Coherent excitation and control of the acoustic vibrations of metal nanoparticles

摘要

The reduction of a material size to a nanometric scale drastically modifies its vibrational properties with the appearance of discrete vibrations. Information on these quantized modes is essential for the understanding of the properties of nanoscopic metallic systems such as their size-dependent heat capacity or electron-lattice interactions. Furthermore, their characteristics, frequency and damping, are sensitive probes of the size, shape and environment of the nanoparticles and thus of the material characteristics. For a R radius nanosphere, the isotropic confined modes are related to the bulk phonons with wavelength /spl lambda/ satisfying the spatial condition /spl lambda/ /spl ap/ 2R/(n + 1). It has been shown that these modes can be coherently excited and probed in metal nanoparticles, using a femtosecond pump-probe technique. The fundamental n = 0 expansion and contraction (breathing) mode dominates the response, masking possible contributions of other modes. We have extended this technique to control and select the vibrational movement of metal nanoparticles by using a pump pulse pair. We have shown that the fundamental mode can be stopped and the n = 1 mode launched, allowing its time domain observation and determination of its frequency and, for the first time, its damping.