Applied Physics / Physics Colloquium

Any competent scientific editor will scribble "redundant terminology!" in the margin next to the phrase "10^{-15} femtometers". But that is indeed the current experimental upper limit on the distance between an electron's center-of-mass and its center-of-charge. There are several good reasons to imagine that these two locations might not be at exactly the same place inside this nominally "point particle." At JILA, we hope to improve the experimental limit by a factor of ten. If we see a nonzero value, that will be evidence for new particle physics at an interesting energy scale.

Held Tuesdays at 4:30 pm in the William R. Hewlett Teaching Center, room 201.

Refreshments in the lobby of Varian Physics at 4:15 pm.

Winter 2015/2016, Committee: R. Blandford (Chair), T. Heinz, L. Hollberg, K. Irwin

Increased demands of batteries for applications in consumer electronics, electric vehicle and grid present opportunities and challenges for rechargeable batteries. This lecture will analyze the nature of energy storage, the existing technology and present the promising future batteries, which can have significantly higher energy density, lower cost, better safety and longer life. Novel battery chemistries and materials are key for a revolutionary change.

Held Tuesdays at 4:30 pm in the William R. Hewlett Teaching Center, room 201.

Refreshments in the lobby of Varian Physics at 4:15 pm.

Winter 2015/2016, Committee: R. Blandford (Chair), T. Heinz, L. Hollberg, K. Irwin

The IceCube Neutrino Observatory at the US South Pole Station is a cubic-kilometer scale neutrino detector, using a billion tons of ultra-pure Antarctic ice as a Cherenkov radiator. IceCube detects over 100,000 neutrinos per year, at energies ranging from a few GeV to several PeV. Most of these are produced in the Earth's atmosphere, but at the highest energies we have discovered a neutrino flux of cosmic origin. Although their exact sources remain unclear, the level of this flux suggests that hadron accelerators produce a considerable fraction of the energy in the non-thermal universe. We will discuss the IceCube Observatory, the observations of high energy neutrinos, and their significance.

Held Tuesdays at 4:30 pm in the William R. Hewlett Teaching Center, room 201.

Refreshments in the lobby of Varian Physics at 4:15 pm.

Winter 2015/2016, Committee: R. Blandford (Chair), T. Heinz, L. Hollberg, K. Irwin

In Pixar's Inside Out, Joy proclaims, "Do you ever look at someone and wonder, what's going on inside?" My group asks the same question about nanomaterials whose function plays a critical role in energy, biology, and information-relevant processes. In this presentation, I will describe new techniques that enable visualization of nanoparticle phase transitions and light-matter interactions with nanometer-scale resolution. First, we explore nanomaterial phase transitions induced by solute intercalation, to understand and improve materials for energy storage applications. As a model system, we investigate hydrogen absorption and desorption in individual palladium nanocrystals. Our approach is based on in-situ electron energy-loss spectroscopy in an environmental transmission electron microscope. By probing hydrogen-induced shifts of the palladium plasmon resonance, we find that sub-30nm single-crystalline nanoparticles do not exhibit phase-coexistence and that surface effects dictate the size dependence of the ​hydrogen absorption pressures. Then, we introduce a novel tomographic technique, cathodoluminescence spectroscopic tomography, to probe optical properties in three dimensions with nanometer-scale spatial and spectral resolution. Particular attention is given to reconstructing a 3D metamaterial resonator supporting broadband electric and magnetic resonances at optical frequencies. Our tomograms allow us to locate regions of efficient cathodoluminescence across visible and near-infrared wavelengths, with contributions from material luminescence and radiative decay of electromagnetic eigenmodes. This tomographic technique could be used to precisely locate radiative recombination centers in light-emitting diodes, to probe the nanoscale distribution of defect states in organic photovoltaics, and potentially to provide new label-free avenues for biological imaging. Taken together, our results provide a general framework for high-resolution visualization of chemical reactions and light-matter interactions, well-below the diffraction limit and in three-dimensions.

Held Tuesdays at 4:30 pm in the William R. Hewlett Teaching Center, room 201.

Refreshments in the lobby of Varian Physics at 4:15 pm.

Autumn 2015/2016, Committee: A, Linde (Chair), S. Chu, P. Hayden, M. Schnitzer, L. Senatore

The past few years have seen a tremendous increase in the scale of galaxy surveys measuring the large scale structure of the universe. Photometric galaxy surveys are now routinely probing the dark matter clustering through the weak lensing technique, while spectroscopic surveys do the same through the redshift space distortions technique. Baryonic acoustic oscillation measurements are determining distances as a function of redshift, allowing one to measure the matter density and the Hubble parameter. In combination with the cosmic microwave background, these data sets are providing invaluable information on the origins of the structure in our universe, its geometry, age, matter content, neutrino properties, and other fundamental properties of our universe. Without trying to be comprehensive, this colloquium will describe some recent advances in the field, and ponder possible directions for the future.

Held Tuesdays at 4:30 pm in the William R. Hewlett Teaching Center, room 201.

Refreshments in the lobby of Varian Physics at 4:15 pm.

Autumn 2015/2016, Committee: A, Linde (Chair), S. Chu, P. Hayden, M. Schnitzer, L. Senatore

The AdS/CFT correspondence from string theory provides a quantum theory of gravity in which spacetime and gravitational physics emerge from an ordinary non-gravitational system with many degrees of freedom. In this talk, I will explain how quantum entanglement between these degrees of freedom is crucial for the emergence of a classical spacetime, and describe progress in understanding how spacetime dynamics (gravitation) arises from the physics of quantum entanglement

Held Tuesdays at 4:30 pm in the William R. Hewlett Teaching Center, room 201.

Refreshments in the lobby of Varian Physics at 4:15 pm.

Autumn 2015/2016, Committee: A, Linde (Chair), S. Chu, P. Hayden, M. Schnitzer, L. Senatore

The development of high brightness blue LEDs and blue laser diodes required many breakthroughs of GaN growth, p-type conductivity control, InGaN growth and device structures using InGaN/GaN double heterostructures. I will discuss the history and background story of the key scientific issues solved in order to realize high efficiency solid state lighting. The fundamental discovery of high quality p-type doping by removing hydrogen passivation, and the role of the InGaN layer in achieving high brightness blue LEDs and Laser Diodes will be described. I will also discuss the latest results of InGaN-based emitting devices at the University of California at Santa Barbara.
There is an intrinsic problem of the LEDs that cannot be easily overcome. When we increase the current densities so high, a reduction in efficiency with increasing the current density is observed. This phenomena, referred to as efficiency droop, forces LED manufactures to operate LEDs at lower current densities (and hence reduced light output) than would be possible to prevent excess heating of the device. An alternative method to produce white light is by using a blue laser, as opposed to an LED, in combination with a phosphor. Above the lasing threshold, the carrier density is clamped at threshold, fixing its density. Increases in carrier density beyond the threshold density immediately contribute to stimulated emission, or lasing. Thus, the carrier density is maintained at the lower, threshold density, prohibiting it from reaching densities where the Auger recombination process becomes the dominant recombination process. Auger recombination, with the resulting efficiency droop, does not appreciably occur in blue laser diodes.

Held Tuesdays at 4:30 pm in the William R. Hewlett Teaching Center, room 201.

Refreshments in the lobby of Varian Physics at 4:15 pm.

Autumn 2015/2016, Committee: A, Linde (Chair), S. Chu, P. Hayden, M. Schnitzer, L. Senatore

For the past 30 years nearly all theoretical physicists have believed that quantum field theories based on Einstein's general relativity necessarily must be ill-defined in the ultraviolet. This is the well-known nonrenormalizability problem of gravity. But is this actually true in general? We describe recent calculations that cast doubt on this simple picture and show that story of quantum gravity is much more interesting than seemed possible. The new calculations are made possible by recent enormous advances in our ability to compute scattering amplitudes based on unitarity, a new relationship between gravity and gauge theories, as well as advanced integration methods. The results show that in certain highly supersymmetric supergravity theories ultraviolet divergences do not appear at the locations predicted by symmetry considerations, at least as far as has been calculated. In less supersymmetric theories, ultraviolet divergences do occur but their structure can be bizarre and appear to be linked to anomaly-like behavior. This is even true in what was thought to be a well understood case: pure Einstein gravity with no matter. Potential explanations and prospects for the future are described.

Held Tuesdays at 4:30 pm in the William R. Hewlett Teaching Center, room 201.

Refreshments in the lobby of Varian Physics at 4:15 pm.

Autumn 2015/2016, Committee: A, Linde (Chair), S. Chu, P. Hayden, M. Schnitzer, L. Senatore