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Physics Conference Room, SB B326
Coffee starts at 12:00 PM and talk starts at 12:15 PM
- Monday, February 7, 2011
ABSTRACT: Crystalline films are often grown or annealed below their roughening temperature. The microscopic physics involves the attachment and detachment of atoms at steps, and the diffusion of atoms across terraces. The macroscopic consequences of these atomic-scale mechanisms are still poorly understood. My talk will discuss recent progress with Hala Al Hajj Shehadeh and Jonathan Weare, concerning the evolution of a one-dimensional step-train separating two facets in the "attachment-detachment-limited" regime. I'll explain why the evolution is asymptotically self-similar, and why its continuum limit is associated with certain fourth-order nonlinear PDE's. The talk will be self-contained, requiring no prior background about crystal growth.
Mordecai-Mark Mac Low
- Monday, February 14, 2011
ABSTRACT: In this talk I consider two questions. First, I investigate the formation of molecular clouds from diffuse interstellar gas. It has been argued that the midplane pressure controls the fraction of molecular hydrogen present, and thus the star formation rate. Alternatively, I and others have suggested that the gravitational instability of the disk controls both. I present numerical results demonstrating that the observed correlations between midplane pressure, molecular hydrogen fraction, and star formation rate can be explained within the gravitational instability picture. Second, I discuss how ionization affects the formation of massive stars. Although most distinctive observables of massive stars can be traced back to their ionizing radiation, it does not appear to have a strong effect on their actual formation. Rather, I present simulations suggesting that stars only ionize large volumes after their accretion has already been throttled by gravitational fragmentation in the accretion flow. At the same time these models can explain many aspects of the observations of ultracompact H II regions.
- Wednesday, February 16, 2011
ABSTRACT: We describe enzymatic systems which involve biocatalytic reactions utilized for information processing ("biocomputing").
Extensive ongoing research in biocomputing, mimicking binary logic gates has been motivated by potential applications in biotechnology. Furthermore, novel sensor concepts have been contemplated with multiple inputs processed biochemically before the final output is coupled to transducing "smart-material" electrodes and other systems. These applications have warranted consideration of networking of biocomputing gates. First few-gate networks have been realized and studied. In order to achieve scalable, stable network design and functioning, considerations of noise propagation and control have been initiated as a new research direction. Optimization of single enzyme-based gates for avoiding analog noise amplification has been explored, as were certain network-optimization concepts. We survey these developments, as well as offer an outlook for possible future research foci. The latter include design and uses of non-binary network elements, specifically, filters, as well as other developments motivated by potential novel sensor and biotechnology applications.
- Wednesday, February 23, 2011
ABSTRACT: Photosynthesis starts when a photon is captured by a photosynthetic reaction center containing a chlorophyll molecule. The photon is converted into an excited state of the molecule, a so-called "exciton." In order for the plant to convert that energy into ATP, the energy must be transferred between molecules via a process called Förster resonant energy transfer (FRET). The same process is employed in the design of photovoltaic materials as well as various exotic kinds of excitonic circuits and is even contemplated as a mechanism for implementing quantum computation. The details of how the energy moves in FRET is both an important problem in understanding biological systems and a crucial issue in engineering efficient solar collectors. One of the ways that scientists have studied this problem is in simulations of the electronic structure of molecules and "artificial molecules" known as nanoparticles. The sophistication of these calculations continues to grow as increasingly powerful computer platforms and algorithms become available. In this talk I will describe the calculation of FRET as an important paradigm in electronic structure calculations and show how future developments might solve previously poorly understood problems, such as how quantum mechanics enters into the photosynthetic process in real plants.
- Monday, February 28, 2011
- Monday, March 7, 2011
- Monday, March 14, 2011
ABSTRACT: There has been considerable interest in curvature, diffusive curvature flows and general relativity on discrete geometries. This has been applied to broad problems from complex networks to solve the greedy routing problem to Regge calculus to solve Einstein's field equations. Hamilton 's Ricci Flow has garnered considerable interest in its application to help prove the Poincare conjecture by Perelman. We describe here our progress in formulating discrete Ricci flow (DRF) on a simplicial geometry of arbitrary dimension, D. In so doing we provide an explicit and geometric construction of the Riemann tensor, Ricci tensor and the scalar curvature. We will then discuss the discrete formulation of Einstein's geometric theory of gravitation known as Regge calculus.
- Monday, March 21, 2011
- Monday, March 28, 2011
ABSTRACT: The next frontier in particle physics is the Large Hadron Collider (LHC), which, having partly recovered from an "incident" two years ago, is restarting now, following a brief preliminary run in 2010 at the European Laboratory for Nuclear and Particle Research (CERN), outside of Geneva, Switzerland. There, two beams of ~ 4 TeV protons (where 1 TeV=1000 GeV, or ~1000 proton masses) will be made to collide head-on, and thereby provide data that will help clarify current puzzles and inadequacies in our conceptual formulation of the nature of the fundamental particles and their interactions. The LHC will respond to specific issues raised by the apparent limitations in the logic of the "standard model (SM)," which is the current, remarkably successful theory of all particle interactions. The development of the SM is arguably the most significant achievement of elementary-particle physics! Its framework accommodates all observed phenomena within a "gauge" quantum-field theory encompassing electroweak (E&M and Weak) and strong (color – QCD) interactions. Despite that it agrees with all observations, the SM is flawed in that it has many free parameters. But even more telling is that it becomes internally inconsistent beyond TeV energies. The LHC is expected to resolve this scientific conundrum through discovery of new kinds of particles or interactions at this "Terascale." I will describe the nature of the currently disappearing energy frontier at the 1 TeV Tevatron Collider at Fermilab, outside of Chicago, and our hopes for the mightier LHC.
- Monday, April 4, 2011
- Wednesday, April 6, 2011
ABSTRACT: We present experimental studies on the generation of pulsed and continuous-wave squeezed vacuum via nonlinear rotation of the polarization ellipse in a (87)Rb vapor. Squeezing is observed for a wide range of input powers and pump detunings on the D1 line, while only excess noise is present on the D2 line. The maximum continuous-wave squeezing observed is -1.4 +/- 0.1 dB (-2.0 dB corrected for losses). We measure -1.1 dB squeezing at the resonance frequency of the (85)Rb F = 3 --> F' transition, which may allow the storage of squeezed light generated by (87)Rb in a (85)Rb quantum memory. Using a pulsed pump, pulsed squeezed light with -1 dB of squeezing for 200 ns pulse widths is observed at 1 MHz repetition rate.
- Monday, April 11, 2011
- Monday, May 9, 2011
- Monday, September 19, 2011
ABSTRACT: Convergence of light toward a desired location in optically diffusive and aberrative media is the most important challenge in optical methods of biomedical imaging, where inhomogeneous media with some degree of opacity are generally present. I will review various approaches towards this goal. The techniques include optical phase conjugation, measuring the transmission matrix of forward and back scattered light, and microscopic wavefront adjustments. I will show examples of applying the new understanding and techniques to control spatial coherence in multimode fibers and laser arrays.
- Wednesday, October 5, 2011
ABSTRACT: Imaging in highly heterogeneous media is a notoriously difficult problem because of the presence of unknown clutter. It is often useful to construct functionals of the data that are as independent of the unknown clutter as possible. Field-field correlations are such functionals, which can often be modeled as solutions to kinetic equations. This talk will review basic aspects of such functionals and present results of reconstructions of inclusions buried in strong clutter.
- Wednesday, October 12, 2011
ABSTRACT: The talk will mainly concentrate on the results of optical and morphological properties of zinc oxide (ZnO) nanostructures and on some recent work on organic polymer based photovoltaic devices. Low dimensional nanostructures of ZnO have potential to improve the efficiency and compactness of electronic and photonic devices including LEDs, optical waveguides and sensors. The growth mechanism and characterization of ZnO nanorod and nanowall systems grown on sapphire and Si substrates using vapour phase transport (VPT) method will be presented. The experimental growth conditions to achieve well-aligned ZnO nanorod and naorod/nanowall morphology are optimized by using different forms of carbon powder as source. For ZnO nanostructures to be effectively utilized in devices, it must be possible to dope these structures effectively and homogeneously. The distribution of In and Al dopants with ZnO nanostructures grown on Si and a-sapphire substrates is investigated using low temperature cathodoluminescence. The photoluminescence (PL) peak seemingly unique to ZnO nanostructure at ~3.367 eV, known as the surface exciton (SX) peak, believed to be due to surface adsorbed species are investigated aimed at elucidation of the nature and origin of the emission and its relationship to the nanostructure morphology. The optical and electronic properties of the active layer (usually a blend of semiconducting polymer and fullerene), contributes to the generation of electron-hole (e-h) pairs (excitons) and their dissociation in an organic photovoltaic cell. To improve the optical and electronic properties of the active layer, dye doping in polymers/fullerene blend and concept of hybrid polymer/inorganic nanorod photovoltaic cells are implemented. Dye molecules with the polymer/fullerene composite are used to enhance absorption at longer wavelengths, which could not be harvested otherwise. In hybrid solar cell concept, ZnO nanorods are used in the active layer to increasing the donor-acceptor interfacial area and creating electron transport pathways toward the negative electrode that possess very high electron mobility.
- Monday, October 17, 2011
ABSTRACT: New materials with potentially new properties and applications can be formed by linking nanocomponents. The assembly of three-dimensional ordered arrays of CdSe and of Fe2O3 nanoparticles by a microfluidics technique is described, along with the formation of hybrids of CdSe nanoparticles and single-walled carbon nanotubes. Coupling between the nanocomponents is important in understanding the new properties these nanomaterials may possess, as is demonstrated by how FRET coupling affects the Stokes shift in photoluminescence from these hybrids. Such systems are also potentially useful in several applications. Nanoparticles are coupled to achieve improved thermoelectric performance. Improved catalytic activity can be achieved using nanoparticles, as is probed by using Raman scattering.
- Monday, October 24, 2011
- Monday, October 31, 2011
ABSTRACT: This presentation focuses on charge/energy transfer modeling starting from nanoscale quantum phenomena and continuously broadening to macroscale, where these quantum effects can be observed and utilized. Methods of quantum field theory and a quantum transport equation based on the Keldysh-Feynman diagrammatic technique are employed to describe quantum phenomena in the electron-phonon energy exchange  and transport phenomena [2,3]. The sophisticated analytical methods used at the nanoscale may be sewed with the numerical Monte-Carlo simulations at larger scales . This approach is used for investigating the charge/energy transfer in nano-materials and for modeling of superconductor and semiconductor nanodevices. I will discuss the design and optimization of next-generation ultra-sensitive detectors, quantum nanocalorimeters, THz mixers, and IR single-photon counters . I will also consider a quantum dot solar cell and its optimization that leads to enhanced harvesting and very efficient photovoltaic conversion of IR energy .
1. Y.-L. Zhong, A. Sergeev et al., Phys. Rev. Lett. 104, 206803 (2010).
2. M. Bell, A. Sergeev, et al., Phys. Rev. Lett. 104, 046805 (2010).
3. A. Sergeev, M. Reizer, and V. Mitin, Phys. Rev. Lett. 106, 139701 (2011); Europhys. Letters 92, 27003 (2010).
4. V. Mitin, A. Sergeev, L-H. Chien, and N. Vagidov, Large-Scale Scientific Computing, Springer, p. 403, 2010.
5. B.S. Karasik, A. Sergeev, and D. Prober, "Nanobolometers for THz photon detection," IEEE Trans. on Terahertz Science & Technology, 1 (Inaugural Issue), 97 (2011).
6. K.A. Sablon, J.W. Little, V. Mitin, A. Sergeev, N. Vagidov, and K. Reinhardt, Nano Letters 11, 2311 (2011).
- Monday, November 7, 2011
ABSTRACT: The localization length (LL) has been derived for one-dimensional bi-layered structures with random perturbations in the refractive indices for each type of layers. Main attention is paid to the comparison between conventional materials and those consisting of mixed right-hand and left-hand materials. It is shown that the localization length is described by the universal expression for both cases, when the widths of layers of right-hand and left-hand materials are different. In a specific case, when the widths are equal, our analytical approach demonstrates that the inverse LL vanishes in the first order of perturbation theory. We were able to develop the expression for the LL with higher order terms, and explain puzzling numerical results, recently discussed in literature.
- Monday, November 14, 2011
- Monday, November 21, 2011
- Monday, December 5, 2011