Vibrational Damping of Single Supported Plasmonic Nanodisks

Achieving mechanical nanoresonators that combine high frequencies with large quality factors remains a central challenge in nanomechanics. Meeting this challenge requires a detailed understanding and mitigation of vibrational damping mechanisms arising from both intrinsic dissipation and acoustic radiation into the surrounding medium. Supported nano-objects are particularly promising in this regard, as recent studies have demonstrated strong, mode-specific reduction of acoustic radiation using tailored simple geometries. Here, we report systematic investigations of the dependence of vibrational quality factors on nano-object morphology and substrate properties. Using single-particle pump-probe spectroscopy, we measured the vibrational dynamics of colloidally synthesized gold nanodisks with varying diameter-to-thickness ratio, deposited on three distinct solid substrates differing in composition and thickness. The acoustic responses of the investigated nanodisks were dominated by the same vibrational mode, whose quality factor, ranging from 10 to 50, was found to depend sensitively on both nanodisk geometry and substrate properties. These trends were well reproduced by finite-element simulations solely accounting for radiative damping, indicating its dominant role in the observed dissipation. Remarkably, high signal-to-noise measurements on a selected nanodisk revealed the presence of a second vibrational mode with a comparable frequency but a lower amplitude and a substantially higher quality factor. Combined with numerical modeling, this observation suggests that the optically launched vibrational dynamics of supported nanoparticles may often involve weakly damped modes, although their detection remains challenging.