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Physics Conference Room, SB B326
Coffee starts at 12:00 PM and talk starts at 12:15 PM
- Monday, December 9, 2013
- Monday, December 2, 2013
- Monday, November 25, 2013
ABSTRACT: The concepts of open and closed channels are useful to understand transport properties in disordered open media. We will review the physics of open channels and introduce a new random matrix ensemble that allows to predict the values of transmission or reflection achievable with wavefront shaping techniques in lossless or weakly absorbing media. This ensemble is parameterized by an effective fraction of controlled channels that we calculate microscopically. Its expression depends on the geometry (waveguide or slab), the illumination protocol (numerical aperture, size and shape of the illumination profile), and the long-range mesoscopic correlations of the medium. We will report measurements of the transmission eigenvalue density and of the total transmission in agreement with theoretical predictions. Finally, we will show that the same theoretical formalism can be used to predict the classical information capacity of a disordered medium as well as the effect of the disorder on the entanglement properties of a given input state of light.
- Monday, November 18, 2013
Barry P. Rand
- Monday, November 11, 2013
ABSTRACT: In this seminar, we will focus on two aspects of our work that look at materials which have been studied for quite some time, but try to utilize them in new and interesting ways. In the first part, we will look at metals, specifically Au and Ag. It turns out that metals, like semiconductors, can be quantized for diameters <2 nm. At such sizes in fact, even relatively efficient quantum yields of emission have been demonstrated. Here, we look at thin films of metal nanoclusters (MNCs), and demonstrate a thin film LED with either Au or Ag MNCs as the emitting element. In both cases, the electroluminescence peak of the LED corresponds with the photoluminescence of the MNCs in solution. In the second part, we will focus on our recent efforts to template the growth of organic semiconductors. Through proper control of crystal phase, molecular orientation, and grain size (from nm to μm), we are able to realize higher solar cell performance from "classical" materials than otherwise possible.
- Monday, October 28, 2013
ABSTRACT: In my talk I discuss several aspects of transport phenomena in the near-integrable multiscale dynamical system. Multi-scale systems naturally arise when a small perturbation is added to an integrable base (or unperturbed) subsystem. Not only are such systems common in various applications, this is the only class of dynamical systems that generically affords a quantitative analytical treatment. Direct brute-force numerical simulation of such systems are possible, but usually are very challenging precisely due to a big separation of time scales. Approximate analytical tools represent an important alternative for studying such systems. An approach that greatly simplifies the description of the mixing dynamics in multi-scale systems is based on the method of averaging: in order to study long-time dynamics, the equations of motion for phase points are averaged over the fast time scale(s). In the present talk I illustrate the glory and the fall of the method of averaging by considering two examples: one from microfluidics and one from plasma physics. In the first part of the talk I consider mixing via resonances-induced chaotic advection in microdroplets. I show that proper characterization of the mixing quality requires introduction of two different metrics. The first metric determines the relative volumes of the domain of chaotic streamlines and the domain of regular streamlines. The second metric describes the time for homogenization inside the chaotic domain. In the second part of my talk, I describe the resonant interaction between monochromatic electromagnetic waves and magnetized electrons in configurations with magnetic field reversals (e.g. in the earth magnetotail). I discuss in two resonant phenomena occurring during slow passages of a particle through a resonance: capture into resonance and scattering on resonance. These processes result in destruction of adiabatic invariant, chaotization and almost free acceleration of particles. We calculate the characteristic times of mixing due to resonant effects and the rates of the acceleration.
- Monday, October 21, 2013
ABSTRACT: Topological Insulators and Superconductors are novel materials with non-trivial energy-band topology. This non-triviallity has important physical consequences, such as the emergence of chiral edge or surface bands at the boundary of the samples, or quantized electric and magneto-electric responses. Depending on their generic symmetries, the topological materials fall in several distinct classes. An ongoing effort is understanding what are the physical properties that remain robust in the presence of strong disorder. Some fundamental questions, which apply to all these distinct classes of topological materials, are: Do the edge and surface modes localize? Are there extended states in the bulk (as in the IQHE)? Does the Magneto-Electric response remain quantized in the presence of strong disorder? In this talk, I will give a brief historical account of the field and bring up-front some of the present challenges and new research directions. In the second part I will introduce a non-commutative geometry program for topological insulators, which enabled us to make analytical and computational progress in the field of strongly disordered topological materials.
- Tuesday, October 15, 2013
- Monday, September 30, 2013
- Monday, September 23, 2013
ABSTRACT: Although lightning is one of the most commonly known and destructive natural phenomena on Earth, it remains poorly understood in terms of the most basic physics. Questions such as how it is created inside thunderstorms and how it manages to travel many tens of kilometers are still being worked out today. Benjamin Franklin is considered to be the pioneer of this research field, propelled by his famous kite experiment that showed lightning to be an electrical discharge. Since that time, lightning experiments have been relatively difficult to achieve due to its seemingly random occurrence, unpredictability and short duration. However, over the last decade, the research groups at Florida Institute of Technology and the University of Florida have made detailed measurements of the lightning discharge using a rocket triggering technique at the International Center for Lightning Research and Testing (ICLRT). A consequence of this study has been the discovery that lightning emits x-rays and gamma-rays as it travels through the atmosphere and down to Earth's surface. In 1994, NASA satellite data from the Compton Gamma-Ray Observatory, originally designed to measure gamma-ray bursts from distant galaxies, discovered intense glows of radiation being emitted from the Earth. These discharges were so bright (up to 40 MeV), that they saturated all the high energy detectors onboard the spacecraft. After many similar events were observed, these phenomena, known as Terrestrial Gamma-Ray Flashes (TGFs), were shown to originate from within the thunderstorm region. Most recently, the potential radiation doses that airline passengers would experience inside the core of a TGF have been calculated. It has also been determined, with remote radio measurements and theoretical modeling, that a new type of electrical discharge, called "dark lightning", is responsible for these high energy events. However, its relationship to a normal lightning discharge still remains a mystery. Many other exotic phenomena have also been observed as a result of thunderstorms including halos, elves, sprites and blue jets. The fact that these lightning related events can affect the upper atmosphere and lower ionosphere has reshaped the scientific field to the study of high energy atmospheric physics. The research done on lightning has thus fused many areas of physics including plasma physics, atmospheric physics, and quantum electrodynamics. The rapidly expanding field has opened many opportunities for both theoretical and experimental studies. Mathematical modeling, including Monte Carlo simulations, has given us a better understanding about the nature of particle acceleration inside the high field regions of thunderstorms. Likewise, instrumentation including photomultiplier tubes, high speed video cameras, and electric field antennas, has helped us gain a better insight into the lightning discharge near the ground. In this talk, we review the history of lightning research, show many of its recent developments, and lay out the questions that are being addressed today by many physicists, meteorologists and engineers around the world. We also hope to expand the importance of lightning safety and the awareness of exciting research opportunities for many young scientists.
- Monday, September 16, 2013