Surface-plasmon control of ultrafast energy-relaxation modes in photoexcited Au nanorods probed by time-resolved single-particle X-ray imaging.

Ultrafast laser excitation can drive materials into exotic states beyond thermodynamic limits, offering alternative ways to control how matter stores and releases energy. Yet, whether light can actively steer energy-relaxation pathways during structural transitions remains unclear due to the lack of direct experimental evidence. Here we show, using single-pulse time-resolved X-ray imaging of gold nanorods, that photoinduced localized surface plasmons control ultrafast energy relaxation into distinct deformation modes, transverse or longitudinal deformation modes, each accompanied by characteristic plasmon-induced oscillatory distortions depending on the laser fluence. Numerical simulations further confirm that localized surface plasmons dictate ultrafast energy relaxation process from photoexcited hot electrons to anharmonic nanocrystal deformations. Our results provide direct evidence that surface plasmon-mediated interactions enable ultrafast, nanoscale control of materials' energetics, opening a pathway for tailoring energy-transfer processes with femtosecond laser fields. This approach lays the foundation for customizing nonequilibrium phase dynamics at the nanoscale and provides a route to tailoring energy-transfer processes using femtosecond laser fields.