Free electron laser based multicolor spectroscopy

The most recent light sources, extreme ultraviolet (EUV) and X-ray free electron lasers (FELs), aim to extended tabletop laser experiments to shorter wavelengths, adding element and chemical state specificity by exciting and probing electronic transitions from core levels. In this article, we intend to discuss how the advances in the performance of the FELs, with respect to multi-color pulse production, may push the development of original experimental strategies to study non-equilibrium behavior of matter at the femtosecond-nanometer time-length scales. This would have a tremendous impact as an experimental tool to investigate a large array of phenomena ranging from nano-dynamics in complex materials to phenomena that are at the heart of conversion of light into other forms of energy.