Seminars Archive

Optically induced structural dynamics: Time resolved Raman spectroscopy.

Abstract
This contribution focuses on optically induced phenomena in the semi-metals bismuth and antimony. The structure of these semi-metals is not the usual cubic one, as found in other simple metals, but rather a distorted one called the arsenic-A7 structure. The origin of this distortion lies in the electron-phonon interaction, and in particular depends on the charge density in the conduction band of these materials. At a high enough density this distortion should vanish, leading to a cubic phase, and one would expect that it is possible to induce a phase transition to this state by photo-doping. Recent experiments, using direct probing of the phonon excitation spectrum via time resolved optical pump/Raman probe spectroscopy, have shown that one can indeed reduce the distortion by photo-doping. Moreover, they yield strong indications for an optically induced phase transition in the semi-metals, albeit that it does not appear to be a transition to the cubic phase. Apart from introducing the novel experimental technique, the observed transient phonon spectra of Bi and Sb will be discussed in terms of thermal and non-thermal effects and screening of the crystal potential by the photo induced charge density in the conduction band. (Ref. F. Parmigiani)