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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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Astronomy Day 2012

Professional and amateur astronomers from across the world will gather to celebrate Astronomy Day on Saturday, April 28, 2012.

Clark Planetarium & the University of Utah will be hosting a series of events during the day and the Salt Lake Astronomical Society (SLAS) features a star party at the Stansbury Park Observatory Complex that evening.

Events start at 12:30 p.m. with half price admission to any show in the Hansen Dome Theatre until 6:45 p.m. Discounted show tickets will be available at the ticket window on the day of the event.

Clark Planetarium’s education department, Salt Lake County Library Services,University of Utah Physics & Astronomy and SLAS will provide free hands on demonstrations and activities at the planetarium from 1 – 4 p.m. with a chance win a free telescope courtesy of Celestron.

Please see below for a complete schedule of Astronomy Day events.

    Show Schedule:

    • 12:30 p.m. Perfect Little Planet
    • 1:30 p.m. Ultimate Universe
    • 2:30 p.m. Perfect Little Planet
    • 3:30 p.m. Black Holes
    • 4:30 p.m. Telescope and observing presentation by Mike Murray
    • 5:30 p.m. Perfect Little Planet
    • 6:45 p.m. Night Vision

    Free Activities:


      More information regarding the events at the Clark Planetarium is available here.

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      Bill Pandit Thesis Defense 04/20/12

      Thesis Defense

      Bill Pandit

      Friday, April 20, 2012
      3:30pm (334 JFB)

      Title: Ultrafast Photophysics of pi-Conjugated Polymers for Organic Photovoltaic Applications"

      Abstract:

      In this work we used the pump and probe photomodulation (PM) spectroscopy technique to measure the transient excitation dynamics in various pi- conjugated polymers (PCPs) films and blends with appropriate molecular acceptors. Using two different ultrafast laser systems, we extended the PM spectrum to cover a broad spectral range from 0.25 – 2.5 eV in the time domain from 100 fs to 1 ns with 150 fs time resolution. We also used continuous wave (CW) photomodulation spectroscopy, photoluminescence, electro-absorption, doping-induced absorption, and x-ray diffraction to study the photoexcitations and doping induced states, as well as other optical properties of PCPs and polymer donor-fullerene acceptor blends. In addition to these measurements, we also fabricated organic photovoltaic (OPV) solar cell devices based on poly(3-hexyl-thiophene) [P3HT]/fullerene [PCBM] blend and estimated their power conversion efficiency (PCE) in relation to the optical studies. In (1.2:1) weight ratio of P3HT/PCBM blend that shows maximum donor (D) and acceptor (A) domain separation we found that although the intrachain excitons in the polymer domains decay within ~10 ps, no charge polarons are generated at their expense. Instead there is a built-up of charge-transfer (CT) excitons at the D-A interfaces, which may dissociate into separated ‘free’ polarons in the D and A domains at a later time. Although the CT excitons are photogenerated much faster in D-A blends with a smaller domain size (such as in P3HT with random order), their dissociation is less efficient because of larger binding energy. Our results elucidate the charge photogeneration mechanism in D-A blends, and unravel the important role of the binding energy in generating ‘free’ charge polarons.

      We also studied the photophysics of a low band gap polymer, namely poly-thienophene-benzodithiophene (PTB7) film and its blend with acceptor [6,6] phenyl C71 butyric acid methyl ester [PC71BM]. In the CW PM spectrum of PTB7/PC71BM blend, clear signatures of polarons are observed. Whereas PA bands related to triplet excitons and trapped polarons are observed in the PM spectrum of pristine PTB7 film. The transient ultrafast PA of PTB7 is dominated by a singlet exciton band at ~0.95 eV. In the transient ultrafast PA spectrum of PTB7/PC71BM blend we found singlet exciton, charge transfer exciton and a polaron band that are generated simultaneously; this is different from the transient PM spectrum of P3HT/PCBM blend. We also found that the charge transfer exciton in PTB7/PC71BM dissociates faster than in P3HT/PCBM blends. This may be one of the reasons for getting higher PCE of ~7.4 % in the PTB7/PC71BM based solar cells compared to PCE ~4 % in P3HT/PCBM based solar cells.

      [1] “Two-step charge photogeneration dynamics in polymer/fullerene blends for photovoltaic applications”
      Bill Pandit*, Sanjeev Singh*, Tek. P. Basel, & Z. V. Vardeny, under review in Phys. Rev. B.* equal contributions.

      [2] “Photoexcitation dynamics in Poly [[4,8-bis[(2-ethylhexyl)oxy] benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl] (PTB7) and its fullerene blend”
      Bill Pandit, Tek. P. Basel, Ella Olejnik, Uyen Hyunh & Z. V. Vardeny, in preparation for Phys. Rev. B.

      [3] “Ultrafast optical studies of ordered poly (3-thienylene-vinylene) films”
      E. Olejnik, B. Pandit, T. Basel, E. Lafalce, C.-X Sheng, C. Zhang, X. Jiang, & Z. V. Vardeny, under review in Phys. Rev. B.

      [4] “Photoexcitation dynamics in isotope exchanged DOO-PPV”
      Ella Olejnik, Bill Pandit, Tek Basel & Z. V. Vardeny, submitted to Appl. Phys. Lett.

      [5] “Photoexcitation dynamics in polythiophene/fullerene blends for photovoltaic applications”
      C.-X. Sheng, T. Basel, B. Pandit & Z. V. Vardeny, Organic Electronics 13, 1031 (2012).

      [6] “Evidence for excimer photoexcitations in an ordered pi – conjugated polymer film”
      K. Aryanpour, C-X. Sheng, E. Olejnik, B. Pandit, D. Psiachos, S. Mazumdar, & Z. V. Vardeny, Phys. Rev. B 83,155124 (2011).

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      RECON: Self-Gated Cardiac Perfusion MRI: Edward DiBella

      Self-Gated Cardiac Perfusion MRI by Edward DiBella

      Thursday Apr. 19, 2:00-2:45pm,
      INSCC Large Conference Room

      RECON (UCAIR REsearch CONnections) provides a scientific forum for Ph.D.–level exchange of ideas, concepts, and research of broad interest in medical imaging.  Meeting 2-3 times monthly, 30 min. presentations are given by UCAIR faculty, postdocs, collaborators, and late-stage graduate students.  Topics will range broadly across medical imaging and the related sciences, and should include general-interest content that is not modality-specific.  The emphasis is on physics and engineering research, with some translational clinical components; more clinically-oriented presentations can be found at the monthly UCAIR Seminar held at SOM Classroom A/B.  Special-interest topics that delve deeper into a specific modality, problem, or theory will also be occasionally presented. RECON attendees are asked to request special-interest topics that would be appealing to a wide audience.  Everyone with an interest in medical imaging, small animal imaging, image processing, or just science in general is invited to attend (attendance by UCAIR students and trainees is mandatory).  Each RECON will begin with a few minutes of general UCAIR announcements and accolades, etc.; and coffee-&-bagel snacks will be provided.

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