Vibrational modes of nanoparticles studied by ultrafast optical spectroscopy.

Abstract

Excitation of metal nanoparticles with an ultrafast laser pulse causes rapid heating. This can excite the vibrational modes of the particle that correlate with the expansion coordinate. Comparison of the experimentally measured periods to continuum mechanics calculations (usually done through finite element modeling) allows us to assign the observed vibrational modes. In most cases the breathing mode is excited, and its frequency depends on the size, shape, and elastic constants of the particles. For typical particle sizes of tens of nanometers, the frequencies are in the range of 10–100 GHz. A range of different shapes has been studied: spheres, cubes, rods, and even hollow particles. The measured and calculated periods are usually in good agreement, which implies that the elastic constants of small metal particles are similar to the bulk material. Experiments on single metal particles have also been performed. These measurements allow us to determine the damping times of the acoustic vibrational modes, which cannot be done in conventional ensemble experiments because of the different sizes and shapes that are invariably present in the samples. These measurements allow us to study energy relaxation and propagation in nanomaterials, and how nanoparticles interact mechanically with their environment.