Ultrafast photoexcitation of plasmonic metal nanostructures results in a cascade of electronic, optical, and structural responses along discrete temporal trajectories. Surface plasmon excitation and rapid electron thermalization couple to coherent, low-frequency (i.e., GHz) acoustic modes over the span of picoseconds, and the precise nature of the coupling and mode excitation are sensitively dependent upon particle-particle and particle-substrate interactions, geometric shape, and elastic properties. Importantly, the electronic and structural responses are interdependent, as the nanoscale transient elastic distortions modulate the dielectric response function, which in turn modifies the surface plasmon resonance, while dependence of plasmonic responses on the proximity and the configuration of neighboring particles could potentially affect the coherent, acoustic modes. Among a variety of time-resolved spectroscopic and structural probes, ultrafast electron microscopy has been used to directly image discrete, nanoscale structural dynamics in plasmonic single-particles and few-particle clusters. Here, we report the direct imaging with UEM of photoexcited structural dynamics in a non-oriented cluster of plasmonic Au nanorods supported by an amorphous silicon nitride substrate. With ultrafast bright-field imaging, the nanoscale structural response of individual particles comprising the cluster is spatially resolved and tracked over the first 100 ps following fs in situ photoexcitation. An initial decrease in counts within the particles is observed immediately during and after photoexcitation for all particles in the cluster, indicative of an ultrafast rise in thermal diffuse scattering (TDS) due to electron-lattice thermalization. Following this, emergence of coherent, low-frequency acoustic modes is observed approximately 10 ps after the onset of TDS, with clear structural oscillations observable in the particles. In addition to the experimental configuration and the results, proposals for the mechanisms underlying the observed behaviors will be presented.

Direct imaging of incoherent-to-coherent structural dynamics in plasmonic nanorods with ultrafast electron microscopy

2019 Fall National Meeting

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