Xiaofei Shen

High-energy ion irradiation deposits extreme energy in a narrow range (∼1–10 nm) along ion trajectories in solid through electronic energy loss, producing unique irradiation effects such as ion tracks. However, intrinsic velocity effects impose an upper limit on electronic energy loss that cannot be overcome by adjusting irradiation parameters. We introduce a method using electric fields during irradiation to enhance nanoscale energy deposition by accelerating ion-excited electrons within subpicosecond timescales. Our extended field-enhanced thermal spike model quantitatively describes this enhancement and predicts a significant reduction in the electronic energy loss required for ion track formation in amorphous SiO 2 , which is in excellent agreement with experimental observations. This work provides an approach to control energy deposition during irradiation and boosts the wide application of ion tracks in material modification and nanoengineering to much broader extents.

Relativistic electrons colliding with intense counterpropagating laser pulses are expected to lose energy through radiation reaction. However, we reveal a counterintuitive regime where reflected leptons (including incident electrons, generated electrons, and positrons) gain significant energies when an ultraintense laser pulse interacts with counterpropagating electrons. Because of strong radiation reaction, these particles can be halted and reflected near the laser peak. The subsequent asymmetric laser field then accelerates the reflected leptons to energies far exceeding their initial values. Using three-dimensional particle-in-cell simulations, we demonstrate the generation and acceleration of quasimonoenergetic positrons to multi-GeV energies with a high number conversion efficiency employing forthcoming multipetawatt lasers, which provide conceptually a simple single-stage solution for positron creation and acceleration. The proposed mechanism of lepton acceleration is scalable to the astrophysical scenario and can be related to the origin of ultrahigh-energy cosmic rays in environments with intense fast radio bursts.

A novel scheme for generating high-flux neutron beams via collisionless shock acceleration is proposed, where a subpicosecond laser pulse interacts with a practical gas cell target composed of uniform deuteron gas and a solid foil covering the front of the cell. When the heated foil expands into the low-density gas, a strong electrostatic field induced by hot electrons rolls up the gas deuterons, compressing them into a high-density peak while accelerating them to supersonic speeds. This process would spontaneously launch a strong collisionless shock wave, without the need for a specific plasma profile. Copious upstream deuterons will be continuously reflected and accelerated to high energies. Once the laser pulse penetrates the thin foil due to relativistic transparency, the shock velocity and the reflected deuteron energies are further enhanced. By combining two-dimensional particle-in-cell and three-dimensional Monte Carlo simulations, we demonstrate that by utilizing a LiF catcher target, a high-flux collimated neutron beam with yield per unit solid angle exceeding 10 10   n sr − 1 is generated via 7 L i ( d , x n ) reactions driven by a subpicosecond laser pulse at an intensity of 8.8 × 10 19   W cm − 2 , six times higher than that from a typical foil target without deuterium gas. This simple approach overcomes the challenges of controlling the density profiles in conventional ablation schemes, paving the way for laser-driven compact neutron sources.

Circularly-polarized (CP) high harmonics (HHs) are pivotal for investigating chirality-sensitive light-matter interactions. While bichromatic counterrotating (BC) CP lasers have proved effective in diverse applications, particularly in producing CP HHs, their generation at relativistic intensities remains challenging because of the damage threshold of conventional optics. Here, we propose a novel and highly integrated approach to generate relativistic BCCP pulses, where a magnetized plasma slab is utilized to serve as an indestructible waveplate and filter simultaneously. When a bichromatic linearly-polarized laser pulse propagates through this plasma optic, each branch of the incident pulse will split into two CP pulses with opposite helicity and different dispersion relations. We identify a parameter window where the plasma optic acts as a filter that allows only the right CP pulse at lower frequency and the left CP pulse at higher frequency to propagate, constructing a BCCP field. Its topological properties can be easily manipulated via adjusting the initial components of the incident lasers. Three-dimensional particle-in-cell simulations demonstrate the feasibility of our approach with currently available laser facilities. This paves the way for producing intense CP HHs and manipulating the topology of materials and particle beams.

We propose a laboratory accelerated test method for study of the atmospheric neutron radiation effects by using a compact laser-driven spallation neutron source (LDSNS). The LDSNS is obtained by injecting a GeV quasimonoenergetic proton beam accelerated by light-sail (LS) radiation pressure of intense lasers into a lead target. According to the LS scaling law, proton energy spectra can be regulated by adjusting the laser and foil target parameters, so that the neutron spectra at various atmospheric altitudes are well reproduced. Integrated PIC and MC simulations have verified the scheme (for that at altitude 12 km, laser intensity of 10 22 W / cm 2 is required) and shown that the neutron fluence per laser shot is as high as 1.73 × 10 8 n / cm 2 , equivalent to 3.7 year flux accumulation in atmosphere. Using the G4SEE toolkit, we estimate that the HM628128 SRAM may undergo an average of ten single event upsets per shot for the proposed laboratory test.

A method for the generation of ultrarelativistic electron beams with high spin polarization is put forward, where a tightly focused linearly polarized ultraintense laser pulse interacts with a nonprepolarized transverse-size-tailored solid target. The radiative spin polarization and angular separation is facilitated by the standing wave formed via the incident and reflected laser pulses at the overdense plasma surface. Strong electron heating caused by transverse instability enhances photon emission in the density spikes injected into the standing wave near the surface. Two groups of electrons with opposite transverse polarization emerge, antialigned to the magnetic field, which are angularly separated in the standing wave due to the phase-matched oscillation of the magnetic field and the vector potential. The polarized electrons propelled into the plasma slab are focused at the exit by the self-generated quasistatic fields. Our particle-in-cell simulations demonstrate the feasibility of highly polarized electrons with a single 10 PW laser beam, e.g., with polarization of 60 % and charge of 8 pC selected at energy of 200 MeV within 15 mrad angle and 10 % energy spread. Published by the American Physical Society 2024

We investigate collisionless shock acceleration of spin-polarized 3 He for laser pulses with normalized vector potentials in the range a 0 = 100 − 200 . The setup utilized in the 2D-particle-in-cell simulations consists of a solid carbon foil that is placed in front of the main Helium target. The foil is heated by the laser pulse and shields the Helium from the highly oscillating fields. In turn, a shock wave with more homogeneous fields is induced, leading to highly polarized ion beams. We observe that the inclusion of radiation reaction into our simulations leads to a higher beam charge without affecting the polarization degree to a significant extent.