Robert Berg
Judith Brown
Theodore W. Ducas
Yue Hu
William W. Quivers, Jr.
Glenn Stark
Go to the Physics Department Home Page
Robert Berg - Experimental Semiconductor Physics (email Professor Berg)
My current research interests are centered on developing new computational tools for use in science education. In 1996 I was a Visiting Professor in the Epistemology and Learning group at the MIT Media Lab and I continue to collaborate closely with the group, working on the creation of a new generation of "programmable bricks" called Crickets. The LEGO Mindstorms product, which was released in the Fall of 1998 by the LEGO company, was inspired by our group's work on programmable bricks.
With Mitchel Resnick (MIT) and Mike Eisenberg (Colorado), I lead an NSF-funded project called Beyond Black Boxes, in which children are using Crickets to design their own instruments for scientific investigations. During the summers of 1997 and 1998 we hosted at Wellesley a Beyond Black Boxes Workshop for more than 30 project participants from around the country. I serve as an advisor for a project that is in progress at the Computer Clubhouse located at the Patriots Trail Girl Scout Council headquarters in Boston. The project, which is being funded by a three year grant from the Massachusetts Cultural Council, is based on our Beyond Black Boxes effort and will involve girls in building their own instruments for scientific investigations.
Optical Spectroscopy. I have a long-standing interest in the optical properties of semiconductors. More recently, working with Tom Bauer and a number of Wellesley students, I have been working on a project that will use narrow band-width diode lasers to trap and cool rubidium atoms. I am also interested in getting started using the "optical tweezers" setup that has been constructed by Ted Ducas and Janet Lee '99.
Nuclear Magnetic Resonance Spectroscopy - Melaine Trecoske, Nina Schwartz '95 and I have built a Fourier transform NMR spectrometer for use in the Physics Department's advanced laboratory course (Physics 349).
Recent Student Projects:
Elaine Ulrich '01 is currently working on using narrow band-width diode lasers to trap and cool rubidium atoms.
During 1998, Ted Ducas and I served as advisors to Gretchen Campbell '01, Jenny Ross '00, Ann Sanders '01 and Tyler Wellensiek who participated in NASA's Reduced Gravity Student Flight Opportunities Program.
Ann Hinztman '99, a cognitive science independent major, built a robot which competed in Trinity College's annual Fire-Fighting robot contest in April, 1999. (See http://www.trincoll.edu/~robot/). I would love for a team of Wellesley students to enter the contest in Spring, 2000.
Ava Erickson '99 worked on many aspects of The Beyond Black Boxes project over the last three years. Most recently she work with Vanessa Collela and Fred Martin at MIT and myself to design a new type of Cricket for use "participatory simulations". (See http://vanessa.www.media.mit.edu/people/vanessa/part-sims/)
I also serve as a mentor at the Computer Clubhouse, an after-school learning center where youth from under-served communities work together on computer-related projects. The goal is for youth to become fluent with new computational media, becoming creators (not just consumers) of computer-based projects.
Return to Physics Faculty List
Judith Brown - Machine Perception of Musical Features (email Professor Brown)
My general interest is in machine perception of music, that is to what extent is the information contained in musical signals and musical scores accessible by computer. My initial interest in this area was in pitch tracking by computer both in the frequency domain and in the time domain and in pitch perception by humans. Other problems I've studied have included the use of autocorrelation to determine musical meter, pitch perception of frequency modulated musical signals by skilled performers, harmonicity of musical instruments, limits of accuracy in performances and in pitch perception by skilled musicians, and the effects of noise on accuracy of calculations using the phasevocoder. Publications are listed on my Wellesley web page.
Recent Research Projects:
Pitch perception of frequency modulated signals by skilled musical performers. Listening experiments were carried out on musicians to determine how sounds played with vibrato are perceived. It was found that it is the mean of the frequency variation which is perceived, and surprisingly the frequency difference limen is identical for sounds with and without vibrato.
Harmonicity of musical instruments Calculations were carried out on instruments including the viola, violin, cello, piano, flute, alto clarinet, and human voice to determine exact frequency ratios of upper harmonics to the fundamental. For the instruments producing sustained notes (all but the piano), exact integer ratios were found to within the accuracy of the calculation.
Accuracy of frequency estimates using the phase vocoder (with Miller Puckette) We examined the effect of varying the window size and function, and the hopsize on the accuracy of calculations using the phase vocoder. Of particular interest is the case of hopsize equal one sample for which we had previously found a computationally efficient approximation.
Musical instrument identification Calculations have been carried out using cepstral coefficients and autocorrelation coefficients as features to distinguish between sounds produced by oboes and by saxophones. These computer calculations had a higher success rate than human perception experiments on the same sounds. This work has recently been extended to include the flute and the clarinet.
Return to Physics Faculty List
Theodore W. Ducas - Laser Physics and Physics of Biological Systems (email Professor Ducas)
My research in Physics has been largely in the area of laser spectroscopy of atoms and molecules. In addition to my work with students in my laboratory at Wellesley, I have collaborated with the group of Prof. Daniel Kleppner at M.I.T. The work at M.I.T. has centered around the study of highly excited atomic states : "Rydberg States". Exciting particular Rydberg levels with tunable dye lasers, we have investigated field ionization, induced radiative transfer, interaction with blackbody radiation, lifetimes and the modification of the interaction between atoms and photons induced by a resonant cavity. Current work is centered on precision spectroscopy on circular Rydberg states of Hydrogen.
At Wellesley, I have worked with students on a wide variety of research projects, reflecting their particular interests and the extensive range of the applications of physics. In atomic physics and modern optics students have worked on such topics as construction of tunable lasers, optogalvanic atomic spectroscopy, atomic radiative lifetimes, and pulse propagation through optical fibers. One astrophysics major worked on modeling propulsion systems for interstellar travel. I supervised an M.I.T. student's undergraduate thesis on mathematical modeling on how porpoises might extract detailed spatial information using echolocation.
A number of students have worked on projects in biophysics or biomedical engineering. Some of this work has been jointly supervised with faculty at M.I.T. or in the Harvard-M.I.T. Health Sciences and Technology Program. For example, an honors thesis student worked with a CO2 laser and phase sensitive detection apparatus to apply photoacoustic spectroscopic techniques in the study of normal and pathological animal tissue. This work was performed in collaboration with a team from M.I.T. and the Boston University School of Medicine. Another honors student worked on the application of multiwire proportional chambers (used in nuclear physics) to quantifying bone mineralization. A recent student project involved construction of an optical system as part of a new technique being developed at the Harvard School of Public Health aimed at measuring oxygenation of hemoglobin during operations. Currently, an honors student is working at the Harvard Medical School investigating possible resonance effects of electric fields on cell membranes.
Return to Physics Faculty List
Yue Hu (email Professor Hu)
My research interests fall into the general category of complex fluids. The systems that I have studied are colloids -- mixtures of small undissolved particles suspended in other surrounding substances. I have conducted experiments, theoretical work, and computer simulations on the dynamic and dielectric properties of colloids in alternating electric fields.
Experimental Observation of Electrohydrodynamic Instabilities - When an alternating electric field is applied to an aqueous suspension of micron-size polystyrene particles, the particles become polarized and are attracted to each other. Earlier theories using simple electric dipole models to explain the interactions between the polarized particles predicted the formation of chains and columns of particles in the direction of an applied electric field. Our experiments, however, have revealed other much more complex polarization possibilities. We have observed anomalous hydrodynamic instabilities in certain frequency ranges of the applied field, where instead of forming chains and columns, particles form zig-zag bands that are oriented in different directions from the applied field, with particles circulating continuously within a given band.
A Phenomenological Model for Observed Electrohydrodynamic Instabilities - In this phenomenological model, we assume that the particles acquire a spontaneous spinning motion about an axis perpendicular to the applied electric field. As a result of the spinning, the polarization of the charge layer around a particle (double layer) becomes asymmetric with respect to the applied field, and the particle dipole moment and the applied field are not in the same direction. Computer simulations based on this model are able to reproduce the experimentally observed instabilities, including the formation of bands tilted relative to the direction of the applied field, and circulation of particles within a band.
Modeling the Polarization of the Double Layer - A double layer consists of a loosely bound diffuse layer and a tightly bound Stern layer. The Stern layer is further divided into inner and outer Helmholtz layers. Most of the recent models of the double layer have focused on the diffuse layer and have studied the dielectric properties of the suspensions and the electrodynamic properties of the particles. The diffuse double layer models work well for smooth particles like silica but fail to predict the dielectric properties of "hairy" particles like amphoteric latex particles.
I have proposed a model that concentrates on the inner Helmholtz layer. Ions in this layer are treated as discrete point charges, and their motion is simulated using a biased random walk method. In the limit where the relaxation frequency of this layer is much smaller than those of the outer layers (outer Helmholtz and diffuse layers), this model produces excellent fits to a set of dielectric dispersion data* that previous theories had been unable to explain.
This model offers insights into the polarization of surface charges at an ion-to-ion level. Furthermore, because linear response approximations -- which are only valid for weak fields -- are not necessary in this model, a series of non-linear phenomena are predicted. This is particularly relevant for the study of electrorheological fluids, where the applied electric fields are generally very strong. Spontaneous Particle Spinning and Its Effects on Electrohydrodynamic Properties of Colloidal Suspensions A direct consequence of the inner Helmholtz layer model is the demonstration that particles can go into a state of spontaneous spinning when the frequency and the strength of the applied field satisfy certain conditions. This provides a theoretical basis for the assumption adopted in my earlier phenomenological model, and this is also consistent with the experimental observation of electrohydrodynamic instabilities occurring over a wide range of frequencies for a given sample. I am currently conducting more experiments to systematically investigate this phenomenon. I also plan to explore the possibility of probing the inner Helmholtz layer on a microscopic level using NMR techniques.
Return to Physics Faculty List
William W. Quivers, Jr. - Laser Physics (email Professor Quivers)
I am a theorist whose general area of research is laser spectroscopy of atoms. Specifically, I have developed a model that describes laser optical pumping in multilevel atomic systems that undergo velocity changing collisions. This model has been employed in the in the sub-field of laser-nuclear science to study the properties of excited nuclei. For example, it's been used in laser-induced nuclear orientation experiments, which produced first-time measurements of the nuclear magnetic and electric quadrupole moments of the 1- ms rubidium-85 isomer. In addition, it has been utilized in atom-atom collision studies.
Most recently, I have been working in the field of cavity quantum electrodynamics. In particular, I've worked with Professor Michael Feld's single-atom laser group at MIT. This group, in fact, was the first to develop such a device. Presently, I'm working on calculating the single-atom laser lineshape.
Return to Physics Faculty List
Kanwal Singh (email Professor Singh)
The main focus of my research has been interacting Bose systems; my doctoral dissertation was primarily on the "dirty Boson" problem. That is, thin films adsorbed on various disordered substrates (such as glasses) are cooled until they undergo a phase transition to the superfluid state. Extrapolating the data to absolute zero indicates there is a finite density required for the system to be superfluid. In other words a sample below some critical density (which is roughly a monolayer) can never be superfluid. This zero-temperature phase transition is entirely driven by quantum effects, including boson-boson interactions and the interaction between the bosons and the random background potential provided by the substrate. These quantum fluctuations also dominate the finite, but low temperature transitions. I have done theoretical work modeling these systems with a Hamiltonian that included not only the standard kinetic energy and boson-boson interaction but also a term that represented the substrate via a random background potential. Not surprisingly, the presence of the random potential is precisely what makes the problem so difficult. Specifically, the interplay between the boson-boson repulsion and the attractive disordered background contains all the subtleties of the phase transition.
Since leaving Berkeley I have also applied some of the techniques developed in my dissertation to other problems. My primary focus recently has been the Bose condensed gases first produced by a group at the Joint Institute for Laboratory Astrophysics (JILA) in 1995, and soon after at MIT and Rice University. The various experimental groups have all used trapping potentials which are different in shape and size, and the precise effects of the trap on the condensate are not well understood. Other interesting questions for the future include the effect of different types of atoms in one condensate, as well as the interaction between condensates of different ground states.
In addition to my work on low-temperature Bose systems, my teaching philosophy has recently led me to think about a new research project. I have long been frustrated by the limited exposure that undergraduates typically have to current problems in research. Hence I have become intrigued by presenting topics of current research interest in physics to the junior or senior level undergraduate, who has not necessarily worked with sophisticated mathematical techniques and/or who has not yet had a course in quantum mechanics. I am now starting to work on the first of a series of articles designed to introduce topics of current research interest to the intermediate/advanced undergraduate.
As of now, the topics I am planning to undertake fall into the area of condensed matter/low-temperature physics, such as superfluidity, superconductivity (both "ordinary" and high temperature), laser cooling and trapping, Josephson effects and the ensuing technological applications, Helium-3, etc. I anticipate that as this project matures, the topics will broaden significantly in scope.
Return to Physics Faculty List
Glenn Stark - Experimental Molecular Spectroscopy (email Professor Stark)
My research is in the field of experimental molecular spectroscopy. My laboratory programs emphasize, but are not limited to, molecular transitions of interest to the astrophysical and aeronomic communities. The bulk of my work has involved the measurement and interpretation of high-resolution absorption spectra of vacuum ultraviolet (100 - 200 nm) and extreme ultraviolet (50 - 100 nm) transitions. I am also active in the field of Fourier transform spectroscopy of diatomic molecules, and I have a continuing interest in laser spectroscopies of diatomics.
Primary Research Projects:
1. In the past ten years I have directed major efforts toward laboratory studies of the EUV and VUV absorption spectra of carbon monoxide, molecular nitrogen, and sulfur dioxide. I have measured and analyzed absolute photoabsorption cross sections (or equivalently, transition probabilities or oscillator strengths), line widths, and wavelengths of molecular features that are needed for the interpretation of the photophysics and photochemistry of both astronomical sources and the Earth's atmosphere. Most recently, I have been concentrating on predissociating transitions in N2 (80-100 nm) and on the complex SO2 absorption bands near 200 nm. Both of these projects are supported by a three-year NASA Planetary Atmospheres grant to Wellesley College. The N2 measurements are being carried out at the Photon Factory synchrotron facility in Tsukuba, Japan, in collaboration with colleagues at the Harvard-Smithsonian Center for Astrophysics, the National Research Council of Canada, and the Photon Factory. The SO2 measurements are performed on the VUV Fourier transform spectrometer at Imperial College, London.
2. I have used Fourier transform emission spectroscopy to study the high-resolution spectra of a number of diatomics, including OH/OD, C2, and N2+. Work in the visible/near uv is carried out on the 1-meter FT spectrometer in the McMath Solar Observatory at Kitt Peak National Lab; VUV transitions are studied on the 0.2-meter FT spectrometer at Imperial College, London.
3. I spent my last sabbatical leave (1996) at the Australian National University, working in the labs of Dr. Brenton Lewis. There, we used high-resolution, tunable VUV laser systems to study transitions in CO and O2. The CO work concentrated on transition strength measurements, while the O2 work focused on the spectroscopy of high-lying Rydberg states in that molecule.
4. I have also been active in the development of a laser lab facility at Wellesley. To date, I have advised three senior theses and a number of summer students in this lab. We have designed and constructed a supersonic expansion jet apparatus, and have studied, mostly as a test case, the laser induced fluorescence of molecular iodine cooled in the expansion.
Return to Physics Faculty List
Phat Vu (email Professor Vu)
Research in experimental low temperature condensed matter physics: using lattice vibrations to study structure-property relationships of thin films; measurements of thermal conductivity and/or internal friction between 0.05 to 500K in various cryostats; sample preparation employs technology of the small (e.g., nanofabrication) as well as technology of the large (e.g., particle accelerators); sample diagnostics include electron microscopy, atomic force microscopy, x-ray diffraction, etc.
Special student project: exploring diffraction phenomena with a two-dimensional array of quantum dots; I have fabricated phonon diffraction gratings and would like to use such gratings for photons, i.e., to check for optical diffraction; the goal is to make some pictures, compare with the catalog of diffraction phenomena, and to publish (with a student) in the American Journal of Physics; some occasional travel to the Cornell Nanofabrication Facility may be necessary, providing exciting exposure to nanotechnology in a clean room environment.
Return to Physics Faculty List