Ultrafast light-induced dichroism in silver nanoparticles

Ultrafast light-induced dichroism in silver nanoparticles is studied. Two differently prepared colloids are examined: one sample has a broad distribution of sizes and shapes of silver nanoparticles, while the second sample, which is processed by laser ablation, presents a more uniform size and shape distribution (greater fraction of spherical particles). The experiments are performed with a femtosecond laser in a two-color, polarization-resolved, pump-probe setup. For the pristine colloid the signal amplitudes for the parallel and perpendicular polarizations present different behaviors as a function of the probe photon energy, which is not observed for the laser ablated samples. The temporal response of the samples is also different, with the laser ablated samples presenting faster electronic cooling rates. A model describing the induced dichroism that takes into account the shape of the colloid particles is presented. The effects of the size distribution are also discussed.