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Physics Conference Room, SB B326
Coffee starts at 12:00 PM and talk starts at 12:15 PM
- Monday, February 5, 2007
ABSTRACT: A simple quantum mechanical model consisting of a discrete level Resonantly coupled to a continuum of finite width, where the coupling can be varied from perturbative to strong, is considered. The particle is initially localized at the discrete level, and the time dependence of the amplitude to find the particle at the discrete level is calculated without resorting to perturbation theory. The deviations from the exponential decay law, predicted by the Fermi's Golden Rule, are discussed.
- Monday, February 26, 2007
ABSTRACT: Time-delayed feedback occurs in many systems and is particularly important at high-speeds, where the time it takes signals to propagate through the device components is comparable to the time scale of the fluctuations. A fascinating feature of such systems is that seemingly simple devices can show exceedingly complex dynamics. This has motivated the use of electronic and photonic time-delay feedback devices in practical applications of chaos, such as ranging and synchronization based chaos communications, for which microwave and radio-frequency oscillators are needed. I will give an overview of our recent work on high-speed chaos generators, synchronization, and chaos communication.
- Wednesday, February 28, 2007
ABSTRACT: Large surface area and adjustable internal surface chemistry of porous carbons are attractive for a wide range of applications, including electrical energy storage, catalyst support, gas separation membranes, hydrogen storage media, adsorption and separation of biomolecules. Major efforts have been directed towards control of carbon structure, pore size, shape and uniformity, specific surface area (SSA) and total pore volume of these materials.
Carbide derived carbons (CDC), produced by selective etching of metals from metal carbides, have up to 80% open pore volume and finely tunable pore size and SSA. This is achieved by "burning out" the metals (and metalloids) in halogen atmospheres at modest temperatures. The resulting carbon retains the original shape of the carbide and shows linear reaction kinetics, allowing conversion of a carbide surface to a carbon layer of any thickness, including the entire monolith, film or particle. Depending on the synthesis conditions, the process allows formation of nanotubes, onions, nanocrystalline diamonds, and nanoporous carbons with a remarkably narrow pore size distribution. The ability of the developed technology to fine tune the pore size, and independently control the microstructure and surface termination in carbon, offers unique opportunities for fundamental studies of adsorption and transport in porous media.
This presentation will provide an overview of the state of the art in the CDC synthesis and describe major breakthroughs in energy and biomedical applications, such as hydrogen storage, electrical energy storage, and adsorption of proteins achieved by rational design of carbon materials.
- Monday, March 5, 2007
ABSTRACT: Silicon Microphotonics is a planar integrated technology for adding optical interconnection and enhancing the signal processing capability of microelectronic chips, using the CMOS fabrication toolset. This talk will present the vision of this emerging technology and primarily focus on a critical roadblock: the absence of a silicon compatible, high quantum efficiency material with light emission in the l~1 mm wavelength range.
Two methodologies for dealing with this materials constraint will be introduced: (i) the design and predicted performance of an ultrahigh gain-efficiency Erbium-doped waveguide optical amplifier, designed to boost the signal power of an on off-chip laser; and (ii) the design and predicted performance of an Erbium-doped ring resonator on-chip laser.
Materials studies in high concentration Erbium-doped glasses, such as silicon nitride and silicon oxynitride, will be presented for approaches (i) and (ii). We report record optical constant values for SiON:Er and Si3N4:Er, and map the influence of the nitride environment on Er optical gain. Si nanostructures have been observed and characterized for optical sensitizer properties. Current impediments with these materials towards the realization of a lasing cavity will be identified.
The talk will conclude with recent work on silicon nitride/oxynitride photonic crystal structures for the design of simpler optically pumped waveguide amplifiers, and a passive device application for chip-to-optical fiber coupling of a light signal.
- Wednesday, March 7, 2007
ABSTRACT: Stars form deep inside clouds of dense dust and gas. Most of the optical light from the nascent stars cannot escape from these clouds, making the star formation process practically invisible to even the most powerful optical telescopes. It was not until recent advances in radio, millimeter, and infrared detectors, over the last 30 years, that important observational progress was made in our understanding of the star-formation process. However, much more progress is needed.
Understanding the star formation process is essential to astrophysics,as it will give us an insight on how the Earth and Sun formed, and will also help us better understand the early universe and the formation of galaxies. I will talk about one of the most important, and yet still poorly understood, stages of the star forming process --the mass outflow stage. As stars form inside molecular clouds, they gravitationally gather gas from their surrounding gaseous environment and disk, while at the same time they energetically spurt out vast amounts of mass in a bipolar flow. Outflows deposit energy and momentum into their surroundings and have a considerable impact on the dynamics, distribution, and chemical composition of the gas in star forming clouds. I will discuss the physical and chemical impact outflows have on the star formation environment.
- Monday, March 12, 2007
ABSTRACT: 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.
PHOTOCHEMISTRY 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 an inert 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.
- Monday, April 16, 2007
- Monday, April 23, 2007
ABSTRACT: Contrary to the common belief that noise and decoherence are detrimental to spectroscopic measurements, we propose and experimentally demonstrate a new method of coherent Raman spectroscopy with spectrally broad incoherent laser pulses. Laser induced molecular vibrations are probed by femtosecond laser pulses with intentionally introduced spectral phase noise, and the vibrational resonances are identified through intensity correlations in the noisy spectrum of the scattered photons. Spectral resolution is not limited by the pulse bandwidth, and is not sensitive to the temporal profile quality of the pulses. The method does not require complicated pulse-shaping setups, spectral multiplexing or spatial beam arrangements. It enables full utilization of the broad bandwidth of femtosecond pulses, and quick scanless retrieval of Raman spectra.
- Sunday, May 27, 2007
ABSTRACT: Q space Nuclear Magnetic Resonance imaging is a well-known non-invasive experimental technique allowing for structural investigations of a variety of complex systems relevant to problems in industry, material science and biology. The technique allows one to accurately measure the morphology of a confining pore and molecular diffusion rate of mobile molecules within interstices of a structurally complex system. In our laboratory we have recently designed a variable temperature NMR microscope capable of delivering gradient pulses on the order of 50,000 G/cm allowing for high resolution (less than 1 micrometer) scattering studies. In this work we apply this hardware to study the rate of molecular diffusion of water in purified bovine nuchal ligament elastin.
Elastin is an insoluble and highly cross-linked protein in the extra cellular matrix responsible for the elastic properties of vertebrate tissues. While all models for elasticity require water as a plasticizer, no direct experimental study of the molecular dynamics of water has yet been performed. In this talk I will discuss q-space imaging, the measurements we performed, and the implications of our findings.
- Monday, September 10, 2007
ABSTRACT: Modern astrophysical observations of the cosmic microwave background, of distant supernovae, and of the distribution of galaxies on very large (billion lightyear) scales has revealed that the Universe is 13.7 billion years old, that the mass in the Universe is mostly in some non-baryonic form ("dark matter"), and that the expansion of the Universe is accelerating due to some unknown physical effect (denoted "dark energy" for convenience). I will discuss this evidence, concentrating on the large-scale distribution of galaxies. In particular, I will discuss the recent detection by the Sloan Digital Sky Survey of the "baryonic acoustic oscillation," the remnants of sound waves in the very early Universe. Finally, I will discuss the prospects for improving our understanding of cosmology by mapping the baryonic acoustic oscillations more precisely and at larger distances.
- Friday, September 21, 2007
- Monday, September 24, 2007
ABSTRACT: Consider the imaging of inclusions buried in heterogeneous media (modeled as random media) from wave field measurements. For strongly disordered random media, classical imaging techniques based on the back-propagation of coherent wave fields are bound to fail when the random media are not known explicitly. Our only hope then rests on our finding a macroscopic model for the random clutter.
I will argue that kinetic equations offer the simplest models to quantify available observables of wave propagation in such random media, namely, wave energy densities and field-field correlations. Moreover, these observables are asymptotically statistically stable quantities, i.e. do not depend on the realization of the random medium. Buried inclusions then become constitutive parameters in the kinetic equations and their imaging becomes a deterministic inverse transport problem.
I will consider several kinetic-based imaging scenarios depending on available measurements (wave energy measurements or field-field correlation measurements). Their theoretical imaging capabilities will be compared for small-volume inclusions. I will present some reconstructions based on numerical simulations as well as on experimental measurements.
- Monday, October 1, 2007
ABSTRACT: It is well documented that a significant number of American and Canadian high school and college graduates are scientifically illiterate and have very little interest in science or mathematics. Moreover, their attitudes toward science are often negatively affected by traditional physics instruction. Physics faculty have been trying to address this problem with variable success for the last few decades. However, proposed solutions (often working on a small scale at upper level courses) were difficult or impossible to implement in large introductory physics courses. The talk will describe how recent findings in Physics Education Research coupled with the effective use of modern technology (Logger Pro and clickers) have been used to implement physics education reform at two large research universities across Canada: The University of British Columbia (700 student-undergraduate introductory physics course for science majors) and Ryerson University (400-student introductory physics course for science majors and 150-student introductory physics course for future architects). The design, implementation and evaluation of Interactive Lecture Experiments was our attempt to reform first year physics teaching and learning.
The preliminary results of the study from Ryerson University and research findings from the University of British Columbia as well as future research directions will be discussed.
- Monday, October 15, 2007
- Monday, October 22, 2007
NOTES: Joint Chemistry/Physics Colloquium, Remsen 101
- Monday, October 29, 2007
ABSTRACT: One-dimensional (1-D) nanomaterials in general and semiconductor nanowires in particular have begun to be explored extensively as promising building blocks for high-performance nanoscale devices. Their promises derive in part from expectations of exceptional electronic properties, such as enhanced charge carrier transport characteristics in 1-D. In this context, quantitative characterizations of carrier transport are desirable to substantiate these high expectations and to establish the performance limitations of nanowire-based devices. In addition, quantitative metrics of carrier transport can also be used to quantify the effects of nanowire surface passivation schemes being developed that aim to improve the device performance.
In this talk, quantitative visualization of carrier transport in semiconductor nanowires using a scanning photocurrent microscopy (SPCM) technique will be demonstrated. Specifically, analysis of the local photocurrent maps obtained by the SPCM technique enables the measurement of carrier diffusion length, a critical transport parameter that controls electronic and opto-electronic device performance, for bipolar and unipolar carrier transport processes in intrinsic (undoped CdS) and extrinsic (n-type Si) nanowires, respectively. In addition, the bipolar carrier diffusion length is found to be enhanced as a result of increased carrier lifetime and the electrostatic repulsion when electrons and holes are spatially separated, and this is supported by local transient (nanosecond) photocurrent measurements and photocurrent mapping under various excitation intensities.
Chee Wei Wong
- Monday, November 5, 2007
- Monday, November 19, 2007
ABSTRACT: The results of study of the optical reflection and contactless electroreflection from a periodic system of multiple GaAs/AlGaAs quantum wells will be presented. The quantum well width was 15 or 20 nm and the barriers were 104 nm thick. In this system, the electromagnetic resonance of the Bragg reflection occurs at the frequency that coincides or is close to the frequency of the exciton-polariton resonance in the wells. The optical measurements were made at various temperatures, angles of the light incidence and polarization. The optical reflection spectra have been found to be a result of the interplay of three different contributions, namely (i) the reflection from the air/semiconductor interface, (ii) the Bragg reflection due to periodic modulation of the background indices of refraction being different for the wells and barriers, and (iii) the resonant reflection from the periodic system of exciton-polaritons in quantum wells. The latter contribution was separately studied by contactless electroreflection technique in the spectral range covering ground states of the heavy-hole and light-hole excitons. A quantitative analysis of the experimental contactless electroreflection line shape has been done along with quantum-mechanical calculations, which revealed the characteristic energies and broadening parameters for different exciton-polariton levels. In particular, the systems of four and thirty two quantum wells exhibit spectral features with the characteristic broadening of 1.8 meV and 2.2 meV at 17 K, respectively. By comparison with theoretical calculations, we discuss the radiative and non-radiative contributions to the total broadening.
- Monday, November 26, 2007
ABSTRACT: Supersolid state of matter - phase combining properties of crystal and superfluid - was proposed by Andreev, Lifshitz and Chester for crystalline He4 about 40 years ago. However, early experiments have failed detecting such state. Recently, Kim & Chan at Penn State have observed predicted by Leggett superfluid decoupling in rotational pendulum - so called non-classical rotational inertia (NCRI) of hcp solid He4. This has caused a strong wave of renewed interest to the supersolid state of matter. I will review main features of supersolid, its various theoretical scenarios and will focus on our most recent results obtained by first principles quantum Monte Carlo large scale simulations. Such simulations impose strong constraints on theories of supersolidity and indicate that structural crystalline defects are responsible for NCRI of He4.
- Monday, December 3, 2007
- Monday, December 10, 2007
ABSTRACT: Nanoplasmonic phenomena are based on resonant excitation of surface plasmons causing highly enhanced and localized optical fields on nanoscale. These nanoscale fields induce a multitude of enhanced optical effects, in particular, surface enhanced Raman scattering (SERS) including single-molecule SERS, enhanced second- and third-harmonic generations, enhanced two-photon electron emission from nanostructured surfaces, and others. There are many existing and prospective applications of nanoplasmonics in nanoprobing, ultrasensitive detection, biomedical monitoring, etc. The talk will include a broad Introduction to the topic and also certain forefront, focus areas based partially on original contributions, including ultrafast, coherent, nonlinear, and stimulated phenomena. Spaser will be one of the focus points of the talk.