Coherent Acoustic\nPhonons in Plasmonic Nanoparticles:\nElastic Properties and Dissipation at Low Temperatures

We studied the frequency and quality factor of mechanical\nplasmonic\nnanoresonators as a function of temperature, ranging from ambient\nto 4 K. Our investigation focused on individual gold nanorods and\nnanodisks of various sizes. We observed that oscillation frequencies\nincrease linearly as temperature decreases until saturation is reached\nat cryogenic temperatures. This behavior is explained by the temperature\ndependence of the elastic modulus, with a Debye temperature compatible\nwith reported bulk values for gold. To describe the behavior of the\nquality factor, we developed a model considering the nanostructures\nas anelastic solids, identifying a dissipation peak around 150 K due\nto a thermally activated process, likely of the Niblett-Wilks mechanism\ntype. Importantly, our findings suggest that external dissipation\nfactors are more critical to improving quality factors than internal\nfriction, which can be increased by modifying the nanoresonator’s\nenvironment. Our results enable the future design of structures with\nhigh vibration frequencies and quality factors by effectively controlling\nexternal losses.