Modelling for Transient Optically Induced Metal - Transitions in Narrow-Gap Semiconductors and Semimetals.

The theoretical work presented in this thesis is based on models developed to interpret a series of optical experiments with short-pulse lasers, which allow a time -domain study of phenomena on a sub-picosecond timescale. By means of a pump-probe technique, we observe large amplitude oscillations in the time domain reflectivity response of a series of narrow-gap semiconductors and semimetals. The oscillations have the frequency of the fully-symmetric optical phonon mode of the system, and are maximally displaced from their midpoint value at zero time delay between pump and probe. These features indicate that a coherent phonon vibration is generated in these materials via an electronic excitation at different points of the Brillouin zone, which displaces instantaneously the equilibrium positions of the atoms. It is precisely this generation of coherent phonons that makes the time-domain technique distinct from conventional frequency domain techniques, such as Raman and neutron scattering. Using a range of theoretical techniques, from nearly free electron models to state-of-the art ab initio calculations, I have made quantitative microscopic evaluations of the coherent phonon phenomenon. The studies focus on two unique aspects of having such coherent atomic vibrations in a narrow gap material, with special emphasis on the group V semimetals Sb and Bi. First of all, I have performed dynamical band structure calculations, as a function of the coherent atomic motion, in order to inspect the possibility of a transient metal-insulator transition at a terahertz frequency. Secondly, I have calculated the evolution of the displaced atoms in quasi-equilibrium with the laser -excited carriers, as the electron-ion coupled system returns to its ground state equilibrium. These calculations are fundamental, insofar they provide a quantitative microscopic description of the coherent phonon phenomenon. Moreover, the predicted magnitude of the atomic displacements, and the resulting band gap shifts in Sb, indicate that this material can undergo a metal-insulator transition at a terahertz frequency, when illuminated by a high power short pulse laser. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.).