Widely tunable two-color x-ray free-electron laser pulses

We demonstrate the generation of widely tunable two-color x-ray free-electron laser (FEL) pulses at Swiss-FEL. In a split-undulator configuration, each color is produced in a different undulator section, and a chicane between the two sections allows for a variable time separation between the two pulses of up to 500 fs. We show an unprecedented photon energy ratio between the two colors of about three (350 and 915 eV), with each individual pulse having a peak power of a few gigawatts and a duration down to the femtosecond level. Moreover, we demonstrate the reduction of the required undulator length via the optical klystron mechanism and the time-resolved diagnostics of the FEL pulses utilizing the same beam setup as for the pulse generation. The unique combination of widely tunable energy and time separation of the two-color pulse pair offers opportunities to study ultrafast x-ray-induced energy transfer and relaxation processes in physics, chemistry, and biology.