow limited by the energy that our high energy accelerators can afford. Up to 4 TeV protons are realized in the Large Hadron Collider (LHC). Leptons, such as electrons and positrons, however gained energies of about 100 GeV or less. Multi-TeV lepton accelerators are still lacking due to the relatively low acceleration gradient of conventional methods, which may induce unbearable cost. On the other hand, plasmas have shown extraordinary potential in accelerating electrons and ions, providing orders of magnitude higher acceleration fields of 10–100 GV/m. In such context, we propose a plasma-based high-energy lepton accelerator, in which a weakly focusing plasma structure is formed near the beam axis. The structure preserves the emittance of the accelerated beam and produces low radiation losses. Moreover, the structure allows for a considerable decrease of the witness energy spread at the driver depletion stage.

With the development of ultra-intense laser technology, MeV ions can be obtained from laser–foil interactions in the laboratory. These energetic ion beams can be applied in fast ignition for inertial confinement fusion, medical therapy, and proton imaging. However, these ions are mainly accelerated in the laser propagation direction. Ion acceleration in an azimuthal orientation was scarcely studied. In this research, a doughnut Laguerre–Gaussian (LG) laser is used for the first time to examine laser–plasma interaction in the relativistic intensity regime in three-dimensional particle-in-cell simulations. Studies have shown that a novel rotation of the plasma is produced from the hollow screw-like drill of an mode laser. The angular momentum of particles in the longitudinal direction produced by the LG laser is enhanced compared with that produced by the usual laser pulses, such as linearly and circularly polarized Gaussian pulses. Moreover, the particles (including electrons and ions) can be trapped and uniformly compressed in the dark central minimum of the doughnut LG pulse. The hollow-structured LG laser has potential applications in the generation of x-rays with orbital angular momentum, plasma accelerators, fast ignition for inertial confinement fusion, and pulsars in the astrophysical environment.

Using plasma mirror injection we demonstrate, both analytically and numerically, that a circularly polarized helical laser pulse can accelerate highly collimated dense bunches of electrons to several hundred MeV using currently available laser systems. The circular-polarized helical (Laguerre–Gaussian) beam has a unique field structure where the transverse fields have helix-like wave-fronts which tend to zero on-axis where, at focus, there are large on-axis longitudinal magnetic and electric fields. The acceleration of electrons by this type of laser pulse is analyzed as a function of radial mode number and it is shown that the radial mode number has a profound effect on electron acceleration close to the laser axis. Using three-dimensional particle-in-cell simulations a circular-polarized helical laser beam with power of 0.6 PW is shown to produce several dense attosecond bunches. The bunch nearest the peak of the laser envelope has an energy of 0.47 GeV with spread as narrow as 10%, a charge of 26 pC with duration of ∼ 400 as, and a very low divergence of 20 mrad. The confinement by longitudinal magnetic fields in the near-axis region allows the longitudinal electric fields to accelerate the electrons over a long period after the initial reflection. Both the longitudinal E and B fields are shown to be essential for electron acceleration in this scheme. This opens up new paths toward attosecond electron beams, or attosecond radiation, at many laser facilities around the world.

A three-dimensional (3D) particle-in-cell (PIC) simulation is used to investigate witness proton acceleration in underdense plasma with a short intense Laguerre-Gaussian (LG) laser pulse. Driven by the LG 10 laser pulse, a special bubble with an electron pillar on the axis is formed in which protons can be well confined by the generated transversal focusing field and accelerated by the longitudinal wakefield. The risk of scattering prior to acceleration with a Gaussian laser pulse in underdense plasma is avoided, and protons are accelerated stably to much higher energy. In the simulation, a proton beam has been accelerated to 7 GeV from 1 GeV in underdense tritium plasma driven by a 2.14 × 10 22 W cm −2 LG 10 laser pulse.