Frontiers of free-electron laser science II

The advent of short wavelength free electron lasers (FELs) has opened new research pathways for investigating ultrafast electronic and structural dynamics in matter. FELs can deliver both transversely and longitudinally coherent pulses across a broad spectrum extending from vacuum ultraviolet to x-ray regimes, combining unprecedented power densities near ∼10²⁰ W cm⁻² and extremely short pulse durations down to a few femtoseconds. Intense FEL pulses focused to ∼1 μm² make single-shot diffractive imaging of nano-crystals or even non-crystalized bio-samples as well as other small objects a reality. Time-resolved spectroscopic and structural studies on the timescale of femtoseconds, with FEL pulses as the 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 highly ionized solids that are novel because the matter is at high energy density, or because it undergoes violent Coulomb explosions as an isolated molecule. Relevant theories dealing with such extreme conditions are also rapidly growing. The special issue 'Frontiers of free-electron laser (FEL) science II' is a collection of these recent activities enabled by fourth generation light sources, i.e., short wavelength FELs in Europe, (FLASH and FERMI), Japan (SACLA and SCSS), and USA (LCLS) as well as relevant theories. In this issue you will find reports on new experimental methods, instrumentation, and theoretical tools. The present compilation of results is by no means complete, but we expect that this collection will be a resource for the rapidly expanding scientific community in this research field.