Events

The optical response of semiconductor quantum-wells often displays excitonic resonances with a typical energy spacing of 1 to 100 meV. To directly access transitions between these resonances, one can apply electro�magnetic fields in the terahertz frequency range. The resulting quantum dynamics and associated nonlinear optical effects are of great interest. They both manifest fundamental physical processes, such as many-body interactions and Coulomb correlations; and also have broad applications for optoelectronic devices.

I will describe recent experimental results, where we realize an asymmetric optical potential barrier for ultracold Rb 87 atoms using laser light tuned near the D2 optical transition. Such a one-way barrier, where atoms impinging on one side are transmitted but reflected from the other, is a realization of Maxwell's demon and has important implications for cooling atoms and molecules not amenable to standard laser-cooling techniques. In our experiment, atoms are confined to a far-detuned dipole trap consisting of a single focused Gaussian beam, which is divided near the focus by the barrier.

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Production of thin film solar cells is growing at over 40% per year. However, advances are limited by a lack of fundamental understanding of the materials. This talk reviews our studies of the growth mechanisms, optoelectronic properties and surface science of one class of materials applied to these cells: Cu(In,Ga)Se2. The work focuses on epitaxial single crystals grown with a variety of orientations to provide controlled surfaces for study. Scanning probe measurements show how surface morphology, atomic dynamics, and surface energy are connected.

Since the 1970s light has been used to trap, control and manipulate particles on the micro and nanoscales. In particular optical tweezer traps have been used in applications in biology, chemistry, engineering and physics. Given the current emphasis on nanotechnology, applications continue to arise where nano and sub nano-precision are desired. The ability to use optical tweezers to confine smaller particles and manipulate these particles with greater precision is of interest.

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The device physics and interface chemistry of conjugated polymers are heavily influenced by the transport of both ionic and electronic charge carriers. During the doping of conjugated polymers, ions are introduced for charge compensation. These ions are typically mobile and substantially alter the electric field distribution within and characteristics of conjugated polymer devices relative to analogues based on more traditional inorganic semiconductors. In this talk, the influence of ions on the device physics and chemistry of conjugated polymers and their junctions will be discussed.

Abstract [1]: Carbon nanotube field effect transistors have been shown to be effective low concentration biosensors. Low detection limits are important because it allows for early detection of diseases like cancer. The response of nanotubes to adsorped protein is still not fully understood, and we are investigating this response as a function of flow rate, concentration of protein, and dimensionality of the nanotube sensor. We have used polylysine adsorption as a model system to compare theoretical predictions with experiment

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Since the early work of Pauling and Corey, the paradigm for thinking about and modeling protein backbone geometry has been that of a single ideal set of bond lengths and angles for the peptide unit. Based on the crystallographically determined structures of proteins, I published work in the mid-90s showing that there is strong evidence for systematic variations in bond angles as a function of the conformational differences only involving rotations around single bonds. We are now surveying protein structures determined at <1 � resolution to better characterize the systematic variations.

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In this presentation I will report a strongly interacting new dendrimer system with an extended spectroscopic unit (coherent domain) beyond the trimer configuration. Strong cooperative enhancement of - two photon absorption cross-section was observed when going from the trimer arrangement to the next generation.

1. Density-functional study of the BaBi(Q)TiO_3 perovskite solid solution
   Using density-functional theory we predict properties of the solid solution perovskites BaBi(Q)TiO_3 where Q is Mg, Ni or Zn. These properties are strongly affected by the presence of both A-site and B-site disorder. We study all distinct orderings within a given 2x2x2 supercell.
2. Electrostatic signatures of single protein dynamics for detection with carbon nanotube sensors