RESEARCH LABORATORY OF ELECTRONICS

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 the Joint Services Electronics Program (JSEP) of the Army, Navy, and Air Force; other Defense Department 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 1996 are mentioned. The interested reader can obtain further information from RLE Progress Report No. 139, which describes research activities during calendar year 1996.

ELECTRONICS AND OPTICS

MATERIALS AND FABRICATION
Professor Jesús del Alamo has developed a physics-based model that predicts off-state breakdown voltage of high-electron mobility transistors over a large temperature range and for different device designs. This model will be useful in the design of high-power transistors in extreme temperature ranges, such as those in satellites. Professor Clifton Fonstad has demonstrated for the first time a high-efficiency indium-gallium arsenide-phosphide light-emitting diode that is monolithically integrated on a custom-designed, commercially processed integrated circuit. In this way, a research foundry for optoelectronic integrated circuits is able to provide high-performance digital electronic circuits that are seamlessly combined with optical emitters and detectors. Professor Leslie Kolodziejski has devised techniques to prepare the gallium arsenide surface prior to regrowth for these optical devices, and has implemented phosphorus-containing materials in the light-emitting device structure. She has also fabricated novel channel-dropping filters and various photonic band-gap structures to manipulate light at optical frequencies in microphotonic systems. This includes the ability to guide light at 90-degree angles with no loss and with curvatures on the order of the wavelength of the light being guided.

QUANTUM-EFFECT DEVICES

Professor Henry Smith demonstrated a new paradigm for electron-beam lithography called spatial phase-locked e-beam lithography. This can achieve a pattern placement precision of approximately 4 nanometers, which is more than an order of magnitude improvement over conventional electron-beam lithographic methods. He also fabricated an array of magnetic posts in nickel with an areal density of 2.5 x 10⁹ per cm². The goal of this project is to make quantized magnetic storage devices with densities as high as 40 billion bits per square centimeter. This project is being conducted in collaboration with Professor Kamal Youcef-Toumi of the Department of the Mechanical Engineering and Professor Caroline Ross of the Department of Materials Science and Engineering. Professor Xiao-Gang Wen has investigated the dynamical properties of a coupled quantum dot that couples to one or more leads. The equation of motion for this system indicates that some electrons can tunnel through a dot via different intermediate states, and that different tunneling paths can interfere with each other. However, in some cases, the coupling to the leads may destroy this quantum interference effect inside the dot.

OPTICS AND DEVICES
Professor James Fujimoto has developed a new medical imaging diagnostic technique called optical coherence tomography. This technique functions as a type of "optical biopsy" and can produce high-resolution images of tissue microstructure. It has been applied to many biological tissues including the retina, coronary arteries, and neural tissue. Professor Fujimoto has also investigated new short-pulse generation techniques based on the use of semiconductors and other materials with a nonlinear optical response that are used to initiate short-pulse generation. Professor Peter Hagelstein demonstrated the ability to focus precisely in long, low-density laser plasmas in gas. This is appropriate for the development of gain in a neon-like laser operating at 47 nanometers, and will be useful in various applications including deep-ultraviolet lithography and new surface analytical techniques. Professor Hagelstein has also been studying anomalies in metal hydrides, where it is important to demonstrate a transfer mechanism for large energy quantum from the lattice to the atoms and nuclei embedded in the lattice. Professor Hermann Haus has been exploring transoceanic optical communication schemes. He has demonstrated a "stretched pulse" communication technique that doubles the repeater distance as compared to existing cable. He has also designed new optical filters that are approximately 10 optical wavelengths long. These filters can be combined to produce improved response. Professor Qing Hu has observed tunable terahertz emission due to intersubband transition in quantum wells. He has also performed cryogenic pump-and-probe measurements with picosecond resolution. In addition, he has built and tested a 3x3 focal plane array using superconducting tunnel junctions. This will be useful in spectroscopy and in remote sensing for astrophysics and environmental studies.

SURFACES AND INTERFACES
Professor Simon Mochrie has developed a theory for the time evolution of grooves which self-assemble on certain stepped silicon surfaces. His theory accounts for earlier observations of how the groove width varies with the one-half power of the time after a temperature jump into the grooved phase.

CIRCUITS AND SYSTEMS
Professor Anantha Chandrakasan has devised several strategies for energy-efficient computing. These include a new technique for video compression that reduces by two orders of magnitude the number of operations performed at a low-power terminal receiving images. Professor Srinivas Devadas has developed a new computer-aided design environment and a language to describe instruction sets of vastly different processor architectures. In this way, mixed hardware-software systems can be designed to include programmable processors as well as application-specific circuitry. Professor Jacob White has formulated precorrected fast Fourier transform algorithms that can rapidly simulate complicated three-dimensional structures. These techniques computed wave-induced loads on a floating airport model, and they use three orders of magnitude less memory than traditional approaches. Professor Berthold Horn has worked on very large-scale integration machine vision systems for intelligent vehicle control and image compression for video conferencing. He introduced a new method to image the interiors of translucent objects and developed a new "time to collision" warning system, based on optical flow techniques, for use in intelligent vehicle control.

LANGUAGE, SPEECH, HEARING, AND SENSORY COMMUNICATION

SPEECH COMMUNICATION
Professor Kenneth Stevens has examined the significant variability that exists in certain consonant sounds in English in different contexts. He has explained this variability in terms of context-conditioned differences in movements of the tongue and other articulatory structures. Dr. Stefanie Shattuck-Hufnagel has been studying the intertwined effects of speech production planning, including the prosody and error patterns in spoken utterances. She conducted a major review of recent prosodic literature and has shown that the information in spoken sentences is substantially different from that in written sentences. She is also building a database that will facilitate the analysis of prosodic constraints on speech error occurrence, detection, and correction. Dr. Suzanne Boyce completed a major study of vocal tract shapes for the American English /r/ by using dimensions measured by magnetic resonance imaging within the context of a vocal tract computer model. Dr. Reiner Wilhelms-Tricarico has been building a computer program to generate a refined and accurate model of the tongue and the mouth floor by finite elements. This project will help to generate finite-element representations of other human organs.

SENSORY COMMUNICATION

Dr. Mandayam Srinivasan has developed high-resolution mechanistic models of the human fingerpad. These models confirmed earlier results, which showed that slowly adapting mechanoreceptors of the fingerpad encode the shapes of objects that indent the skin by responding to strain energy density at their spatial location.

AUDITORY PHYSIOLOGY

Professor Dennis Freeman has developed a computer microvision system that measures nanometer motions of micrometer-sized targets. In this system, a light microscope projects magnified images of a moving target onto a charge-coupled device camera. Initially, the system was used to study the motion of stereocilia in cochlear hair cells, but it has also proven useful in the study and characterization of manufactured microelectromechanical systems such as miniature gyroscopes. In order to integrate our understanding of signal processing in the ear across vertebrate species, Professor William Peake studied the structure and acoustic function of the middle ear for all species of the cat family. He measured the acoustic properties of a lion ear in a postmortem specimen obtained from a local zoo. Dr. John Rosowski studied the changes in middle-ear mechanics produced by perforations in the human tympanic membrane, such as those introduced by tympanostomy tubes that are used to alleviate middle-ear infections.

FOCUS AREAS

ATOMIC, MOLECULAR, AND OPTICAL PHYSICS
Professor Shaoul Ezekiel and Dr. Selim Shahriar have developed spectrally multiplexed holographic memories aimed at increasing the processing and storage capacities of computers. The projected capacity of such a memory is 10¹⁶ bits per cubic centimeter. Professor Wolfgang Ketterle has produced an output coupler for Bose-Einstein condensed atoms. This was an essential step towards the realization of an atom laser. In addition to observing interference between two Bose condensates, he demonstrated that these condensates could be released from their magnetic trap and still produce interference. This production of coherent atom beams was the achievement of a basic atom laser. The atom laser provided the first direct evidence for the coherence of Bose condensates and proved the existence of long-range correlations. Professor Daniel Kleppner has been extending our understanding of the connections between quantum mechanics and classical motion by using a new technique involving recurrent spectroscopy in a microwave field. These experiments provide evidence for one case in which quantum mechanics can describe detailed classical motion, a result that had been anticipated but not justified in theory or experiment. Professor David Pritchard demonstrated new scientific and technical applications for atom interferometers. These include the studies of fundamental physics and the measurement of atomic and molecular properties. He has shown that these atom interferometers are extremely sensitive to rotations. It is expected that an atom interferometer designed specifically for rotation sensing could perform better than the best laboratory laser gyroscope currently available.

PLASMA PHYSICS
Professor Abraham Bers and Dr. Abhay Ram continue to explore the coherent acceleration of ions in a magnetized plasma that is subjected to multiple electrostatic waves. A new nonlinear acceleration mechanism has been discovered. Among the several applications for this discovery is the possibility to slow down energetic ions, which may provide a means to extract energy from fusion-generated alpha particles. Professor Bruno Coppi has been studying a variety of plasma phenomena, and proposed the Ignitor experiment to study magnetically confined ignition. The production of an ignited plasma would be an important achievement, and would be best accomplished by using machines that operate at high magnetic field and employ cryogenic normal conducting magnets. The theory for this experiment was developed by Professor Coppi's research group.

RADIO ASTRONOMY
Professor Jacqueline Hewitt has been studying gravitational lenses. She has also been conducting searches for radio astronomical transients and for radio counterparts to gamma-ray bursts. Recent evidence of coincident events at multiple detectors is now being verified with additional detectors. In order to obtain profiles of temperature and moisture in the atmosphere, Dr. Philip Rosenkranz has modeled microwave transmittance in the atmosphere and the inversion of microwave radiometer measurements taken from satellites or aircraft. His findings have led to the construction of a microwave instrument for new high-altitude research aircraft that will provide accurate temperature and precipitation measurements.

DIGITAL SIGNAL PROCESSING
Professor Gregory Wornell has introduced new classes of error-correcting coding techniques for use in wireless communication applications. One is a novel class of analog error-correcting codes based on nonlinear dynamical and chaotic system theory that are well suited to broadcast and cellular applications. The other is a powerful class of ultralow-complexity adaptive codes for use in low-power wireless communication links. Professor Wornell has also developed new, computationally efficient techniques to jointly exploit time, frequency, and space diversity in wireless communication systems.

ADVANCED TELEVISION AND SIGNAL PROCESSING
Professor Jae Lim continues his research on the design of a digital high-definition television system. A design specification was created by the Grand Alliance, which is a consortium of organizations, and its design was accepted by the Federal Communications Commission. Plans are now being made for the implementation of this design, and receivers will be manufactured for public sale soon.

ELECTROMAGNETICS
Professor Jin Au Kong continues his research in many areas of electromagnetic theory and applications. These include: remote sensing of the Earth and its environment, computer simulation of synthetic aperture radar returns from Earth terrain, electromagnetic interference and compatibility, microwave and millimeter-wave integrated circuits and interconnects, and simulation and analysis applied to precision aircraft landing systems.

OPTICAL COMMUNICATIONS
Professor Jeffrey Shapiro and Dr. Ngai Chuen Wong continue their research in quantum optics and radar-based automatic target recognition. They completed a study of local oscillator selection to optimize quadrature noise squeezing in homodyne detection. This study predicts a new regime of nonclassical light generation in optical fiber (Raman squeezing) and has led to a search for a comparable squeezing regime for Brillouin scattering. Professor Shapiro has also developed a wavelet-based approach for maximum likelihood estimation of laser radar range imaging.

INDIVIDUAL RESEARCH
Professor Donald Troxel continues research on distributed design and manufacture techniques. The goal in this area is to develop tools that will enable cooperative design and manufacture at multiple fabrication facilities. In one example, a personal computer at one facility remotely controls a microscope at another facility so that the results of fabrication processing can be examined at a distance.

AFFIRMATIVE ACTION
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 and to being more open to suggestions in this area. During the past year, four searches were conducted, which resulted in the hiring of two female employees.

More information about the Research Laboratory of Electronics can be found on the Worldwide Web at the following URL: http://rleweb.mit.edu /

Jonathan Allen

MIT Reports to the President 1996-97