The relaxation processes of electrons and spins systems following the absorption of femtosecond optical pulses in ferromagnetic nickel have been studied using optical and magneto-optical pump-probe techniques. The magnetization of the film drops rapidly during the first picosecond, but different electron and spin dynamics are observed for delays in the range 0-5 ps. The experimental results are adequately described by a model including three interacting reservoirs (electron, spin, and lattice).[S0031-9007(96)00167-6] PACS numbers: 75.40. Gb, 78.20.Ls, 78.47.+p Ultrafast optical spectroscopy is an ideal technique to investigate the electronic relaxation processes in metallic materials [1][2][3]. A femtosecond optical pulse can induce a nascent nonequilibrium electron gas which subsequently thermalizes to a Fermi distribution. This thermalization, which takes place within about 500 fs as measured for instance in noble metals, is due to electron-electron interactions [4][5][6]. In the next step, the hot electron gas relaxes its energy to the lattice due to electron-phonon interactions, a process which occurs within 1-10 ps depending on the incident laser intensity. During this time, the temperature exchange between the electron bath (temperature T e ) and the lattice (temperature T l ) can be investigated. From such measurements, one can deduce the characteristic times of the microscopic interactions which govern the basic metallic properties like electron transport and superconductivity [7].In spite of the large amount of work performed in this field, little attention has been paid to magnetic effects occurring in the femtosecond time scale. An important challenge is to know whether the initial hot electron distribution can induce a spin dynamics associated with a spin temperature T s different from T e and T l . This would lead to an ultrafast demagnetization, a relevant effect in regards to applications in magneto-optic devices. From a conceptual point of view, such a fast magnetization change is not trivial since the optical transitions preserve the electronic spin. However, one can expect that, during the transient hot electron regime, spin populations are modified due to spin dependent electron scattering. The characteristic time of this process has not been investigated with a femtosecond temporal resolution. Previous experiments performed with picosecond pulses on Ni [8] or Fe [9] have shown no demagnetization effect up to the melting point of the samples. The authors concluded that the spin-lattice relaxation times of these metals are larger than 30 ps. A more sophisticated pump-probe experiment, using 10 ns pump and 60 ps probe pulses, made on Gd films could deduce a spinlattice relaxation time of 100 6 80 ps [10,11]. All these experiments were performed on a time scale where electrons and lattice temperatures are in equilibrium and therefore could not resolve separately the effects of electron-spin and spin-lattice relaxation mechanisms on the demagnetization process.The aim of this paper is to study b...

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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...