Light beams with helical phase-fronts are known to carry orbital angular momentum (OAM) and provide an additional degree of freedom to beams of coherent light. While OAM beams can be readily derived ...from Gaussian laser beams with phase plates or gratings, this is far more challenging in the extreme ultra-violet (XUV), especially for the case of high XUV intensity. Here, we theoretically and numerically demonstrate that intense surface harmonics carrying OAM are naturally produced by the intrinsic dynamics of a relativistically intense circularly-polarized Gaussian beam (i.e. non-vortex) interacting with a target at normal incidence. Relativistic surface oscillations convert the laser pulses to intense XUV harmonic radiation via the well-known relativistic oscillating mirror mechanism. We show that the azimuthal and radial dependence of the harmonic generation process converts the spin angular momentum of the laser beam to orbital angular momentum resulting in an intense attosecond pulse (or pulse train) with OAM.
A stable relativistic ion acceleration regime for thin foils irradiated by circularly polarized laser pulses is suggested. In this regime, the "light-sail" stage of radiation pressure acceleration ...for ions is smoothly connected with the initial relativistic "hole-boring" stage, and a defined relationship between laser intensity I0, foil density n{0}, and thickness l{0} should be satisfied. For foils with a wide range of n{0}, the required I0 and l{0} for the regime are theoretically estimated and verified with the particle-in-cell code ILLUMINATION. It is shown for the first time by 2D simulations that high-density monoenergetic ion beams with energy above GeV/u and divergence of 10 degrees are produced by circularly polarized lasers at intensities of 10;{22} W/cm;{2}, which are within reach of current laser systems.
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The description of the dynamics of an electron in an external electromagnetic field of arbitrary intensity is one of the most fundamental outstanding problems in electrodynamics. Remarkably, to date, ...there is no unanimously accepted theoretical solution for ultrahigh intensities and little or no experimental data. The basic challenge is the inclusion of the self-interaction of the electron with the field emitted by the electron itself—the so-called radiation reaction force. We report here on the experimental evidence of strong radiation reaction, in an all-optical experiment, during the propagation of highly relativistic electrons (maximum energy exceeding 2 GeV) through the field of an ultraintense laser (peak intensity of4×1020W/cm2). In their own rest frame, the highest-energy electrons experience an electric field as high as one quarter of the critical field of quantum electrodynamics and are seen to lose up to 30% of their kinetic energy during the propagation through the laser field. The experimental data show signatures of quantum effects in the electron dynamics in the external laser field, potentially showing departures from the constant cross field approximation.
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The dynamics of energetic particles in strong electromagnetic fields can be heavily influenced by the energy loss arising from the emission of radiation during acceleration, known as radiation ...reaction. When interacting with a high-energy electron beam, today’s lasers are sufficiently intense to explore the transition between the classical and quantum radiation reaction regimes. We present evidence of radiation reaction in the collision of an ultrarelativistic electron beam generated by laser-wakefield acceleration (ϵ>500MeV) with an intense laser pulse (a0>10). We measure an energy loss in the postcollision electron spectrum that is correlated with the detected signal of hard photons (γrays), consistent with a quantum description of radiation reaction. The generatedγrays have the highest energies yet reported from an all-optical inverse Compton scattering scheme, with critical energyϵcrit>30MeV.
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Electron-positron pair plasmas represent a unique state of matter, whereby there exists an intrinsic and complete symmetry between negatively charged (matter) and positively charged (antimatter) ...particles. These plasmas play a fundamental role in the dynamics of ultra-massive astrophysical objects and are believed to be associated with the emission of ultra-bright gamma-ray bursts. Despite extensive theoretical modelling, our knowledge of this state of matter is still speculative, owing to the extreme difficulty in recreating neutral matter-antimatter plasmas in the laboratory. Here we show that, by using a compact laser-driven setup, ion-free electron-positron plasmas with unique characteristics can be produced. Their charge neutrality (same amount of matter and antimatter), high-density and small divergence finally open up the possibility of studying electron-positron plasmas in controlled laboratory experiments.
The acceleration of ions from ultrathin foils has been investigated by using 250 TW, subpicosecond laser pulses, focused to intensities of up to 3 × 10(20) W cm(-2). The ion spectra show the ...appearance of narrow-band features for protons and carbon ions peaked at higher energies (in the 5-10 MeV/nucleon range) and with significantly higher flux than previously reported. The spectral features and their scaling with laser and target parameters provide evidence of a multispecies scenario of radiation pressure acceleration in the light sail mode, as confirmed by analytical estimates and 2D particle-in-cell simulations. The scaling indicates that monoenergetic peaks with more than 100 MeV/nucleon are obtainable with moderate improvements of the target and laser characteristics, which are within reach of ongoing technical developments.
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Ultraintense laser pulses with a few-cycle rising edge are ideally suited to accelerating ions from ultrathin foils, and achieving such pulses in practice represents a formidable challenge. We show ...that such pulses can be obtained using sufficiently strong and well-controlled relativistic nonlinearities in spatially well-defined near-critical-density plasmas. The resulting ultraintense pulses with an extremely steep rising edge give rise to significantly enhanced carbon ion energies consistent with a transition to radiation pressure acceleration.
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The acceleration of ions from ultrathin (10-100 nm) carbon foils has been investigated using intense (∼6×10^{20} W cm^{-2}) ultrashort (45 fs) laser pulses, highlighting a strong dependence of the ...ion beam parameters on the laser polarization, with circularly polarized (CP) pulses producing the highest energies for both protons and carbons (25-30 MeV/nucleon); in particular, carbon ion energies obtained employing CP pulses were significantly higher (∼2.5 times) than for irradiations employing linearly polarized pulses. Particle-in-cell simulations indicate that radiation pressure acceleration becomes the dominant mechanism for the thinnest targets and CP pulses.
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We report on the experimental studies of laser driven ion acceleration from a double-layer target where a near-critical density target with a few-micron thickness is coated in front of a ...nanometer-thin diamondlike carbon foil. A significant enhancement of proton maximum energies from 12 to ∼30 MeV is observed when a relativistic laser pulse impinges on the double-layer target under linear polarization. We attributed the enhanced acceleration to superponderomotive electrons that were simultaneously measured in the experiments with energies far beyond the free-electron ponderomotive limit. Our interpretation is supported by two-dimensional simulation results.
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We report the experimental generation of highly energetic carbon ions up to 48 MeV per nucleon by shooting double-layer targets composed of well-controlled slightly underdense plasma and ultrathin ...foils with ultraintense femtosecond laser pulses. Particle-in-cell simulations reveal that carbon ions are ejected from the ultrathin foils due to radiation pressure and then accelerated in an enhanced sheath field established by the superponderomotive electron flow. Such a cascaded acceleration is especially suited for heavy ion acceleration with femtosecond laser pulses. The breakthrough of heavy ion energy up to many tens of MeV/u at a high repetition rate would be able to trigger significant advances in nuclear physics, high energy density physics, and medical physics.
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