Xichen Hu

Relativistic interaction of ultra-intense laser pulses with cluster targets is of particular interest for high-energy-density physics, compact X-ray source development, and laboratory astrophysics. Understanding the dynamics of such plasmas requires precise control of their parameters, in particular temperature, on subpicosecond timescales. In this study, X-ray spectral methods were used to diagnose the laser plasma of krypton cluster targets, created at laser pulse intensities of the order of 10²⁰–10²¹ W/cm². The use of a time-dependent detailed radiation-collisional kinetic model made it possible to describe the results of the observed X-ray spectra in the femtosecond laser plasma of a cluster target. We present a method for diagnosing the non-stationary plasma of high-atomic-number (krypton) clusters using resonance spectral lines 1s²2s²2p⁵3s ¹P₁–1s²2s²2p^{6 1}S₀ and 1s²2s²2p⁵3s ³P₁–1s²2s²2p^{6 1}S₀ of the Ne-like Kr XXVII ion, allowing one to determine the plasma temperature at the moment of “plasma channel” formation. In the experiment, this temperature was shown to be 55 ± 5 eV. The same spectroscopic approach can be extended to other cluster species (for example, Ar, Xe) for non-stationary plasma diagnostics in the relativistic regime.

The polarization of gamma rays produced in strong-field quantum electrodynamics (SFQED) is a fundamental and long-standing prediction, the verification of which has remained elusive, limiting both foundational tests and applications. Here, we report the first experimental measurement of gamma-ray polarization generated via all-optical nonlinear Compton scattering. Colliding a laser-wakefield-accelerated electron beam with an intense counter-propagating laser pulse reflected from a plasma mirror, we produce bright gamma rays in the strong-field regime (a ₀ > 1). For gamma rays with a ₀ ≈ 3, a linear polarization degree of ≈ 50% is measured via the azimuthal asymmetry of photoneutrons from a deuterium target, and independently verified by a Compton polarimeter. The results show excellent agreement with SFQED calculations employing the locally monochromatic approximation, while diverging from predictions based on the locally constant field approximation, highlighting the importance of quantum interference effects in this regime. Our work provides experimental evidence for polarization dynamics in SFQED, supports a key prediction of nonperturbative QED, and paves the way for compact, laser-driven sources of polarized gamma rays.

Vortex γ photons carrying orbital angular momenta (OAM) hold great potential for various applications. However, their generation remains a great challenge. Here, we successfully generate sub-MeV vortex γ photons via all-optical inverse Compton scattering of relativistic electrons colliding with a subrelativistic Laguerre-Gaussian laser. In principle, directly measuring the OAM of γ photons is challenging due to their incoherence and extremely short wavelength. Therein, we put forward a novel method to determine the OAM properties by revealing the quantum opening angle of vortex γ photons, since vortex particles exhibit not only a spiral phase but also transverse momentum according to the quantum electrodynamics theory. Thus, γ photons carrying OAM manifest a much larger angular distribution than those without OAM, which has been clearly observed in our experiments. This angular expansion is considered as an overall effect lying beyond classical theory. Our method provides the first experimental evidence for detecting vortex γ photons and opens a new perspective for investigating OAM-induced quantum phenomena in broad fields.

Efficient generation of high-energy γ-rays from laser-electron interactions is crucial for advancing nuclear photonics and strong-field quantum electrodynamics (QED). However, the energy-conversion efficiency of plasma-mirror-based all-optical inverse Compton scattering (ICS) has been fundamentally constrained by the limited overlap between the reflected laser pulse and the wakefield-accelerated electron beam. Here we experimentally demonstrate a coated plasma mirror that spontaneously focuses the reflected pulse, substantially enhancing the local field intensity and driving strongly nonlinear multiphoton ICS. Experiments show that the coating improves the laser-to-radiation energy conversion efficiency by nearly an order of magnitude compared with uncoated mirrors, at the level of 10^-4-10^-3, with photon energies extending from tens to hundreds of MeV. This simple yet robust approach overcomes the intrinsic energy-utilization limit of plasma-mirror ICS and offers a scalable route toward compact, high-brightness γ-ray sources for table-top strong-field QED studies and nuclear photonics applications.

Although “ghost” imaging has seen remarkable advances since it was first studied 30 years ago, there have been few studies on x-ray ghost imaging (XGI) due to technical difficulties. A chief potential advantage of XGI is that imaging can be achieved with much lower radiation intensity. However, in previous XGI experiments with continuous wave x-ray illumination, the total sampling time was actually quite long, leading to additional radiation dose accumulation. In this paper, we report a proof-of-concept experiment using a high-sensitivity single-pixel detector together with a simple tabletop x-ray source followed by an adjustable mechanical shutter, which could reduce the total exposure time to as little as 5 ms. Imaging was realized with a spatial resolution of 100 μm under a low radiation dose. The influence of x-ray tube current, sampling frequency, and exposure times on image quality was examined. Our experiment illustrates a practical method for low-radiation dose ghost imaging and provides valuable reference data for the future development of medical XGI cameras.

High‐charge beams remain a key research focus in laser plasma acceleration, driven by the demands of extreme experimental conditions in nuclear physics, laboratory astrophysics, and fusion research. A distinct acceleration mechanism is presented in which the interaction between petawatt femtosecond laser pulses and the subcritical density (SCD) plasma produces electron beams with charges of hundreds of nanocoulomb (nC), far exceeding those typically achieved via conventional laser wakefield acceleration. The synergy between the multiwakefield structure in SCD plasma and the laser self‐steepening effect significantly enhances the electron injection rate to 100 nC ps ⁻¹ , experimentally achieving the electron beam charge up to 245 nC. This mechanism operates by reducing the phase velocity of plasma waves and increasing electron momentum, allowing large numbers of electrons to exceed the wave‐breaking threshold and undergo efficient injection. Experiments and simulations consistently confirm the dependence of charge on plasma density and laser energy, with charge reaching a maximum at a specific plasma density. Such high‐charge, high‐density electron beams offer promising applications in high‐flux particle beam physics, large‐field‐of‐view industrial imaging, advanced light sources, and radiation‐hardness testing of electronic systems.

A dual-beam platform is developed for all-optical Thomson/Compton scattering, with versatile parameter tuning capabilities including electron energy, radiation energy, radiation polarization, etc. By integrating this platform with a 200 TW Ti:sapphire laser system, we demonstrate the generation of inverse Compton scattering X-/gamma-rays with tunable energies ranging from tens of keV to MeV. The polarization of X-/gamma-rays is manipulated by adjusting the polarization of the scattering laser. In the near future, by combining this platform with multi-PW laser facilities, our goal is to explore the transition from nonlinear Thomson scattering to nonlinear Compton scattering, ultimately verifying theories related to strong-field quantum electrodynamics effects induced by extreme scattering.