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Ph. D. Thesis
Photograph by Tim
Ford
Paul D Cunningham
TitleNational Research Council Research Associate at Naval Research Laboratory Education
Ph.D. Physics - University of Maryland Baltimore County, 2010
Previous ExperienceDuring my undergraduate studies at Towson University, I researched colossal magnitoresistance in manganite thin films under the mentoring of Dr. M. Raj Rajeswari and the Materials Research Group. Prior to entering graduate school, I worked for a year with the Reliability Branch of the Logistics Analysis Division of the US Army Materiel Systems Analysis Activity performing Physics-of-Failure analysis at Aberdeen Proving Ground. In my early graduate studies at Univ. of Maryland Baltimore Co., I used atomistic modeling to examine electric field poling of guest-host electro-optic polymers under the mentoring of Dr. L. Michael Hayden. As a graduate research assistant within the Dept. of Physics at UMBC I examined the conductive properties of materials using THz spectroscopy under the mentoring of Dr. L. Michael Hayden. Specifically, my Ph. D. research involved studying carrier and exciton dynamics on picosecond time scales in semiconducting polymer films as well as charge transfer processies in bulk heterojunction photovoltaic cells using time-resolved THz spectroscopy. The aim of this research was to improve the understanding of charge carrier generation and transport in polymeric materials to further organic solar cell and organic electronics technology. During this research I was a member of the National Science Foundation Science and Technology Center for Materials and Devices for Information Technology Research and collaborated with numerous researchers around the country. I was also a Graduate Student and Postdoctoral Council member for STC-MDITR. Research InterestsTime-resolved THz studies  Terahertz radiation, which lies in the far-infrared, corresponds to collective oscillations in crystals and organic molecules. This characteristic combined with its low, non-ionizing, energy makes THz spectroscopy well suited for security, biomedical, and other spectroscopic applications. Time-resolved THz spectroscopy, i.e. optical-pump THz-probe, is a special case of ultrafast pump-probe spectroscopy where the THz (low energy) probe does not perturb the system. The high frequency, as compared to DC methods, is sensitive to carrier transport over the nanoscale. Coherent methods, such as the electro-optic effect, allow for the detection of THz electric field transients. The fact that the inverse of carrier scattering times correspond to THz frequencies makes THz radiation well suited for studying the evolution of the dielectric properties of a material after excitation. Such studies can aid in determining how charge generation and transport in nanocrystalline materials differ from their bulk counterparts, examining quasi-particle dynamics in superconductors, exploring multi-exciton generation in quantum confined structures, and elucidating charge generation and transport in organic crystals and polymers.
Charge transport in organic materials
 Organics present a low-cost alternative to inorganic materials for photovoltaic, light-emitting diode, and opto-electronic applications with the benefits of roll-to-roll fabrication and tunable properties. Despite a wealth of research and commercially available devices, fundamental properties such as primary photogenerated products, photo-induced charge generation and subsequent charge transport are not yet fully understood. These intrinsic properties are further convoluted by their dependence on morphology and processing conditions. Systematic studies utilizing a variety of techniques are necessary to elucidate the nature of these properties that impact device performance. Carrier Multiplication in Lead Chalcogenide Nanostructures
 In photovoltaic cells, incident photons with energy larger than the band gap of the active material excite "hot" carriers/exctions. The excess energy is typically dissipated via phonon assisted carrier cooling as charge carriers return to the band edge and is thus wasted. Carrier multiplication allows the excess energy of a "hot" exciton to be utilized to excite an additional exciton, in a sense yielding 2 charge carriers for the price of 1 photon. This process could dramatically incease the photocurrent and thus the power conversion efficiency of photovoltaic cells. Lead chalcogenide nanocrystals have shown the most promise in this area, while nanorods and nanowires remain relatively unexplored. Additionally, the larger aspect ratio of rods and wires may improve the charge transport properties of these material systems over spherical crystals. Electro-optic polymers for THz applications
 Nonlinear optical guest-host polymer systems are a promising class of materials for opto-electronic devices e.g. electo-optic modulators for photonics-based telecommunications owing to their wavelength tunable properties and large nonlinearity. These systems also present promise as broadband sources and sensors for THz radiation due to their low loss and good phase-matching characteristics. Potential applications include nondestructive device evaluation, noninvasive imaging for medical and security application, chemical sensing, chemical fingerprinting, and time-resolved studies of photoconductive material systems.
Publications
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