Wavelength-tunable split-and-delay optical system for hard X-ray free-electron lasers.

We developed a hard X-ray split-and-delay optical (SDO) system based on Bragg diffraction in crystal optics for generating two split pulses with a variable temporal separation. To achieve both high stability and operational flexibility, the SDO system was designed to include variable-delay and fixed-delay branches. As key optical elements, we fabricated high quality thin crystals and channel-cut crystals by applying the plasma chemical vaporization machining technique. The SDO system using Si(220) crystals covered a photon energy range of 6.5-11.5keV and a delay time range from a negative value to > 45 ps over the photon energy range (up to 220 ps at 6.5 keV). A simple alignment method for realizing a spatial overlap between the split pulses was developed. The SDO system was tested at a SPring-8 beamline in combination with a focusing system. We achieved an excellent overlap with an accuracy of 30 nm for ∼ 200 nm focused beams in both the horizontal and vertical directions. This achievement is an important progress towards the realization of time-resolved studies using multiple X-ray pulses with a time range from femtosecond to subnanosecond scales at X-ray free-electron laser facilities.