A Pinch of Light and a Dash of Negative Refraction: Recipes for Making a Perfect Lens and a Cloak of Invisibility
Negative index materials (NIMs) are characterized by an electric permittivity and a magnetic permeability that are simultaneously negative, resulting in a negative index of refraction. Unlike all naturally-occurring materials, the group velocity of electromagnetic signals in a NIM is oriented opposite to the phase velocity. This antiparallel velocity flow gives rise to many exotic effects, including negative refraction, a reversed Doppler effect, and electromagnetic cloaking. NIMs also amplify evanescent electromagnetic Fourier components, creating the possibility for a "perfect lens" that can resolve arbitrarily-small feature sizes. Recent studies have shown that artificial materials (metamaterials) can be engineered to give negative indices in the microwave and infrared frequency range. However, scaling metameterials for operation at visible wavelengths - where a perfect lens and an invisibility cloak would have the most pronounced impact - has remained elusive. In this seminar, I will present the first experimental demonstration of visible-frequency negative refraction. Negative indices are achieved by exploiting the unique properties of surface plasmons - coupled electronphoton waves at the interface between a metal and dielectric. By exciting surface plasmon waves between two closely spaced metallic mirrors, we can engineer materials with negative indices as large as -5 in the blue-green region of the visible. My talk will present both the theory and implementation of negative index materials, drawing on advances enabled by the emerging field of plasmonics. Progress toward a sub-diffraction limited optical microscope and an electromagnetic cloak will also be discussed.
Symmetry and Simulation: How Geometry Affects Scientific Computing from the Solar System to your Microwave Oven
How do we model change in physical systems? Ever since Newton proposed his laws of motion, the answer has been differential equations. Solving these equations lets us make predictions about the future, or even the past, but for most complex systems in science and engineering, it is impossible or impractical to obtain exact solutions. Therefore, we must rely on numerical simulation to compute approximate answers. With computing power cheap and ubiquitous, these simulations can now be carried out better and faster than ever before. However, even some of the "best" numerical methods have a serious problem. In trying to simulate the laws of motion accurately, they end up breaking other physical laws, such as the conservation laws for momentum and energy. For many problems - from long-time simulations of the solar system, to molecular dynamics, to computing the resonant frequencies of a microwave oven - failure to preserve these sorts of features can result in a major loss of predictive power. These properties can best be understood in terms of geometric mechanics, a powerful branch of classical mechanics that incorporates mathematical tools from differential geometry and calculus of variations, but which has until recently seen very little use among applied mathematicians. Recent work (including my own) has shown that by bringing together geometric mechanics and numerical analysis- symmetry and simulation- we can develop numerical methods that accurately model dynamics while preserving the underlying geometric structures. In this talk, I hope to bring the audience on a tour of recent advances at this rich intersection of geometry, physics, and computation. Along the way, we will pick up many important threads that run back through the history of mathematics and physics, from Kepler and Newton to the present day. This will be accompanied by a variety of examples and simulations, from planetary orbits to electromagnetic waves.
Seeing is Believing: Visualization of Condensed Matter Structure in Four Dimensions
The power of being able to see the three-dimensional structure of a molecule or condensed matter at the atomic level can never be underestimated. One perfect example is the celebrated double-helical structure of DNA, whose discovery had a huge impact on biology and medicine and marked a new chapter in our understanding of life. However, in order to understand the nature of physical, chemical or biological functions, not only do the relevant static structures need to be determined but also their dynamics on different time scales must be visualized. For complex systems comprised of hundreds to thousands of atoms, four-dimensional (spatiotemporal) structural information becomes indispensable to unraveling structure-dynamical behavior. As a result, the development of experimental techniques that can directly provide snapshots of transient structures has become an area of intense activity within this decade. In this talk, I will describe our achievements in developing time-resolved electron diffraction for the direct probing of structural dynamics in condensed matter. From ultrafast crystallographic snapshots, we uncovered concealed transitional structures and thereby elucidated the elementary steps during phase transformations. Examples range from the recent discovery of a new nonequilibrium phase in a cuprate superconductor to studies of interfacial assemblies of water, fatty acid and phospholipid molecules on hydrophobic and hydrophilic surfaces, to the most recent visualization of solid-solid phase transformation. We will conclude with a perspective on future explorations in material science and biology.
The High-Energy Universe in Focus: A New Telescope of High-Energy X-Rays
Many exotic celestial phenomena produce X-rays---supernovae, black holes and neutron stars are but a few examples. Study of the X-ray sky has brought much new knowledge to astronomy, especially in the past decade, with the launch of new X-ray telescopes in space. These telescopes employ focusing technology---X-ray mirrors to concentrate large amounts of light onto compact `X-ray digital cameras'---which results in unprecedented sharpness, colors and sensitivity. While these focusing telescopes detect low-energy X-rays (analogous to red-colored light in the visible band), no equivalent instrument exists today for high-energy X-rays---the `orange' through `violet' of the X-ray band. Our experiment, the High Energy Focusing Telescope (HEFT), is developed to satisfy exactly this need.
HEFT is among the first generation of focusing telescopes sensitive to high-energy X-rays. The instrument consists of several coaligned telescopes---X-ray mirrors and `X-ray digital cameras'---on a balloon-borne pointing platform. Both the X-ray mirrors and cameras are new technologies developed by our collaboration and by others over the past decade. If placed on a satellite, a high-energy X-ray telescope with HEFT technologies will be a thousand times more sensitive than any high-energy X-ray instrument currently in space, enabling new scientific studies that are not feasible with current space technologies. After a decade of hardware development, HEFT was launched for the first time in May, 2005.
In this talk, I shall explain the significance of high-energy X-rays in observational astronomy, describe the new technologies for focusing high-energy X-rays, and present the maiden flight of HEFT, including our latest scientific results.
Stayin' Alive: How Bacteria Survive in Biofilm Communitites
Bacteria have ruled the planet for billions of years, yet only in the last few years have we learned that these organisms spend much of their lives as surface-associated communities, or biofilms. These biofilms are extremely resilient to changes in environmental conditions and can be up to a hundred times more resistant to antibiotics and antimicrobial agents. This lifestyle strategy allows the bacteria to survive well in diverse environments. However, they can also adversely affect humans, forming biofilms in pipelines, as plaque on teeth, on the surfaces of medical implants and causing life-threatening infections in the lungs of patients with cystic fibrosis. We are only starting to understand how bacteria live in these communities, how the communities are organized, how they get enough nutrients and what makes them so resilient. Bacteria in biofilm communities are nutrient limited, but instead of dying as free-swimming cells might, we have found that cells in all regions of the biofilm continue to live and produce energy. This means that all cells are potentially capable of responding to changes and interacting with their environment over long periods of time. In this seminar, I will discuss how we are working to understand the metabolic state of these quiescent cells, a growth state that has important implications for bacterial survival in natural and man-made environments, yet one that has remained largely unexplored.
Metals with Memory: How these amazing materials remember their shape
As the science of materials has evolved, we have gained the ability to make unusual alloys that do not exist in nature but have properties advantageous in practical applications. As we enter a generation where greater demands are being put upon material functionality, particularly in biomedical and small-scale applications, it is increasingly important that we are able to characterize and understand how these materials work. Establishing a relationship between the microscopic behavior of a material and its macroscopic properties facilitates the development of these types of new materials and the prediction of their behavior in practical applications.
One prominent example of this type of engineered material is nitinol. Nitinol is a mixture of Nickel and Titanium that has the ability to elastically "stretch" far beyond most metals (superelasticity) and also has the ability to remember a previous shape when heated (the shape memory effect). It achieves this by changing the geometry of its crystalline structure. This transformation between phases is the fundamental mechanism behind its unique properties, but there is little experimental data characterizing how this transformation proceeds. By using a relatively new technique called Digital Image Correlation (DIC), we have succeeded in getting the first quantitative understanding of how this transformation mechanism nucleates and proceeds. This talk will introduce the shape memory effect and superelasticity, describe the physics behind these phenomena, and discuss the current and future applications of these materials.
Decisions, Decisions: The Ellsberg Paradox and the Neural Foundations of Decision-Making Under Uncertainty
Decision-making under uncertainty is a fundamental activity at every societal level with examples as diverse as people saving for retirement, companies pricing insurance, and countries evaluating military, social, and environmental risks. The choices can vary greatly in the amount of information available to the decision-maker about the outcome probabilities. Standard decision theory, however, precludes agents from taking into account uncertainty about probabilities. This prediction was rejected decisively by the introduction of a thought experiment called the "Ellsberg Paradox."
Much work has been done in the ensuing decades to resolve the paradox by relaxing the axioms underlying the standard model, with little attention paid to the actual decision-making apparatus -- the brain. Recent advances in technology have allowed us to peer inside the minds of decision-makers. We used a combination of data from functional brain imaging and patients with focal brain damage to study the Ellsberg paradox. Our study shows that standard theory is wrong on both the behavioral and neural levels. More generally, it shows the value of combining ideas and tools from social and biological sciences.
The Quantum Internet: How Einstein's objection to quantum mechanics leads to a whole new field in physics
Einstein never did accept quantum mechanics. In a paper co-authored by Rosen and Podolsky in 1935, he objected to the role of quantum mechanics as a complete description of nature. But it was that paper which established the concept of quantum entanglement, a purely quantum mechanical correlation among various components of a physical system. For two components of a physical system linked by quantum entanglement, acting on one will affect the state of the other, irrespective of the distance between them. Quantum entanglement turns out to be the key resource in the new science of quantum information.
At the heart of quantum information science are distributed quantum networks. In quantum networks, quantum information is generated, processed, and stored locally in quantum nodes. These nodes are linked by quantum channels that transport quantum states from site to site with high fidelity by way of quantum teleportation. Even with relatively modest processing capabilities, the envisioned quantum "internet" could be used to accomplish tasks that are otherwise impossible within the realm of classical communication and computation, such as secret sharing, secure multi-party computation, and quantum metrology. Besides an introduction to the concepts of quantum entanglement and teleportation, I will talk about the effort in our group towards the realization of a quantum repeater architecture for large-scale quantum networks (to nettle Einstein, if he is watching us).
Darwin's Dumpster: How cellular sloppiness governs the rate of evolution
The astonishing diversity of life reflects the ongoing evolution of myriad molecular components such as genes and the proteins they encode. Strangely, even within the same organism, some proteins evolve rapidly while others barely change. For decades, functional differences between proteins have been thought to cause evolutionary rate differences, but our work reveals that protein function plays at best a minor role. Instead, we have uncovered a surprising alternative: the cellular machinery that synthesizes proteins from genes is sloppy, defective proteins cost dearly, and the speed of a gene's evolution depends on how costly it is to change. At a time when challenges to Darwinian evolution abound, our attack on this controversial problem provides an eye-opening view of how evolution science really works.
Were Microbes the Architects of Ancient Shorelines?
Stromatolites—laminated, enigmatically shaped cones and domes that dominated carbonate reefs for the first 80% of Earth's history—may be the oldest macroscopic record of life on Earth. If and how microbes contributed to the formation of many ancient stromatolites has puzzled geologists for decades. To develop criteria for the recognition of uniquely microbial signatures in rocks, we precipitated calcium carbonate in the presence and absence of various modern microorganisms under chemical conditions relevant for the early Earth. Using this approach, we showed that anoxygenic photosynthetic bacteria could have built stromatolites even before the rise of modern stromatolite builders—cyanobacteria. Our work disproved the paradigm that microbial sulfate reduction, a metabolism important for the formation of modern stromatolites, was responsible for the formation of similar structures billions of years ago. We also developed criteria to constrain the microbial origin of a previously unconfirmed biosignature—sub-micron-sized pores that occur in rapidly precipitating carbonate rocks. These insights into how microbes shape rocks are critical to the search for microbial biosignatures on the early Earth and other planets.
The Atom Chip
Central to the advancement of our understanding of quantum physics and to the development of the tantalizing quantum computer is the quest to devise ever better ways to manipulate atoms and photons in our laboratories. The atom chip—a device reminiscent of a computer circuit board but designed for cold neutral atoms—is an important new addition to the toolboxes of quantum physics and nanotechnology. We have fabricated and employed these devices to couple single atoms to photons, to investigate the newly discovered dilute-gas Bose-Einstein condensates, and for creating an atom mirror from an ordinary computer hard drive. Quantum computer hardware and atom laser manipulation are but a few promising atom chip applications.
The Sweeter Side of Cell Signaling
Fewer than 30,000 genes orchestrate the workings of human cells. These genes encode the proteins which conduct the myriad activities necessary for life. Chemical modifications of these proteins play a critical role in regulating their cellular functions. We study one such modification, known as O-GlcNAc glycosylation. The addition of this single sugar, β-N-acetylglucosamine (GlcNAc), to proteins has been linked to nutrient sensing and gene expression. Moreover, the enzyme that transfers the sugar is abundant in the brain, and the modification has been linked to neurodegenerative disease. Understanding this dynamic modification has been hampered by the difficulty in detecting it. To address this challenge, we developed a strategy which incorporates a mutant enzyme and unnatural substrate for rapid and sensitive detection of O-GlcNAc-modified proteins. Coupling our approach to mass spectrometry yielded the first direct, wide-scale identification of O-GlcNAc proteins in the brain. Our studies have identified O-GlcNAc on proteins associated with neurotransmitter release and the formation of nerve cell structures important for learning and memory. Currently, we are investigating the dynamic regulation of O-GlcNAc in the brain in an attempt to understand its role in nerve cell function and neurodegeneration.
The Interplanetary Transport Network: Space Transportation Architecture for the 21st Century
The competing gravitational pull between celestial bodies creates a vast array of low-energy passageways that winds around the sun, planets and moons. Space travel along these corridors would slash the amount of fuel needed to explore and develop our solar system. We have shown how to identify and traverse these passageways, which are associated with Lagrange points, regions of balance near a planet or moon. We have laid the groundwork for a new kind of space transportation architecture for the exploration of Mars, the asteroids, and the outer solar system, including a mission to assess the possibility of life on Jupiter's icy moons.
Stalking the Exciton Condensate: Exotic Behavior of Electrons under Extreme Conditions
For over 40 years, scientists have sought the Bose-Einstein condensation (BEC) of excitons. BEC is responsible for many novel effects when achieved in liquid helium (creating superfluidity), in electron pairs (producing superconductivity—the lack of electrical resistance in metals), and most recently in alkali atoms (for which the 2001 Nobel prize was awarded). But the BEC of excitons has remained elusive. Excitons are fragile particles, bound pairs of electrons and holes with a predilection for annihilating one another, making the formation of a BEC difficult. In our lab we have created stable excitons in neighboring layers of two-dimensional electron gases embedded in highly-engineered semiconductor crystals. We have observed superfluid-like flow of these excitons, evidence that the long-sought exciton condensation has finally been achieved.
Cancerous Stem Cells: Insights into the Origins of Human Brain Tumors
Brain tumors are the most common solid cancer and most frequent cause of cancer death in children. Despite decades of research, the regimen of surgery, radiation, and chemotherapy remains the principle therapy available to patients. The deleterious effects of these traditional treatments, along with the lack of effective tumor-specific treatments and diagnostic tools, lead to high morbidity and mortality rates. Recent advances in leukemia research have identified bone marrow-derived stem cells as a source for those cancers. Based on this work, we have taken a novel approach to identifying the origins of brain tumors. We found that brain tumor-derived stem cells share similar properties with neural stem cells--they both give rise to neurons and glia, produce similar proportions of cell types, and self-renew--but differ in that they proliferate extensively and differentiate into abnormal cells that may contribute to tumorigenesis. These findings suggest that targeting tumor-derived stem cells is a promising approach to treating brain tumors.
Eitan Grinspun (Computer Science): "Multiresolution in Graphics and Simulation: From Wrinkles in Valentines Day Balloons to Folds in the Brain"
Sarah Heilshorn (Chemistry and Chemical Engineering): "Protein Engineering: A Novel Approach to Creating New Biomaterials"
Ian Swanson (Physics): "Tangled Science: Unraveling the Superstring Mystery"
Joshua Bloom (Astrophysics): "Toward the Origin of Gamma-Ray Bursts, the Biggest Bangs in the Universe"
Benjamin Weiss (Planetary Science): "Ancient Magnetism, Panspermia, and the Evolution of Mars"
Ramesh Srinivasan (Chemistry and Chemical Engineering): "Catching Molecules in the Act: Ultrafast Diffraction of Transient Structures in Real Time"