Scientific Interests and Work:
Professor Susan Dexheimer has a broad research background in experimental condensed matter physics, chemical physics, and molecular biophysics, and is interested in research problems at the interfaces of these fields. Her research expertise includes the development and application of spectroscopic techniques to study electronic materials and molecular systems. Her current work focuses on studies of the ultrafast dynamics of fundamental excitations in condensed matter systems and of carrier dynamics in amorphous and nanocrystalline electronic materials for photovoltaic applications, using a combination of femtosecond optical, terahertz, and x-ray techniques.
Professor Dexheimer received an S.B. in physics from the Massachusetts Institute of Technology, where she carried out research in AMO physics under the supervision of David E. Pritchard, and received a Ph.D. in physics from the University of California, Berkeley, where she carried out research in molecular biophysics under the supervision of Melvin P. Klein. She carried out postdoctoral research in femtosecond laser spectroscopy as a University of California President’s Postdoctoral Fellow under the supervision of Charles V. Shank. She is the recipient of a National Science Foundation Faculty Early Career Development (CAREER) Award, the WSU College of Sciences Young Faculty Achievement Award, and the WSU College of Arts and Sciences Thomas E. Lutz Teaching Excellence Award.
Ph.D. (Physics) University of California, Berkeley
S.B. (Physics) Massachusetts Institute of Technology
RESEARCH: ULTRAFAST DYNAMICS OF MATERIALS
Current research areas
Dynamics of quasiparticle formation
The formation of localized electronic quasiparticle states reflects the fundamental physics of coupling between electronic and lattice dynamics and has a profound impact on the properties of a wide range of materials: polaron formation determines the charge transport properties of many electronic materials, and formation of self-trapped excitons, or exciton-polarons, dramatically changes optical properties and energy transport mechanisms. While the equilibrium properties of quasiparticles are well-established in many systems, the development of a full understanding of the dynamics of quasiparticle formation is an active area of experimental and theoretical investigation. In addition to its fundamental significance, this knowledge promises a means to understand, and thereby exploit, the fast electronic and optical response of materials.
Our femtosecond time-resolved studies probe the dynamics of the localization process, focusing on the formation and evolution of self-trapped excitons and polarons. The experiments are carried out in quasi-one-dimensional materials in which the strength of the electron-phonon coupling that drives the dynamics can be systematically tuned by varying the material composition. Experiments using femtosecond vibrationally impulsive excitation, in which the system is excited with an optical pulse short compared to the periods of the relevant vibrational modes, allow us to time-resolve the coupled electronic and lattice dynamics as the system evolves from the initially photoexcited delocalized electronic state to form a self-trapped exciton. Polaron dynamics are probed using time-resolved terahertz spectroscopy, in which short pulses of far-infrared light are used to monitor the fast photoinduced carrier response, and time-resolved x-ray experiments probe the local changes in structure and electronic distribution. Our work to date has revealed critical aspects of the localization dynamics, including observations of formation times for self-trapped excitons and polarons on the order of a single lattice vibrational period and involvement of both optical and acoustic phonons in the self-trapping process.
Carrier dynamics in amorphous and nanocrystalline semiconductors
The long-range order of the lattice plays a fundamental role in determining the electronic properties of crystalline semiconductors, and the understanding of the physics as this order is reduced is a central issue in condensed matter physics. Amorphous semiconductors have also attracted considerable interest as a result of their important technological applications, in addition to the intriguing physics of their electronic states. In particular, hydrogenated amorphous silicon, amorphous silicon-germanium alloys, and thin-film nanocrystalline silicon are promising materials for practical, inexpensive solar cells, as well as for a variety of optoelectronic applications, in addition to being prototype materials for understanding the electronic properties of disordered systems. Our work involves studies of the initial carrier relaxation, trapping, transport, and recombination processes, focusing on the relation between the degree of disorder and the carrier dynamics.
Support for this work includes grants from the National Science Foundation Condensed Matter Physics Program and Major Research Instrumentation Program, the Department of Energy National Renewable Energy Laboratory, and the American Chemical Society Petroleum Research Fund, and user agreements with the Center for Integrated Nanotechnologies at Los Alamos National Laboratory, the Advanced Light Source at Lawrence Berkeley National Laboratory, and the Advanced Photon Source at Argonne National Laboratory.
Selected recent publications
F.X. Morrissey, J. G. Mance, A. D. Van Pelt, and S. L. Dexheimer, Femtosecond dynamics of exciton localization: Self-trapping from the small to the large polaron limit, J. Phys. Condensed Matter 25, 144204 (2013).
Highlighted in the Institute of Physics news feature “Ultrafast Dynamics of Polaron Formation”
J.G. Mance, J. J. Felver, and S. L. Dexheimer, Observation of structural relaxation during exciton self-trapping via excited-state resonant impulsive stimulated Raman spectroscopy, J. Chem. Phys.142, 084309(2015).
J.G. Mance, J. J. Felver, and S. L. Dexheimer, Optical and acoustic phonon dynamics of exciton self-trapping in a nondegenerate quasi-one-dimensional charge density wave system, J. Phys. Chem. C 118, 11186-11192 (2014).
F.X. Morrissey and S.L. Dexheimer, Coherent acoustic phonon generation in exciton self-trapping, Phys Rev B 81, 094302 (2010).