|University of Michigan|
Department of Electrical Engineering and Computer Science
Current research in our group comprises two quite different areas: cooperative nonlinear optics and ultra high-resolution laser spectroscopy of point defects in wide-bandgap semiconductors and nanocrystals.
Cooperative nonlinear optics focuses on the interaction of light within clusters of neighboring atoms. Near-neighbor impurity atoms in solids form coupled oscillator systems which exhibit nonlinear emission or absorption at unexpected frequencies in a manner quite different from conventional nonlinear optics. We study coupled pairs and trios of atoms in rare-earth-doped solids, and have demonstrated several types of visible upconversion laser and novel nonlinear devices based on them.
Cooperative Upconversion lasers are distinctive in providing output frequencies higher than their excitation or pump frequency. The interactions which cause this unusual behavior can also form the basis for bistable devices which exhibit unusually low power requirements for intrinsic switching. Recently we observed bistability and multi-stability of the luminescence and stimulated emission from a rare-earth-doped solid at room temperature with no external feedback. The mechanism is under study and a search for chaotic luminescent behavior is underway. Hence, fundamental aspects of pair interactions with light are a principal concern, and we are developing quantum theory for phenomena such as, avalanche upconversion and intrinsically bistable luminescence, as well as exploring the implications of coherence and energy migration in such processes.
Nonlinear laser spectroscopy emphasizing hole-burning, four-wave mixing and coherent transient techniques is also used extensively in our group to study the physical and electronic structure of point defects and dopants in solids. Deep centers the wide bandgap semiconductors in diamond and boron nitride influence the electrical properties in important ways. They can also be candidate centers for short-wavelength tunable injection lasers and detectors. However, very little detailed knowledge is available regarding luminescent centers in these novel electronic materials, and virtually no information is available on quantum size effects in nanocrystals of these and other wide bandgap materials. Information of this kind is important to acquire in order to understand for example, the dynamics of rare earth laser phosphors, a recent invention by our group, and how light becomes localized in highly scattering nancrystals. Hence we are applying precision spectroscopy to develop the knowledge base necessary for novel applications of new semiconductors and dielectric nanophoshors for hybrid displays, communication devices and short wavelength lasers.Recent Courses Taught: