Confined acoustic modes in nanoparticles

Brillouin scattering from aggregates of monodisperse polystyrene spheres has been measured as a function of their diameters (20 - 990nm). In this range of sphere sizes studied, confined acoustic modes and two types of bulk longitudinal acoustic modes are observed. One type exists within larger spheres (diameter > 700 nm), while the other propagates in aggregates of smaller spheres (diameter ≤ 80 nm) which are considered as polystyrene-air composites. The frequencies of the confined acoustic modes are found to be proportional to inverse sphere diameter, in accordance with Lamb's theory.