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Su Liu Thesis Defense 05/09/12

Thesis Defense

Su Liu

Wednesday, May 9, 2012
9:30am (334 JFB)

Title: Manipulation of Exciton Dynamics in Macrocycle Molecules and Inorganic Semiconductor Nanocrystals

Abstract:

The temporal dynamics of excitons and the evolution of excited states of a material system reflect both the excitation conditions and the final destination of the excitation energy. Precise control of material structure through modern nanofabrication provides nanostructures with well-defined relaxation paths of excitons, which can be manipulated and probed using external stimulation. In particular, electrostatic manipulation of exciton dynamics with external electric fields can be used to study electronic properties of novel material systems such as semiconductor nanocrystals and pi-conjugated molecules, which may be well suited for future applications in optoelectronic devices.

In this work, electric field induced quenching of photoluminescence through generation of indirect excitons is performed on colloidal tetrapod heterostructure nanocrystals and a multichromophoric model molecular system. The dependence of quenching on optical excitation density, which shows opposite trends in these two material systems, reflects the specific origin of quenching in each system. The large reduction in decay lifetime of indirect excitons in the tetrapods also enables storage of optical information with external electric field, which can be observed using time-resolved spectroscopy. As a model light-harvesting system with efficient energy funneling from the arm to the core, the tetrapod is an ideal system to study the impact of electric field on multiexciton states in the core and the “hot” excitons in the arm, thus providing insight into the effects of an electric field on intraparticle energy transfer. While energy transfer in the heterostructure tetrapods is through direct charge carrier thermalization, it is coherent and incoherent dipolar energy transfer that couple chromophores in the multichromophoric molecules which mimic the intermolecular interactions in organic electronics. Both single molecule spectroscopy and time-resolved spectroscopy were employed to probe the structurally dependent coherent and incoherent energy transfer.

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Fanxiang Jiao Thesis Defense 05/08/12

Thesis Defense

Fanxiang Jiao

Tuesday, May 8, 2012
10:00am (3780 WEB)

Title: Uncertainty Analysis and Visualization of Diffusion Tensor Images

Abstract:

Diffusion magnetic resonance imaging (dMRI) has become a popular technique to detect brain white matter structure. However, imaging noise, imaging artifacts, and modeling techniques, etc., create many uncertainties, which may generate misleading information for further analysis or applications, such as surgical planning. Therefore, how to analyze, effectively visualize, and reduce these uncertainties become very important research questions. In this dissertation, we present both rank-k decomposition and direct decomposition approaches based on spherical deconvolution to decompose the fiber directions more accurately for high angular resolution diffusion imaging (HARDI) data, which will reduce the uncertainties of the fiber directions. By applying volume rendering techniques to an ensemble of 3D orientation distribution function (ODF) glyphs, which we call SIP functions of diffusion shapes, one can elucidates the complex heteroscedastic structural variation in these local diffusion shapes. Furthermore, we quantify the extent of this variation by measuring the fraction of the volume of these shapes, which is consistent across all noise levels, the certain volume ratio. To better understand the uncertainties in white matter fiber tracks, we propose three metrics to quantify the differences between the results of diffusion tensor magnetic resonance imaging (DT-MRI) fiber tracking algorithms: the area between corresponding fibers of each bundle, the Earth Mover's Distance (EMD) between two fiber bundle volumes, and the current distance between two fiber bundle volumes. Based on these metrics, we discuss an interactive fiber track comparison visualization toolkit we have developed to visualize these uncertainties more efficiently. Physical phantoms, with high repeatability and reproducibility, are also designed with the hope of validating the dMRI techniques. In summary, this dissertation provides a better understanding about uncertainties in diffusion magnetic resonance imaging: where and how much are the uncertainties? How to reduce these uncertainties? How to possibly validate our algorithms?

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2012 List of Graduates & Awards

Congratulations to our 2012 graduates and scholarship recipients!


2012 Awards & Scholarships Baccalaureate Degrees Masters Degrees Ph. D Degrees
Outstanding Graduate: Monica Allen
Swigart
– Kapildeb Ambal
Outstanding Undergrad RA
: Dan Filler & Ben Czaja
Outstanding Graduate TA
: Shirin Jamali & Ren Pankovich
Outstanding Postdoc:
Tom Strohman
Outstanding Undergrad (Sr):
Jessica Johnston
Outstanding Undergrad (Jr)
: Eric Peterson
Outstanding Undergrad (Soph):
Evangelia Papadopoulos
Tyler Soelberg Memorial (Sr):
Dylan Gregerson

Hans-Paul Frederick Baehr
Michael S. Bentley
Dieter Alexander Bevans
Michael John Bigelow
Matthew A. Blackmon
Cierra Anne Block
Justin Lamar Boyer
Jordan R. Brown
Mason Paul Childs
Kevin Ray Davenport
Alexander T. Derrick
Elena Deryusheva
Dillon Blake Ely
Mark McKay Feil
Alex Hilton Gibbs
Jeffrey Michael Helotes
Thomas David Higgs
Jason Lynn Hoggan
Scott Michael Karren
Jason T. Martineau
Debra Lynn Smail Mitchell
Julia Rose Nielson
Andrew Scott Perry
Michael Robert Price
Joseph Timothy Rowley
Nirmal I Shah
Matthew Roy Shaw
John David Shifflet
Jenna Marie Whippen

Josh Coon
Nora Hassan
Fei Teng
Bill Pandit
Jose Cardoza
Saskia Innemee
Luca Visinelli

Monica Allen
Will Baker
Josh Coon
Priti Shah
Paul Nunez
Su Liu
Gary Finnegan
Rachel Glenn
Zayd Ma
Kipp vanSchooten
Fangxiang Jiao
Hyunjeong Kim
Luca Visinelli


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Rachel Glenn Thesis Defense 05/04/12

Thesis Defense

Rachel Glenn

Friday, May 4, 2012
3:00pm (110 INSCC)

Title: Many-Body and Spin-Orbit Aspects of the AC Phenomena

Abstract:

The thesis reports on research in the general field of light interaction with matter. According to the topics addressed it can be naturally divided into two parts: (Part I) many-body aspects of the Rabi oscillations which a two-level systems undergoes under a strong resonant drive, and (Part II) absorption of the ac field between the spectrum branches of two-dimensional fermions that are split by the combined action of Zeeman and spin-orbit (SO) fields.

The focus of Part I are the following many-body effects that modify the conventional Rabi oscillations: (i) coupling of a two-level system to a single vibrational mode of the environment, and (ii) correlated Rabi oscillations in two electron-hole systems coupled by tunneling with strong electron-hole attraction. In (i) a new effect of Rabi-vibronic resonance is uncovered. If the frequency of the Rabi oscillations, ΩR is close to the frequency, ω0, of the vibrational mode, the oscillations acquire a collective character. It is demonstrated that the actual frequency of the collective oscillations exhibits a bistable behavior as a function of ΩR-ω0. The main finding in (ii) is, that the Fourier spectrum of the Rabi oscillations in two coupled electron-hole systems undergoes a strong transformation with increasing ΩR. For ΩR smaller than the tunneling frequency the spectrum is dominated by a low-frequency (<<ΩR ) component and contains two additional weaker lines; conventional Rabi oscillations are restored only as ΩR exceeds the electron-hole attraction strength.

The highlight of Part II is a finding that, while the spectrum of absorption between either Zeeman-split branches or SO-split branches is close to a δ-peak, in the presence of both, it transforms into a broad line with singular behavior at the edges. In particular, when two splitting are equal, absorption of very low (much smaller than the splitting) frequencies become possible. The shape of the absorption spectrum is highly anisotropic with respect to the exciting field. This peculiar behavior of the absorption is also studied in wire geometry, where the interplay between two couplings (Zeeman and spin-orbit splitting) affects the shape of numerous absorption peaks.

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