Femtosecond Material Science: Phase Transitions Induced by Photo-Excited Carriers

Harvard University, Cambridge MA

Investigators

Abstract

This research uses optical techniques to study electron and lattice dynamics in solids that undergo semiconductor-to-metal transitions after excitation with intense femtosecond laser pulses. In addition to investigating the dynamics of highly excited materials, the proposed work will determine the degree to which their phase can be controlled with multiple-pulse excitations by studying the interplay between free carrier dynamics and ionic motion during a semiconductor-to-metal transition. A novel aspect of the work is the measurement of the dynamics of the spectral dielectric function with femtosecond time resolution, which fully characterizes the linear optical response of the material and from which one can infer changes in band structure and electronic configuration. Through quality research-based education of undergraduate and graduate students, this research also contributes to science and engineering education and the training of future scientists and engineers. The advent of femtosecond-pulse laser sources offers the opportunity to manipulate solid materials both by novel means and on unprecedented time scales. This research is aimed at studying the response of semiconductors to intense femtosecond pulse excitations that drive the material from a semiconductor to a metal. The goals of this work are to determine the mechanisms by which the transition occurs and the degree to which the phase of the material (semiconductor vs. insulator) can be controlled on a femtosecond time scale. A novel experimental technique that measures the optical properties with femtosecond time resolution will be applied to these materials, which enables one to distinguish between the effects of electronic excitation and changes in crystal structure. Through quality research-based education of undergraduate and graduate students, this research also contributes to science and engineering education and the training of future scientists and engineers.