Physics Colloquia – Spring 2008 

(Fridays 2:00 PM. PS 109)

Titles link to the abstracts.
Date Speaker Title
Jan 18
 Viktor K. Jirsa (FAU)
Jan 25
 William T. Rhodes (FAU)
Feb 1
 Angela M. Guzman (FAU)
Feb 8
 James A. Harvey (UCF/CREOL)
Feb 15
 Warner Miller (FAU)
Feb 22
 Mukesh Dhamala (Georgia State Univ.)
Feb 29
 Andy Lau (FAU)
Mar 14
 Pedro Prieto Pulido (U. del Valle, Colombia)
Mar 21
 Vasudevan (Vengu) Lakshminarayanan (U. of Waterloo, Canada).
Mar 28
 Hernando Garcia (Southern Illinois U.)
Apr 4
 Naser Qureshi (CCADET/UNAM, Mexico)
Apr 11
 Marcelo Del Castillo-Mussot (Institute of Physics, UNAM, Mexico)
Apr 18
 Robert W. Boyd (Institute of Optics, U. of Rochester)
Apr 25
 Florencio E. Hernandez (CREOL)
     

 Colloquium Abstracts

Noise during rest explores the brain’s dynamic repertoire
Viktor K. Jirsa (FAU), Jan 18
Traditionally brain function is studied through measuring physiological responses in controlled sensory, motor and cognitive paradigms. However, even in absence of all goal-directed behaviour, a collection of cortical regions consistently shows temporally coherent rest activity. In humans, this default network has been shown to greatly overlap with functional architectures present during consciously directed activity, which motivates the interpretation of rest activity as day dreaming, free association, stream of consciousness and inner rehearsal. In monkeys, it has been shown though that similar coherent fluctuations are present during deep anaesthesia at levels of no consciousness8. Here, we show that the same rest state networks emerge from a stability analysis of the network dynamics using biologically realistic primate brain connectivity, although anatomical information alone does not identify the network. We specifically demonstrate that noise and time delays via propagation along connecting fibres are essential for the emergence of the coherent fluctuations of the default network. Our results indicate that brain noise transiently activates the rest networks and gives rise to the dynamics observed with associated mental processes.
 
Four Centuries of Imaging Technology, 1607 – 2007
William T. Rhodes (FAU), Jan 25
The history of the development of imaging technology combines important elements from scientific research, materials development, technology generally, and economic forces. All are examined in this talk, which describes important developments from the time of Galileo through the most recent developments in microscopy, television, and ultrasound imaging.
 
Quantum states of light and quantum tomography
Angela M. Guzman (FAU), Feb 1
Quantum optics deals with describing, measuring, and finding applications for quantum states of light. How do we distinguish between classical and quantum states of light? The Wigner representation provides a valuable tool for visualizing and measuring different quantum states of light. Quantum tomography has been applied experimentally to reconstruct the quantum state of light from a complete set of measured quantities in a way similar to classical tomography, where one builds up a picture of a hidden object by making observations from different angles.
 
A Global View of Diffraction: Re-visited
James E. Harvey (UCF/CREOL), Feb 8
Radiometry is a neglected stepchild in the physics curriculum at most universities, and electrical engineers learn even less about radiometry as there is not even a quantity analogous to radiance in classical electrical engineering fields. The fact that electrical engineers have been writing most of our optics textbooks for the last few decades has thus done little to advance the understanding of radiometric nomenclature and principles. The recent revelation that diffracted radiance is the fundamental quantity predicted by scalar diffraction theory, and is shift-invariant in direction cosine space, has lead to the development of a generalized linear systems formulation of non-paraxial scalar diffraction phenomena. Thus simple Fourier techniques can now be used to predict a variety of wide-angle diffraction phenomena. These include: (1) the redistribution of radiant energy from evanescent diffracted waves into propagating ones, (2) the angular broadening (and apparent shifting) of wide-angle diffracted orders, and (3) diffraction efficiencies predicted with an accuracy usually thought to require rigorous electromagnetic theory. In addition, a unified surface scatter theory has been shown to be more accurate than the classical Beckmann-Kirchhoff theory in predicting non-intuitive scatter effects at large incident and scattered angles, without the smooth-surface limitation of the Rayleigh-Rice scattering theory. This new understanding of non-paraxial diffraction phenomena is becoming increasingly important in the design and analysis of optical systems, particularly those dealing with nano-structures or the increasingly popular field of nano-photonics.
 
Measuring the Scalar Curvature with Clocks and Photons: Voronoi-Delaunay Lattices in Regge Calculus
Warner Miller (FAU), Feb 15
The Riemann scalar curvature plays a central role in Einstein’s geometric theory of gravity. We describe a new geometric construction of this scalar curvature invariant at an event (vertex) in a discrete spacetime geometry. This allows one to constructively measure the scalar curvature using only clocks and photons. Given recent interest in discrete pre-geometric models of quantum gravity, we believe is it ever so important to reconstruct the curvature scalar with respect to a finite number of communicating observers. This derivation makes use of a new fundamental lattice cell built from elements inherited from both the original simplicial (Delaunay) spacetime and its circumcentric dual (Voronoi) lattice. The orthogonality properties between these two lattices yield an expression for the vertex-based scalar curvature which is strikingly similar to the corresponding hinge-based expression in Regge calculus (deficit angle per unit Voronoi dual area). In particular, we show that the scalar curvature is simply a vertex-based weighted average of deficits per weighted average of dual areas.
 
Analyzing Information Flow in Brain Networks with Nonparametric Granger Causality
Mukesh Dhamala (Georgia State Univ.), Feb 22
Multielectrode neurophysiological recording and high-resolution neuroimaging generate multivariate data that are the basis for understanding the patterns of neural interactions in the brain. These interactions are directional in the sense of signal transfer among neural systems. Extracting such directional information (or effective connectivity) from measured brain data is a key to our understanding of a brain function. We have recently proposed a nonparametric (model-free) approach to Granger causality for assessing directional influences. This new nonparametric approach, which is based on widely used Fourier and wavelet spectral methods, not only overcomes some of the existing problems in parametric techniques, but also opens up new applications in neuroscience. In this colloquium, the utility of the nonparametric Granger causality techniques will be illustrated with applications to some real-world data and the associated findings will be discussed.
 
Non-equilibrium fluctuations and mechanochemical couplings of a molecular motor
Andy Lau (FAU), Feb 29
Molecular motors are nano-machines that convert chemical energy into mechanical work and motion. They are responsible for a variety of functions carried out in a living cell. Understanding the mechanochemical transduction mechanisms behind these motors remains a significant challenge. In this talk, we disscuss the out-of-equilibrium features of a single motor protein which moves along a linear filaments that polar and periodic. The physics of the operation of such motors can be described by a simple discrete stochastic model which is coupled to a chemical reaction. Within this model, we discuss the violations of Einstein and Onsager relations and the efficiency for a single processive motor operating far from equilibrium. With the aid of the Fluctuation Theorem, we analyze the general features of these violations and this efficiency and link them to mechanochemical couplings of motors. In particular, an analysis of the experimental data of kinesin using our framework leads to interesting predictions that may serve as a guide for future experiments.
 
Structural and magnetic properties of manganite superlattices
Pedro Prieto Pulido (U. del Valle, Colombia), Mar 14
Superlattices of ferromagnetic (F) and antiferromagnetic (AF) oxide materials have attracted increased attention given the exchange bias or exchange anisotropy phenomenon. All these issues have taken on technological importance as well, given the use of exchange bias systems in today’s magnetoresistance (GMR) sensors for hard disk drive applications. Research on magnetic oxide superlattices has focused towards the technological goals of increasing magnetoresistance in lower applied magnetic fields and more useful temperatures ranges for various applications. Of particular interest are La2/3Ca1/3MnO3/ La1/3Ca2/3MnO3 (F-LCMO/AF-LCMO) superlattices, due to the very high structural compatibility of the AF and F layers in this system, permitting epitaxial coherent growth. Results of a study of the temperature dependence of magnetization and magnetotransport properties for a series of [AF-LCMO/F-LCMO]N superlattices as a function of the antiferro- and ferromagnetically doped layer thickness will be presented.
 
Waveguiding in retinal photoreceptors
Vasudevan (Vengu) Lakshminarayanan (U. of Waterloo, Canada), Mar 21
The rods and cones of the human eye behave like classical fiber optic elements in their light capture and transmission to sites of absorption. In other words, specific waveguide modes can be observed. This also implies that the photoreceptors are oriented and have specific alignment properties. In terms of visual performance, this implies that we are more sensitive to light coming in through the center of the entrance pupil of the eye than the periphery. In this talk I will discuss various aspects of waveguiding in vertebrate photoreceptors, methods of measurement, as well as theories of such waveguiding phenomena.
 
Optical Properties of Multi-Metal Nanocomposites
Hernando Garcia (Southern Illinois U.), Mar 28
Plasmonic based nanophotonics may be the bridge between high-impedance electronic devices to the low-impedance in electromagnetic light propagation. This, if possible, will render plasmonics as the future technological revolution. The high bandwidth capacity (THz) and large group velocity will make plasmonic-based devices as ideal power converters. Therefore the understanding of the effective permittivity of these nanostructured materials is of fundamental importance for future plasmonic based nanophotonic devices. Bergman-Milton, and Hashin-Shtrikman, bounds have provided limits on the effective permittivity of a composite material comprising two isotropic dielectric materials. We have extended this formalism to include more than two components in the composite [Garcia et al. Phys. Rev. B, 75, 045439 (2007)], and calculated the linear as well as the nonlinear optical susceptibilities of the mixture. In this talk I will show how to use this formalism to calculate the linear and nonlinear optical susceptibility of glass matrices containing noble metal nanoparticles (Ag, Al, Cu, and Au). Also I will outline the physical realization of these systems and compare the theory with experiments. A brief introduction to plasmonics will be given at the beginning of the talk with emphasis on current application such as: memory storage, sensing, information processing, and solar harvesting.
 
Mixing optics and nanomechanics
Naser Qureshi (CCADET/UNAM, Mexico), Apr 4
We review a selection of recent advances in optical microscopy and lithography, and discuss some of the remarkable opportunities that localized optical probes and actuators have created recently in experimental physics. This talk will focus on aspects of nanoscale magnetism that have been studied with scanning optical probes, as well as micromechanical systems that involve scanned optical beams.
 
Interdisciplinary models of collective behavior: Three-body interactions in coalition forming, and thermodynamical model of the distribution of wealth in the USA
Marcelo Del Castillo-Mussot (Institute of Physics, UNAM, Mexico), Apr 11
An study of the effects of three-body interactions in the process of coalition formation is presented. In particular, we modify a spin glass model of bimodal propensities in order to include a particular three-body Hamiltonian that reproduces the main features of the required interactions. The model can be used to study conflicts, political struggles, political parties, social networks, wars and organizational structures. As an application, we analyze a simplified model of the Iraq war. Also we present the model of Silva and Yakovenko for the distribution of wealth, that shows that personal income distribution in the USA has a well-defined two-class structure. The majority of population (97-99%) belongs to the lower class characterized by the exponential Boltzmann-Gibbs ("thermal") distribution, whereas the upper class (1-3% of the population) has a Pareto power law ("superthermal") distribution. By analyzing income data for 1983-2001, they show that the "thermal" part is stationary in time, save for a gradual increase of the effective temperature, whereas the "superthermal" tail swells and shrinks following the stock market.
 
Fundamentals and Application of Slow and Fast Light
Robert W. Boyd (Institute of Optics, U. of Rochester), Apr 18
Research performed over the past several years has demonstrated new methods for controlling the velocity of propagation of pulses of light through material systems. Ultra slow velocities (tens of meters per second) and ultra fast velocities (including negative velocities) have been demonstrated. This talk will commence with an overview of this field and will include a discussion of some new ideas for applications of fast and slow light based on the use of room temperature solids.
 
Plasmonics in Linear and Nonlinear Optics: New Advances!
Florencio E. Hernandez (CREOL), Apr 25
The current need for superior biological and chemical sensors, as well as bioimaging and photodynamic therapy in medicine, has awakened the interest of scientists in multiphoton excitation processes. Two-photon absorption has been widely used in fluorescence spectroscopy and imaging in recent years because of its large effective Stokes’ Shift and high spatial resolution. However, the irradiation penetration depth is limited in medical and biological applications because of the unwanted absorption and scattering when two red photons are used. To resolve these limitations, we have recently proposed three-photon absorption processes that can afford minimization of the scattered light losses, and a reduction of unwanted linear absorption in the living organism transparency window. However, the extraordinary intensities typically required for higher order excitation has limited the progress in this field. In order to surmount the required high irradiances in nonlinear optics, we have considered the use of metal nanoparticles to locally enhance the electric-field, thus assisting the excitation process. Herein we present the study of the effect that gold nanoparticles with different shapes have on the two- and three-photon absorption and decay rates of organic molecules with different dipole moment orientation with respect to the metal surface. Also, new approaches for chemical and biological sensing using metal nanoparticles are discussed.