Spectral dependence of the ultrafast optical response of nonspherical gold nanoparticles

The extension of the discrete dipole approximation (DDA) model to the calculation of the ultrafast transient optical response of metal nanoparticles, whatever their shape, is presented. For this, the DDA is combined with the resolution of the Boltzmann equation for the conduction electrons within the low-perturbation limit and the Rosei model for the metal band structure. This approach thus takes into account the athermal regime for the electron gas. It is then illustrated in the cases of gold nanospheres and nanopeanuts. The spectral dependence of the ultrafast response is shown to be very sensitive to the particle morphology, the surface plasmon resonance enhancing the transient modification of the interband transitions. These results are used to interpret spectrally resolved pump-probe measurements performed on a dense nanoparticle assembly, demonstrating selective-excitation and selective-detection processes.