The ultrafast electron dynamics in optically excited metallic nanostructures is of great importance for both fundamental studies and technological applications to materials science. Thin metal films of submicron thickness are typical examples of nanostructures and are widely used in modern high-speed electronic and opto-electronic devices. In such systems, the switching time can nowadays approach the femtosecond time domain. On this time scale, the electron distribution is out of thermal equilibrium. In order to control the energy consumption, it is therefore important to develop a better understanding of electron transport and energy relaxation in the femtosecond temporal regime.
First, we have performed self-consistent simulations of the electron dynamics and transport in thin metal films, using a semiclassical Vlasov-Poisson model. The dynamical properties are strongly influenced by the finite size of the system and the presence of surfaces. Our results showed that: (i) heat transport is ballistic and occurs at a velocity close to the Fermi speed; (ii) after the excitation energy has been absorbed by the film, slow nonlinear oscillations appear, with a period T proportional to the film thickness: these oscillations are due to nonequilibrium electrons bouncing back and forth on the film surfaces; (iii) except for trivial scaling factors, the above transport properties are insensitive to the excitation energy and the initial electron temperature.
Secondly, we have analyzed the impact of quantum effect on the dynamics. The quantum evolution was simulated using the Wigner equation. We observed a classical-quantum transition at low enough excitation energies (see figure): above a certain threshold the evolution is classical and the above Vlasov results are recovered, particularly the ballistic oscillations; below the threshold quantum effects play a role and the nonlinear oscillation period differs from the ballistic value. the effect of dissipation and decoherence was also investigated.