UC Berkeley

This dissertation attempts to illuminate and expand our understanding of charge dynamics in a number of different semiconductor materials, in particular the behavior of electron-hole pairs which are correlated via the long-range Coulomb interaction. Terahertz spectroscopy gives us direct access to probe the fundamental excitations of quasiparticles in a wide range of materials systems. By probing the low-energy excitations of materials on short time scales, one can learn a great deal about their quasiparticles and dynamics as well as explore new tools for their manipulation.

Carriers in semiconductors, existing either as bound electron-hole pairs or “free” carriers (or both), are particularly amenable to this avenue of study. The internal structure of excitons in semiconductors lies energetically in the far- to mid-infrared. Free carriers moving under the influence of a free-space electric field reveal much about themselves. By exploiting this unique window, one can both characterize and manipulate these systems in novel ways, as well as seek to understand the physics of new materials.

I study the ferromagnetic semiconductor gallium manganese phosphide (GaMnP) to better understand the role of charge carriers in the origins of ferromagnetism. By applying the tools of terahertz spectroscopy, I am able to make contact free measurements of hole populations in a novel GaMnP film.

I exploit THz spectroscopy to provide a direct gauge of bound and unbound pair densities in gallium arsenide quantum wells, enabling the observation of the excitonic fine structure as it evolves under high-density conditions. And finally I have observed for the first time stimulated emission of far-infrared radiation from internal transitions in excitons in the semiconductor Cu2O. Broadband THz spectroscopy is employed to directly map out the ultrafast dynamics, spectral positions, and line broadenings of intra-excitonic transitions. This work has opened up new possibilities for the quantum manipulation of excitons. With these studies, I continue to test and expand the atomic analogy of excitons.