Ultrafast dynamics in solids
The ultrafast response of metals and semiconductors to electronic excitations is analysed. Time resolved two-photon emission and non-linear optics allow us to study the relaxation of systems at non-equilibrium. Representative results for the spin dependent relaxation of excited electrons in transition metals, in ferromagnetic metals such as Ni, Co, Fe and in high T c superconductors are given. Also ultrafast dynamics of semiconductors such as Si, Ge and C is analysed. In particular, in covalently bonded solids, electronic excitations from the valence to the conduction band cause a change of the bond character and thus strong changes of various properties and induce coherent phonons, ablation, ultrafast melting and phase transitions at non-equilibrium. The ultrafast relaxation processes are controlled by energy and angular momentum conservation, of course, and this is particularly important for magnetism at non-equilibrium. The time dependence of the magnetic relaxation in nanostructures reflects in a characteristic way the atomic structure. The ultrafast Coulomb explosion of clusters in intense electric fields may serve as a demonstration of non-equilibrium dynamics resulting from electric field induced solid–plasma transition. Dynamics in small clusters can test the validity of concepts used in thermodynamics. Optically controlled electronic population dynamics may be expected to be relevant for ultrafast switching devices and recording.