Fall 2020 Colloquia

When light interacts with complex materials, it can give rise to a variety of interesting phenomena. In this talk, I will first give a short explanation of how light interacts with materials. This will include a discussion of plasmon polaritons, a type of quasiparticle that arises from the strong interaction of a photon with the electrons in a material. Plasmon polaritons can be used for a variety of applications including focusing and imaging below the diffraction limit of light, subdiffraction waveguiding, gas sensing, and many more. I will discuss our work on exciting and coupling plasmon polaritons in topological insulator thin films and layered structures. Topological insulators have two-dimensional surface states that house massless electrons. The plasmon polaritons in these materials therefore show unusual properties. I will discuss the dispersion of these modes and show record high mode indices and extremely long polariton lifetimes. I will close by discussing our work on semiconductor hyperbolic metamaterials. These materials act optically metallic in one direction and transparent in the other direction. I will show our work demonstrating negative refraction (bending light backward) in these materials as well as their ability to house their own complex plasmon polariton modes.
Recorded Video Link

Bio: Prof. Stephanie Law received her B.S. in Physics from Iowa State University and her Ph.D. in Physics from the University of Illinois Urbana Champaign. She then held a postdoctoral position in the Electrical Engineering department at UIUC before moving to the University of Delaware as the Clare Boothe Luce Assistant Professor in Materials Science and Engineering. She is now an Associate Professor in Materials Science and Engineering and holds an affiliate appointment in the Department of Physics and Astronomy. She is also the co-director of the UD Materials Growth Facility and an Associate Editor for the Journal of Vacuum Science and Technology. Prof. Law has won the North American Molecular Beam Epitaxy Young Investigator award, the Department of Energy Early Career award, the AVS Peter Mark Memorial Award, and the Presidential Early Career Award for Scientists and Engineers (PECASE).

Neutron beta decay is an archetype for all semi-leptonic charged-current weak processes. A precise value for the neutron lifetime is required for consistency tests of the Standard Model and is needed to predict the primordial 4He abundance from the theory of Big Bang Nucleosynthesis. Other parameters from neutron beta decay, in combination with the neutron lifetime, can be used to extract the Vud parameter in the quark mixing matrix of the Standard Model, providing a useful test of new physics. These are all quantities that have been in flux in recent years, with the advent of high precision experiments. The status of the field as well as recent and upcoming measurements will be presented.
Recorded Video Link

A year ago, the Mines physics department led a student team to compete in Morocco in the inaugural Solar Decathlon Africa. The engineering and science that lead to the victory in this project will be discussed, along with the tale of the international adventure that involved two physics departments and three universities. While this may be the only time that a basic science department formed the two lead institutions, it highlights the importance and future role of science in the broader energy and building science.
Recorded Video Link

Bio: Prof. Ohno graduated from the University of Maryland in experimental surface science in 1993, under the direction of Prof. Ellen Williams, who served as director of ARPA-E. His work at the University of Minnesota strengthened his interest in materials science before coming to Mines in 1992. As the oldest member of the department, he has seen the growth of the department beyond its original research focus at that time, which included photovoltaics. Leading that program led to service as the director of the campus Energy Minor, and ultimately support for student organizations involved in energy topics.

Understanding how the brain’s complex neural networks perform critical functions and govern behavior, cognition and intuition is a key goal of neuroscience and can lead to improved treatment for various neurological disorders. The development of new tools for studying the brain is critical in this effort. Light Microscopy has greatly expanded the capabilities for minimally invasive cellular-level biological studies and in combination with genetically encoded fluorescent indicators allows unprecedented real-time imaging of action potentials in individual neurons in a network. In this talk, I will discuss recent work in my lab on the development of miniature fiber-coupled microscopes for 3-D imaging using adaptive optics and their applications for studies in freely moving and behaving animals. Additionally, I will discuss how adaptive optics for control of light patterning combined with optogenetics makes it possible to modulate neuronal activity allowing new studies of how neural circuits govern behavior.
Recorded Video Link

Bio: Dr. Gibson is an associate professor in the department of Bioengineering at the University of Colorado Anschutz Medical Campus with a joint appointment in the Neuroscience program. She earned her PhD in Physics from the University of Colorado at Boulder with a specialization in nonlinear optics. She was subsequently a National Research Council/National Academy of Sciences postdoctoral fellow in biophysics, studying protein dynamics with nonlinear optical spectroscopy. Since becoming a faculty member, she has focused on development of optical technologies for clinical applications and biomedical research.

We describe a compact and inexpensive computational microscope that encodes 3D information into a single 2D sensor measurement, then exploits sparsity to reconstruct the volume with good resolution across a large volume. Our system uses simple hardware and scalable software for easy reproducibility and adoption. The inverse algorithm is based on large-scale nonlinear optimization with self-calibration of aberrations and we discuss computational optical design approaches for optimizing the system’s performance. We demonstrate applications in whole organism bioimaging and neural activity tracking in vivo.
Recorded Video Link

When the dimensionality of an electron system is reduced from three dimensions to two dimensions, new behavior emerges. This has been demonstrated in gallium arsenide quantum Hall systems since the 1980’s, and more recently in van der Waals (vdW) materials, such as graphene. This talk will discuss the behavior of electrons in reduced dimensions with a focus on their spin properties. We highlight our recent study of vdW materials with intrinsic magnetic order. These materials are at the forefront of condensed matter physics research. We use a materials informatics (machine learning applied to materials research) approach to study the magnetic properties and chemical stability of vdW materials. Crystal structures based on monolayer Cr2Ge2Te6, of the form A2B2X6, are studied using density functional theory (DFT) calculations and machine learning methods. Magnetic properties, such as the magnetic moment are determined. The formation energies are also calculated and used to estimate the chemical stability. We show that machine learning methods, combined with DFT, can provide a computationally efficient means to predict properties of two-dimensional (2D) magnetic materials. In addition, data analytics provides novel insights into the microscopic origins of magnetic ordering in two dimensions. Analysis of DFT data highlights that the X site strongly affects the magnetic coupling between neighboring A sites – driving magnetic ordering. This novel approach to materials research paves the way for the rapid discovery of magnetic 2D materials that are chemically stable.Trevor David Rhone,1,3 Wei Chen,1 Shaan Desai,1 Steven B. Torrisi1, Daniel T. Larson1, Amir Yacoby,1 and Efthimios Kaxiras1, 2
1. Department of Physics, Harvard University, Cambridge, Massachusetts
2. School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts
3. Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, Troy, New York

Recorded Video Link

Silicon is an attractive materials platform for developing large-scale quantum computers because of its compatibility with classical silicon electronics and its potential for scalability. This talk will discuss qubits made from quantum dots with multiple electrons in silicon/silicon-germanium heterostructures. These qubits can be manipulated on nanosecond time scales, and their coherence can be extended greatly by appropriate manipulation protocols. They can be tuned so that additional quantum resonances appear that can be driven coherently, which we show is consistent with effects arising form strong electron-electron interactions. Thus, these multi-electron qubits are interesting both as building blocks for quantum computers and as testbeds for investigating strongly interacting electrons.
Recorded Video Link

Many lab courses include a final project that spans multiple weeks. Such projects serve several purposes, including nurturing students’ sense of project ownership. Project ownership refers in part to students’ control over and responsibility for an experiment. Research in science education suggests that ownership, motivation, and persistence are interrelated, and that feelings of ownership can fluctuate in time. Building on prior work at a single institution, we have conducted a multi-site study of students’ sense of ownership of multi-week final projects in upper-level physics lab courses. Using survey and interview data, we propose a model that describes ownership as a relationship between student and project characterized by particular student-project and interpersonal interactions during three temporal phases: choice of topic, execution of methods, and creation of deliverables. In our presentation, we will describe implications for the design and implementation of final projects whose goals include fostering a sense of project ownership among students.

Bio: Dr. Dimitri Dounas-Frazer is an Assistant Professor of Physics and Astronomy and of Science, Mathematics, and Technology Education at Western Washington University. He has interdisciplinary expertise in experimental atomic physics and education research. He primarily studies three aspects of physics laboratory coursework: students’ use of model-based reasoning in experimental physics contexts, instructors’ beliefs and practices regarding teaching and learning laboratory skills, and classroom factors that cultivate student ownership of research projects. Additionally, Dr. Dounas-Frazer is an active member of local and national physics diversity initiatives. He is a Mines alum (classes of ’06 and ’07). He completed his Ph.D. in 2012 at the University of California Berkeley, where he performed high-precision measurements of weak nuclear effects in atomic systems. His postdoctoral experience includes teacher preparation at the California Polytechnic State University San Luis Obispo and education research at the University of Colorado Boulder.

Bio: Ira Ché Lassen is an undergraduate student at Western Washington University (WWU) and Fairhaven College. He expects to complete a BS in Physics and a BA in Interdisciplinary Studies by June 2022. Lassen’s interests include acoustics, rhetoric, and physics education research (PER), and he has professional experience with 3D sign manufacturing, CNC laser operation, and IT support. In his roles as a Teaching Assistant in the WWU Physics & Astronomy Department and Research Assistant in the WWU PER Group, Lassen is building expertise in both teaching and studying physics laboratory courses.

Photovoltaic (PV) devices based on metal halide perovskite (MHP) absorbers have reached outstanding performance over the past few years, surpassing power conversion efficiency of over 25% for lab cells and with large area devices in excess of 18%. For the solar application stability, the most demanding requirement to assess for PV and remains the outstanding issue for MHP based devices. The problem of stability motivates basic science driven work on MHP based PV at NREL and work by industrial partners. Material and device insight can enable MHP PV stability along with the associated opportunities to further improve efficiency with multijunction while maintaining scalability and manufacturability is critical. This talk will highlight the latest work at NREL to develop understanding of critical roadblocks, aspects of solar cell performance, device architectures, stability and operational dynamic to enable the next generation of photovoltaics.
Recorded Video Link

Bio: Joseph Berry is a senior scientist at the National Renewable Energy Laboratory working on halide perovskite solar cells. His PhD for work was on spin transport and physics in semiconductor heterostructures from Penn State University. His efforts at NREL emphasize relating basic interfacial properties to technologically relevant device level behaviors in traditional and novel semiconductor heterostructures including oxides, organics and most recently hybrid semiconducting materials. He leads the US Department of Energy (DOE) Solar Energy Technology Office’s SETO core technology program, “De-risking Halide Perovskite Solar Cells” at NREL. He is a principle investigator on the NREL lead Department of Energy, Center for Hybrid Organic Inorganic Semiconductors for Energy (CHOISE) Energy Frontier Research Center, exploring basic aspect of hybrid materials and is the director of the newly formed U.S. Manufacturing of Advanced Perovskites (U.S. MAP) consortium a collaboration between industry academia and the national labs to bring perovskite technologies to market.

Web:
www.nrel.gov/pv/perovskite-solar-cells.html
www.choise-efrc.org
www.usa-perovskites.org

 Hydrodynamic whirlpools have fascinated scientists for centuries, seeking to understand their individual structure, stability, and the ways in which they interact with one another. Who hasn’t marveled at tornadoes or watched as soap bubbles get sucked into the vortex of a bathtub drain? To reduce ideas to their essence, such fluid vortices are often considered in a two-dimensional setting where they amount to current swirling around a singularity. These, in turn, bear a striking resemblance to cross-sections of optical vortices that can be created with lasers, but with the propagation axis now treated as time. The vortex center is a then a dark spot about which the phase of light rotates like a barber shop sign. Such engineered light can therefore be interpreted as a two-dimensional, compressible fluid, and the vortices it harbors exhibit all sorts of odd and potentially useful behavior. For instance, optical vortices can attract, repel, scatter, and even annihilate one another. Even more intriguing, these two-dimensional topological objects have a lot in common with the macroscopic quantum states of Bose-Einstein condensates and fractional quantum Hall systems. Pairs can even be used in Bell tests to demonstrate lack of local realism. This motivates a serious consideration of optical vortices as quantum objects that might be harnessed in emerging quantum information technologies. With these deeper issues in mind, our colloquium lecture is intended to serve as an introduction to optical vortices and their classical few-body dynamics. We tag-team an experimentalist and a theorist to provide a fuller perspective of what makes this form of light so interesting.
Recorded Video Link