Coherent and transient states studied with extreme ultraviolet and X-ray free electron lasers: present and future prospects

The most recent light sources, extreme ultraviolet (EUV) and X-ray free electron lasers (FELs), have extended tabletop laser experiments to shorter wavelengths, adding element and chemical state specificity by exciting and probing electronic transitions from core levels. Through their unique properties, combining femtosecond X-ray pulses with coherence and enormous peak brightness, the FELs have enabled studies of a broad class of dynamic phenomena in matter that crosses many scientific disciplines and have led to major breakthroughs in the last few years. In this article, we review how the advances in the performance of the FELs, with respect to coherence, polarization and multi-color pulse production, have pushed the development of original experimental strategies to study non-equilibrium behavior of matter at the femtosecond–nanometer time–length scales. In this review, the emphasis is placed on the contribution of the EUV and soft X-ray FELs on three important subjects: (i) the new regime of X-ray matter interactions with ultrashort very intense X-ray pulses, (ii) the new potential of coherent imaging and scattering for answering questions about nano dynamics in complex materials and (iii) the unique possibility to stimulate and probe nonlinear phenomena that are at the heart of conversion of light into other forms of energy, relevant to photovoltaics, femtosecond magnetism and phase transitions in correlated materials.