Long Lifetime and Coupling of Acoustic Vibrations of Gold Nanoplates on Unsupported Thin Films.

Radiation losses due to energy transfer to acoustic waves in the surroundings lead to short vibrational lifetimes of plasmonic nanoresonators and severely limits their applications in quantum technologies and mass sensing. Here we demonstrate a general method for improving the acoustic vibration lifetimes of Au nanoplates (flat structures with edge lengths of several microns and thicknesses of 10-40 nm) by using unsupported thin films instead of thick substrates, which blocks the outgoing acoustic waves. We experimentally confirm the vibrational quality factor improvement of Au nanoplates on Si3N4 and Al2O3/Si3N4 thin films with thicknesses less than 100 nm. Through this substrate phonon engineering, we successfully observe strong vibrational coupling between two Au nanoplates. Furthermore, the increased vibrational lifetimes allow us to study the vibrations of the nanoplates relative to the substrate and, for stacked nanoplates, relative to each other. The frequencies of these relative vibrational modes are well described by a simple coupled mass model, which provides the effective spring constants between interfaces separated by surface bonded molecules. We also demonstrate that the continuous thin films can be used for small plasmonic nanostructures (dimensions of several hundred nm), with similar increases in the vibrational lifetimes.