Roberto Luzzi

Transient analysis of thermal distorsion in a silicon substrate on incidence of a single soft x-ray FEL pulse

We discuss the dynamics of a silicon surface after incidence of a short, high energy pulse in the soft X-ray range. We focus on time-delays long enough after pulse incidence, so that the absorbed energy can be seen as a nonuniform time-dependent heat distribution in the solid. A model is developed using techniques of non-equilibrium hydro-thermodynamics, considering just the longitudinal and transverse acoustic phonon systems in the excited solid. The general theory leads to Maxwell-Cattaneo partial differential equations for the material medium n(r,t) and the energy h(r,t) volume densities; these reduce to the diffusion equation for the temperature T(r,t) and the usual thermo-mechanical elastic equation for the strain u(r,t) on further simplification. Here we solve the Maxwell-Cattaneo equation for T(r,t) and compare to previous results where the diffusion equation was used instead; the Maxwell- Cattaneo equation predicts faster cooling at short (dozens of fs, say) time delays. Previously obtained results for the strain field are briefly recalled.

Resonant raman scattering of electrons in quantum wells : Identification of elementary excitations

Inelastic scattering of light by electrons in a quantum well was studied. We considered the resonant Raman scattering associated with intersubband electronic excitations. Evidence is provided that when the two lowest subbands are considered four types of excitations are present. They give rise to two scattering lines associated with single-particle excitations and another two associated with collective charge-density excitations, while spin-density excitations are not considered. The shape of the Raman spectrum strongly depends on several characteristics in the experiment, namely, the quantum well width, the concentration of the carriers, many-body effects, the temperature, and, mainly, the momentum-transfer dependent on the laser-photon frequency and the experimental geometry. Particular experimental results are satisfactorily described.