The Department of Physics and Astronomy Colloquium is held on Monday afternoons at 4:15 pm in room SLH 102 unless otherwise noted. Refreshments are served at 4:00 pm.

Fall 2013

August 26
 No Colloquium

September 2
No colloquium

September 9

Semiconductor nanowire photonics

Carsten Ronning

View Abstract

The miniaturization of light sources, the confinement and manipulation of light on a sub-wavelength scale as well as the detection of single photons are key challenges for the realization of future photonic circuits. Here, semiconductor nanowires are of major interest as a serving material platform, since they do not only offer superior photonic properties, but can also bridge the interface to electronic circuits enabled by their semiconducting properties. The former one includes efficient waveguiding [1], light emission at electrical excitation [2], stimulated emission and lasing [3,4], as well as the possibility of coupling with plasmonic structures [5]. I will cover several of those aspects in my presentation, and we will show a route for efficient doping of semiconductor nanowires with tunable luminescence centers [6].

[1] T. Voss et al., Nano Letters 7 (2007) 3675
[2] M. Zimmler et al., Nano Letters 8 (2008) 1695
[3] S. Geburt et al., Nanotechnology 23 (2012) 365204
[4] M. Zimmler et al., Semi. Sci. & Tchn. 25 (2010) 024001
[5] V. Sorger et al., Nano Letters 11 (2011) 4907
[6] C. Ronning et al., Mater. Sci. Eng. R 70 (2010) 30

September 16

Laboratory Investigations of Plasma Interactions and Charging on the Lunar Surface.

Joseph Wang

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The lunar surface is directly exposed to solar radiation and various space plasma environments. The resulting plasma-lunar surface interactions are relevant to almost all aspects of human and robotic lunar exploration activities as well as many natural processes on the lunar surface. A laboratory simulator for the lunar exospheric plasma and dust environment is currently being developed at USC to investigate the meso-scale plasma-dust-surface interactions at the lunar terminator region, where the combined effects from the low sun elevation angle and the localized plasma wake generated by plasma flow over a rugged terrain can generate strongly differentially charged surfaces and complex dust dynamics. This talk will present results from a correlated experimental and modeling investigation to characterize lunar regolith surface charging under different plasma environments and address several long standing questions in lunar surface charging and dusty plasmas.

September 23

Quantum mechanics in the deep interior of the Sun

Werner Däppen
Physics & Astronomy

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The search for the origin of the energy (and lifetime) of the Sun has been a challenge for nearly two hundred years. Important intermediate steps were taken by some of the most famous physicists (among others, Kelvin, Einstein, Gamov, Bethe, von Weizsäcker and Salpeter). Although broadly speaking, the problem has been fully solved (the source of energy being nuclear fusion of hydrogen), there still remain unresolved subtleties. These details matter thanks to the great diagnostic tools of the solar interior, which are helioseismology and neutrino detection.
In my talk, I will concentrate on a particular open issue, the reaction enhancement of nuclear fusion due to Coulomb screening in a plasma.
Recent results have shown that a proper understanding of this enhancement involves the basic tenets of quantum mechanics and quantum measurement. In passing, and as a prerequisite for a proper statement of the problem, I will review the history of our understanding of the energy of the Sun and the stars.

September 30

Structural Predictions of Peptide-MHC for Rational HIV Vaccine Designs

Ha Youn Lee
Keck School of Medicine

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Humoral immunity has been a significant focus of HIV vaccines since the Thai Phase III clinical trial showed 31% vaccine efficacy in reducing HIV acquisition in 2012. Properly arming the other branch of the adaptive immune system, cytotoxic T lymphocytes, is an important task to ensure full protection against breakthrough viruses. Viral escape has been considered to be one of the major roadblocks for T cell based HIV vaccines, as evidenced in the failure of the STEP HIV vaccine trial. One of the most promising strategies for mitigating viral escapes is inducing broad T cell responses by vaccination. Diverse T cell receptor repertoires have been convincingly shown to be advantageous in elimination of viral variants during infections of HIV, SIV, and HCV. The surface morphology of the peptide-MHC (pMHC) complex is one of the primary characteristics responsible for controlling T cell diversity. This talk will introduce our pMHC conformations prediction algorithm, which employs homology-based models and all-atom molecular dynamics simulations. The signature of conformational transitions was found to be effective in predicting pMHC structures, yielding high prediction accuracy of 1.58 Å RMSD for 17 test structures. The accuracy can be attributed to the fact that the transition rate from a meta-stable, local energy minimum conformation to the energetically-favored conformation is greater than the reverse transition rate. The algorithms were further examined by a blind test, which predicted more prominent conformations for two significantly more immunogenic mutant peptides, KVAEIVHFL and KVAELVWFL, compared to a wild-type peptide, KVAELVHFL. Our model predictions were then corroborated by high resolution X-ray crystallography. A prominent morphology can be considered as one of key parameters for optimal T cell responsiveness, and our computational tools can be used to prioritize HIV vaccine candidates based on MHC binding antigen structure.

October 7

Simulation of abelian and non-abelian lattice gauge theories with cold atoms

Benni Reznik
Tel Aviv University

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Quantum simulations of High Energy Physics, and especially of gauge theories, is an emerging and exciting direction in quantum simulations. In simulations of such theories, compared to simulations of condensed matter physics, extra restrictions, such as local gauge and Lorentz invariance must be implemented. I shall describe recent progress towards simulation of lattice gauge theories using ultracold atoms, and new methods that allows to implement local gauge invariance using the fundamental symmetries that arise from natural atomic interactions and conservation laws.
This opens up the way for implementing elementary gauge invariant interactions theories such as compact QED (U(1)), SU(N) and Z_N, and suggests a new way for exploring fundamental HEP phenomena, such as dynamical quark confinement, and exotic QCD phases, that currently are inaccessible computationally, by using “table-top” experiments with cold atoms.

October 14

Closing in on the nature of dark matter with astroparticle observations


Jennifer Siegal-Gaskins

View Abstract

Dark matter makes up roughly 80% of the matter in the universe, yet the details of its particle nature remain unknown. Many particle dark matter candidates can pair annihilate or decay to produce Standard Model particles, including gamma rays, charged particles, and neutrinos. The detection of these indirect signals of the annihilation or decay of dark matter in our Galaxy and beyond is a promising method for identifying dark matter, understanding its intrinsic properties, and mapping its distribution in the universe. Recent indirect searches have yielded several tantalizing hints of dark matter signals, however a confident detection remains elusive. I will review recent results of dark matter searches using data from the Fermi Gamma-ray Space Telescope and other experiments and discuss the constraints these place on particle dark matter models. I will also discuss new approaches and prospects for robustly identifying a dark matter signal with upcoming experiments.

October 16

USC Science Film Competition

October 16, 2013: Deadline for competition registration. Deadline for application for small grants.

View Abstract

The competition is open to teams of USC students from any program,
on any of the USC campuses.
Entries must be from interdisciplinary teams of students.
These interdisciplinary teams enter a short film, in any genre or style,
that explains and illustrates a scientific concept, principle, or issue,
for a wide non-expert audience.

October 21
No Colloquium

October 28

Mechanics-driven Self-organization in Tissue-scale Tubulogenesis

Chin-Lin Guo

View Abstract

In our bodies, the size of a single cell is around few microns, while the size of organs can be up to tens of centimeters. These cells are normally surrounded by extracellular matrix (ECM) molecule such as collagen, with an amazing ability to self-organize with ECM into organ-scale structures, in the absence of preexisting spatial cues. While the molecular signaling in cell-ECM interaction has been extensively studied, the physical mechanisms by which cells interact with ECM to create organ-scale coordination are not well understood. Understanding the underlying biophysical mechanisms can advance technology such as scaffold-free tissue engineering for regenerative medicine, as well as the construction of disease models for clinical studies. Here, we study how the dimensionality in cell-ECM interactions influences cellular behaviors. We find two distinct behaviors. When mammalian epithelial cells are surrounded by 3-D ECM (i.e., in a solid phase), they can develop long-range mechanical interactions (up to 600 microns), which, however, is used to enhance tumor invasion. When these cells and ECM molecules are cultured in a liquid phase, they can self-organize into centimeter-long, hundreds of micrometer-wide tubules with highly organized architectures, mimicking the real tissues. Based on these results, we discuss how the dimensionality and complexity in cell-ECM interactions influence the spatiotemporal coordination in the self-organization of organ-scale structures.


Chin-Lin Guo graduated from National Taiwan University, with a M.D. and a M.S. in Electrical Engineering (advisor: Jynpyng Wang). He obtained his Ph.D. in Physics at University of California, San Diego (advisor: Herbert Levine, con-mentor: Shu Chien, Terence Hwa), and postdoctoral training at Harvard University, Molecular and Cell Biology department (advisor: Andrew W Murray). He is now an assistant professor at California Institute of Technology, Bioengineering. His research is to explore the physical mechanisms governing cell polarization and tissue-scale self-organization.

November 4

iPodia:Borderless Interactive Learning

Stephen Lu
Department of Aerospace and Mechanical Engineering

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With rapid changes of technology and globalization, the 21st century learning will have a vastly different shape and form than that of today.
We have already witnessed how much learning has changed when campus classrooms are networked to distance students at home and work. Now, try to imagine what could happen when campus classrooms are networked together for students to interact and collaborate globally ? campus learning will forever be different! Your imagination of this future is actually happening within the iPodia Alliance at this very moment. Walk into an iPodia classroom on an Alliance member's campus today, you will see students are learning interactively and working collaboratively with their global classmates across physical, institutional, and cultural boundaries. iPodia links the best faculty and students together in a virtual world-classroom to enrich their learning opportunities in each physical classroom. By flattening the landscape of learning, iPodia brings out healthy competition and productive collaboration between learners around the world. It empowers teachers to do more and students to learn more, better preparing everyone for the future.

However, this exciting future presents great challenges to all learning institutions today. For examples, many universities have launched MOOC as a response to the call for change in education. While its long-term viability is still being debated, what could be the new value proposition of campus education in the MOOC-era? Ubiquitous technologies have maxed-out the delivery distance from teachers to students. While distance education remains valuable to some, can these platforms be used to also eliminate the learning distance between students? International experiences have become an important requirement for today's students facing global challenges. Although many overseas studies serve this need as a privilege of the few, is it possible to provide global learning experience on local campuses for it to become the right of the many?

The iPodia program, where "i" stands for inverted, interactive, and international, was established to address these challenges. This presentation introduces the iPodia pedagogy and the iPodia Alliance where a team of elite universities on different continents are working together to demonstrate borderless interactive learning as a future paradigm of global education.


Professor Stephen Lu is a world renowned researcher, teacher, and an education entrepreneur. He has excelled in both academic and business worlds, holding a permanent endowed chair professorship at University of Southern California (USC) and serving the executive/advisory roles at multiple international organizations. He founded the IMPACT Research Laboratory, directs the Product Development Engineering, and the Viterbi iPodia (ViP) programs at USC. In 2012, he founded the iPodia Alliance - a global consortium among ten leading universities to promote the "borderless interactive learning" paradigm. Dr. Lu's academic interests and technical expertise are in design thinking, collaborative engineering, technological innovation, and education reform. He has published over 330 technical papers, books, and chapters, and served on many high-quality journals' editorial boards, including the Chief Editor of the International Journal of Collaborative Engineering. He was a visiting professor at MIT, Technical University, Berlin, Shanghai Jiao-tong University and KAIST. Dr. Lu was the Chairman of the Technical Advisory Committee of the SAVE/JAST program at Lockheed-Martin Co. which designed/developed the next generation fighter aircrafts for the U.S.
military. He is a senior Fellow of ASME (American Society of Mechanical
Engineers) and CIRP (International Academy of Production Engineering), and has chaired CIRP's Design Scientific and Technical Committee and Chairperson of CIRP-ECN Working Group. For his technical contributions, he has received many prestigious domestic and international awards, including honorary professorships from six foreign universities and two Presidential recognitions in the USA and Germany.

November 11

Stochastic Bistability and Switching in Biochemical Networks

Elizabeth Read
UC Irvine

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Complex networks of biochemical reactions encode the cellular processes that are necessary to life. In recent years, theoretical modeling of biochemical reaction networks has shed light on the connections between network topology, network dynamics, and biological function. At the same time, the pervasiveness of stochastic fluctuations in cellular processes has been amply demonstrated. These fluctuations can fundamentally influence dynamics—for example, we found that models of virus evolution coupled to cellular immune responses have stochastic steady-state distributions that are qualitatively different from those predicted by deterministic equations.
I will discuss how the features of the network that give rise to this disparity are related to those underlying stochastic bistabilities encountered in gene regulation and cell signaling. I will also discuss recent advances in numerical methods for inferring dynamical switching mechanisms of complex networks, which have the potential to inform strategies for perturbing cell dynamics.

November 18

Dark Matter and First Galaxies Light Up

Marusa Bradac
UC Davis

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The cluster of galaxies 1E0657-56 has been the subject of intense research in the last few years. This system is remarkably well-suited to addressing outstanding issues in both cosmology and fundamental physics. It is one of the hottest and most luminous X-ray clusters known, and is unique in being a major supersonic cluster merger occurring nearly in the plane of the sky, earning it the nickname "the Bullet Cluster". The newest HST/WFC3 data has revealed further interesting properties of this cluster. We have also studied a couple of new Bullet-like cluster, MACSJ0025-1222, A520, and DLSCL J0916.2+2951. Although they do not contain a low-entropy, high density hydrodynamical `bullet,' these clusters exhibits many similar properties to the Bullet Cluster.

Furthermore, the advent of Wide Field Camera 3 (WFC3) on HST enabled us to detect galaxies at z~7. They are likely beacons of the epoch of reionization, which marked the end of the so-called ``Dark Ages'' and signified the transformation of the universe from opaque to transparent. Clusters of galaxies, when used as cosmic telescopes, can greatly simplify the task of studying and finding these galaxies.
With a massive cluster one can gain several magnitudes of magnification over a typical observing field, enabling imaging and spectroscopic studies of intrinsically lower-luminosity galaxies than would otherwise be observable, even with the largest telescopes. In this talk I will present the new results we have obtained with Spitzer space telescope to study these galaxies behind some of the most powerful cosmic telescopes and discuss our future goals.

November 25

Plasmonic Nanostructures for Nanophotonic and Biomedical Applications

Anatoliy Pinchuk
University of Colorado, Colorado Springs

View Abstract

Noble metal nanoparticles exhibit collective coherent electronic excitations known as Surface Plasmon Resonance (SPR), resulting in a wide range of interesting phenomena. Plasmonics has become a vibrant field of research with multiple potential applications in nanooptics, biosensing, photovoltaics, to name just a few. Metal anisotropic nanoparticles exhibit multiple SPR resonances depending on their geometry and mutual arrangement. Optical extinction spectroscopy provides an insight into multipolar SPR excitations in gold anisotropic nanoparticles. Regular arrays of gold nanoparticles can couple through electromagnetic near- and far-fields, which leads to collective SPR modes. The near- and far- field electromagnetic coupling between plasmonic nanoparticles shifts the collective SPR mode and can be used in optical biochemical sensors. Laser light can be used to deposit fluorescent silver nanoparticles in a solution on a substrate, which can potentially be developed into 3D mesoscale printing technology. The light energy of a laser can be efficiently transformed into heat by using gold nanoparticles with their SPR tuned to the excitation wavelength of the laser.

December 2

Large-Scale Structure: Surveys, Systematics, and Intensity Mapping

Anthony Pullen

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Abstract: Large-scale structure (LSS) surveys have produced powerful probes of cosmological physics. Current and upcoming LSS surveys will allow us to answer several fundamental questions about our Universe, but systematic effects are of great concern to these missions. We present our latest results considering systematic effects in upcoming LSS surveys, which show that identifying these effects will be critical to future surveys. We also discuss "intensity mapping", a new LSS survey strategy where we map fluctuations in the intensity signal from star-forming galaxies and the intergalactic medium in order to probe cosmology.
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