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

Abstract

The dynamics of hot electrons, induced by femtosecond pulses, is investigated in silver nanoparticles embedded in a glass matrix. A detailed temporal and spectral study of the plasmon resonance is reported when the laser excitation density is varied over three orders of magnitude. It is compared in the same spectral range with the electron dynamics in silver thin films measured in transmission and reflection. From these measurements the dynamics of the experimental complex dielectric function of the thin films is determined. This dynamics is well explained with the model dielectric function $\varepsilon \mathrm{}\left(t\right),$ described in the random phase approximation, including optical transitions 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 associated 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 matrix. The effective dielectric function ${\varepsilon }_{\mathrm{eff}}\mathrm{}\left(t\right)\mathrm{}$ of the nanoparticles is calculated using the same parameters as the ones used in $\varepsilon \mathrm{}\left(t\right)\mathrm{}$ and a constant surface scattering rate. With this model, the dynamical spectral shift of the plasmon mode is well reproduced. It is shown to be mostly related to the effect of the electron-electron scattering on the real part of ${\varepsilon }_{\mathrm{eff}}\mathrm{}\left(t\right).$ However, the model is shown to be insufficient to explain the electron relaxation at the plasmon resonance in the regime of high-excitation densities where, in average, more than one photon per nanoparticle is absorbed.