A femtosecond time-resolved Bragg coherent diffraction imaging experimental setup at PAL-XFEL for nanoscale lattice dynamics studies.

We report on the design and operational characterization of a femtosecond time-resolved Bragg coherent X-ray diffraction imaging (tr-BCDI) setup at the XSS beamline of PAL-XFEL, aimed at enabling direct nanoscale visualization of ultrafast lattice dynamics in individual nanocrystals. The pump-probe temporal resolution was evaluated via time-resolved X-ray diffraction measurements on a Bi (003) epitaxial thin film, confirming a timing precision better than 60 fs through the observation of coherent phonon oscillations. The static coherent imaging capability was benchmarked by reconstructing three-dimensional displacement fields of Au and zeolite nanocrystals, achieving spatial resolutions of 32.3 nm and 50.3 nm, respectively. Crucially, the time-resolved imaging capability was directly demonstrated by tracking the ultrafast structural response of a zeolite nanocrystal under femtosecond laser excitation, successfully capturing the transient shift of the Bragg peak and the corresponding projection of the displacement fields through two-dimensional phase retrieval at a fixed angular condition. These results collectively establish the XSS tr-BCDI setup as a reliable and complementary framework within the global landscape of XFEL facilities, providing direct access to femtosecond-scale structural dynamics in crystalline nanomaterials.