Sound Wave Propagation Anisotropy in Silver Nanoprisms: Characterization of Photoinduced Multiple Modes Using the Symmetric Molecular Dynamics Method

Abstract

We present both theoretical and experimental studies of the sound velocity anisotropy and laser-induced multiple acoustic phonon oscillations in silver nanoprisms. A large-scale simulation method based on molecular dynamics and group theory for nanoprisms with C-3v symmetry was developed. We observed the most prominent planar modes and the less studied vertical modes and found that the anisotropic vibrational properties of silver nanoprisms strongly depend on the particle's aspect ratio (bisector length over thickness). For the case of a smaller aspect ratio, we identified these modes as the A, modes with a larger in phase radial atomic displacement These modes can be classified into three main types: breathing modes and totally symmetric modes which are planar modes and thickness-related vertical modes. Due to the strong coupling with laterally confined modes, a broad band of thickness-related modes is found (in contrast to a single sharp peak for thin films), which leads to a fast decaying component in the oscillation spectrum. The calculated results and the sound velocity anisotropy along different crystal axes are in good agreement with our transient optical absorption experiments.