The ultrafast transient optical response of gold nanorods presents a complex spectral signature that is very sensitive to the nanoparticle aspect ratio. This stems from the different electronic contributions to the photoinduced dynamics of the metal dielectric function, which modify the transverse and longitudinal localized plasmon modes. Here, we analyze the physical origins of the ultrafast optical response of ensembles of nanorods over the whole visible range. Using broadband time-resolved spectroscopy, we determine within the first picoseconds after pump excitation the transient response of colloidal solutions containing gold nanorods with different mean aspect ratios. Supported by model calculation, it is shown that the contribution of interband electron transitions dominates at ultrashort times, even for photon energies far below their threshold. At longer times, a slower intraband transition component linked with the nanoparticle heating appears. We then describe how the ensemble effect modifies the global spectral profile. The initial athermal regime for the conduction electron gas is demonstrated to affect the first instants of the dynamics. Finally, the influence of the shape distribution is experimentally evidenced and analyzed through a double selection process.

5 pagesInternational audienceWe present an all-optical method to investigate the GHz dynamics of the elastic contact between a single metallic nanoparticle and a substrate. A resonant excitation mechanism driven by the 82-MHz Dirac comb of the femtosecond oscillator is associated with femtosecond pump-probe experiments performed in a transient reflectivity configuration. This scheme allows us not only to detect the known breathing mode of the nanoparticle but also to unravel the existence of an axial oscillation of the nanoparticle through an intrinsic common-path interferometer.We measured the eigenfrequency and the lifetime of this vertical motion, which are related to the contact stiffness and hysteresis, and to the acoustic leakage at the nanoparticle-substrate interface. A modeling of the axial oscillation in the framework of classical adhesion theories predicts a simple power law dependence of the axial eigenfrequency with respect to the breathing mode frequency. Measurements performed for single particles with radii ranging from 60 to 700 nm are in strong agreement with this prediction