Broadband Spectral Signature of the Ultrafast Transient Optical Response of Gold Nanorods

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.