Colloquium Fall 2006
This talk will focus on two interesting new physical possibilities made possible by current short, intense laser pulses: (1) Laser acceleration of electrons to GeV energies, and (2) Control of electron detachment or ionization spectra by means of the carrier-envelope phase of a few-cycle laser pulse. In introductory material I will include information on the new 100 terawatt laser facility being built at Nebraska.
An outstanding problem in astrophysics is to explain the origin of the almost featureless cosmic ray spectrum extending up to energies of some 1020 eV. A very small feature is apparent at between about 1013 - 1015eV, the "knee." In the late 1970's, a suite of papers was published establishing the idea of diffusive shock acceleration for cosmic rays, essentially a first-order Fermi mechanism, which appeared to provide an explanation for the observed cosmic ray spectrum up to the knee. Diffusive shock acceleration is probably the most widely used particle acceleration mechanism in astrophysics and space physics, yet the theory is based on some stringent simplifications. The detailed [plasma] physics of the acceleration mechanism requires elucidation. We are fortunate in that very detailed observations of particle acceleration at shock waves, particularly in the guise of Space Weather, are providing considerable experimental insight into the basic physics of particle acceleration at a shock wave. Indeed, understanding the problem of particle acceleration at interplanetary shocks is assuming increasing importance, especially in the context of understanding the space environment. Detailed interplanetary observations are not easily interpreted in terms of the simple original models of particle acceleration at shock waves. Three fundamental aspects make the interplanetary problem much more complicated than the typical astrophysical problem: the time dependence of the acceleration and the solar wind background; the geometry of the shock; and the long mean free path for particle transport away from the shock wave. An interplanetary shock is not steady, as it decelerates and expands into an expanding, temporal solar wind. Furthermore, the shock geometry varies from quasi-parallel to quasi-perpendicular along a shock front, and multiple shocks can be present simultaneously in the solar wind. Consequently, the shock itself introduces a multiplicity of time scales, ranging from shock propagation time scales to particle acceleration time scales at parallel and perpendicular shocks, and many of these time scales feed into other time scales (such as determining maximum particle energy scalings, escape time scales, etc.). We will discuss the basic physics of particle acceleration via scalings, their relationship to particle acceleration models, observations and geometry in both an astrophysical and space physics context. This will include discussing the physics of perpendicular and parallel shocks, upstream turbulence, particle spectra, and particle injection and the seed population. After acceleration of particles at an interplanetary shock, the transport of energetic particles is non-diffusive because of their large mean free path in the quiet solar wind. The complications of coupling diffusive (at the shock) to non-diffusive transport will be addressed. In particular, we will address the coupled acceleration and transport of heavy ions, Fe/O ratios, the variability among individual events, and seed particle populations. We will discuss theoretical models and address recent modeling efforts.
Two-dimensional photonic crystal devices take advantage of our ability to pattern the dielectric, through nanofabrication techniques, on a scale that is shorter than the optical wavelength at which the device operates. Patterning on this length scale allows us, in principle, to engineer the electromagnetic properties of photonic devices in microscopic detail. It is a serious challenge, however, to understand how to utilize this freedom to improve device performance, and this photonic crystal device technology is still relatively immature. Nevertheless, a great deal of progress in photonic crystal device development has been made in the past few years. In this presentation I will discuss photonic crystal lasers with particular emphasis on devices capable of room temperature CW operation and devices with quantum dot active regions. The CW lasers have 3 dB bandwidths of just under 10 GHz with approximately 30 dB of side mode suppression. The photonic crystal lasers with quantum dot active regions have absorbed powers at threshold of under 10 microwatts. The presentation will describe the optical loss mechanisms in photonic crystal resonant cavities and I will also discuss efforts to model the electromagnetic properties of these devices in the near and far field using both finite-difference time-domain and finite element methods and compare these predictions to the experimental data. The presentation will also address device issues associated with passive photonic crystal components such as optical loss, waveguide dispersion, and the design of waveguide junctions. Demonstrations of Mach-Zehnder interferometers and directional couplers will be presented and again results from experiments will be compared to numerical predictions.
The recent proof of the Poincaré Conjecture, a century-old problem in 3-dimensional geometry, has made many headlines this summer. I will explain what the Poincaré Conjecture was, and also introduce its less talked about (but perhaps more important) extension the Geometrization Conjecture. I will try to convey why mathematicians cared about these problems, why physicists may be interested in their solution, and what the current and future impact of these breakthroughs is likely to be. The talk will assume no mathematical background beyond calculus.
The 2006 Nobel Prizes: Who, What and Why! Lin Chen, Gage Crump, Elena Pierpaoli
University of Southern California
We'll each talk for 15 minutes about the Nobel prizes in Chemistry, Physics, and Physiology/Medicine, respectively. Who got the prizes? What work did they do? Why was the work prize-worthy? We may also reflect briefly upon the impact of the work on our own research. Come along and find out. We'll have a generous question and answer session afterwards, so please bring questions!
Lin Chen, Department of Molecular and Computational Biology, Department of Chemistry and Biochemistry, USC.
Elena Pierpaoli, Department of Physics and Astronomy, USC.
Gage DeKoeyer Crump, Center for Stem Cell and Regenerative Medicine, Keck School of Medicine, USC.
The classical behavior of hamiltonian systems can be classified as (at one extreme) "integrable" or (at the other) "chaotic". How these properties are reflected in the quantum behavior of the system, and the nature of the physical consequences, has been the subject of intensive investigation. I will summarize some of the known results, and then show how "quantum chaos" sheds light on the ancient question of how a closed, isolated, many-body quantum system (in a pure state) comes to thermal equilibrium without outside influences.
Patent Law and Patent Attorneys: What it is, What we do
Pillsbury Winthrop Shaw Pittman LLP
I'll cover the basics of patent law and a basic understanding of what patent attorneys do. You will come away with a better understanding of patents than most people (even engineers) have, and maybe even a new career possibility.
Watering the Earth
Institute of Physics of Interplanetary Space, Rome - Planetary Science Institute, Tucson
Water is one of the key molecules of life, and a fundamental solvent of our own human life form. The planet that spawned our watery origins, Earth, presently carries enough surface water in vapor or liquid form to cover the entire planet to a depth of about 3 km. The fact that nearly three-quarters of the Earth's surface is covered by seas triggered writer Arthur C. Clarke to question why our planet is called *Earth*, when it could more aptly be called *Ocean*. Driven by our watery origins, we naturally look for other life forms in the universe at the "water hole" (wavelengths 18-21 cm). Simultaneously, we search, and find, water in planetary atmospheres, comets, asteroids, interplanetary dust, and molecular clouds. Water drives our questions about terrestrial and extraterrestrial life and we wonder how we came to exist on a planet so rich in water in the first place. So then, how _did_ our planet Earth get its water? The short answer is: 'we still don't know'. Despite our living embedded in the Earth environment, the origin of our atmosphere is one of the most puzzling enigmas in the planetary sciences. The processes and sources that contributed to its formation require knowledge of the formation of the solar nebula, Earth and its planetary neighbors, and each of their subsequent interactions, including the smaller members of the clan: asteroids, meteorites and comets. Timing and location is everything in this story and our main tool for trying to understand the puzzle will be elemental isotopic abundance measurements.
Quantum fluids flow through narrow straits. A pore no larger than a few nanometers allows superfluid helium to rock back and forth between two vessels, for example. We will give a physical picture of how spin waves and superfluids percolate through a porous medium.
The Large Hadron Collider (LHC) now being built at CERN, Geneva, Switzerland promises discoveries of great scientific importance. This will be the first particle accelerator with sufficiently high energy to enable the US High Energy Physics (HEP) groups to study fundemental interactions of a kind never before observed-those responsible for giving particles mass. The LHC will collide protons at sqrt(s) = 14 TeV and lead ions at sqrt(s) = 5.5 TeV. The CMS (Compact Muon Solenoid) detector features a 4 Tesla solenoid, electromagnetic and hadronic calorimeters covering a pseudorapidity region up to 6.7 (0.14 degrees), a large silicon tracker surrounding the interaction point and excellent muon detection capabilities. CMS is a massive undertaking that involves over 2700 scientists and engineers from 175 institutions in 39 countries. The status of the construction of CMS is reviewed, with emphasis on the forward detectors. The physics reach of the CMS detector is presented .
Observations of neutrinos from the sun and neutrinos produced in the Earth's atmosphere are consistent with the neutrinos changing from one flavor to another - neutrino oscillations. Muon storage rings offer the possibility of intense neutrino beams to measure neutrino oscillation parameters and possible CP violation in the leptonic sector. Muon storage ring colliders may be the best way to discover and study Higgs bosons in supersymmetry. The ultimate goal is a multi-TeV muon collider. A muon beam must be cooled to increase the phase space density and allow as many muons as possible to pass through the downstream apertures of an accelerator system. Since muons decay, ionization cooling is used to cool the beam rapidly. Progress on research and development towards neutrino factories and muon colliders will be discussed, as well as an international Muon Ionization Cooling Experiment (MICE) that has been approved by the Rutherford Laboratory in the U.K.