2019 Seminars

Vinh Ho (Physics, Virginia Tech)

“Fast and High Responsivity Graphene-based Photodetectors at Room-temperature by Engineering Dielectric Films”

The realization of low-cost photodetectors with high quantum efficiency, high sensitivity, and fast photo-response in the visible and infrared remains one of the challenges in optoelectronics. Ideally, these photodetectors should be based on Complementary Metal-Oxide-Semiconductor (CMOS) compatible platform for monolithic integration with read-out electronics to provide for high-density, high-throughput and low-cost manufacturing. Graphene is ideally suitable for optoelectronic photodetectors sensitive from visible to infrared frequencies, and have proved to fulfil those requirements. Here, we have engineered the interface between graphene and dielectric Ta2O5 and Ti2O3 thin-films by e-beam evaporation method to introduce quantum dots as absorption centers from visible to infrared region. Our graphene-based photodetectors have showed a high responsivity up to 2×105 A/W as well as a fast response time in the nano-second time scale at room temperature. These results address key challenges for broadband photodetectors from visible to infrared region, and are promising for the development of graphene-based optoelectronic applications.

Host: Vinh Nguyen

Dr. Patrick Dennis (Air Force Research Lab, Wright-Patterson Air Force Base, Ohio)

"Protein Hydrogels from Marine Invertebrates: A Platform for Tunable Functionality”

Sclerotized, proteinaceous structures in marine invertebrates are used for predation by facilitating grappling, piercing and tearing of prey. These structures must have robust mechanical properties that are tailored to the size, shape and function of the specific predatory tool. Two such structures are the squid sucker ring teeth (SRT) assembly and jaws from the North Atlantic sandworm, Nereis virens. Both structures are not mineralized and are primarily comprised of proteins. Intriguingly, these sclerotized acellular structures are formed in a constitutive marine environment without the benefit of evaporation to aid in removal of bulk water. We have studied this phenomenon in hydrogels created from two proteins, suckerin and Nvjp-1, derived from the squid SRT assembly and sandworm jaw, respectively. Upon exposure of the protein-based hydrogels to aqueous salt solutions, a significant decrease in hydrogel size occurs where bulk water is driven out and a condensation of the protein hydrogel occurs. Interestingly, the contraction rate as well as the mechanical properties of the condensed hydrogels are greatly dependent on the type of cation and anion present in the salt, and the trends differ among the two proteins. The final size and mechanical properties of the condensed structures is dependent on both the initial concentration of the hydrogels as well as the ions used for condensation. Together, the results suggest that spatially controlled casting densities coupled with a selective exposure to ions can create features in the final condensed structure with tunable mechanical properties, similar to what is observed in the marine organisms.

Host: Vinh Nguyen

No Seminar Holiday (Columbus Day)

Host:

Prof. Qi-Huo Wei (Kent State University)

“Printing Molecular Orientations as You Wish”

Liquid crystals consisting of rod-shaped molecules are a remarkable soft matter with extraordinary responsivity to external stimuli. Techniques to control molecular orientations are essential in both making and operating liquid crystal devices that have changed our daily lives completely. Traditional display devices are based on uniform alignments of molecules at substrate surfaces. In this talk, I will present a new photopatterning approach for aligning molecules into complex 2D and 3D orientations with sub-micron resolutions. This approach relies on so-called plasmonic metamasks to generate designer polarization direction patterns and photoalignments. I will present the basic principles behind this approach and a number of intriguing applications enabled by it, including micro-optical devices for laser beam shaping, commanding chaotic motions of bacteria, and creating topological defects with designer structures.

Host: Shengfeng Cheng

Dr. Greg Quiroz (Johns Hopkins Applied Physics Lab)

“Deep Reinforcement Learning for Quantum Control: Learning to Optimally Navigate in Complex Noisy Environments”

Quantum control seeks to establish control over a quantum system in such a way so that logical operations are implemented while simultaneously mitigating unwanted interactions between the system and its environment. From the point of view of quantum computation, quantum control can potentially provide significant improvements in computational accuracy when quantum logic operations are tailored for the particular noise plaguing the hardware. Specifically tailoring each controlled operation can be quite demanding if one wishes to perform this task for every instantiation of a quantum algorithm. Here, we examine how one can leverage reinforcement learning to learn and predict quantum gates in the presence of noise; thus, providing a streamlined method for gate design for generic quantum algorithms.

Host: Sophia Economou

Dr. Vivek Amin (NIST)

"Anatomy of spin-orbit torque"

Information and communications technology ispredicted to account for 10% to 20% of the world’s power consumption within adecade. Alleviating this rise in power consumption requires rethinkingthe way we electronically process and store information. Spintronics, orspin electronics, offers a possible solution to this problem by using spincurrents or spin waves rather than conventional charge currents to manipulateinformation. A key ingredient in spintronics is spin-orbit coupling: therelativistic coupling between a particle’s spin and orbital moments. Spin-orbit coupling permits conduction electrons to extract a virtuallyunlimited amount of angular momentum from the crystal lattice, potentiallyenabling energy efficient information processing. In this talk, I willdiscuss the electrical manipulation of a ferromagnet’s magnetization throughspin-orbit coupling. This phenomenon, known as spin-orbit torque, couldhelp harness all the advantages of different electronic memories (e.g. speed,nonvolatility, radiation hardness) into one device. The presentunderstanding of spin-orbit torque is incomplete because there is no consensusamong theory and experiment over the important mechanisms. We review thetraditional spin-orbit torque mechanisms and then show that novel interfacialor bulk effects are needed to explain recent experiments. Shedding lighton these mechanisms will help clarify the nature of spin-orbit torque, creatingexciting new possibilities for current-controlled magnetization dynamics withattractive applications for information processing.

Host: Satoru Emori

No Seminar

Host:

Prof. Xinqi Gong (Renmin University of China)

"Mathematical Intelligence Applications for Bio-Marcromolecular Problems"

The intersection among mathematics, information and biology has becoming more and more interesting and important. Many studies in this direction have led to developments of theories, methods and applications. But the too fast advancing of nowadays forefront information technology and biology knowledge, have triggered two obviously emerging phenomena, tremendous brand-new peaks accessible by new kinds of efforts, randomly meaningless results by in-correct intersections. Here I will show some of our recent results in developing and distinguishing efficiently intelligent approaches and applications for computational molecular biology and medical problems, such as protein structure-function-interaction prediction and pancreas cancer CT image analysis using algorithms like Fast Fourier transform, Monte Carlo, and deep learning, and some new designed physical and geometrical features.

Host: Shengfeng Cheng

Dr. Rui Zhang (University of Chicago)

"Structure and Dynamics of Topological Defects in Active Liquid Crystals"

Topological defects in nematic liquid crystals exhibit unique optical and physicochemical properties that have led to emerging applications in directed self-assembly of colloids and macromolecules. Recent experiments have demonstrated that active matter that consists of a dense collection of self-propelled rods can form an active nematic liquid crystal in which defects bind and unbind in a chaotic-like manner. Abundant examples of active nematics are found in different animate and inanimate systems, including flocking animals, bacteria, tissue cells, biopolymer suspensions, and even vibrating granular rods. However, the material properties of and seemingly chaotic-like defect dynamics in these non-equilibrium systems are poorly understood, limiting their applications. In this talk, I will discuss our recent work on unraveling defect behavior in active nematics. Specifically, we have adopted a hydrodynamic model to explain how the morphology, structure and dynamics of defects are determined by the interplay between elasticity and activity. Our model predictions are successfully confirmed by actomyosin-based experiments, shedding light on understanding and further control of topological defects in active liquid crystals.

Host: Shengfeng Cheng

Sam Carter (Naval Research Lab)

"Spins in InAs quantum dots: qubits, sensors, and photon sources "

Over the past few decades a number of exciting applications of quantum coherence and entanglement have been developed that promise fundamental improvements in computing, secure communications, and sensing. A team of scientists at the Naval Research Laboratory are working to develop a physical implementation for these quantum information applications using semiconductor indium arsenide quantum dots (QDs). This system has the advantages of a robust solid state host, strong optical transitions, mature device fabrication, tunable properties, and a scalable, monolithic architecture. A single electron or hole spin within a QD acts as a stationary quantum bit that can be optically controlled on a picosecond timescale. In this presentation, I will discuss how a spin in a QD or in a pair of coupled QDs can also be used for sensing mechanical motion and for generating tunable single photons. To sense motion, QDs have been incorporated into mechanical resonators, which couple to the dots through strain. When mechanical resonators are driven, the optical transitions of QDs shift significantly [1], and the spin states shift as well [2]. In single QDs, the hole spin shows much stronger coupling to strain than electrons spins, due to the stronger spin-orbit interaction. In coupled QDs, a pair of interacting electron spins can be made highly sensitive to strain gradients that change the relative QD energies. To generate photons, we make use of the Raman spin-flip process, which has the advantage of generating photons with properties determined by the drive laser and the spin properties. In this way, we are able to demonstrate spectral and temporal control over single photon wavepackets [3], with very low two photon emission probability and high indistinguishability. Finally, I will briefly discuss efforts that combine these topics, in which highly localized strain is used to tune multiple QD photon emitters into resonance within nanophotonic waveguides [4]. This work is supported by the U.S. Office of Naval Research and the OSD Quantum Sciences and Engineering Program. [1] Carter, S. G. et al. Sensing flexural motion of a photonic crystal membrane with InGaAs quantum dots. Appl. Phys. Lett. 111, 183101 (2017). [2] Carter, S. G. et al. Spin-mechanical coupling of an InAs quantum dot embedded in a mechanical resonator. Phys. Rev. Lett. 121, 246801 (2018). [3] Pursley, B. C., Carter, S. G., Yakes, M. K., Bracker, A. S. & Gammon, D. Picosecond pulse shaping of single photons using quantum dots. Nat. Commun. 9, 115 (2018). [4] Grim, J. Q. et al. Scalable in operando strain tuning of multiple quantum dots within a photonic waveguide architecture. arXiv 1810.05195 (2018).

Host: Sophia Economou

Dr. Cihan Nadir Kaplan (Harvard University)

"Morphing hard and soft matter by reaction-transport dynamics"

Engineering next-generation materials that can grow into efficient multitasking agents, move rapidly, or discern environmental cues greatly benefits from inspiration from biological systems. In the first part of my talk, I will present a geometrical theory that explains the biomineralization-inspired growth and form of carbonate-silica microarchitectures in a dynamic reaction-diffusion system. The theory predicts new self-assembly pathways of intricate morphologies and thereby guides the synthesis of light-guiding optical structures. The second part is dedicated to a soft matter analog of controlled actuation and complex sensing in living systems. Specifically, I will introduce a continuum framework of a simple hydrogel system that is activated upon transport and reaction of chemical stimuli. The hydrogel exhibits unique cascades of mechanical and optical responses, suggesting that common gels have a much larger sensing space than currently employed. The theoretical work presented in my talk is intimately connected to modern materials science. The effective convergence of theory and experiment paves the way for optimized hard or soft biomimetic materials for applications ranging from bottom-up manufacturing to soft robotics.

Host: Rana Ashkar

Dr. Trung Dac Nguyen (Northwestern University)

"Engineering materials from bottom up for bioremediation applications"

The accumulation of mismanaged organic waste and toxins in insecticides and warfare agents poses serious environmental threats to the ecosystem and human health. Bioremediation provides an effective means to address these issues, for instance, by using cleaning agents containing enzymes that break down the pollutants into more bio-friendly products. The ability to maximize the catalytic activity of those enzymes out of their native media will enable industry-scale bioremediation applications for a wider variety of pollutants. In this talk, I will discuss a recently proposed bottom-up approach for stabilizing enzymes in organic solvents, where the enzymes are mixed with copolymers that are composed of hydrophobic and hydrophilic monomers arranged in disordered sequences. These so-called random heteropolymers possess a rich diversity in monomer sequences, which plays a vital role in reducing the enzymes’ exposure the unfavorable solvent. This helps explain why the proposed approach is superior to the often-used reverse-micelle techniques in retaining the activity of numerous types of enzymes in toluene. Furthermore, the collectivity in the interaction between the polymers and enzymes upon assembly is predicted to lead to the uniform size of the assembled complexes, which is important for their solubility and delivery. These outstanding features of the random heteropolymer approach suggests new possibilities for engineering protein-based materials for applications much beyond bioremediation.

Host: Daniel Capelluto

Prof. Nuno Araujo (University Lisbon, Portugal)

"Dynamics of Colloidal Particles on Surface and Interfaces"

Colloidal particles are considered ideal building blocks to produce materials with enhanced physical properties. The state-of-the-art techniques for synthesizing these particles provide control over shape, size, and directionality of the interactions. In spite of these advances, there is still a huge gap between the synthesis of individual components and the management of their spontaneous organization towards the desired structures. The main challenge is the control over the dynamics of self-organization. In their kinetic route towards thermodynamically stable structures, colloidal particles self-organize into intermediate structures that are much larger than the individual particles and become the relevant units for the dynamics. To follow the dynamics and identify kinetically trapped structures, one needs to develop new theoretical and numerical tools. In this seminar, we will discuss the self-organization of colloidal particles under confinement. Experiments with suspensions of ellipsoidal colloidal particles suggest a transition in the statistical properties of the stain left by an evaporating drop, depending on the eccentricity of the particles [1]. We proposed a stochastic model to show that the very-strong anisotropic capillary attraction between particles stemming from the deformation of the interface can be responsible for such transition [2,3]. We will discuss the main mechanisms involved and compare the quantitative results with experiments. With the experimental groups of Erika Eiser (Univ. Cambridge) and Jasna Bruijc (New York University), we have shown that the long-range capillary attraction and consequent formation of kinetically trapped structures of colloidal particles at interfaces can be avoided using DNA-coated colloids on complementary functionalized interfaces (oil droplet) [4], where we keep the irreversible interfacial binding but suppress the strong attraction, resulting in a fully ergodic colloidal dynamics. We will discuss how the coverage of the oil droplet by colloidal particles and the self-assembled structures depend on different system parameters, such as, temperature and bulk concentration of colloidal particles. [1] P. J. Yunker et al. Physical Review Letters 110, 035501 (2013). [2] C. S. Dias et al., Soft Matter 14, 1903 (2018). [3] C. S. Dias, N. A. M. Araújo, M. M. Telo da Gama, EPL 107, 56002 (2014). [4] D. Joshi et al. Science Advances 2, e1600881 (2016). * with M. M. Telo da Gama, C. S. Dias, A. S. Nunes, and D. Pinto

Host: Uwe Tauber

APS Meeting (No Seminar)

Host:

Spring Break

Host:

Prof. So Takei (Queens College, NY)

“Macroscopic Quantum Spintronics Devices”

We propose two platforms for realizing macroscopic spintronics qubits. The first prototype magnetic quantum information processing device, based on spin superfluidity and spin Hall phenomena, realizes the spin-supercurrent analog of the superconducting phase qubit, and allows for full electrical control and readout. The second device stores a quantum state in a topological defect of a magnetic insulator and realizes the magnetic analog of the three-level rf-SQUID qubit. We propose non-invasive methods to coherently control and readout the quantum state using ac magnetic fields and magnetic force microscopy, respectively. Various physical estimates for both devices, e.g., operational temperatures and decoherence times, will be made and discussed.

Host: Satoru Emori

Prof. Shawn Cui (Math Dept. , Virginia Tech)

“4-dimensional topological quantum field theories from fusion categories ”

We give a construction of a family of 4D topological quantum field theories (TQFT). By the Atiyah-Segal axiomatic formulation, a 4D TQFT assigns vector spaces to 3D space manifolds and linear maps to 4D spacetime manifolds (cobordisms) satisfying some compatibility conditions. If the spacetime manifold has no boundary, then the corresponding linear map is a scalar, called the partition function. We will focus on the construction of partition functions. The data used in the construction involves some interplay between group actions and fusion categories. The resulting TQFTs generalize simultaneously most known ones in the literature such as Dijkgraaf-Witten TQFT, Crane-Yetter/Walker-Wang TQFT, etc. Further generalizations using higher categories will also be discussed briefly.

Host: Djordje Minic

Dr. Ulrich Dobramysl (University of Cambridge)

"How do cells sense direction?"

In many biological processes, in particular embryonic and brain development, cells need to follow chemical gradients to arrive at a precise location. To this end, they need to be able to determine the direction and position of a source releasing diffusing guidance cues from information gathered by receptor clusters located on the cell membrane. We develop an analytical model and an efficient numerical simulation procedure to calculate the particle fluxes to receptors and determine the limits of direction sensing in different environments. We find that the cell needs three or more receptor clusters to enable reconstruction of the exact source position from measured particle fluxes. We develop simulations of cells navigating in a chemical gradient based on these findings and show that cells are indeed able to find their targets.

Host: Uwe Tauber

Prof. Peter Schauss (University of Virginia)

“Quantum gas microscopy of many-body dynamics in Fermi-Hubbard and Ising systems”

The ability to probe and manipulate cold atoms in optical lattices at the atomic level using quantum gas microscopes enables quantitative studies of quantum many-body dynamics. While there are many well-developed theoretical tools to study many-body quantum systems in equilibrium, gaining insight into dynamics is challenging with available techniques. Approximate methods need to be benchmarked, creating an urgent need for measurements in experimental model systems. In this talk, I will discuss two such measurements. First, I will present a study that probes the relaxation of density modulations in the doped Fermi-Hubbard model. This leads to a hydrodynamic description that allows us to determine the conductivity. We observe bad metallic behavior that we compare to predictions from finite-temperature Lanczos calculations and dynamical mean field theory. Second, I introduce a new platform to study the 2D quantum Ising model. Via optical coupling of atoms in an optical lattice to a low-lying Rydberg state, we observe quench dynamics in the resulting Ising model and prepare states with antiferromagnetic correlations

Host: Ed Barnes

Prof. Sebastian Deffner (University of Maryland)

“Quantum thermodynamics: An introduction to the thermodynamics of quantum computers”

We are the verge of a technological revolution. Over the last couple of years the first computational devices have become commercially available that promise to exploit so-called quantum supremacy. Even though the thermodynamic cost for processing classical information has been known since the 1960s, the thermodynamic description of quantum computers is still at its infancy. This is due to the fact that many notions of classical thermodynamics, such as work, do not readily generalize to quantum systems in the presence of thermal and quantum noise. In this colloquium, we will outline a novel conceptual framework of an emerging theory, Quantum Thermodynamics, and its application to quantum computers.

Host: Sophia Economou