Electron Dynamics in Copper Metallic Nanoparticles Probed with Femtosecond Optical Pulses

Abstract

The thermalization of electrons in copper nanoparticles embedded in glass is investigated using femtosecond pump-probe spectroscopy.The time dependent induced transmission is enhanced near the surface plasmon resonance of the nanoparticles, as opposed to the static one obtained with thermomodulation measurements.In addition, a slowing of the process of electron cooling to the lattice temperature is observed at the plasrnon resonance. These observations show the importance of quasiparticle collisions in confined metallic structures. PACS numbers: 78.47.+p The scattering and absorption of light by metallic nanoparticles embedded in a transparent matrix depend on both the complex dielectric function of the metal and the boundary conditions at the metal surface. In particular, in the spectral region corresponding to the first order plasmon mode obtained from the theory of Mie [1], the optical susceptibility is resonantly enhanced [2].Regarding the electron dynamics in metal spheres, the surface scattering of electrons leads to a linewidth of the plasma resonance inversely proportional to the particle size. It can be taken into account by an effective mean free path of electrons propagating at the Fermi velocity [3 -5]. This geometrical effect is, a priori, not related to the thermodynamics of the electron distribution. The aim of the present work is to investigate the dynamics of hot electrons in copper spheres when a nonequilibrium quasiparticle population is created by ultrashort optical pulses. The dominant scattering processes involved in this temporal domain are electron-electron (e-e) and electron-phonon (ep) collisions. Unique information on the dynamics of metallic confined structures is obtained by comparison with measurements in thin metal films.The dynamics of the electron gas in the nanoparticles is investigated with femtosecond pump-probe spectroscopy as reported in the study of metal films [6 -8]. The pump pulse initially heats the electron gas which subsequent thermalization and relaxation to the lattice is temporally and spectrally resolved with the probe pulse. Thermalization refers here to the temporal regime during which e-e collisions are very effective in redistributing the initial nonequilibrium quasiparticle distribution.An important aspect in the case of copper spheres is the transition d~EF from the filled d band to the Fermi level of copper (energy FF) and the plasmon resonance which energy E&&" is close to this transition. Two main results are reported. First, the time dependent transmission spectra are compared with those obtained with a static thermomodulation technique on the same sample. These measurements show that the differential transmission is enhanced near the plasmon resonance during several hundred femtoseconds. This confirms the results of Farm et al. [9] who have shown that in gold films the initial non-Fermi-Dirac electronic distribution persists for a very long time (-1 ps) after excitation.It clearly establishes that the electron dynamics in metal nanoparticles must include...

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