MIT Reports to the President 1998-99
The Research Laboratory of Electronics (RLE), the Institute's oldest interdisciplinary research laboratory, was founded in 1946 as the natural evolution of the wartime MIT Radiation Laboratory. Initially, RLE was formed to bring together interests in physics and electrical engineering to work on problems in electromagnetic radiation, circuits, and specialized vacuum tubes. Over the years, RLE's research interests have branched out in many directions and, in fact, several of these interests have precipitated the formation of additional laboratories. Research within RLE is conducted by approximately 55 faculty members who are affiliated with the Departments of Electrical Engineering and Computer Science, Physics, Chemistry, Materials Science and Engineering, Aeronautics and Astronautics, and Linguistics. During the past year, approximately 200 graduate students and 75 undergraduates have worked on research projects within RLE. Major support is derived from Department of Defense (DoD) agencies; the Department of Energy (DOE); the National Science Foundation (NSF); the National Institutes of Health (NIH); and the National Aeronautics and Space Administration (NASA). In addition, many research projects are funded through industry and private foundations. Although RLE has a very heterogeneous character, its organization is composed of two major thrusts and several smaller focus areas. One major thrust is centered on electronics and optics, and the other is centered on language, speech, hearing, and sensory communication. Each of the smaller focus areas often has substantial overlap with other research in RLE.
In the following remarks, several research highlights from 1998 are mentioned. The interested reader can obtain further information from RLE Progress Report No. 141, which describes research activities during calendar year 1998.
ELECTRONICS AND OPTICS
Professor Jesús del Alamo studied the hydrogen-induced degradation of indium phosphide high-electron mobility transistors. He found that hydrogen reacts with titanium in the gate structure of these devices to produce titanium hydride. This introduces a tensile stress on the semiconductor underneath the gate and, in turn, leads to a shift in the threshold voltage that affects all device characteristics.
Professor Leslie Kolodziejski used several epitaxial growth techniques to produce devices with innovative behaviors. Novel rectangular patterned surfaces were used in new distributive feedback lasers. A bipolar quantum cascade laser design with high external efficiency was investigated. A new photonic bandgap microcavity waveguide with an airbridge geometry may be helpful in demonstrating "threshold lasing."
Professor Marc Kastner continues to study electronic transport through single-electron transistors close to the localized-delocalized transition. His ongoing study of the Kondo effect in these transistors will be important in attempts to use quantum dots for quantum computing.
Professor Henry Smith continues a wide range of research activities in the NanoStructures Laboratory that focuses on nanolithography, electronic devices, and optical devices. He developed a new maskless form of lithography called zone plate array lithography. This will greatly reduce the cost of ultraviolet, deep ultraviolet, and x-ray lithography techniques. It will also enable a wider variety of users to fabricate nanostructures.
Professor Xiao-Gang Wen studied tunneling current through quantum dots under various experimental conditions. He found that scattering in the quantum dot is a "many-body scattering," which is very different from the scattering of free electrons. Thus, an incoming electron can be scattered into an electron as well as many particle hole pairs. A quantum-dot system behaves much like an atom and, because of the new probes and controls available only in quantum dots, many new physical phenomena can be studied using these systems.
OPTICS AND DEVICES
Professor James Fujimoto continues to develop optical coherence tomography (OCT) and its applications in biomedical imaging. By increasing resolution, he demonstrated cellular-level imaging in OCT. For his work in this area, Professor Fujimoto received the 1999 Discover magazine award for Technology Innovation in Medical Diagnosis. In collaboration with Professor Hermann Haus, Professor Erich Ippen, and Professor Franz Kaertner, Professor Fujimoto also demonstrated ultrashort laser pulses of less than 5.5 femtoseconds, which correspond to approximately two optical cycles of light. These ultrashort pulses will be used in future OCT systems to measure extremely short distances in biomedical systems.
Professor Peter Hagelstein collaborated with Professor Hermann Haus on new configuration space models for quantum-optic soliton propagation in optical fibers. These models have led to a new interpretation of the Gordon-Haus effect, which describes the center of mass jitter that results from the loss or gain of a single discrete photon. Their work shows that the new configuration space models use a simpler formulation to solve nonlinear quantum-optic problems than currently used models.
In new computer modeling studies, Professor Hermann Haus demonstrated that photonic bandgap-like structures can be used in high-density integrated optics. Additionally, he has shown that the proper design of photonic bandgap-like structures can make leakage radiation negligible, thus enabling high-density optical integration. He also demonstrated that ninety-degree bends in photonic bandgap-like structures are possible with negligible radiation. Furthermore, waveguide crossovers can be designed with very low crosstalk (less than 30 dB). For example, a 100x100 optical routing network could be placed on a standard wafer.
As mentioned previously, Professor Erich Ippen collaborated with Professor James Fujimoto and Professor Hermann Haus to set a new record for the shortest optical pulses obtained directly from a laser. Professor Ippen also developed new ultrashort-pulse optical fiber lasers used in optical networks and precise measurement systems.
Professor Qing Hu built and tested a 3x3 170 to 210 gigahertz micromachined superconducting sensor array. This array demonstrated high-performance receiver technology in the millimeter-wave, terahertz, and infrared regimes. He continues to develop sources in the terahertz range and has observed a narrow-linewidth, terahertz intersubband emission from a three-level system.
SURFACES AND INTERFACES
Professor John Joannopoulos continues to use theoretical total energy calculations to explain various phenomena. For example, oxygen diffusion in silicon can be enhanced by introducing hydrogen. Because of these studies, a two-step process showing the migration of the oxygen and hydrogen complex was demonstrated. The initial step shows the oxygen atom jumping between two adjacent body-center configurations and the simultaneous formation of an intermediate metastable state that corresponds to a hydrogen-saturated silicon broken bond. In the second step, the defect is annealed and the hydrogen atom jumps to a stable body-center configuration, which is in agreement with experimental observations of the activation energy.
Using theoretical techniques, Professor Joannopoulos has also shown how to design a novel class of optical materials for potential monolithic integration with silicon. He designed a new semiconductor alloy that should eliminate problems of charge mismatch, provide the desired lattice constant, and possess a direct bandgap at optical fiber communication bandwidths. This is particularly important since it is the discovery of a new class of materials that have not yet been fabricated, but which have highly desirable properties for several applications.
CIRCUITS AND SYSTEMS
Professor Anantha Chandrakasan developed an ultralow-power processor for embedded sensors intended for medical monitoring. The goal is to minimize energy dissipation so that self-powered techniques can be used. His new processor provides approximately six orders of magnitude improvement in energy utilization by optimizing algorithms, architectures, and circuits.
Professor Jacob White developed several fast numerical algorithms to characterize integrated circuit interconnect and packaging, micromachined devices, and offshore structures. For example, the FAST MEMs program is useful for coupled electromechanical simulation. He also developed a new version of WAMIT, an offshore structure analysis widely used in industry.
Professor John Wyatt continues fundamental studies for an implantable ocular prosthesis that aims to provide some useful vision to blind individuals with outer retinal degeneration, such as macular degeneration or retinitis pigmentosa. A detailed study shows that the electrical stimulation threshold for retinal axons increases when the applied electric field is perpendicular to the axons. This finding may provide a mechanism that allows targeted cells to be stimulated while avoiding the stimulation of peripheral cells through their axons.
LANGUAGE, SPEECH, HEARING, AND SENSORY COMMUNICATION
Professor Kenneth Stevens studied the acoustic characteristics of dysarthric speech and how it introduces deviations of vowels from normalcy. This research isolates certain acoustic attributes that are specially marked for speakers with these neurogenic disorders. Professor Stevens also examined patterns of respiration during speech production. He developed models that describe how the acoustic patterns of speech are constrained by the requirements of respiration.
Dr. Joseph Perkell continues to study strategies for speech motor control, how they are influenced by properties of the production mechanism, and the role of self-hearing. Evidence was found to support the hypothesis that speech movements are programmed to achieve auditory or acoustic goals.
Dr. Stefanie Shattuck-Hufnagel investigated several effects in speech related to prosody. In collaboration with Dr. Anne Cutler, she showed that word-level errors in spontaneous speech are significantly more likely to be corrected with marked prosody than are individual sound-level errors. She also illustrated how the boundary between two content words is cued by the relative durations of the surrounding syllables. Ongoing studies are also consistent with the view that the perception of prominence in a spoken utterance is not determined solely by the fundamental frequency pattern, but rather by a combination of factors. These factors include fundamental frequency, perceived word organization, and knowledge of the language spoken.
Professor Louis Braida, Senior Scientist Nathaniel Durlach, and Principal Scientists Charlotte Reed, Mandayam Srinivasan, and Patrick Zurek continue to conduct basic perceptual experiments on hearing and touch. They continue to develop applications in the areas of aids for the handicapped, human-machine interfaces, and virtual environment training systems. Major foci of the group's efforts include: the analysis and modeling of normal and impaired-hearing; the psychophysics of touch; the study of way finding and navigation; spatial filtering using multi-microphone adaptive arrays; the perceptual integration of stimuli received through different sensory channels; and hardware and software that enable human users to manually sense and manipulate virtual (computer-generated) objects.
Professor Dennis Freeman developed a new form of synthetic aperture microscopy. The target is illuminated by finely structured light that results from the interference of many laser beams. The structured light allows high-resolution pictures to be obtained from a sequence of images gathered by a low-resolution imager. Thus, the resolution of a conventional imager can be boosted by hundreds to thousands of subpixels per physical pixel. This work is significant because it changes the scaling laws for optical imaging. In particular, it is important for applications that require long working distances and large depths of field, such as semiconductor manufacturing.
Dr. Bertrand Delgutte studied the underlying mechanisms that explain why people with normal hearing excel at hearing a sound of interest (such as a voice or musical instrument) among competing noises, particularly when the sound and the noise originate from different positions. He found a class of neurons in the auditory midbrain that can better detect a signal in noise when the signal and noise are spatially segregated, rather than when they occupy the same position. Such neurons are likely to play an important role in hearing sounds of interest among competing noise.
Using two laser beams, Dr. Selim Shahriar and Professor Shaoul Ezekiel demonstrated experimentally that the spin-based quantum bits in a diamond crystal can be rotated by any desired angle. This is a major step toward realizing a many-bit quantum computer. Dr. Shahriar used an optical blazed-grating potential to split an atomic wave generated by trapped rubidium atoms. This technique is the basis for atom interferometry used in nanolithography and rotation sensing. The interference signal can be used to provide rotational sensitivity more than four orders of magnitude better than the best optical or atomic gyroscopes currently used. Thus, measurement of the general relativistic frame dragging effect may be possible.
Professor Wolfgang Ketterle continues to study the properties of Bose-Einstein condensates in various environments. Light scattering at small angles does not impart enough momentum to the condensate to create a recoiling atom. Instead, a sound wave is created by optically imprinting phonons into the gas. A sound wave is a collective excitation of all atoms in the system. Therefore, atoms cannot act as individual atoms, but instead must show correlated motion. This was verified by observing a significant decrease in the rate of light scattering in the phonon-regime.
Professor David Pritchard focuses on making the most accurate atomic mass measurements in the world. He achieved accuracies (typically 10-10) one to three orders of magnitude beyond previously accepted values. He also achieved new determinations of the molar Planck constant, new determinations of the fine structure constant, and reference masses for mass measurements of radioactive nuclei that are important for testing models of astrophysical, heavy element formation. The accurate determination of the atomic weight of silicon opens the way for an atomic standard of mass by replacing the "artifact" kilogram mass standard with a crystal of pure silicon.
Professor Abraham Bers studies heating and current drive in magnetic confinement fusion glasses, ion energization, in space plasmas, and intense laser-plasma interactions in inertial fusion experiments. A new mechanism for observed ion energization (from the ionosphere to the magnetosphere) was proposed and is being evaluated using current rocket and satellite data.
Professor David Staelin developed a neural network that estimates precipitation rates based on passive microwave images obtained by the Advanced Microwave Sounding Unit. In this way, the sounding unit can determine precipitation accuracy of approximately 1.1 millimeter per hour and 50-kilometer spatial resolution, as well as excellent agreement with respect to precipitation location. Over most of the globe, precipitation can now be mapped twice daily with comparable accuracy by each observation satellite.
Professor Gregory Wornell developed a new iterative class of robust digital watermarking algorithms called quantization index modulation. These algorithms are useful for copyright notification and enforcement of multimedia audio and video content distributed in digital formats. Professor Wornell also developed a powerful, new class of iterative equalization techniques that compensates for the severe multipath propagation effects that occur in increasingly wider band digital wireless systems.
The need to broadcast high-definition television at resolutions higher than those permitted by the current standard was addressed by Professor Jae Lim. He investigated several methods to transmit video at these higher resolutions. In addition, he studied new video compression methods that use preprocessing techniques, which increase the efficiency of the video compression.
Professor Jin Au Kong's research is focused on application areas related to electromagnetic wave theory. These include the remote sensing of the Earth, computer simulation of synthetic aperture radar returns from Earth terrain, electromagnetic interference and compatibility, and microwave and millimeter-wave integrated circuits and interconnects.
Professor Jeffrey Shapiro conducted several studies in quantum optics and examined the interplay between nonlinearity, dispersion, and noise in optical fiber squeezed state generation. Dr. Ngai-Chuen Wong studied novel nonlinear optical devices and their applications to quantum optics, optical frequency metrology, and optical communication networks. Recent theoretical results were applied to the design of an optical frequency divider based on self-phaselocking, which can be used in high-precision optical frequency metrology.
Professor Donald Troxel designed a new user interface for the microvision system developed by Professor Dennis Freeman. He also worked with Professor Carl Thompson to develop a tool for very large system integration (VLSI) designers. This tool would assess the reliability of the integrated circuit design in relation to electromigration problems, and would be useful in other types of other types of integrated circuit reliability problems.
RLE has worked and will continue working to increase the number of women and minorities in career positions in the laboratory, in the context of the limited pool of qualified technical applicants and the unique qualifications of RLE's sponsored research staff. Specific measures will include: (1) maintaining our high standards for recruitment procedures that include sending job postings to minority colleges and organizations; (2) working closely with the RLE faculty/staff supervisor at the beginning of each search to identify ways of recruiting minority and women candidates for the new position; and (3) being committed to finding new techniques to identify more effectively women and minority candidates. During the past year, due to limited turnover in RLE's research staff, success in affirmative action for research staff has been limited. Of the two research staff appointments made this year, one was made to a woman.
More information about the Research Laboratory of Electronics can be found on the World Wide Web at http://rleweb.mit.edu/.
MIT Reports to the President 1998-99