Colloquia held Spring Semester, 2008

February 5 , 2008
DVD Produced by AAPT
"The World of Enrico Fermi"
This DVD produced in limited edition depicts rare original footage of Enrico Fermi, his research teams, and colleagues. He is seen in Rome, Columbia University, Chicago, and Los Alamos. The first part brings Fermi’s life up to 1938 when Fermi received the Nobel Prize for his basic work on nuclear reactions. He and his family then fled to America to escape fascism. The second part includes the war years and his development of nuclear reactors. After the war he was increasingly concerned with applications of nuclear energy and with public policy.

February 12 , 2008
Dr. Zhiming M. Wang
Institute of Nanoscale Science and Technology
 University of Arkansas
Fayetteville, Arkansas
"Epitaxial Quantum Dots: A Playing Field for Nanoscale Physics"
Experimental observation of vertical and lateral interaction among epitaxial quantum dots is reviewed with particular emphasis on the control of growth parameters. By stacking two layers of quantum dots vertically, asymmetric dot pairs are realized and their sizes and consequently their energy level can be separately adjusted in different layers. The strength of their electronic coupling is controllable by varying the barrier thickness in between. The lateral interaction among quantum dots is usually weak due to the intrinsic large separation. We are able to introduce one-dimensional post-wetting layers to connect the quantum dots and therefore to form a network to enhance the lateral interaction among quantum dots. With controllable vertical and lateral interactions, self-organization of quantum dots provides an excellent opportunity to fabricate three-dimensional quantum dot crystals for novel optoelectronic devices.
**Special Note: Dr. Zhiming Wang interviewed for a faculty position at UNT.

February 20 , 2008
Dr. Juraj Topolancik
Rowland Institute at
Harvard University
Cambridge, Massachusetts
"High-Quality Optical Resonators and Their Applications"
High-quality (Q) optical resonators confine significant optical powers in small spaces for extended time periods. Interactions of light with matter are greatly enhanced in these microstructures. They have attracted considerable interest due to their potential applications in optical signal process-ing, sensing and nonlinear optics. First, I will focus on whispering-gallery microresonators (silica microspheres) and will describe how we have employed them to monitor molecular transformations in complex biomembranes. Second, I will consider nanofabricated cavities in photonic crystals (PhCs). Two-dimensional PhCs are periodic dielectric that inhibit light propagation in bands of frequencies commonly referred to as photonic bandgap. Intentional breaking of the lattice periodicity introduces local defects in which light is trapped by the total internal and Bragg reflections. The poss-ibility of a highly-efficient photon confinement has established PhCs as a platform for designing optical nanocavities. We have used a different approach to photon localization in these structures. The design concept applies random structural perturbations uniformly throughout the artificial crystal by deliberately changing the shapes and orientations of the lattice elements. The disorder created in this way represents random scatterers which impede propagation of Bloch-waves through the underlying periodic lattice. Optical modes guided along line-defects in disordered crystals experience strong backscattering which gives rise to localization. We have observed optical resonances with ultra-small modal volumes and the effective Qs of up to ~250,000. These Qs are comparable to the values measured on nanocavities engineered by meticulous parametric tuning of local defects. I will briefly discuss applications of these random nanocavities for ultra-sensitive biodetection.
**Special Note: Dr. Juraj Topolancik interviewed for a faculty position at UNT.

February 26, 2008
Dr. Usha Philipose
Department of Electrical Engineering
University of Toronto
Toronto, Canada
"Semiconductor Nanowires for Nanoscale Optoelectronic Devices"
Nanotechnology, the development and engineering of useful devices, systems and materials with dimensions less than 100nm, is based on the concept that materials behave in unusual, counterintuitive ways when their dimensions fall within the nanometer-scale size regime. In the “nanoscale” realm, the laws of physics are no longer only dependent on the composition of the structure, but also on its physical size and shape. In particular, semiconductor nanostructures such as nanowires and quantum dots show unique and interesting properties with great potential for novel applications. In this talk, I will present an overview of the Vapor-Liquid-Solid (VLS) growth of semiconductor nanowires. I will describe the synthesis of defect free crystalline nanowires required for successful device applications. Growth process optimization and post processing techniques for control of stoichiometry and defects will be presented. Optical, electrical and magnetic properties of pure and doped II-VI and III-V nanowires will be presented. Finally, potential applications of thesenanowires as photodetectors, spintronic devices and as efficient light emitting devices will be outlined.
**Special Note: Dr. Usha Philipose interviewed for a faculty position at UNT.

March 4, 2008
Dr. Thomas Planchon
Department of Physics,
Colorado School of Mines
Golden, Colorado
"Manipulating Photons: Toward Control Over Ultrashort Light Pulses with Active Optics"
Adaptive optics and pulse shaping methods were originally developed for astronomical and telecommunication applications. In the last decades, they have been recognized as preeminent methods to control  the spatial and temporal shapes of ultrashort (femtosecond) laser  pulses. This presentation will review the use of these active control methods in the field of ultrafast optical sciences. By manipulating the light distribution, the first interest for physicists is to maximize the  photon density in the focus of nonlinear optics experiments, by obtaining diffraction-limited focal spot and shortening the pulse duration. These versatile controls also allow more complex changes  over the light distribution that favor the physical process of  interest. Previously, temporal and spatial aspects of a laser pulse have been considered separately and research has focused on the active control of only one of these at a time. One of the most exciting directions in this research area is the possibility to act on both aspects simultaneously. This presentation will describe the different optimization schemes, from closed loop control to genetic algorithms, and experimental results  obtained in three different laboratories.
**Special Note: Dr. Thomas Planchon interviewed for a faculty position at UNT.

March 6, 2008
Dr. Bart Willems
Department of Physics and Astronomy
Northwestern University
Evanston, Illinois
"Compact Object Binaries: Stellar and Binary Evolution in the Gravitational Wave Era"
The construction and planning of ground- and space-based laser interferometers has led to a massive surge of interest in sources of gravitational wave radiation during the past decade. Direct detection of these ripples in space-time will provide unprecedented tests of Einstein's theory of general relativity and open a brand new window on the universe unhindered by the main obstacles affecting electromagnetic radiation. Among the rich set of anticipated gravitational wave sources, binary star systems made up of compact remnants of stellar evolution are expected to be the most numerous class of gravitationally radiating objects. In this colloquium, I will look into the future and discuss the physics that will become accessible with the launch of the Laser Interferometer Space Antenna (LISA), the most ambitious gravitational wave observatory planned to date. I will particularly focus on binary star systems consisting of one or two white dwarfs, the evolutionary endpoints of more than 90% of the stars in the galaxy (including the Sun). Recent theoretical advances and observations by the Spitzer infrared space telescope show that the formation of circumbinary disks during mass-transfer episodes between the binary components can drastically affect the binary evolution, potentially resolving longstanding problems and raising questions about neutron star and black hole binary formation. I will also show the unique opportunity offered by globular clusters and LISA to unveil white dwarf physics inaccessible through electromagnetic observation.
**Special Note: Dr. Bart Willems interviewed for a faculty position at UNT.

March 25, 2008
Dr. Ohad Shemmer
Department of Astronomy & Astrophysics
The Pennsylvania State University
University Park, Pennsylvania
"Tracing the History of Black-Hole Growth Across Cosmic Time"
Active galactic nuclei (AGN) are among the most luminous sources in the Universe, and it is widely accepted that they are powered by mass accretion onto supermassive black holes (BHs) in their centers. The tight relation observed between supermassive BHs and their host galaxies indicates that BH growth is linked with galaxy evolution, and hence probing BH growth in AGN should lead to insights into how galaxies have formed and evolved. Utilizing new XMM-Newton X-ray Observatory spectra of luminous AGN at high redshift, I will show that the X-ray spectral slope and X-ray luminosity can provide accurate determinations of the BH mass and accretion rate in AGN. This newly developed X-ray-based method may provide a useful probe for tracing the history of BH growth across cosmic time. It may also prove to be the best way to probe BH growth in optically-obscured AGN, whichcomprise a significant fraction of the AGN population. I will show an application of this method to a large archival dataset of X-ray-selected AGN and place constraints on current cosmological models of BH growth. Finally, I will describe how the next generation of space-based observatories such as JWST and Constellation-X may shed light on BH formation and evolution during the reionization epoch.
**Special Note: Dr. Ohad Shemmer interviewed for a faculty position at UNT.

March 27, 2008
Dr. Jiming Bao
School of Engineering and Applied Sciences
Harvard University
Cambridge, Massachusetts
"Nanophotonics, Nanofabrication and Nanomaterials"
This talk will present my recent work at the intersection of photonics, nanofabrication, and materials design. I will start with an overview of the light emission properties of semiconductor nanowires, including single-nanowire light-emitting diodes, and optically pumped ultraviolet nanowire lasers. Then, I will introduce nanoskiving as a technique for engineering the optical response of metallic nanostructures. Finally, I will discuss two novel approaches for the design of new nanomaterials: point-defect engineered silicon for silicon photonics and rotationally twinned nanowires as a new type of superlattice.
**Special Note: Dr. Jiming Bao interviewed for a faculty position at UNT.

April 1, 2008
Professor Wolfgang Schleich
Institut für Quanten Physik
University of Ulm
Ulm, Germany
"Bose-Einstein Condensation in Microgravity"
This talk is an introduction into the activities of the QUANTUS collaboration consisting of several German universities. The goal is to create and observe Bose-Einstein condensation (BEC) in free fall. We report the first observation of the time evolution of a wave function of a BEC prepared in a small capsule during the extended free fall of one second in the hundred meter drop tower at Bremen. This realization of Einstein's Gedanken experiment of a freely falling elevator opens up new avenues for tests of the equivalence principle, the foundations of quantum mechanics, and precision measurements at the frontiers of quantum mechanics, gravity, and relativity. This talk will focus mainly on theoretical aspects. It will be followed a few weeks later (May 13, 2008) with a talk by E. M. Rasel of the University of Hannover (Germany) oriented more towards experimental questions.

April 8, 2008
Dr. Paolo Grigolini
Center for Nonlinear Science and Department of Physics
University of North Texas
Denton, Texas
"1/f Noise as an Interdisciplinary Key: from Neurophysiology to Sociology with Excursions into Arts and Linguistics"
The 1/f noise is ubiquitous in physics, but its origin is not yet understood. We review the recent work of on the brain dynamics showing that the brain is a generator of 1/f noise. This condition becomes evident when the subjects are performing cognitive tasks. We show that 1/f noise appears in many experiments on body movements and heartbeats. We discuss the results of statistical analysis of music and painting that shows music and painting are generators of 1/f noise. Some recent studies show that 1/f noise property of music is related to the Zipf law of linguistics. The Pareto law in economics is proven to be equivalent to the Zipf law. These observations and the joint work of psychologists suggest that the 1/f noise is the manifestation of a phase transition generated by the self-organization of many interacting units. It is possible to create an approach to 1/f noise that may have a wide range of applications, from neuro-physiology to sociology. Experimental evidence shows that there seems to be a connection between aesthetic values and 1/f noise. A conjecture of why the brain is attracted by music and painting is that 1/f noise systems are sensitive to the stimuli exerted on them by other 1/f noise system. There is already experimental evidence of this property, ranging from the observation of brain response to 1/f stimuli, to fractal ventilators designed to help the breathing process, to rocking chairs whose swinging change according to the heartbeat fluctuation of the users, thereby producing relaxation. We mention a new fluctuation-dissipation that can be used to support these experimental results.

April 15, 2008
Professor Guenter W. Gross
Dept. of Biological Sciences
Center for Network Neuroscience
University of North Texas
Denton, Texas
"Nerve cell networks on microelectrode arrays: Investigations of complex network dynamics"
This presentation will highlight basic methods, recent advances, and strategies for collaboration between Physics and Neuroscience. The combination of our joint capacities will allow us to answer fundamental neuroscientific questions and inspire new, realistic theories of network function. The collaboration should also lead to designs for software and hardware, such as distributed sensor networks and parallel computer architectures using biologically inspired principles of computation.
(1) Bettencourt L.M, Stephens, G.J.,Ham, M., and Gross, G.W. (2007). Functional structure of cortical neuronal networks grown in vitro. Physical Reviews E 75, 021915-1-10.

April 22, 2008
Professor Sergio E. Ulloa
Department of Physics and Astronomy
 Nanoscale and Quantum Phenomena Institute
Ohio University
Athens, Ohio
"Quantum Optics with Quantum Dots: Coherent Exciton and Spin Dynamics"
Coherent manipulation of quantum states for quantum computation and information processing is receiving a great deal of attention these days in different physical systems. Among those, semiconductor quantum dots are among the most versatile structures in which to explore physical problems associated with different aspects of information processing. Quantum dots are nearly ideal to implement quantum system control and study their behavior under external probes and dephasing environments. In this talk I will discuss how to monitor coherent excitonic oscillations in single self-assembled quantum dots, how to monitor their quantum state via photocurrent measurements, and how we understand the different dephasing mechanisms present in these systems. We further explore the production of spin-polarized carriers by resonantly exciting states in the quantum dots with appropriate optical polarization. We find that it is possible to achieve significant polarization even in random distributions of dots in self-assembled arrays. I will also present the Aharonov-Bohm effect analogue in neutral but polarized excitons that exist in quantum ring structures, and how an applied magnetic field can be used to manipulate the luminescence of the ground state of pairs of coupled rings.
* Work done in collaboration with Jose Villas-Boas, Sasha Govorov, and Luis Dias. See Phys. Rev. B 75, 155334 (2007), Phys. Rev. Lett. 94, 057404 (2005), Phys. Rev. B 72, 125327 (2005), and Phys. Rev. B 76, 155306 (2007).

May 13, 2008
Dr. E.-M. Rasel
Department of Physics
University of Hannover
Hannover, Germany
"Inertial sensors with classical and quantum degenerate gases on ground and in microgravity"
Quantum matter gives unique insights into a broad range of phenomena in fundamental physics as well as offering prospects for novel quantum sensors. Reaching ever-new frontiers in low temperature physics and achieving full control of these elementary quantum systems are part of the central motivations for research on cooling and manipulation of atoms. The breaking of temperature records opened the way to many new scientific achievements, like atom interferometers and atomic clocks with highest accuracy, novel phase transitions and atom lasers. Low temperatures are essential for the control of the systematic errors in such sensors. Microgravity will help to extend the science of quantum gases and sensors at very low temperatures and extended durations of the unperturbed evolution of these distinguished quantum objects. These conditions set the stage for the study of the physics of ultra-dilute gases and giant matter-waves and the control of these macroscopic quantum objects and mixtures in an environment unbiased by gravity. Microgravity is also expected a decisive ingredient for better tests in fundamental physics of gravity and relativity. In particular, microgravity is of high relevance for matter-wave interferometers and experiments with quantum matter (Bose-Einstein Condensates or degenerate Fermi gases) as it permits the extension the unperturbed free fall of these test particles in a low-noise environment. This is a prerequisite for fundamental tests in the quantum domain such as the equivalence principle or the realisation of ideal reference systems. The QUANTUS team, formed by a consortium of the Leibniz University of Hannover, and the Universities of Hamburg, Berlin, Ulm and ZARM, as well as the Max-Planck Institute and ENS, realised a compact facility to study a Rubidium Bose-Einstein Condensate in the extended free fall at the drop tower in Bremen and during parabolic flights. The facility will permit to study the generation and outcoupling of BEC in microgravity, the study of decoherence and atom interferometry. The remote controlled and miniaturised facility, which produces Bose-Einstein condensates of Rubidium, has been in operation since November 2007. 
*In collaboration with:
W. Ertmer,T.v. Zoest
Leibniz Universität Hannover, 30167 Hannover, Germany
K. Bongs
Midlands Ultracold Atom Research Centre, Birmingham B15 2TT, United Kingdom
T. Könemann, Hj. Dittus, C. Lämmerzahl,
ZARM, University of Bremen, 28359 Bremen, Germany
 E. Kajari, W. Schleich, R. Walser
Universität Ulm, Institut für Quantenphysik, 89081 Ulm, Germany
 A. Vogel, K. Sengstock,
Universität Hamburg, Institut fuer Laser-Physik, 22761 Hamburg, Germany
W. Levozko, A. Peters,
Humboldt-Universitaet zu Berlin, Germany
T. Steinmetz, J. Reichel
Laboratoire Kastler Brossel de l'E.N.S. 75231 Paris, France