Professor Lauren Lowman, Assistant Professor, Department of Engineering, Wake Forest University
George P. Williams, Jr. Lecture Hall, (Olin 101)
Wednesday, October 3, 2018, at 4:00 PM

There will be a reception with refreshments at 3:30 PM in the lounge. All interested persons are cordially invited to attend.

ABSTRACT

Predicting how plant canopy density and greenness respond to changing climate conditions is key for monitoring ecosystem health. Variability in plant canopy properties of leaf area index (LAI) and fraction of photosynthetically active radiation (FPAR) reflect concurrent atmospheric and soil conditions. During the growing season, favorable temperatures and longer periods of sun exposure are conducive to plant growth and canopy development. However, intermittent changes in water availability lead to reductions in plant health and function. Changes in vegetation canopy are often modeled through parameters representing the optimal growing conditions and include temperature, light, atmospheric water demand and soil water availability. Determining optimal biophysical parameters that capture the nonlinearity of plant water relations and the non-stationarity of the underlying climatological condition poses a challenge, especially when long periods of the climate record are dominated by either wet or dry conditions. Further, even if a stationarity assumption holds over the long-term climatological record, often only relatively short records of surface and atmospheric properties are available which can exhibit intermittent trends.

Using limited climate records, parameters representing favorable plant growth conditions are estimated within a Bayesian framework. This presentation demonstrates how uncertainty introduced by the inherent non-stationarity of model inputs propagates from the parameter estimates through a land-surface hydrology model coupled to a predictive phenology model. Parameters estimated from different analysis periods effectively calibrate a plant water-use strategy within the land-surface hydrology model. A specific application of the coupled hydrology-phenology modeling framework for assessing and quantifying the impacts of wildfires on the Southeast U.S. carbon budget will be described with preliminary results.

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Professor Keith Bonin – WFU Department of Physics Professor and Associate Provost for Research and Scholarly Inquiry
George P. Williams, Jr. Lecture Hall, (Olin 101)
Wednesday, September 19, 2018, at 4:00 PM

There will be a reception with refreshments at 3:30 PM in the lounge. All interested persons are cordially invited to attend.

ABSTRACT

Here we create a series of optical corrals and calculate their potential energy profile. A standing-wave Bessel beam is used to form traps in one dimension (along the optical axis of the laser beam) and corrals in two dimensions, in planes perpendicular to the optical axis at the antinodes of the standing waves. These optical corrals are formed by an axicon-generated Bessel beam that is reflected back onto itself.

We report on Mie calculations of the 2D optical corrals and then compare the resulting probability distributions to those observed for latex beads of diameters 100, 200, and 300 nm. The experimental radial probability density function of tracked particles closely mimics the theoretical optical structure of a Bessel standing-wave corral. The Bessel standing-wave corrals we have characterized can be used to measure rotational diffusion and torques on micro- and nanorods to help understand microfluidic behavior.

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WFU Physics Career Advising Event

SPEAKER:  Professor Dava Newman, Apollo Program Professor of Astronautics, Department of Aeronautics and Astronautics at Harvard-MIT Health, Sciences, and Technology, as well as Phi Beta Kappa Visiting Scholar

PLACE: Olin Physical Laboratory, Room 105

Lunch will be provided. All interested persons are cordially invited to attend.

Professor Dava Newman has specialization and research interests in aerospace biomedical engineering: Biomechanics, control, and dynamics; advanced suits and life support systems; human factors; design; space policy; and leadership development.

Her teaching interests include Introduction to Aerospace and Design, space biomedical engineering, exploration from oceans to space, and leadership development.

Professor Newman has had an amazing career path.  She has many society memberships, has received multiple honors and awards, has held numerous positions at MIT, and was the NASA Deputy Administrator from May 2015-2017.  For more information on her career, please use this link – http://aeroastro.mit.edu/faculty-research/faculty-list/dava-j-newman).

For this SPS Meeting/Career Advising Event, Professor Newman will discuss her career trajectory and will close the event with an interactive Q&A session.

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WFU Physics Career Advising Event

SPEAKERS:  Returned Peace Corps Volunteer and Wake Forest Physics Alum Billy Nicholson and Returned Peace Corps Volunteer and Wake Forest Physics Professor Dany Kim-Shapiro

PLACE: Olin Physical Laboratory, Room 105

Lunch will be provided. All interested persons are cordially invited to attend.

Billy Nicholson recently returned from two years of service in Ghana and Dany Kim-Shapiro served in Zaire (Democratic Republic of the Congo) from 1984-1986.  They will talk about their experiences and also provide general information on different opportunities to serve in the Peace Corps.

Although there may be some emphasis on opportunities for Physics or STEM students, there will be a lot of information for students from all majors as well.   The Peace Corps is a great way to serve your country and the world and doing so can greatly benefit your career.  Come find out more.

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Dr. Richard Williams – Reynolds Professor, Department of Physics, Wake Forest University and recipient of the 2017 Physics Department Distinguished Alumni Award
George P. Williams, Jr. Lecture Hall, (Olin 101)
Wednesday, September 5, 2018, at 4:00 PM

There will be a reception with refreshments at 3:30 PM in the lounge. All interested persons are cordially invited to attend.

ABSTRACT

Last week marked the 54^th anniversary of my entry to Wake Forest as a freshman. After Prof. Shields, I may have the second longest (if slightly broken) span of active involvement in Wake Forest Physics of anyone present.  That means part of my talk has to seem like your grandpa ruminating about the old days and old ways from then to now.

Speaking of time, the scientific core of my remarks will look at the evolution and applications of time-resolved spectroscopy in 50 years – that’s 9 orders of magnitude from nanoseconds to attoseconds.  Fascination with that core technique of “excite/probe” cause-and-effect physics eventually brought me and my research group at Wake Forest to our current focus on gamma rays interacting with materials.  I want to share why that is interesting.

Dr.  Metin Gurcan – Director of the Center for Biomedical Informatics and Professor in the Department of Internal Medicine, Wake Forest School of Medicine
George P. Williams, Jr. Lecture Hall, (Olin 101)
Wednesday, September 12, 2018, at 4:00 PM

There will be a reception with refreshments at 3:30 PM in the lounge. All interested persons are cordially invited to attend.

ABSTRACT

Increased interest in medical imaging has resulted in development of a variety of image analysis systems. Many of these systems follow the ‘computer-aided diagnosis’ paradigm. In this paradigm, the main function of the image analysis system is to help medical professionals (e.g. radiologists, pathologists, dermatologists) in their decision-making, instead of making decisions on their behalf. If a system is designed to help medical professionals, its logic, development methodology and evaluation should make sense to the medical professionals who use them.

In this talk, we will describe how to develop an image analysis system, its modular blocks, how to translate medical knowledge into algorithms, how to supplement this knowledge with pattern recognition methods, and how to evaluate them with carefully designed reader studies in which medical professionals with varying levels of experience participate as readers.

Eric Henderson, Masters Candidate
Public Presentation in Olin Physical Laboratory, Room 101
Thursday, July 26, 2018, at 3:00 PM
Dave Carroll, PhD, Advisor

The defense will follow.

ABSTRACT

Light is essential to our modern world. To keep up with the demands of power efficiency and broadened applicability, new lighting technologies must be invented. Organic light emitting varactors (OLEVs) represent a promising future in this regard, and these are explored in this study.  OLEVs have a semiconducting gating layer, as well as the typical light emitting and buffer layers of an OLED. Under AC driving, this gate allows control of current flow during the lighting part of the power cycle thereby allowing the maximum theoretical efficiency of the OLEV to exceed that of the DC driven OLED. However, there are numerous factors that remain to be understood about the morphology of the gate-emitter complex, before such maximum efficiencies can be approached.

In this work, a simple capacitive model for the OLEV is introduced and a direct correlation between gate layer thickness, resonant frequency and brightness is demonstrated. Trends in brightness with the gate layer properties are directly in line with expectations from this simple model. Surprisingly, luminance with field strength measurements showed little variation with thin film morphology. This suggests that field concentration across the films (due to hillock formation during gate film growth) was effectively planarized, thereby yielding a length scale for acceptable roughness in device fabrication. Morphological effects where, however, observed in the critical failure fields of thicker films.

While this work focused on magnetron sputtered ZnO layers with organic emitters, the gate-emitter paring of ZnO with a hybrid inorganic-organic perovskite, MAPbBr₃, was attempted. In this system charge accumulation at the gate – emitter interface should be screened leading to lower barrier heights during the power cycle. This allowed for a direct study of the effect of these barrier heights in systems with high carrier mobility.  Indeed, the ZnO/Perovskite OLEV did not produce light as might be expected with extreme leakage currents. In contrast, the OLED counterparts do light up suggesting that this leakage current is the primary source of failure.

Drew Onken, PhD Candidate
Public Presentation in ZSR Library Auditorium, Room 404
Monday, July 23, 2018, at 2:00 PM
Richard Williams, PhD, Advisor

The defense will follow.

ABSTRACT

To address outstanding issues in the growth and performance of crystals for radiation detection, I develop and employ several material characterization techniques not previously implemented in the field. Two main, interconnected issues are addressed: the cracking of certain crystals during the growth process and spatial inhomogeneity in the defect distributions of radiation detector crystals. Although the purpose of these materials is to detect high-energy gamma rays, no gamma rays were used in these studies. Instead, much can be learned from the more precise interactions of low-energy photon and neutron beams.

The growth of single-crystal boules of certain semiconductors and scintillators can be plagued by cracking during post-growth cool-down. I have conducted proof-of-concept studies on a survey of laser techniques both to map thermal and chemical non-uniformity in situ as well as to perform laser ablation to separate material, to produce cleaner cuts, and to drive dislocation multiplication. In addition, neutron diffraction at high temperatures approaching the melting point can characterize the thermal and chemical stresses which can be exacerbated to the point of cracking by thermal gradients in the furnace.

After fabrication, asymmetries in the growth process can result in an inhomogeneous distribution of defects and dopants in the crystal. These non-uniformities in the crystal can have a significant impact on the energy resolution and degradation of the radiation detector. I construct a two-photon “multi-scope” and implement position-sensitive spectroscopic techniques to map the inhomogeneity of a crystal’s response, considering the origin of the inhomogeneity and the impact it has on detector performance. Finally, with an understanding of how spatial and energy density non-uniformity of response can affect energy resolution, digitized scintillation pulses are analyzed in an effort to extract extra information from the pulse shape in hopes of improving non-proportionality and energy resolution.

Jiajie Xiao, PhD Candidate
Public Presentation in Olin Physics Building, Room 101
Friday, June 15, 2018, at 1:oo PM
Fred Salsbury, PhD, Advisor

The defense will follow.

ABSTRACT

Thrombin is an attractive drug target for antithrombotic therapy and chemotherapeutic development due to its critical role in blood coagulation, as well as in cancer cell growth and metastasis. Many experiments have demonstrated that thrombin is a multifunctional allosteric enzyme, whose functions are regulated by the binding of effector molecule at a site other than the active site of the enzyme. However, the exact mechanism of thrombin’s allostery remains unclear and widely debated.

This work describes my application of molecular dynamics simulations and various quantitative methods to uncover thrombin’s allostery. It discusses thrombin’s allosteric responses to different factors including ion conditions, disease-associated mutations, and ligation statuses. My in-depth atomic-level investigation presents experimentally consistent results and provides mechanistic insights into thrombin’s functional switch. The generalized allostery should be the main mechanism of thrombin’s functional regulations. Several novel testable predictions further the understanding of thrombin’s substrate recognition process and allosteric pathways. The allosteric responses revealed in this work may be exploited in further drug discovery and development.