Acoustic Vibrations in Nanosized Gold-Shell Particles

We demonstrate and discuss acoustic vibrations in spherical shell particles. The particles consist of 38-nm-thick gold shells, sandwiched between an inner silica core of 456 nm in diameter and an outer silica shell of 10 nm in thickness. Employing standard and asynchronous optical sampling (ASOPS) ultrafast pump-probe experiments, we excite acoustic vibrations of the shells and detect the oscillations via modulations of the optical reflectivity. Two kinds of vibrations were excited and observed. One oscillation with 400 ps period corresponds to the prediction of Lamb theory for breathing modes of free thin gold shells. The other oscillation with 25ps-period corresponds to resonant longitudinal thickness vibrations of the gold shells. At later times, echoes appear corresponding to the travel time through the silica core.