An overview of XFEL and SACLA is presented. Typical experimental methods for XFEL sources are introduced. Following a historical review, unique features of SACLA and future perspectives are summarized.

X-ray free electron laser (XFEL) produces femtosecond X-ray pulses with 10¹¹-10¹² photons/pulse and nearly full spatial coherence to offer new research opportunities in various scientific fields. To fully utilize the characteristics of XFEL, beamline optics, photon diagnostics systems, and experimental instruments have been developed at SPring-8 Angstrom Compact free-electron LAser (SACLA). This article gives an overview of the beamlines and experimental instruments at SACLA.

Intense, femtosecond X-ray pulses from X-ray free electron laser （XFEL） have enabled acquisition of diffraction patterns from protein microcrystals before the onset of radiation damage. Although a single-shot exposure of XFEL destroys a crystal, diffraction by XFEL terminates before movements of atoms become significant. Thus, XFEL allows determination of damage free structures at room temperature. In serial femtosecond crystallography, fresh microcrystals are continuously delivered by an injector to the XFEL interaction zone. When combined with an excitation laser in a pump-probe setting, this technique is of great utility for time-resolved experiments in measuring ultra-fast reactions in proteins. We will describe current situation of SFX experiments at SACLA and future prospects.

X-ray free-electron lasers （XFELs） with femtosecond pulse duration offer an innovative solution to transcend the spatial resolution limitation in conventional X-ray imaging for biological samples and soft matters by clearing up the radiation damage problem using the “diffraction-before-destruction” strategy. Building on this strategy, the authors are developing a method to image solution sample under controlled environment, pulsed coherent X-ray solution scattering （PCXSS）, using XFELs and phase retrieval algorithms in coherent diffractive imaging （CDI）. This article describes the basics of PCXSS and examples of PCXSS measurement, for a living cell and self-assemblies of gold nanoparticles, performed by the authors using SACLA. An attempt toward the industrial application of PCXSS is also described.

Pump-Probe XAFS（X-ray absorption fine structure） using SACLA（XFEL） has been applied to the investigations of the dynamic structure and electronic state change of WO₃ photocatalyst in the photoabsorption process. We found that the ultrafast electron transfer followed by the structure changes. We describe our trial to investigate the photoabsorption process and discuss the future directions.

We have investigated ultrafast electronic and nuclear dynamics in atoms, molecules and clusters induced by very intense（～50 μJ/μm²）, ultrashort（～10 fs） pulses generated by SACLA. At photon energy of 5.0～5.5 keV, we could identify that Xeⁿ⁺ with n up to 26 is produced, evidencing occurrence of deep inner-shell ionization and sequential electronic decay cycles repeated multiple times in the xenon atom within ～10 fs pulse duration. The results for momentum-resolved multiple ion coincidence study on iodine-contained organic molecules（iodomethane and 5-iodouracil）illustrates that the charges are produced by the cycles of deep inner-shell ionization of the iodine atom and sequential electronic decay and spread over the entire molecule within 10 fs, leading to Coulomb explosion. The measured momentum distributions and correlations are well reproduced by the model calculations. The results for electron spectroscopy on argon and xenon clusters, with help of model calculations, illustrate that nanoplasma is formed by the XFEL pulse in tens of fs, and continuous thermal emission from the plasma occurs in ps.

Recent X-ray free electron lasers（XFEL） open new scientific field of broad area. Quantum optics in hard x ray photon are one of the advanced field of them. Specially, after success of XFEL pumped Kα laser, many quantum processes come within the range of research and scientific applications. In this article, simple physical model for understanding these activities is introduced with actual experimental results. Prospect of next possibilities is also briefly mentioned.