Laser-accelerated electrons are promising in producing gamma-photon beams of high peak flux for the study of nuclear photonics, obtaining copious positrons and exploring photon–photon interaction in vacuum. We report on the experimental generation of brilliant gamma-ray beams with not only high photon yield but also low divergence, based on picosecond laser-accelerated electrons. The 120 J 1 ps laser pulse drives self-modulated wakefield acceleration in a high-density gas jet and generates tens-of-MeV electrons with 26 nC and divergence as small as $1.51{}^{\circ}$ . These collimated electrons produce gamma-ray photons through bremsstrahlung radiation when transversing a high-Z solid target. We design a high-energy-resolution Compton-scattering spectrometer and find that a total photon number of $2.2\times {10}^9$ is captured within an acceptance angle of $1.1{}^{\circ}$ for photon energies up to $16\;\mathrm{MeV}$ . Comparison between the experimental results and Monte Carlo simulations illustrates that the photon beam inherits the small divergence from electrons, corresponding to a total photon number of $2.2\times {10}^{11}$ and a divergence of $7.73{}^{\circ}$ .

人类在实验室可实现的激光强度极限是强场量子电动力学（QED）的重要问题。在非理想真空条件下，极端超强激光与残留的电子相互作用触发伽马光子辐射与正负电子对产生的QED级联效应，从而显著消耗激光能量，大幅降低可实现的激光峰值强度。考虑到QED级联效应与激光偏振、焦斑尺寸、脉宽长度有着密切的关系，基于囊括QED过程的粒子网格模拟方法（Particle-in-cell, PIC）对上述参数的效应进行分析，同时构建了激光场演化的自洽方程来进行解释，二者结果基本保持一致，获得的强度极限在考虑的参数范围内为10²⁶～10²⁷ W/cm⁻²。结果表明，同等情形下，圆偏振激光可激发更强的QED级联，使得激光强度上限略低于线偏振。此外，紧聚焦激光由于QED级联发生的时空间尺度更小，从而激光的吸收效应被显著抑制，进而可以实现更强的聚焦强度。对于更长脉宽的激光，由于正负电子对吸收的能量区域更加弥散，使得可实现的激光强度上限阈值有所提升。但对于超短脉宽情形（如单周期），由于QED级联的种子源电子束不能很好地被约束在激光区域，理论分析耗散的激光能量偏高。此外，在高真空度的情形下，残余电子的随机性也会对可实现激光强度产生一定的影响。研究结果可为后续开展极端强场QED实验和数100 PW级超强超短激光装置建设提供指导。

Spatiotemporal optical vortex (STOV) pulses carrying purely transverse intrinsic orbital angular momentum (TOAM) are attracting increasing attention because the TOAM provides a new degree of freedom to characterize light–matter interactions. In this paper, using particle-in-cell simulations, we present spatiotemporal high-harmonic generation in the relativistic region, driven by an intense STOV beam impinging on a plasma target. It is shown that the plasma surface acts as a spatial–temporal-coupled relativistic oscillating mirror with various frequencies. The spatiotemporal features are satisfactorily transferred to the harmonics such that the TOAM scales with the harmonic order. Benefitting from the ultrahigh damage threshold of the plasma over the optical media, the intensity of the harmonics can reach the relativistic region. This study provides a new approach for generating intense spatiotemporal extreme ultraviolet vortices and investigating STOV light–matter interactions at relativistic intensities.

The upper limit of the laser field strength in a perfect vacuum is usually considered as the Schwinger field, corresponding to ～1029W/cm2. We investigate such limitations under realistic nonideal vacuum conditions and find that intensity suppression appears starting from 1025W/cm2, showing an upper threshold at 1026W/cm2 level if the residual electron density in chamber surpasses 109cm-3. This is because the presence of residual electrons triggers the avalanche of quantum electrodynamics cascade that creates copious electron and positron pairs. The leptons are further trapped within the driving laser field due to radiation reaction, which significantly depletes the laser energy. The relationship between the attainable intensity and the vacuity is given according to particle-in-cell simulations and theoretical analysis. These results answer a critical problem on the achievable light intensity based on present vacuum conditions and provide a guideline for future hundreds of petawatt class laser development.

The acceleration of polarized electrons, positrons, protons and ions in strong laser and plasma fields is a very attractive option for obtaining polarized beams in the multi-mega-electron volt range. Recently, there has been substantial progress in the understanding of the dominant mechanisms leading to high degrees of polarization, in the numerical modeling of these processes and in their experimental implementation. This review paper presents an overview on the current state of the field, and on the concepts of polarized laser–plasma accelerators and of beam polarimetry.

The improved laser-to-pedestal contrast ratio enabled by current high-power laser pulse cleaning techniques allows the fine features of the target survive before the main laser pulse arrives. We propose to introduce the nano-fabrication technologies into laser–plasma interaction to explore the novel effects of micro-structures. We found out that not only laser-driven particle sources but also the laser pulse itself can be manipulated by specifically designed micro-cylinder and -tube targets, respectively. The proposal was supported by full-3D particle-in-cell simulations and successful proof-of-principle experiments for the first time. We believe this would open a way to manipulate relativistic laser–plasma interaction at the micro-size level.