Ultrafast structural phase transitions and coherent phonons in solids and nanostructures

Harald Jeschke
Institut für Theoretische Physik, Goethe-Universität Frankfurt am Main, Germany

Laser induced femtosecond structural changes in covalent materials are
studied theoretically. In an attempt to treat both ionic motion and
the changes in the electronic structure with sufficient accuracy, we
employ molecular dynamic simulations on the basis of a time-dependent
potential energy surface derived from a tight-binding Hamiltonian. The
shape and spectral composition of the laser pulse is explicitly taken
into account. We show applications of this approach to the laser
excitation of bulk diamond and ultrathin graphite and silicon
films. By implementing a nonorthogonal tight binding scheme for
germanium, we can address the softening of phonon modes in laser
induced nonequilibrium and the time scales of the structural response.
Recent experiments are in good agreement with predictions from our
theoretical approach. Finally, we discuss the possibilities of
extending the method from optical to XUV frequencies and beyond.