Electron dynamics in silver nanoparticles: Comparison between thin films and glass embedded nanoparticles

The dynamics of hot electrons, induced by femtosecond pulses, is investigat ed in silver nanoparticles embedded in a grass matrix. A detailed temporal and spectral study of the plasmon resonance is reported when the laser exci tation density is varied over three orders of magnitude. It is compared in the same spectral range with the electron dynamics in silver thin films mea sured in transmission and reflection. From these measurements the dynamics of the experimental complex dielectric function of the thin films is determ ined. This dynamics is well explained with the model dielectric function ep silon(t), described in the random phase approximation, including optical tr ansitions from the d bands to the p and s conduction bands and the electron -electron scattering in these bands. For the nanoparticles, the dynamics of the plasmon resonance reveals the different temporal regimes, which are as sociated to the nonthermal component of the electron gas. to the cooling of the electrons to the lattice and to the heat transfer to the surrounding m atrix. The effective dielectric function epsilon(eff)(t) of the nanoparticl es is calculated using the same parameters as the ones used in epsilon(t) a nd a constant surface scattering rate. With this model, the dynamical spect ral shift of the plasmon mode is well reproduced. It is shown to he mostly related to the effect of the electron-electron scattering on the real part of epsilon(eff)(t) However, the model is shown to he insufficient to explai n the electron relaxation at the plasmon resonance in the regime of high-ex citation densities where, in average, more than one photon per nanoparticle is absorbed. [S0163-1829(99)04039-4].