arXiv subjects

Hanghua Xu

Publications and source records attributed to Hanghua Xu.

6 recordsLinked to original sources

arXiv2026

Self-Synchronized Terahertz and X-Ray Free-Electron Lasers from a Single Pre-Bunched Electron Beam

Ultrafast pump-probe spectroscopy combining intense terahertz (THz) and X-ray pulses is a critical tool for investigating complex structural and electronic dynamics in materials. However, current setups combining THz sources and X-ray free-electron lasers (FELs) often suffer from high system complexity, inherent timing jitter, or limited THz pulse properties. Here, we experimentally demonstrate the generation of intrinsically synchronized, strong-field, narrow-band THz and X-ray FELs from a single pre-bunched electron beam. Sequentially passing the beam through X-ray and THz amplifiers reveals a highly synergistic process: the initial periodic THz density modulation notably boosts the X-ray FEL pulse energy, while robustly surviving the intense X-ray emission to drive high-power, narrow-band THz radiation. Originating from the same electron bunch, the two pulses inherently maintain a precise, constant time delay. This jitter-free scheme establishes a highly reliable platform tailored for both X-ray-pump/THz-probe and THz-pump/X-ray-probe experiments.

physics.acc-ph↗

arXiv2026

Demonstration of High-Gain Harmonic Lasing in a Terahertz Free-Electron Laser

Compact Free-Electron Lasers (FELs) offering broad, continuous spectral tunability are traditionally constrained by fixed-parameter magnetic structures and the necessity for high-energy electron beams. High-gain Harmonic Lasing (HL) has long been proposed as a solution to overcome these limitations; however, a robust experimental verification of this principle has remained absent. Here, we report the first experimental demonstration of high-gain HL. By employing a frequency-tunable electron beam density modulation to dominate the fundamental instability, we achieved sustained FEL amplification at the 3rd and 5th harmonics of the wiggler. The HL mode generated output power comparable to conventional fundamental operation with enhanced stability and narrower spectral bandwidth. Notably, we demonstrate that HL extends the spectral coverage by a factor of two under fixed facility constraints, achieving pulse energies up to 540 {\mu}J. These results establish high-gain HL as a versatile mechanism for advancing compact, wavelength-flexible FEL facilities.

physics.acc-ph↗

arXiv2025

High-Precision Measurement of D($\gamma$, $n$)$p$ Photodisintegration Reaction and Implications for Big-Bang Nucleosynthesis

We report on a high-precision measurement of the D($\gamma$,\,$n$)$p$ photodisintegration reaction at the newly commissioned Shanghai Laser Electron Gamma Source (SLEGS), employing a quasi-monochromatic $\gamma$-ray beam from Laser Compton Scattering. The cross sections were determined over $E_\gamma$=2.327--7.089 MeV, achieving up to a factor of 2.2 improvement in precision near the neutron separation threshold. Combined with previous data in a global Markov chain Monte Carlo (MCMC) analysis using dibaryon effective field theory, we obtained the unprecedentedly precise $p$($n$,\,$\gamma$)D cross sections and thermonuclear rate, with a precision up to $\approx$4 times higher than previous evaluations. Implemented in a standard Big-Bang Nucleosynthesis (BBN) framework, this new rate decreases uncertainty of the key cosmological parameter of baryon density $\Omega_b h^2$ by up to $\approx$16\% relative to the LUNA result. A residual $\approx$1.2$\sigma$ tension between $\Omega_b h^2$ constrained from primordial D/H observations and CMB measurements persists, highlighting the need for improved $dd$ reaction rates and offering potential hints of new physics beyond the standard model of cosmology.

nucl-ex↗

arXiv2025

First systematic experimental 2D mapping of linearly polarized $\gamma$-ray polarimetric distribution in relativistic Compton scattering

The interaction of photons with relativistic electrons constitutes a fundamental electromagnetic process whose polarization transfer mechanics remain incompletely characterized. We report the first systematic measurement of spatial polarization distribution for $\gamma$-rays generated via \SI{45}{\degree} slant inverse Compton scattering (ICS) between linearly polarized \SI{0.117}{\eV} photons and \SI{3.5}{\GeV} electrons, performing full 2D mapping of intensity, polarization angle (AOP), and degree of polarization (DOP). Measurements reveal an asymmetric beam profile along the laser's polarization direction that resembles \SI{180}{\degree} backward ICS observations. The central beam region exhibits DOP $\approx$ 1.0 with AOP rigidly aligned at \SI{45}{\degree}, while peripheral regions display complex non-uniform polarization distributions. These findings confirm quantum electrodynamics predictions of near-complete polarization transfer along the beam axis in slant geometries, thus establishing slant scattering as a viable alternative to head-on configurations for generating high DOP $\gamma$-rays.

nucl-ex↗

arXiv2025

Enabling Continuous THz Band Coverage via Precise Electron Beam Tailoring in Free-electron Lasers

High-power, continuously tunable narrowband terahertz (THz) sources are essential for advancing nonlinear optics, THz-driven material dynamics, and ultrafast spectroscopy. Conventional techniques typically impose a trade-off between pulse energy and frequency tunability. Here, we introduce a novel free-electron laser approach that overcomes these limitations by pre-modulating a relativistic electron beam with a frequency-beating laser pulse and leveraging bunch compression along with collective effects to enhance microbunching. Experimental results demonstrate that this technique generates narrowband THz emission with continuous frequency tunability from 7.8 to 30.8THz, achieving pulse energies up to 385{\mu}J while maintaining spectral bandwidths between 7.7% and 14.7%. Moreover, the method exhibits exceptional robustness and scalability, highlighting its unique ability to bridge the long-standing THz gap and offering a promising solution for diverse cutting-edge scientific applications.

physics.acc-ph↗

arXiv2025

Zeptosecond Gamma-Ray Pulses Generation via FEL-Driven Microbunching and Laser-Compton Scattering

We introduce a novel and reliable approach to generate zeptosecond(10^{-21} s,zs), high-energy photon pulse bursts by synergistically exploiting the inherent characteristics of Free-Electron Lasers (FELs) and laser-Compton scattering. The feasibility of this scheme is validated through comprehensive numerical simulations. In a representative simulation, a 4 GeV electron beam, undergoing the FEL process, emits radiation at 0.7 nm and develops a microbunched structure. These microbunches subsequently interact with a 2 ps, 9 micrometers laser pulse in a head-on Compton scattering configuration. Our simulation results demonstrate that under conditions of well-established electron beam microbunching, the proposed method successfully yields high-brightness gamma-photon pulse bursts with durations approaching 800 zeptoseconds and exhibiting exceptional signal-to-noise ratios, consistent with theoretical expectations. This technique opens up avenues for exploring ultrafast nuclear dynamics and provides unprecedented perspectives for delving into the Quantum Zeno and Anti-Zeno effects.

physics.acc-ph↗