Imaging Intra- and Interparticle Acousto-plasmonic Vibrational Dynamics with Ultrafast Electron Microscopy

We report real-space, time-resolved imaging of coherently excited acoustic phonon modes in plasmonic nanoparticles via femtosecond electron imaging with an ultrafast electron microscope. The particles studied were cetyl trimethylammonium bromide stabilized Au nanorods (40 × 120 nm), and the particular specimen configurations for which photoinduced vibrational modes were visualized consisted of a single, isolated nanocrystal and a cluster of four irregularly arranged and randomly oriented particles, all supported on an amorphous Si₃N₄ membrane. In both configurations, we are able to resolve discrete intraparticle acoustic phonon modes via diffraction-contrast modulation with bright-field femtosecond electron imaging. For the single nanorod, we spatiotemporally mapped the intraparticle vibrational energy distribution and decay times. With Fourier filtering, acoustic phonons ranging from 4 to 30 GHz (250 to 33 ps periods, respectively) were visualized, corresponding to bending, extensional, and higher-order modes. Furthermore, heterogeneously distributed intraparticle decay times, ranging from 3 to 10 ns, were spatially mapped, indicating a strong dependence on coupling of the mode to the underlying substrate. For a cluster of four randomly oriented nanorods, we are able to image acoustic phonon modes that are strongly localized to particular particle-particle contact regions within the aggregate. A vibrational mode occurring at 27 GHz (37 ps period) was observed to occur at a 10 nm side-to-end contact region, with other intraparticle points at distances of 20 and 50 nm from the region showing no such dynamics, although the initial few-picosecond diffraction-contrast response was observed changing sign in moving from the end to the center of the particle. Excellent agreement is found between the spatiotemporally mapped vibrational-mode symmetries and finite-element simulations of supported modes in a polymer-coated Au nanorod supported on a Si₃N₄ membrane. This experiment resolves both the structure and dynamic properties of the plasmonic assembly, providing insight into the characteristics of complex plasmonic assemblies that ultimately determine their response to ultrafast excitation.