Wednesday, January 31^st, 11 am, Room 301, Research I,

Changan Xie, Post-doctoral Research Associate, University of Utah

Detection of single biological complexes in liquids using Raman-tweezers

Abstract:

A biological particle is the complex mixture of a large number of biomolecules, such as nucleic acids, proteins, polysaccharides, and lipids. Identification of biomolecules within the complex particle is important to understand various cellular processes. However, due to Brownian motion and cell motility, the biological complex in solution may randomly move away from the confocal excitation volume. Therefore, the complex particles have to be immobilized either physically or chemically. Raman-tweezers is a confocal microscopy system that combines the optical tweezers and Raman spectroscopy. Optical tweezers permits the manipulation and identification of a motile biological particle in solution without physical contact. Raman spectroscopy can provide fingerprint information about species, structures, and molecular conformation within the confined objects. The techniques have been used for identification, characterization and sorting of single biological objects at different size scales, such as cells, bacteria and organelles. Recently, Raman-tweezers has been combined with other techniques such as microfluidic system (lab-on-a-chip) for dynamic process of single biological complexes. I suggest exploring a combination of Raman-tweezers and nano-optics to address the interaction between the various biomolecules. This new capabilities provide a new means to elucidate the molecular processes.

Thursday, February 1^st, 2 pm, Room 302, Research I

John Cressman, Research Associate in the Center for Neural Engineering, Penn State

Unbalanced networks and seizure dynamics

Abstract:

Normal neuronal activity is produced through a balance of excitatory drive and inhibitory control. In this way brains can be characterized as driven steady state systems maintained far from equilibrium. Seizures are the result of a break down in this balance leading to uncontrolled activity. I will present the results from computer models and in vitro experiments that illuminate some of the basic mechanisms responsible for the destabilization of neural networks and seizure dynamics.

Monday, February 5^th, 10:30 am, Room 302, Research I

Rhonda Dzakpasu, Research Fellow, Department of Physics, University of Michigan

Causal Entropy as a Measure of Temporal Relationships and Direction of Information Transfer in Neural Systems

Abstract:

There are billions of neurons in the brain, each of which participates in the execution of various functions. How does the brain organize the operations of these fundamental units? What relationships exist, on a temporal scale, between neurons? Is there an ordering between the temporal patterns of neurons? In order to begin to address these questions, we have developed a novel analytical tool that measures temporal interdependencies between coupled neurons. The technique involves the real time monitoring of inter-event intervals between the coupled neurons. We demonstrate the feasibility of the measure on a mathematical model consisting of two, coupled nonidentical Hindmarsh?Rose models of thalamo-cortical neurons. We show that the measure may be better than more conventional methods at detecting changes in asymmetrical temporal patterns. Finally, we demonstrate how the technique can be modified to study networks of coupled neurons and discuss the application of the measure in the analysis of experimental data.

Thursday, February 8^th, 12:30 pm, Room 301, Research I

Dan Wasserman, Postdoctoral Research Fellow, Department of Electrical Engineering, Princeton University

Novel Sources for Mid-Infrared Emission

Abstract:

The past decade has seen the emergence and rapid development of the Quantum Cascade (QC) Laser, whose power, wavelength flexibility, and compact size has spurred both exciting new fundamental research and technological applications in the mid-Infrared (IR) spectral range. Because numerous species of interest for medical, environmental, and security applications have strong vibrational resonances in the mid-IR, high performance sources and detectors are essential to the development of next-generation sensing systems. Requirements for these applications include not only high power, single-mode, and room temperature operation, but the demonstration of all of the above across the entire mid-IR and THz spectrum. While high-power InP-based QC lasers are being developed for commercial applications in the 6-10μm range, we are also working the next generation of mid-IR emitters, in an effort to extend the wavelength range and increase the efficiency of these devices.

The integration of optical nonlinearities into QC laser active regions allows us to extend the wavelength range of our current QC laser technology, in both the long and short wavelength directions. Results demonstrating Electronic Anti-Stokes Raman Emission from Quantum Cascade lasers offer one method for extending QC Lasers to short wavelengths, for applications such as breath analysis, CO₂ emissions monitoring and industrial process monitoring. Additionally, recent exciting results suggesting the first demonstration of Difference Frequency Generation (DFG) in QC lasers have been obtained. The use of nonlinearities for the generation of DFG light provides an avenue towards room-temperature THz emission, a result which would have significant impact for explosives sensing and medical imaging applications.

Finally, I will discuss efforts to utilize InAs self-assembled quantum dots (QDs) for mid-Infrared emission. The unique properties of these nanostructures offer the possibility of both higher efficiency and surface-emitting mid-IR lasers. The development of surface-emitting QC lasers would open the door to mid-IR “lab on a chip” devices, ideal for high-throughput sensing applications. The first demonstration of mid-IR emission from QDs in unipolar cascade-like structures, and examples of multiple-wavelength, anisotropically polarized emission from such structures, will be discussed.

Friday, February 9^th, 11 am, Room E on the 3rd Floor of the Johnson Center

Ingrid Stairs, Assistant Professor, Department of Physics and Astronomy, University of British Columbia

Relativistic Binary Pulsars

Abstract:

Radio pulsars in double-neutron-star systems provide the best tests of strong-field gravitational theories. I will present recent observations of PSR B1534+12 and the double-pulsar system J0737-3039A/B. In both cases, the pulsar timing observations are sensitive to multiple relativistic corrections to the basic Keplerian orbit, and therefore yield multiple tests of general relativity, with the double pulsar providing the strongest test to date. I will also discuss observations of geodetic precession and implications for the formation of the two systems.

Monday, February 12, NOON, Room 301, Research I

Mikhail Zamkov, postdoctoral fellow, School of Chemical Sciences, University of Illinois, Urbana-Champaign

Ultrafast electron dynamics on a nanoscale unveiled by optical and photoelectron spectroscopies

Abstract:

I would like to touch two topics in my talk:

FORMATION OF TUBULAR ELECTRONIC STATES AROUND CARBON NANOTUBES. Using two-color photoelectron emission we can populate and subsequently observe the special group of electronic states with wave functions enclosing a carbon nanotube. These cylindrical “electronic rings” constitute a new class of “image” states due to their quantized angular motion. The electron rotation about the axis of the nanotube gives rise to a centrifugal force that virtually detaches the electron charge-cloud from the tube's body. By experiencing the lattice structure parallel to the tube's axis these rings can act as powerful scanning probes of nanotube electronic properties.

VIBRATIONAL SPECTROSCOPY OF NANOENERGETIC MATERIALS.

Mixing of reactants on the nanometer length scale represents a new frontier for energetic materials, providing an increased performance in terms of energy release, stability, sensitivity and mechanical properties. Our group is exploring new ways for optimizing these nanoenergetic materials, by using novel fabrication and optical characterization approaches. Specifically, we incorporate metal nanoparticles coated with a 3-nm-thick oxide layer into thin films of a polymer. This mixture exhibits high potential for an efficient conversion of photoenergy into heat or mechanical energy. The dynamics of chemical changes in investigated systems is monitored in real time through time-resolved vibrational spectroscopy.

Tuesday, February 13, 12:30-1:30 PM, Room 301, Research I

Mingzhen Tian, Assistant Research Professor, Physics Department, Montana State University

Rare earth based solid state quantum computing and quantum memory

Abstract:

In the past decade, quantum information has been one of the most active research areas in science and technology. Quantum computing holds the unprecedented computational power for exact simulation of physical processes in quantum systems and efficient solution of some computational hard problems, such as larger number factorization. Quantum memory is an important element in quantum communication over long distant. Physical implementation of quantum computing and quantum memory has been the main focus in developing practical quantum information technology. Rare earth ions doped in solids is one of the promising candidates for building quantum computing hardware and quantum memory. The basic ideas of rare-earth based approach and recent results from experiments and simulations will be presented in the talk.

February 23: Adrian Parsegian, Laboratory Chief, Laboratory of Physical and Structural Biology, National Institute of Child Health and Development

Van der Waals forces: new properties and practices

Abstract:

Despite the breadth of their occurrence, even with the many kinds of science that use them, van der Waals -- charge fluctuation, electrodynamic -- forces are only now beginning to be computed, measured, and applied in ways appropriate to their power to organize materials. This will be a review of the new ways in which regard these forces together with a set of examples showing the ways they behave and the strategies now emerging to learn more about them.

March 23: Frederic Galliano, Observational Cosmology Lab, NASA Goddard Space Flight Center

Dust Evolution in Galaxies

Abstract:

The UV-to-radio spectral energy distribution (SED) of a given region inside a star forming galaxy is the intricate combination of the escaping stellar light and of the radiation reprocessed by gas and dust in various phases of the interstellar medium (ISM). The spectrum emerging from such a region is therefore shaped by the stellar populations, the gas conditions and the dust properties in each phase, as well as the morphology of the ISM. The star formation history of the considered region controls most of these properties, since stars are responsible for the production of metals that will condense into dust, for the formation and evolution of HII regions, for the molecular cloud dissipation and for the grain heating rate. Consequently, the SED will evolve with time, reflecting at each instant the evolutionary state of the region and its physical conditions. This SED therefore contains valuable information on the region's star formation history, and its chemical evolution.

I will present recent results, based on the modeling of star forming regions and galaxies of various types, observed by the infrared satellites Spitzer and ISO. I will discuss the effect of massive star formation on the interstellar medium, which can be considered as a short timescale evolution (a few million years). Then, I will show how the evolution of the dust content in galaxies can be related to the enrichment of the ISM in heavy elements by stars, on long timescales (a few billion years).

March 30: Dmitri Klimov, Assistant Professor, Department of Bioinformatics and Computational Biology, George Mason University

Monte Carlo simulations of cotranslational protein folding

Cotranslational folding of proteins occurs in the course of polypeptide biosynthesis on a ribosome. During cotranslational folding a nascent protein chain is only partially synthesized and extruded and is still tethered to a ribosome. It is conceivable that cotranslational folding differs from a “traditional” in vitro refolding of a full length chain, but so far little is known about the mechanisms of in vivo cotranslational native assembly. This talk discusses our recent Monte Carlo simulations of cotranslational folding of single domain proteins. We show that this class of proteins indeed folds cotranslationally via distinct mechanisms, which are not observed during in vitro refolding. We use free energy landscape perspective to rationalize our thermodynamic and kinetic findings. Comparison with recent experimental and computational studies is provided.

April 13: Stacy McGaugh, Associate Professor, Department of Astronomy, University of Maryland

The Pros and Cons of Invisible Mass and Modified Gravity

There is overwhelming observational evidence for mass discrepancies in the universe. When we look at extragalactic systems, we see motions that are greater than can be accounted for by the action of gravity and the mass we can see. The obvious and widely accepted inference is that there must be additional unseen mass, presumably in a novel type of particle. An alternative interpretation is that the equations governing the motions in such systems need to be generalized. I will discuss some of the pros and cons of these ideas.

April 20: Sandu Popescu, HH Wills Physics Laboratory, Bristol, UK

Limits to Non-locality

Abstract:

Non-locality was first discovered (over four decades ago) in the context of quantum mechanics. But for a couple of years it has been known that non-local correlations stronger than those arising from quantum mechanics are theoretically possible. Are there such correlations in nature and if not, why not? In my talk I will discuss the limits on quantum non-locality and some of the implications of non-locality beyond quantum mechanics, in particular the issues of non-local computation and communication complexity.

Friday, April 27^th, 11 a.m., Room C, Johnson Center

April 27: Stephen Maran, American Astronomical Society

Fifty Years of Great Discoveries in Astronomy

Abstract:

The past half-century in Astronomy is marked by the discoveries of quasars, celestial X-ray sources, the cosmic microwave background radiation, pulsars, brown dwarfs, planetary systems beyond our own, and dark energy. The quasars and many celestial X-ray sources, are black holes. Pulsars, and many others of the X-ray sources, are neutron stars. Astronomers have learned how stars and planets form and where gamma-ray bursts originate in the distant universe. Fifty years ago, few astronomers outside of California concentrated on the study of galaxies and it was not unusual to go through graduate school in Astronomy without taking a course in them. Today, the details of how galaxies form and evolve are starting to take shape and a huge new NASA telescope may find the first ones. During this period, astronauts walked on the Moon, and unmanned scientific spacecraft visited every one of the planets but Pluto. Then Pluto got demoted, making NASA’s record perfect (but a probe is under way).

May 4: Frederick Rothwarf 1, and Sisir Roy 2,3

1 Department of Physics, George Mason University, Fairfax, VA 22030 USA

2 Center for Earth Observing and Space Research, College of Science, George Mason University, Fairfax, VA 22030 USA

3 Physics and Applied Mathematics Unit, Indian Statistical Institute, Calcutta, INDIA

Quantum Vacuum and a Matter - Antimatter Cosmology

Abstract:

A model of the universe as proposed by Allen Rothwarf (1935 -1998) based upon a degenerate Fermion fluid composed of polarizable particle-antiparticle pairs, e.g., electron-positron pairs, leads to a big bang model of the universe where the velocity of light varies inversely with the square root of cosmological time, t. This model is here extended to predict a decelerating expansion of the universe and to derive the Tully-Fisher law describing the flat rotation curves of spiral galaxies. The estimated critical acceleration parameter, aoR, is compared to the experimental, critical modified Newtonian Dynamics (MOND) cosmological acceleration constant, ao, obtained by fitting a large number of rotation curves. The present estimated value is much closer to the experimental value than that obtained with other models. This model for aR(t) allows the derivation of the time dependent radius of the universe R(t) as a function of red shift z, R(z). Other cosmological parameters such as the velocity of light, Hubble’s constant, the Tully-Fisher relation, and the index of refraction of the aether can also be expressed in terms of z. R(z) is compared with the statistical fitting for Veron-Cetty data (2006) for 86,000 quasar red shifts and good agreement is found. This model also determines the time and/or z dependence of certain electromagnetic parameters, i.e., the permittivity; the permeability; and index of refraction of free space. These are found to be useful in various cosmological theories dealing with light passing through media in motion.

Questions about the physics seminar? Contact Karen Sauer at ksauer@physics.gmu.edu

Other interesting seminar series at GMU:

College of Science Seminars

Seminar in Bioinformatics and Computational Biology

Chemistry and Biochemistry Seminar

COLLOQUIA of the Computational Materials Science Center

Krasnow Monday seminar series

Quantum Information Science (QIS) Seminar

Math Colloquium

Time:	Fridays, 11 AM - noon (except where noted)
Place:	Room 302, Research I (except where noted)

Physics and Astronomy Seminars: Spring 2007 Schedule

Department of Physics and Astronomy
George Mason University, Fairfax

Driving Directions to GMU

Physics and Astronomy Seminars: Spring 2007 Schedule

Department of Physics and Astronomy George Mason University, Fairfax

Driving Directions to GMU

Department of Physics and Astronomy
George Mason University, Fairfax