Light beams with a helical phase-front (optical vortices) carry orbital angular momentum (OAM), which provides an attractive degree of freedom for controlling light and developing a series of applications, such as optical tweezers, super-resolution imaging, optical communication, and quantum information processing. Recently, the versatile metasurfaces, two-dimensional arrays of subwavelength structures with space-variant phase responses, provide powerful and convenient approaches for OAM generation. We propose and experimentally demonstrate an efficient method to generate and focus the vortex beam with a planar Cassegrain metasurface. The required helical phase profile is imparted via the secondary reflective metasurface of the planar Cassegrain metasurface. The topological charge of the vortex beam is revealed by measuring the interference pattern at the focal plane of the planar Cassegrain metasurface with a reference Gaussian beam. The combination of OAM generation, Cassegrain system, and ultrathin flat metasurface renders this scheme very attractive for diverse applications at miniature scales.

In this paper, we present a new setup for the measurement of element-specific ultrafast magnetization dynamics in ferromagnetic thin films with a sub-15-fs time resolution. Our experiment relies on a split and delay approach which allows us to fully exploit the shortest X-rays pulses delivered by X-ray Free Electrons Lasers (close to the attosecond range), in an X-ray pump – X-ray probe geometry. The setup performance is demonstrated by measuring the ultrafast elemental response of Ni and Fe during demagnetization of ferromagnetic Ni and Ni₈₀Fe₂₀ (Permalloy) samples upon resonant excitation at the corresponding absorption edges. The transient demagnetization process is measured in both reflection and transmission geometry using, respectively, the transverse magneto-optical Kerr effect (T-MOKE) and the Faraday effect as probing mechanisms.

Coherent terahertz control of magnetization dynamics is an area of current interest due to its great potential for the realization of magnetization control on ultrafast timescales in commercial devices. Here we report on an experiment realized at the THz beamline of the free electron laser FLASH at DESY which offers a tunable terahertz radiation source and spontaneously synchronized free-electron laser X-ray pulses to resonantly probe the magnetization state of a ferromagnetic film. In this proof-of-principle experiment, we have excited a thin Permalloy film at different THz wavelengths and recorded the induced magnetization dynamics with photons resonantly tuned to the Ni $M_{2,3}$ absorption edge. For THz pump pulses including higher orders of the undulator source we observed demagnetization dynamics, which precise shape depended on the employed fundamental wavelength of the undulator source. Analyzing the shape in detail, we can reconstruct the temporal profile of the electric field of the THz pump pulse. This offers a new method for the realization of an in-situ terahertz beamline diagnostic which will help researchers to adjust the pulse characteristics as needed, for example, for future studies of THz induced coherent control of magnetization dynamics.

Synthesizing intense terahertz (THz) waveforms with user-defined electric field profiles represents a pivotal challenge for controlling matter at ultrafast timescales. While relativistic electron beams can serve as a versatile source of coherent radiation, conventional schemes lack the ability to deterministically sculpt the temporal structure of the emitted light. Here, we experimentally demonstrate a paradigm for generating tailored THz waveforms by manipulating the longitudinal phase space of the electron beam. By controlling the nonlinear longitudinal correlation of an energy-chirped electron beam, we generate a microbunch train with a chirped density modulation. When coupled to a tapered undulator, this structured beam produces coherent radiation, whose spectral-temporal properties are tunable via adjustment of the undulator taper. Autocorrelation measurements confirm the emission of chirped pulses spanning 8–13 THz, with spectral characteristics that align with simulations of the beam structure. Our approach, establishing a direct link between beam dynamics and light-field synthesis, opens a pathway toward customizable high-field waveforms for advanced coherent control experiments.

The gas attenuator (GAT) is a crucial device to attenuate the intensity of the free-electron laser for Shenzhen Superconducting Soft X-ray Free-electron Laser (S ³ FEL). In most current GATs, high-pumping-speed mechanical pumps have been employed as the first stage of the differential pumping system (DPS) to achieve the working pressure transition. Thus, a compact triple differential pumping chamber (TDPC) based on 3D printing was designed in our preliminary research of GATs to realize a compact, rapid and cost-effective pressure transition. The simulation results reveal that it yields good differential capability with three low-speed scroll pumps (8 L s ⁻¹ ) and efficient differential performance from 200 Pa to 22.3 Pa, which facilitates operation of the next differential pump (turbomolecular pump). Moreover, the experimental results show a vacuum leakage rate of 1.69 ×10 ⁻¹² Pa m ³ s ⁻¹ and a relative error of less than 25% compared with the simulated pressure. Furthermore, the gas injection position of the active gas cell and the heating temperature (600°C) near the differential pipe have a negligible effect on the differential performance of the TDPC. This study demonstrates the novel concept of a differential pumping chamber, reflects its application prospects in DPS, and provides a useful reference for the DPS design of GATs.