Physics & Astronomy Department
2401 Vanderbilt Place
Nashville, TN 37240-1807
Justin Gregory, Interdisciplinary Materials Science, Vanderbilt University
Optical structure modification by ion implantation in single-crystal diamond studied by coherent acoustic phonon spectroscopy
Ion implantation in diamond crystals is a promising approach to photonic device application. In this talk, I will describe experiments wherein single-crystal CVD diamond specimens were implanted with 1-MeV He+ ions at doses ranging from 1014—1016 cm–2 and analyzed using Coherent Acoustic Phonon (CAP) spectroscopy. The CAP response varies greatly with implantation dose, and provides depth-dependent information about the implantation defect-induced modification of the diamond optical structure. The results indicate an increase in the real and imaginary refractive index of diamond, as well as a sign reversal of the photoelastic coefficients at higher levels of implantation damage. These studies will assist in the application of ion implantation to the fabrication of diamond-based photonic devices.Presentation
Charles Brau, Department of Physics and Astronomy, Vanderbilt University
Channeling radiation as a source of coherent, hard x-rays
Channeling radiation as a source of coherent, hard x-rays. C. A. Brau, B.-K. Choi, J. D. Jarvis, J. W. Lewellen, and P. Piot. In a crystal, the ions in each crystal plane form a sheet of positive charge. When a relativistic electron travels through the crystal parallel to the crystal plane, Lorentz contraction increases the charge density by the factor and the electron oscillates about the crystal plane in quantum states normal to the plane. Radiation from transitions between the quantum states is called channeling radiation. The transverse forces experienced by an electron traveling along a crystal plane are comparable to those in a 10-kT magnetic undulator or a 1-TW laser undulator focused to a 10-um spot. Compared with a conventional undulator, channeling radiation requires only a 40-MeV electron beam, rather than a 10-GeV beam to reach the hard x-ray region. Compared with a laser undulator, a channeling radiation source comprises a small diamond chip ratherthan a complex laser system circulating a kilowatt of laser power. The coherence of the X-radiation depends on how tightly the electron beam can be focused, and this depends on the emittance of the beam. The innovation that we propose is to use a single field-emitting tip as the current source. If we focus this beam at the critical angle for channeling radiation in diamond (about 1 mrad), we get a spot diameter of 40 nm. The transverse coherence of this beam will exceed that of all other X-ray sources, and will be useful for phase-contrast imaging of soft tissue and for X-ray microscopy.
Robert Kaita, Plasma Physics Laboratory, Princeton University
Scientific and Engineering Challenges for First Wall Materials in Magnetic Confinement Fusion
The development of plasma-facing component (PFC) materials that can withstand the high heat loads in fusion reactors is an example of the constructive interplay between science and engineering. Carbon is the most common PFC material in present-day fusion devices, because its low Z minimizes radiation losses if it enters the plasma. Among high Z materials, tungsten is the primary candidate because of its good thermal properties and low sputter yields. However, no solid materials that are suitable for steady state operation have yet been developed. An alternative is to use liquid metals for PFCs. The first toroidal confinement device with alarge liquid lithium free surface was the Current Drive Experiment-Upgrade (CDX-U) at the Princeton Plasma Physics Laboratory. The success of CDX-U not only contributed to our knowledge of plasma confinement, but also motivated the engineering needed for lithium walls in the new Lithium Tokamak Experiment and the National Spherical Torus Experiment (NSTX). The complex plasma-facing components in NSTX, in turn, were found to interact with lithium in ways that required new models for their interpretation. The lecture will discuss the complementary roles of science and engineering in a key area of fusion research and development.
Hadley Lawler, Department of Physics and Astronomy, Vanderbilt University
The opto-acoustic spectra of Si(100): remarkable agreement between experiment and theory
The Si(100) CAP spectrum is measured for energies in the range 1.6-3.4 eV. The spectrum is calculated from first-principles evaluation of the complex photoelastic spectrum, and application of the Thomsen model to the interaction of light with the acoustic pulse. Comparisons of calculated and measured spectra show excellent agreement. Points of agreement include: 1) a signal onset near 1.7 eV, 2) a resonance near 3.1 eV, and 3) the magnitude of the interferometric reflectivity oscillation. The results indicate predictive applications of first-principles methods to CAP measurements on more complex systems are feasible.
Richard Haglund, Department of Physics and Astronomy, Vanderbilt University
Vanadium Dioxide: The Never-Ending Story?
I will describe two recent experiments focused on key scientific issues related to the potential for making ultrafast optical modulators with VO2 micro- and nanostructures: (1) detailed analysis of the spectral signature of the insulator-to-metal transition in both the insulating and metallic states; and (2) plasmonic initiation of the insulator-to-metal transition in VO2 by irradiation of a thick gold film, showing a bias-voltage effect.
Alexander Slepko, Department of Physics, University of Texas at Austin
Theory of biomineral Hydroxyapatite: Looking at bones with density functional theory
Hydroxyapatite (HA) [Ca10(PO4)6(OH)2] is the main mineral component of human bone. Its gives bone strength and its ability to regenerate. Due to HA's complexity the existing body of work is mainly experimental and macroscopic in nature. However, many applications of synthetic HA require understanding of the material at the microscopic level. In this talk I will examine how the microscopic complexity of HA controls material's interaction with the physiological environment. Within the framework of density functional theory I will present a detailed investigation of the energetics, electronic, vibrational and surface properties that explain several experimental observations. I will discuss an unexpected peculiarity in the vibrational spectra (that would be unlikely to be revealed without the theoretical approach) that potentially allows novel preventive medical screening for bone mineral loss.
Host: S. Pantelides
Bin Wang, Department of Physics and Astronomy, Vanderbilt University
The Electronic and Structural phase transition in VO2
The well-known phase transition in VO2 near room temperature can be used for various applications such as data storage, but the corresponding mechanism remains unresolved. Motivated by recent ultrafast measurements, we performed density functional calculations and first-principles molecular dynamics simulations to address the phase transition mechanism. We find that a metallic phase can exist in the monoclinic structure at the transition temperature. We further show that a 6 THz phonon is critical for the photo-induced structural phase transition. I will talk about how to tune the phonon mode to achieve controllable phase transition. The mechanism of structural phase transition will be discussed as well.
Jason Valentine, Mechanical Engineering Department, Vanderbilt University
Zero Index Metamaterials
Ohmic loss in metal based metamaterials continues to be one of the primary impediments to their application at infrared and visible frequencies. Dielectric metamaterials offer one potential solution to this issue by eliminating ohmic loss as well as avoiding saturation in the magnetic response at high frequencies. In this work, we will discuss our recent efforts to develop three-dimensional purely dielectric metamaterials at optical frequencies based on structured silicon. These resonant dielectric metamaterials are employed for achieving near-zero refractive index at optical frequencies. Formed from stacked silicon rod unit cells exhibiting both electric and magnetic dipole Mie resonances, these metamaterials exhibit impedance matched near-zero refractive index at optical frequencies allowing near unity transmission. We present experimental evidence of a nearly isotropic low-index response including angular selectivity of transmission and directive emission from quantum dots placed within the metamaterial. These isotropic low-loss metamaterials can be applied for a variety of applications including directional emitters, filters, and compact lens systems.
Keith Warnick, Department of Physics and Astronomy, Vanderbilt University
*Oxidation catalysis and molecular sensing: DNT and Fe-porphyrin
Heat released by reactions between target molecules and atmospheric oxygen can in principle be exploited for molecular detection. I will describe first-principles calculations demonstrating how Fe-porphyrin can catalyze an exothermic reaction between 2,4-dinitrotoluene (DNT) and atmospheric oxygen at room temperature. This reaction breaks molecular oxygen, incorporating one atom into DNT and leaving the other attached to the Fe catalytic site. A second reaction with a fresh DNT molecule can remove the adsorbed oxygen atom from the Fe site at room temperature, cleaning the catalyst. I will also describe a possible sensing mechanism that involves using the heat released by these reactions to thermally drive the insulator-metal phase transition in a nanostructured VO2 substrate.
Henry Everitt, Chief Scientist, Army AMRCEC WSD, Redstone Arsenal, AL
The most commonly studied metallic plasmonic nanostructures are composed of silver or gold, whose resonant behaviors in the visible or near-infrared wavelengths are used in sub-wavelength photonic devices and surface-enhanced Raman spectroscopy (SERS). There is increasing interest to develop plasmon-based technologies in other spectral regions, and this presentation focuses on our work to develop ultraviolet nanoplasmonic structures to enhance optical properties. New materials must be considered, and improved theoretical analyses are required, in order to identify and achieve optimal performance in this spectral region. This presentation will focus on our work to fabricate, characterize, model, and use gallium nanoparticles whose tunable plasmonic spectra spans the visible and UV regions.
Host: N. Tolk
Xiao Shen, Department of Physics and Astronomy, Vanderbilt University
Cation Sublattice Ordering in Wurtzite-like CuInS2 Nanocrystals
CuInS2 is one of the best candidate materials for solar energy harvesting. Its nanocrystals with a hexagonal lattice structure that is different from the bulk chalcopyrite phase have been synthesized by many groups. The structure of these CuInS2 nanocrystals has been previously identified as the wurtzite structure in which the copper and indium atoms randomly occupy the cation sites. Using first-principles total energy and electronic structure calculations based on density functional theory, UV-vis absorption spectroscopy, X-ray diffraction, and atomic resolution Z-contrast images obtained in an aberration-corrected scanning transmission electron microscope, we show that CuInS2 nanocrystals do not form random wurtzite structure. Instead, the CuInS2 nanocrystals consist of several wurtzite-related crystal structures with ordered cation sublattices, some of which are reported for the first time here. The implications of this result will be discussed as well.
Host: S. Pantelides
Thomas Orlando, School of Chemistry and Biochemistry and School of Physics , Georgia Institute of Technology
Formation of graphene via epitaxial growth on SiC and stimulated reduction of graphite-oxide
Epitaxial graphene layers thermally grown on Si-terminated 6H-SiC (0001) have been probed using Auger electron spectroscopy, Raman microspectroscopy and scanning tunneling microscopy (STM). The average multilayer graphene thickness is determined by attenuation of the Si (L23VV) and C (KVV) Auger electron signals. Systematic changes in the Raman spectra are observed as the film thickness increases from 1 to 3 layers. The most striking observation is a large increase in the intensity of the Raman 2D-band (overtone of the D-band and also known as the G' band) for samples with a mean thickness of more than ~1.5 graphene layers. Correlating this information with STM images, we show that the first graphene layer imaged by STM produces very little 2D intensity, but the second imaged layer shows a single-lorentzian 2D peak near 2750 cm-1, similar to spectra acquired from single-layer micromechanically cleaved graphene (CG). The 4-10 cm-1 higher frequency shift of the G peak relative to CG can be associated with charge exchange with the underlying SiC substrate and the formation of finite size domains of graphene. The much greater (41-50 cm-1) blue shift observed for the 2D-band may be correlated with these domains and compressive strain. I will also discuss a scalable and a relatively inexpensive excimer (248 nm) laser approach that fully reduces graphite oxide and oxidized epitaxial graphene. The resultant domains/patterns are analyzed using Raman micro-spectroscopy, X-ray photoelectron spectroscopy as well as four point probe sheet resistance measurements. The reduction process is initiated by electronic transitions involving states that correlate with the presence of oxygen within/on the lattice but also likely involves phonon mediated transient heating. Since the efficacy of the laser reduction method also depends upon the amount of oxygen initially present in the gas-phase, the best results require use of inert background gases such as nitrogen, argon, helium, etc. This excimer based lithography also allows direct patterning of micro-capacitors on/within the graphite-oxide films using conventional masking technologies.
Host: N. Tolk
Zsuzsa Marka, Department of Physics, Columbia University
Advanced generation of interferometric gravitational wave detectors
I will present an overview of the advanced generation interferometric gravitational wave detectors that will come online in the upcoming years. I will also briefly present the key improvements and technologies that will be employed to reach the design sensitivity, with a particular attention to the advanced LIGO detectors. Finally I will discuss one of the contributions of the Columbia Experimental Gravity group to the advanced LIGO project, the advanced LIGO timing system.
Host: N. Tolk
Stephanie Gilbert Corder, Interdisciplinary Materials Science, Vanderbilt University
Ultrafast Carrier Dynamics of Nanoparticle-embedded GaAs Systems
We present recent work characterizing the ultrafast dynamics of self-assembled nanoparticles in a GaAs matrix. The dynamics are markedly different from semi-insulating GaAs systems; charge transfer, sub-picosecond recombination times and magnetic responses are observed. The results shown here indicate the systems may be candidates for designer electronics compatible with existing semiconductor technology.
Host: N. Tolk
Jie Zjhao and Jon Ehrman, Department of Physics and Astronomy, Vanderbilt University
(1)Topological Influence On Network Of Coupled Chemical Oscillators, and Predicting the Emergence of the Resistant Phenotype Using Quantitative in vivo Imaging Data; and (2)Information Theory Applied to Directed Cell Migration .
Host: E. Rericha
Erin Rericha, Department of Physics and Astronomy, Vanderbilt University
Biophysics at Vanderbilt
Host: N. Tolk
Vasiliki (Vicky) Zorbas Poenitzsch, Materials Engineering Department, Southwest Research Institute
Overview of Plasma Engineering Efforts at Southwest Research Institute
This talk will provide an overview of how Southwest Research Institute, SwRI, applies plasma science to solve engineering problems. The surface engineering group at SwRI is engaged in R and D and pilot scale processing of thin films for various applications. SwRI has developed a portfolio of plasma technologies including a plasma-enhanced magnetron sputtering (PEMS) technology to produce the tailored Ti-Si-C-N nanocomposite coatings, a plasma immersion ion deposition to produce diamond-like carbon films, and a plasma-enhanced chemical vapor deposition technology for carbon nanotube synthesis. Last, but hopefully not least, we will discuss the current development of novel high power impulse plasma source (HiPIPS) for high power, high pressure glow discharges towards a DARPA Local Control of Chemistry non-thermal thin film deposition approach.
Host: N. Tolk
James Butler, Smithsonian National Museum of Natural History
The Standard Model of Diamond CVD Growth
Host: N. Tolk
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