In recent years, short wavelength free electron lasers (FELs) have opened up access to ultrafast electronic and structural dynamics in matter. Currently, four FEL facilities are in operation in the world. FLASH  in Germany and FERMI  in Italy cover the range from extreme ultraviolet (EUV) to soft X-rays, while LCLS  in the U.S. and SACLA  in Japan provide pulses in the hard X-ray regime. In addition, an upgrade version of SCSS , nicknamed SCSS+, has also just started user operation as a beamline of SACLA . These FELs deliver coherent pulses combining unprecedented power densities up to ~1020 W/cm2 and extremely short pulse durations down to a few femtoseconds. The intense coherent FEL pulse focused down to ~1 μm2 makes single-shot diffractive imaging of nano-crystals or even non-crystallized bio-samples as well as other small objects a reality. Time-resolved spectroscopic and structural studies on the timescale of femtoseconds, having FEL pulses as a probe, allow us to probe electrons and atoms in action. Additionally, since FEL pulses are in a new regime of intensity, they are opening up new research fields that exploit the interaction between intense short wavelength pulses and matter, leading to matter at extremely high energy. Relevant theories dealing with such extreme conditions are also rapidly growing.
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