Special Issue on Latest Trends in Free Electron Lasers

In the last decade, free electron laser (FEL) sources operating from the extreme ultraviolet (EUV) up to the hard X-ray photon energy range [1,2,3] have swiftly become an invaluable experimental tool for frontier scientific research [4,5,6,7,8,9,10,11,12,13], complementing and extending the properties of synchrotron sources. Since the first operation of FLASH in 2005 [14], X-ray FEL technology has made impressive progress [15,16,17], driving a rapid worldwide diffusion of FEL large scale facilities. Following a physiological period of intense development and commissioning of the new FEL sources, today the FEL technology can be considered to be reliable for routine user experiments, thus facilitating the formation of a competent FEL users community. The progressive comprehension of the FEL sources physics, gained developing diverse FEL schemes and accompanied by promising results in many different classes of novel experiments, has allowed the FEL community to envision further steps forward [18,19,20,21,22,23].

This Special Issue encompasses outstanding contributions from variegated research groups involved in the development of FELs science and technology, highlighting new emerging and promising trends. A total of eleven papers (eight research articles, one perspective article, one review and one project report) are included in this Special Issue.

Many FEL facilities are already considering to undergo radical upgrades aimed at perfecting their FEL sources and possibly extending their operating range, for example in terms of spectral range and repetition rate. The major scope of those upgrades is to offer users a more flexible FEL source suitable for a wider variety of experiments. A thorough presentation of future developments of the FLASH FEL facility is provided by Schaper et al. [24]. The FLASH2020+ project will make available a MHz fully coherent EUV seeded FEL source. On the other hand, Chen et al. [25] explore the possibility of operating the European X-ray FEL (EuXFEL) in the sub-ångström working regime (hard X-rays) with a repetition rate of 10 Hz. Eom et al. [26] discuss recent upgrades of the Pohang Accelerator Laboratory (PAL-XFEL) involving both the FEL source (increase of the photon energy and pulse intensity, enablement of multicolor and self-seeding operation) and experimental environment (laser/pump-FEL/probe schemes, time-resolved XAS and scattering setups, liquid-jet). Liu et al. [27] outline the construction status of the Shanghai soft X-ray Free-Electron Laser facility (SXFEL). The test facility delivers 100 fs photon pulses in the water window range, while the user facility is under commissioning. Geloni et al. [28] present the results of a machine physics study carried out at the European XFEL aimed at investigating the generation of X-ray pulses in frequency mixing mode.

Promising emerging experimental techniques requiring FEL sources are also presented. De Wijn et al. [29] review the potential of serial femtosecond crystallography at high repetition rate focusing on advantages and limitations of the technique and envisioning possible developments to make it accessible to a broader user community. Blachucki et al. [30] explore the possibility of performing simultaneous X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) measurements using stochastic SASE X-ray FEL beams without monochromatization, beam splitting and seeding. In addition, Blachucki et al. [31] propose an original methodology, which they termed “X-ray chronoscopy”, for exploration of ultrafast processes in matter with attosecond time resolution. The method is based on measuring, through a THz streaking detection system, the X-ray pulse temporal profile change driven by interaction of the pulse with a medium. Another innovative FEL experimental approach is described by Maroju et al. [32], which succeeded in generating attosecond pulse trains using the beating of multiple phase-locked harmonics of the FERMI seeded FEL. The resulting attosecond waveform was monitored and characterized by independently manipulating the amplitudes and phases of the FEL harmonics.

X-ray FEL radiation is also fruitfully used to conduct experiments originally developed with other light sources, like synchrotrons and high harmonic generation (HHG) facilities, taking advantage from the peculiarities of FEL light. Tkachenko et al. [33] simulate a X-ray/pump-X-ray/probe X-ray diffraction imaging experiment, investigating ultrafast processes in condensed matter driven by exposure to intense X-ray pulse, like the nonthermal melting. Liu et al. [34] describe a proof-of-principle experiment using a THz pump and a X-ray FEL probe resonant with a magnetic sample. Both the pump and probe are generated by the same electron bunch in the FEL undulator hall, thus minimizing the relative time-jitter. THz-induced demagnetization ultrafast dynamics is monitored through time-resolved Transverse Magneto-Optical Kerr Effect (T-MOKE).

The high level of quality and innovation of the articles of this Special Issue reflects the remarkable progress made by the FEL community in recent years. The presented contributions indicate important trends in FEL research and development that can play a pivotal role in future advancement in science.