Damping of acoustic vibrations of immobilized single gold nanorods in different environments.

We present measurements of the acoustic vibrations of single gold nanorods deposited on a glass substrate immersed in air and water by ultrafast pump-probe spectroscopy. The nanorods display two vibration modes, the breathing mode and the extensional mode. The damping time of the two modes is influenced by the environment, and a reduction of the quality factor is observed when the particles are immersed in water. The reduced quality factor of the breathing mode is in good agreement with a model that takes into account viscous damping and radiation of sound waves into the medium. The extension mode, however, is heavily damped when the particles are immersed in water, which is attributed to hydrodynamic lubrication forces between the nanoparticle and the glass substrate. Our results identify a new mode of damping in supported nanoparticles and indicate that the immersion medium can have different effects on different modes of vibration.