Departmental Colloquia are given by leading scientists from both outside and inside Tufts. They are intended to be accessible to graduate students and advanced undergraduates, and offer an ideal opportunity to learn about cutting-edge physics and astronomy from the experts. All graduate students are expected to attend.

Spring 2008 Schedule:

 

Thursday, Feb. 7  (note special time) Pavlos Protopapas, The Harvard Smithsonian Center for Astrophysics   A joint Computer Science / Physics and Astronomy Colloquium The IIC Time Series Center: How Astronomers, Computer Scientists and Statisticians are working together to tackle hard problems in astronomy New astronomical surveys such as Pan-STARRS and LSST are under development and will collect petabytes of data.  These surveys will image large areas of sky repeatedly to great depth, and will detect vast numbers of moving, variably bright, and transient objects. The data product of these surveys is series of observations taken over time, or light-curves. The IIC has established an inter-disciplinary Center for Time Series with an immediate focus on astronomy.  I will present three research topics currently being pursued at the IIC that require expertise from astronomy, computer science and statistics.  These are: identifying novel astronomical phenomena in large light-curve datasets, searching for rare phenomena such as extra-solar planets, and efficiently searching for significant events such as occultations of stars by small objects in the outer reaches of our solar system. Pavlos Protopapas is a senior scientist at the IIC and  Harvard-Smithsonian Center for Astrophysics. His research interests spans the outer solar system, extra-solar planets and gravitational lensing. He specializes in analyzing large collections of astronomical data, with a toolbox drawn from data-mining, computer science and statistics. 3:15 pm, Nelson Auditorium, Anderson Hall.  Refreshments in Burden Lounge, Anderson 108, at 2:30 pm.

Wednesday, February 13 Jigang Wang, Materials Sciences Division, E.O. Lawrence Berkeley National Laboratory Spin Manipulation in Magnetic Semiconductors: Ultrafast, Non-thermal and Cooperative Phenomena 4:00 pm, 250 Robinson Hall.  Refreshments served in Robinson 251 at 3:30 pm.

Friday, February 15 Mark Kachanov, Department of Mechanical Engineering, Tufts University Connection between the Conductive and Elastic Properties of Heterogeneous Materials 3:15 pm, Nelson Auditorium, Anderson 112.  Refreshments served at 2:30 pm in Burden Lounge, Anderson 108.

Wednesday, March 5 Weining Man, Princeton University Geometry and Symmetry in Condensed Matter Physics: Photonic Quasicrystals and Ellipsoids Packing Building block geometry and overall structural symmetry play an important role in the design of new materials. In particular this talk investigates ellipsoid packings and photonic quasicrystals, problems in which the geometry of the building blocks and the structural symmetry determine the physical properties. Photonic quasicrystals are constructed from dielectric material arranged in a quasiperiodic pattern whose rotational symmetry is forbidden for periodic crystals. Because quasicrystals have higher point group symmetry than ordinary crystals, they can have more uniform bandgaps. Since calculating the band structure of 3D photonic quasicrystals is fundamentally challenging, and to date beyond the range of computation in a reasonable time, we decided to answer the question experimentally. We constructed the world's first and largest (in terms of the number of units) 3D icosahedral Photonic quasicrystal (compose of polymer) using stereolithography. With our novel method to make polar plots of its microwave transmission vs. frequency and incident angle, we obtained the first-ever visualization of the Brillouin zone of a quasicrystal. Before our experimental work it was not at all clear that Brillouin zones existed or had physical meaning in quasicryatals. We proved that the nearly spherical Brillouin zones of 3D icosahedral quasicrystals make them one of the most promising candidates for complete photonic bandgaps found to date. For ellipsoidal granular material packing, we found in both experiments and simulations that ellipsoids can pack randomly more densely than spheres because of their extra degree of freedom associated with their rotational axes. Discovering the fact that the packing fraction has a cusp-like minimum for spheres and increases rapidly with aspect ratio differ from unity, is important for both theoretical modeling and practical applications. 4:00 pm, 250 Robinson Hall.  Refreshments served in Robinson 251 at 3:30 pm.

Friday, March 7 Mikhail A. Belkin, School of Engineering and Applied Sciences, Harvard University Quantum Cascade Lasers – Bridging the THz Gap with Semiconductor Lasers The frequency range 1-100 THz ( =3-300µm) has long been devoid of compact semiconductor sources of coherent radiation, similar to diode lasers in near-infrared and visible. A breakthrough in this area occurred with the demonstration of a quantum cascade laser (QCL) in 1994. QCLs are unipolar devices based on intersubband transitions in a repeated stack of semiconductor superlattices. As a result, their emission frequency can be widely tailored within the same materials system. Currently, these devices can operate at room temperature in mid-infrared spectral range and at cryogenic temperatures in terahertz spectral range. There is a strong interest to utilize QCLs for a variety of applications, including chem/bio and environmental sensing, terahertz security screening, and spectroscopy. I will give an introduction to the principles of QCLs, provide examples of QCL-based systems for chem/bio sensing, developed in our group, and then describe our progress towards developing a room-temperature terahertz QCL source. In particular, I will talk about our “traditional” THz QCLs, which currently operate at a record temperature of 178K, and a novel type of THz QCL source, operable at room temperature, based on intra-cavity terahertz difference-frequency generation in dual-wavelength mid-infrared QCLs engineered to possess giant second-order nonlinear susceptibility associated with intersubband transitions in the active region. 2:30 pm, 250 Robinson Hall.  Refreshments served in Robinson 251 at 2:00 pm.

Friday, April 4 Art Greene, New England Wire Technologies Corporation Use of Superconductor Materials for Construction of High Energy Accelerators and Fusion Reactors The speaker will give an overview of high energy accelerators worldwide that rely on the use of superconductor materials for their operation.  There will be descriptions of the designs of superconducting magnets and the methods to fabricate them, also including the manufacturing of the key wire and cable components.  There will also be a very brief introduction of the International Thermonuclear Experimental Reactor (ITER) to be constructed in France by seven international partners, including the U.S., and of one of the key cables to be used to construct it. Arthur F. Greene is currently the Engineering Director at New England Wire Technologies Corporation, a specialty wire and cable manufacturer located in Lisbon, New Hampshire.  Art received his Ph.D. from TuftsUniversity in 1967.  Early in his career he did research in particle physics at Argonne National Laboratory and at Fermi National Accelerator Laboratory.  While at Fermilab, he was Assistant Director for Program Planning where he was responsible for the scheduling and operation of experiments that used the accelerator beams.  Prior to joining New England Wire, he was a Senior Scientist at Brookhaven National Laboratory (BNL) where he participated in the design and construction of magnets for several international accelerators.  At BNL he headed the Magnet Division during the manufacturing phase of the superconducting magnets for the Relativistic Heavy Ion Collider (RHIC). Time/ Location TBA

Friday, April 18 --  A joint Philosophy / Physics and Astronomy Colloquium Peter Galison, Pellegrino University Professor, Harvard University   The Image of Objectivity When scientific objectivity became a goal in the early 19th century it was by no means obviously something to be desired.  Natural philosophers had to invert the old epistemic virtues that involved finding ideal forms that lay behind the variations of this or that individual.  Where genius was, plain-sight observation came to dominate.  I will here track how the images and image-making technologies of scientific atlases helped define the modern scientific category of mechanical objectivity-and the new quieted and transparent scientific self that accompanied it.  The fate of objectivity kept turning: twentieth century scientists questioned image-based, mechanical objectivity; they demanded more interpretation and modification of images than mechanical objectivity ever allowed.  With that shift came a new view of the right scientific self, one that explicitly made use of intuition, expertise, and the unconscious.  Now, in the early twenty-first century new kinds of scientific images are demanding quite unexpected ways of being a scientist-selves perched uneasily between scientific, engineering, and entrepreneurial forms of life. 3:00 pm, Nelson Auditorium, Anderson 112.  Refreshments served at 2:30 pm in Robinson Hall 251.