GENERATION OF COHERENT PHONONS IN CONDENSED MEDIA

In this paper the generation of coherent phonons by absorption of femtosecond laser pulses through interband transitions in different materials is summarized. Starting with layered III-VI semiconductors, where optical excitation is performed with amplified pulses, the generation mechanism via strong interband transitions is investigated. In Ge stimulated resonant Raman scattering is found as the decisive driving force. In GaAs, however, the ultrafast screening of surface space charge fields, launches LO-phonons via electrostatic coupling. In the metallic state of high temperature superconductors, the generation of highly symmetric A(g) modes is assigned to displacive processes induced by a nonequilibrium electronic carrier distribution. In the superconducting state the amount of coherent displacement is strictly correlated to the number of optically broken superconducting pairs. In addition to the comparison of various generation processes, the dephasing of coherent phonons is addressed.