Hard x-ray intensity autocorrelation using direct two-photon absorption

An intensity autocorrelation measurement is demonstrated to characterize a pulse duration of 9-keV x-ray free-electron laser (XFEL) pulses from a split-delay optical (SDO) system with four-bounce silicon 220 reflections in each branch. XFEL pulse replicas with variable time delays are generated by the SDO system itself. High intensity of >2 x 10(16) W/cm(2) achieved in a self-seeding operation and careful data analysis allow the measurement with direct two-photon absorption. The autocorrelation trace gave a duration of 7.6 +/- 0.8 fs in full width at half maximum for a Gaussian assumption. Furthermore, the trace shows good agreement with a simulation of the XFEL pulse shape propagating through the SDO system, irrespective of spectral chirps in the original XFEL pulses. Our results open the door toward direct temporal characterization of narrowband XFELs at the hard x-ray regime, such as self-seeded and future cavity-based XFELs, and indicate a solid way for temporal tailoring of ultrafast x-ray pulses with perfect crystals.