X-ray Free Electron Lasers

High-brightness self-seeded X-ray free-electron laser covering the 3.5 keV to 14.6 keV range

This study successfully demonstrates a high-brightness self-seeded X-ray FEL at PAL-XFEL across a broad energy range from 3.5 keV to 14.6 keV. By utilizing thin diamond crystals as monochromators, the research achieved a peak spectral brightness approximately 40 times higher than that of Self-Amplified Spontaneous Emission (SASE) and a narrow bandwidth of 0.19 eV, which is close to the Fourier transform limit. Notably, the study proved that the high stability of the self-seeding system and the suppression of pedestal effects significantly improved the data quality for serial femtosecond crystallography (SFX) experiments.

I. Nam, et al. "High-brightness self-seeded X-ray free-electron laser covering the 3.5 keV to 14.6 keV range" Nature Photonics 15, 435–441 (2021).

Generation of time-synchronized two-color X-ray free-electron laser pulses using phase shifters

This research presents a novel method for generating time-synchronized two-color XFEL pulses by adjusting phase shifters between undulators to create a $\pi$ phase difference (out-of-phase) between the electron beam and the radiation pulse. Theoretical analysis and simulations revealed that this configuration suppresses the resonant frequency while amplifying the side-band spectrum. In experiments at PAL-XFEL, the team successfully generated pulses with two wavelengths separated by approximately 60 eV at 12.38 keV (250 µJ), providing a valuable light source for pump-probe experiments.

M.-H. Cho, et al. "Generation of time-synchronized two-color X-ray free-electron laser pulses using phase shifters" Sci. Rep. 13, 13786 (2023).

XFEL new XFEL modes and diagnostics

Attosecond XFEL: Enables the generation of sub-femtosecond pulses, crucial for exploring quantum electron motion and correlated dynamics on their natural timescales. This opens a direct window into attosecond quantum physics.

TW-scale X-ray pulses: Provide terawatt-level peak powers that allow access to the extreme nonlinear regime of X-ray–matter interactions, where multi-photon absorption and strong-field quantum electrodynamics (QED) effects become observable.

Phase-locked two-pulse XFEL: Generates two coherent X-ray pulses with fixed phase relation, a powerful tool for coherent control of matter at atomic length and ultrafast time scales.

Coaxial VMI (Velocity Map Imaging): To fully utilize these advanced modes, equally sophisticated diagnostics are required. The development of coaxial VMI enables direct measurement of attosecond X-ray pulses by mapping photoelectron momentum distributions with high precision.

This allows researchers to resolve the temporal and spectral structure of ultrashort pulses, ensuring accurate characterization and control of next-generation XFEL beams.

EUV-FEL source for semiconductor industry

Extreme ultraviolet (EUV) light is indispensable for next-generation semiconductor manufacturing, where shorter wavelengths enable finer chip patterning. Current laser-produced plasma (LPP) sources, operating near 500 W, are reaching limits in efficiency and scalability, making it difficult to achieve the kilowatt-class power required for future high-volume production. A promising solution is the adoption of free-electron lasers (FELs) driven by relativistic electron beams. Passing such beams through undulators can generate EUV radiation at 13.6 nm with high stability, and by simply adjusting undulator conditions, even shorter wavelengths such as 6.7 nm become accessible. Unlike LPP, FEL sources provide much cleaner beams with easier transport, reducing system complexity and contamination risks. This positions EUV-FELs as the clear path forward for scaling semiconductor lithography.