Direct Imaging of Incoherent-to-Coherent Structural Dynamics in Plasmonic Nanorods with Ultrafast Electron Microscopy

Abstract

Ultrafast photoexcitation of plasmonic metal nanostructures results in a cascade of incoherent and coherent electronic, optical, and structural responses along discrete temporal trajectories. Surface plasmon excitation and rapid electron thermalization couple to coherent, low-frequency 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 [1-4]. Importantly, the electronic and structural responses are interdependent, as the atomic and 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 amplitudes of the coherent modes. In light of this complexity, detailed insights into the temporallyand spatially-dependent responses in imperfectly-arranged plasmonic assemblies may be gained via direct-imaging methods capable of accessing combined nanometer-picosecond scales for single-particle measurements [4]. Among a variety of time-resolved spectroscopic and structural probes, ultrafast electron microscopy (UEM) has been used to directly image discrete, nanoscale structural dynamics in plasmonic single-particles and few-particle clusters [5]. With UEM, the onset of specific acoustic modes was imaged, and the excitation of isolated, nanoscale vibrational hot spots at discrete particle-particle contact points was observed using Fourier frequency maps generated from the real-space picosecond image series. These vibrational hot spots match the spatial positions of the plasmonic hot spots generated from concentrated electromagnetic fields.