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Sunday, May 20, 2012


Priti Shah Thesis Defense 05/10/12

Thesis Defense

Priti Shah

Thursday, May 10, 2012
3:00pm (206 JFB)

Title: Monocular Measurement of the Ultra High Energy Cosmic Ray Spectrum while Relaxing the Profile Constraint


Cosmic rays are charged particles of galactic and extragalactic origin. In the ultra high energy regime, due to their extremely low flux, cosmic rays can only be observed indirectly via an extensive air shower induced when they interact in the Earth's atmosphere. The Telescope Array (TA) experiment, the largest experiment in operation in the northern hemisphere, observes the longitudinal profile of the fluorescence light from these extensive air showers via telescopes. The Middle Drum fluorescence telescope station utilizes the same equipment as the HiRes-I site of the High Resolution Fly's Eye (predecessor to Telescope Array) experiment. The equipment has simply been reconfigured. As HiRes-I, the telescopes viewed 3-17o in elevation and nearly 360o in azimuth. As a result of this, the track length in the cameras tended to be short. In the Telescope Array configuration, the telescopes view 3-31o in elevation, but only about 120o in azimuth, however, the resulting track lengths are significantly longer. With the short track lengths, one needed to make an assumption about the shape of the profile; that it had a Gaisser-Hillas shape. The longer track lengths make this unnecessary. I have analyzed the data using a Time vs Angle geometry method. The results show an ultra high energy cosmic ray energy spectrum that is consistent with the previous results of the HiRes experiment as well as that measured by the TA surface detectors.


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


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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