A scheme to produce mega-electron-volt- (MeV) level gamma rays with a high level of brilliance, a small divergence angle, and controllable angular momentum from Laguerre-Gaussian (LG) laser-pulse interactions with underdense plasma is proposed. Three-dimensional particle-in-cell simulations show that the gamma photon beam acquires angular momentum from the helically distributed relativistic electrons driven by the LG laser and the self-generated fields in the plasma bubble created by the laser. The divergence angle and the orbital angular momentum (OAM) of the gamma photons can be controlled by manipulating the laser parameters. It is found that a 1-MeV helical gamma-ray pulse with peak brightness $2.23\times {10}^{24}\mathrm{photons}/\mathrm{s}/{\mathrm{mm}}^2/{\mathrm{mrad}}^2/0.1\mathrm{\%}\mathrm{BW}$ and angular momentum $7.1\times {10}^{-15}\mathrm{kg}{\mathrm{m}}^2/\mathrm{s}$, as well as a $<{10}^{\circ }$ divergence angle, can be generated by a $2\times {10}^{22}\mathrm{W}/{\mathrm{cm}}^2$ right-hand circularly polarized LG laser pulse. Such bright gamma rays with OAM offer an additional degree of freedom, which is relevant for understanding quantum-electrodynamics phenomena involving angular momentum, astrophysical phenomena, time-resolved probing of the nucleus, the generation of a vortical positron beam, etc.

Reaching gigagauss magnetic fields opens new horizons both in atomic and plasma physics. At these magnetic field strengths, the electron cyclotron energy ℏω _c becomes comparable to the atomic binding energy (the Rydberg), and the cyclotron frequency ω _c approaches the plasma frequency at solid state densities that significantly modifies optical properties of the target. The generation of such strong quasistatic magnetic fields in laboratory remains a challenge. Using supercomputer simulations, we demonstrate how it can be achieved all-optically by irradiating a micro-channel target by a circularly polarized relativistic femtosecond laser. The laser pulse drives a strong electron vortex along the channel wall, inducing a megagauss longitudinal magnetic field in the channel by the Inverse Faraday Effect. This seed field is then amplified up to a gigagauss level and maintained on a sub-picosecond time scale by the synergistic effect of hydrodynamic flows and dynamos. Our scheme sets a possible platform for producing long living extreme magnetic fields in laboratories using readily available lasers. The concept might also be relevant for applications such as magneto-inertial fusion.

The detection of the 11.3$\, {\rm \mu m}$ emission feature characteristic of the Si–C stretch in carbon-rich evolved stars reveals that silicon carbide (SiC) dust grains are condensed in the outflows of carbon stars. SiC dust could be a significant constituent of interstellar dust since it is generally believed that carbon stars inject a considerable amount of dust into the interstellar medium (ISM). The presence of SiC dust in the ISM is also supported by the identification of pre-solar SiC grains of stellar origin in primitive meteorites. However, the 11.3$\,\mu {\rm m}$ absorption feature of SiC has never been seen in the ISM, and oxidative destruction of SiC is often invoked. In this work, we quantitatively explore the destruction of interstellar SiC dust through oxidation based on molecular dynamics simulations and density functional theory calculations. We find that the reaction of an oxygen atom with SiC molecules and clusters is exothermic and could cause CO-loss. Nevertheless, even if this is extrapolable to bulk SiC dust, the destruction rate of SiC dust through oxidation could still be considerably smaller than the (currently believed) injection rate from carbon stars. Therefore, the lack of the 11.3$\,\mu{\rm m}$ absorption feature of SiC dust in the ISM remains a mystery. A possible solution may lie in the currently believed stellar injection rate of SiC (which may have been overestimated) and/or the size of SiC dust (which may actually be considerably smaller than submicron in size).

When a dynamic medium in which the laser propagates changes its refractive index in time, the laser changes its frequency while keeping its wavevector unchanged to fullfil the dispersion relation. This is usually applied to upshift the laser frequency with ionizing plasma. We propose an alternative technique to modify light frequency. A transient plasma grating can be generated by two identical counterpropagating laser pulses via strongly coupled stimulated Brillouin scattering (SC SBS). The rapid evolution of the plasma grating affects the wave-dispersion relation and a band gap develops around the laser frequency, dependent on the grating amplitude. As a result, the lasers convert their frequency downward to the low edge of the band gap, while a free-traveling laser converts its frequency to both the upper and lower edge of the band gap. Depending on the considered setup, practical applications of this technique include either laser-frequency downshift or spectral splitting can be exploited. The former can be used for Raman amplification in plasma and the latter for dual-color x-ray generation by Thomson and/or Compton scattering.

The dynamics of electron injection in the direct laser acceleration (DLA) regime is investigated using three-dimensional particle-in-cell simulations and theoretical analyses. It is shown that, as an ultraintense laser pulse propagates into a near-critical density or relativistically transparent plasma, the longitudinal charge-separation electric field excites ion density spikes, which modulate the local electric field. The corresponding electric field acts as a series of potential wells to guide the electrons on the edge of the plasma channel into its center where the DLA can take place. On the other hand, an azimuthal magnetic field is self-generated, and it can deflect the injected electrons from the intense laser-field region. Understanding these physical processes paves the way for further optimizing the properties of direct-laser accelerated electron beams and the associated x- and $\gamma$-ray sources.