Acoustic Vibrations and Energy Dissipation Mechanisms for Lithographically Fabricated Plasmonic Nanostructures Revealed by Single-Particle Transient Extinction Spectroscopy

Acoustic vibrations in plasmonic nanostructures provide deep insight into mechanical properties at the nanoscale for potential applications including optomechanical devices. Lithographic fabrication of plasmonic nanostructures allows precise control over shape and size as well as position. Here, we present a summary of our recent ultrafast studies of lithographically fabricated Au and Al nanostructures using single-particle transient extinction spectroscopy to measure the size- and shape-dependent acoustic frequencies and homogeneous damping times. Electronbeam lithography coupling with single-particle measurements necessitate the presence of a substrate, which we found to cause a blue shift in the acoustic vibration frequencies. This frequency shift enables the determination of the binding strength between Au nanostructures and the substrate. The substrate furthermore facilitates vibrational coupling between adjacent Au nanostructures. Electron-beam lithography also makes it possible to explore other plasmonic metals such as Al, which as the Earth's most abundant metal creates a sustainable pathway toward applications. We compared the ultrafast relaxation dynamics and acoustic properties of Al nanodisks to similar Au nanostructures. For both Au and Al nanostructures, we found an acoustic vibration quality factor which we ascribed to internal defects in the polycrystalline nanostructures that dominate the energy dissipation pathway. These findings provide significant insight into the optomechanical properties of nanostructures fabricated by electron-beam lithography.