California NanoSystems Institute
CNSI
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March 19, 2013

Chee Wei Wong, Sc.D.
Center for Integrated Science and Engineering, Solid-State Science and Engineering, and Mechanical Engineering / Optics and Quantum Electronics IGERT, Columbia University
Controlling light in mesoscopic systems: new frontiers in nonlinear, ultrafast, quantum and precision measurements
Abstract:

Recent advances in sub-wavelength nanoscale platforms have afforded the control of light from first principles, with impact to optoelectronics and high-density secure communications. In this talk I will highlight three coherent examples where emerging chip-scale architectures can make a difference. First, I will describe graphene-silicon hybrid circuits for ultrafast optical signal processing, enabled by the two-dimensional Dirac fermionic structure with its unique linear and massless dispersion. The single atomic layer not only enables an absorption defined by the fine structure constant but spectroscopically exhibits dramatically-large nonlinearities for chip-scale frequency conversion and advanced data formats.
Secondly, I will report on chip-scale frequency comb oscillators, where we have recently observed one of the shortest (74-fs) pulse mode-locking on-chip till date. The CMOS 115-GHz oscillators have potential to link the RF and optical domains with low single-sideband phase noise, with implications to precision optical clocks and astrophysical spectrography. Thirdly, I will describe coherent phase-stable interactions on-chip of single excitons and correlated photons for multiple-qubit per photon in fundamentally quantum-secure communications. The dispersion-engineered photonic crystals allow observations of localized states at the band edge, with resulting polariton states. Semiconductor phase stability allows higher-dimensional Hilbert space correlations in these emerging chip-scale architectures.

March 05, 2013

Z. Hong Zhou, Ph.D.
Director, Electron Imaging Center for Nanomachines (EICN)
Professor, Microbiology, Immunology & Molecular Genetics, UCLA
High resolution cryoEM: a new method for atomic modeling of macromolecular complexes
Abstract:

Single-particle cryo-electron microscopy (cryoEM) is a technique for determining three-dimensional (3D) structures from the projection images of molecular complexes preserved in their “native”, non-crystalline state by plunge-freezing. Recently, atomic or near-atomic resolution structures of several viruses and protein assemblies have been determined by single-particle cryoEM, allowing ab initio atomic model building by following the amino acid side chains or nucleic acid bases identifiable in their cryoEM density maps. In particular, these cryoEM structures have revealed extended arms contributing to molecular interactions that are otherwise not resolved by the conventional structural method of x-ray crystallography at similar resolutions. I will present a few macromolecular complexes we recently studied by cryoEM to the UCLA nano science and biomedical research community. These complexes range from large viruses, multi-component enzymes to engineered nano-scale assemblies of pharmaceutical interest. The cryoEM structures of these complexes up to atomic resolution illustrate the power of cryoEM as a major tool (and sometimes the preferred one) for the studies of molecular interactions in supramolecular assemblies or biological nanomachines.

February 26, 2013

Eva Nogales, Ph.D.
Howard Hughes Medical Institute; University of California, Berkeley
Professor of Biochemistry and Molecular Biology and Senior Faculty Scientist at the Lawrence Berkeley National Laboratory
Visualizing Key Structures and Functional Assembly States in Human Gene Regulation
Abstract:

We have used electron microscopy and image reconstruction to characterize the architecture and interactions of protein complexes involved in human gene silencing (PRC2) and transcription initiation (TFIID and the PIC). My talk will emphasize: 1) the use of systematic genetic labels to define the path of polypeptide chains through a complex structure; 2) the use of cryo-EM to visualize protein-DNA complexes; and 3) the capacity of the methodology to detect and characterize coexisting conformations of a large macromolecule in a quantitative and biologically insightful manner.

February 05, 2013

Qihua Xiong, Ph.D.
School of Physical and Mathematical Sciences
School of Electrical and Electronic Engineering
Nanyang Technological University, Singapore



Laser Cooling of Semiconductors

Abstract:

Optical irradiation accompanied by spontaneous anti-Stokes emission can lead to cooling of matter, a phenomenon known as laser cooling or optical refrigeration proposed in 1929 by Peter Pringsheim. In solid state materials, the cooling is achieved by annihilation of lattice vibrations (i.e., phonons). Since the first experimental demonstration in rare-earth doped glasses, considerable progress has been made particularly in ytterbium-doped glasses or crystals with a recent record of ~110 K cooling from ambient, surpassing the thermoelectric Peltier cooler. On the other hand, it would be more tantalizing to realize laser cooling in direct band-gap semiconductors. Semiconductors exhibit more efficient pump light absorption, much lower achievable cooling temperature and direct integrability into electronic and photonic devices. However, so far no net-cooling in semiconductors has been achieved despite of many experimental and theoretical efforts in the past few decades, mainly on III-V group gallium arsenide quantum wells. Here we demonstrate the first net laser cooling in semiconductors using cadmium sulfide (CdS) nanobelt facilitated by multiple longitudinal optical phonon assisted upconversion due to strong and enhanced Fröhlich interactions. Under a low power excitation, we have achieved a ~40 K and ~20 K net cooling in CdS nanobelts starting from 290 K pumped by 514 nm and 532 nm lasers, respectively. The cooling effect is critically dependent on the pumping wavelength, the blue shifting parameters and the absorption, the latter of which can be evaluated from photoconductivity measurement on individual nanowire level. Detailed spectroscopy analysis suggests that cooling to even lower temperature is possible in CdS nanobelt if thermal management is optimized. Our findings suggest alternative II-VI semiconductors for laser cooling compared to III-V GaAs-based heterostructures and may find promising applications in the field of cryogenics with the advantage of compactness, vibration- and cryogen-free, high reliability and direct integrability into nanoscale electronic and photonic devices.

January 15, 2013

Ron Naaman, Ph.D.
The Weizmann Institute of Science
Professor, Chemical Physics Department
The Chirality Induced Spin Selectivity (CISS) Effect- From Spintronics to Electron Transfer in Biology

Abstract:

Spin based properties, applications, and devices are commonly related to magnetic effects and to magnetic materials. Hence, most of the development in spintronics is currently based on inorganic materials. Despite the fact that the magnetoresistance effect has been observed in organic materials, until now spin selectivity of organic based spintronics devices originated from an inorganic ferromagnetic electrode and was not determined by the organic molecules themselves. However in our studies we found that chiral organic molecules can act as spin filter for photoelectrons transmission, in electron transfer, and in electron transport.1

Results will be presented from several recent experiments, a theoretical model will be discussed as well as some implications and applications.

1 For a review discussing some of the results please see: R. Naaman, D.H. Waldeck, The Chiral Induced Spin Selectivity Effect, J. Phys. Chem. Lett. (feature) 3, 2178 (2012).