Research Laboratory of Electronics
The Research Laboratory of Electronics (RLE), founded in 1946, is the Institute's first interdisciplinary research laboratory. Today it is the largest such laboratory at MIT in terms of faculty and student participation. RLE grew out of the wartime MIT Radiation Laboratory and was formed to bring together physicists and electrical engineers to work on problems in electromagnetic radiation, circuits, and specialized vacuum tubes. Over the years, RLE's research interests have branched in many directions and have led to the creation of additional laboratories. Research within RLE is conducted by approximately 40 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 250 graduate students and 60 undergraduates pursued research within RLE. The research is supported primarily by: the 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, numerous projects are funded through industry and private foundations. RLE research is widely varied and consists of two major thrust areas and several smaller focus areas. One thrust area is centered on electronics and optics; the other is centered on language, speech, hearing, and sensory communication.
Detailed information about RLE research in the calendar year 2000 can be found in the RLE Progress Report No. 143. The report can be accessed electronically at http://rleweb.mit.edu/Publications/prog.htm. We summarize here research highlights during the past year.
Materials And Fabrication
Professor Leslie Kolodziejski has demonstrated a 100x enhancement in the extraction efficiency of a light-emitting diode by incorporating a two-dimensional photonic bandgap (PBG) structure into the top cladding layer of the diode. This work, which was done in collaboration with Professors John Joannopoulos, Erich Ippen and Henry Smith of RLE, is part of a suite of PBG structures that are being developed for applications such as optical switches, low-loss waveguide corners, and super-prisms.
Professor Henry Smith has developed high-performance, in-line grating-based optical resonators that presage a new era of highly integrated, chip-based micro-photonic systems for communication and computation. He has also developed lithographic and pattern-transfer processes for magnetic random access memory (MRAM), and observed proper switching of the magnetic layers down to 50-nm-wide elements. This is an important step on the path to very high density, non-volatile memory to replace DRAMs.
Dr. James Goodberlet is developing near-field photolithography, a low cost method for patterning features with dimensions well below 100 nm, and using this technology to fabricate integrated vacuum-electronic and integrated-optical devices. In recent experiments, he has reliably patterned 45-nm-diameter holes and posts, a dimension substantially below the diffraction limit of the 220-nm-wavelength exposing radiation.
Professor Vladimir Bulovic is in the process of developing a versatile materials growth system that will be capable of depositing molecular organics, polymers, metals, metal oxides, inorganic nanodots, and colloids in a controlled layer-by-layer fashion. This system will include in-situ capabilities for fabrication of complex patterned structures and for electrical testing.
Optics and Devices
Professor James Fujimoto has continued to pioneer optical coherence tomography (OCT), an emerging medical imaging technology that is analogous to ultrasound imaging except that it uses light rather than sound. During the past year, he has developed techniques that enhance the image resolution of OCT by almost 10x, from 10-15 µm to 1 µm. Professor Fujimoto is currently performing preliminary clinical studies of various OCT techniques with the Brigham and Women's Hospital, the National Cancer Institute, and the Lahey Clinic.
Professor Erich Ippen has continued his work on the development novel femtosecond lasers and their application to precision-measurement systems. Together with Professors James Fujimoto, Hermann Haus, and Frans Kaertner, he has demonstrated octave-wide optical comb generation in a phase-locked sub-2-cycle Ti:sapphire laser. This source will find increasing application in optical frequency standards and optical clocks. Professors Ippen, Fujimoto, and Kaertner have also established records for the shortest duration pulses produced from a Cr:fosterite laser (13 fs) and a Cr:YAG laser (20 fs). These sources will greatly expand the spectral coverage for optical frequency standards.
Professors Hermann Haus and Erich Ippen have achieved a record soliton noise squeezing of 6.1 dB. This experiment, which confirmed theoretical predictions concerning the evolution of a quantized soliton, is the forerunner of precision measurement systems whose sensitivity exceeds the conventional shot-noise limit.
Professors Haus and Ippen also characterized, both theoretically and experimentally, the timing jitter of mode-locked fiber lasers. Low-jitter fiber lasers are promising candidates for timing sources in broadband analog-to-digital converters.
Professor Qing Hu is continuing research on lasers and electronics for the THz frequency region. During the past year he has studied the role of complex phonon spectra on intersubband scattering rates, and obtained one of the narrowest linewidths (0.18 THz) ever seen from intersubband emission.
Surfaces and Interfaces
Professor John Joannopoulos has engaged in surface and interface structure calculations aimed at solving the problem of combining opto-electronics, that rely on direct-bandgap III-V semiconductor alloys, with indirect-bandgap silicon integrated electronics. His calculations show that a new semiconductor alloy, Zn0.5Si0.5P0.25As0.75, should eliminate the charge mismatch encountered at III-V/Si interfaces, have a lattice constant that is within 0.1 percent of Si, and should possess a direct bandgap at optical fiber communication wavelengths. Additional research has characterized 27 novel material compounds that span a region of bandgap/lattice-constant phase space that was hitherto inaccessible. Experimental efforts are currently underway at MIT to fabricate some of the most promising of these materials. Both U.S. and International Patents are pending for this new class of materials.
Circuits and Systems
Professor John Wyatt is continuing his research on the microchip retinal implant. As part of this project, blind patients have had electrode arrays temporarily placed in their eyes via surgical procedures done under local anesthesia. The patients have then been asked to describe their perceptions when these arrays are electrically stimulated with spatial patterns. Every patient has seen something resulting from the stimulation, but thus far the perceptions do not closely match the stimulation patterns. The ultimate goal of this work is to restore some level of useful vision to those suffering from macular degeneration or retinitis pigmentosa.
Professor Rahul Sarpeshkar is working on an ultralow power analog bionic ear for the deaf, pulse-based hybrid analog-digital circuits, and the design of ultralow-noise electronics for MEMS capacitive sensors. During the past year, he successfully implemented a 3 µW, 70 dB dynamic range, active bandpass filter for the front-end of the bionic ear.
Professor Kenneth Stevens is developing a model of the process by which human listeners identify words in running speech, as well as methods for using acoustic data to aid in quantifying the diagnosis of disordered speech production. The results of his work will have application in the development of improved algorithms for speech synthesis from text and for automatic speech recognition.
Dr. Joseph Perkell has been gathering data in a multi-faceted experiment that addresses the hypothesis that speakers use the same set of auditory goals in both production and perception. His work could have long-term applications in methods for diagnosing and treating communication disorders.
Dr. Stefanie Shattuck-Hufnagel is studying the effects of prosodic structure and prominence on the phonetic implementation of the sounds of words along with the cognitive process of phonological encoding that speakers use to generate these effects. Her work provides a way of thinking about the generation of prosodic structure that can address the challenges of natural spontaneous speech, something that is missing from previous linguistic, cognitive, and engineering approaches to speech production.
Dr. Thomas Wiegand has initiated a program on visual psychophysics via uncontrolled channels. Its aim is to administer tests of sensory performance (initially visual performance) via the World Wide Web. He has developed a color vision test that can be viewed using common Web browsers, and provides usable results even on miscalibrated monitors. Not only is this test applicable to routine screening of school children, but it also has implications for the control and tracking of environmental pollutants. This connection arises because of the tendency of certain toxins to cause progressive yellow/blue color blindness.
Dr. Charlotte Reed is developing tactual aids for persons who are profoundly deaf or deaf-blind. Her recent work has shown that a two-channel tactual display provides sufficient information for distinguishing voiced from voiceless speech segments, both through the tactual display alone and when used in combination with lipreading. These results may lead to major improvements in tactual speech communication aids for the deaf.
Dr. Andrew Oxenham is continuing research on auditory perception and cognition. In collaboration with Drs. Christopher Shera and John Guinan of the Massachusetts Eye and Ear Infirmary, he has developed a new measure of inner ear function that relies on otoacoustic emissions, i.e., the sounds produced by the ear itself. This noninvasive technique has enabled comparisons to be made across many different species, including human. The results demonstrate that frequency tuning in the human cochlea is considerably sharper than that in the animals typically used as physiological models for hearing, i.e., the cat and the guinea pig.
Professor Dennis Freeman has measured the motions of sensory cells in the inner ear of a lizard in response to sound stimulation. He also measured the mechanical properties of the tectorial membrane-a key tissue that mechanically stimulates hair cells—obtaining the first quantitative measurements of its bulk modulus, fixed-charge concentration, and point impedance.
Dr. Bertrand Delgutte is studying the neural mechanisms for perception of sounds. He has found that most neurons in the auditory midbrain respond preferentially to pairs of tones forming dissonant musical intervals than to pairs of tones forming consonant intervals. These findings, which show that percepts generally considered to be cognitive have direct correlates in the responses of auditory neurons, may help design better hearing aids and cochlear implants.
Dr. Donald Eddington is working on a variety of issues related to cochlear implants. During the past year he has developed a measure of the interaction between the electrodes in the stimulus array of such implants. Using this measure, he has demonstrated a new electrode design that reduces the interaction, thus allowing the number of information channels to be nearly doubled. Tests of speech reception in one subject showed a dramatic improvement using the new electrode system.
Atomic, Molecular, and Optical Physics
The Center for Ultracold Atoms (CUA) was established at MIT and Harvard under NSF sponsorship. RLE's Associate Director, Professor Daniel Kleppner, is the Director of CUA. The CUA's research program encompasses experimental and theoretical work on Bose-Einstein condensates (BECs), atom optics, cryogenic sources for BECs.
Professor Wolfgang Ketterle is continuing to study the properties of Bose-Einstein condensates, and to further develop the atom laser, an intense coherent beam of atoms. During the past year, he has observed the formation of highly-ordered vortex lattices in a rotating Bose-condensed gas. Because these observations were free from distortions, even near the boundary, they may provide a model system for the study of vortex matter. Such a system would be important because quantized vortices play key roles in superfluidity and superconductivity.
Professor David Pritchard has been applying atom interferometry toward both fundamental and applied problems such as studying quantum mechanics and making better gyroscopes. In recent measurements, he has quantitatively explored wave-particle duality, perhaps the most counter-intuitive aspect of quantum mechanics. Scattering multiple photons off atoms inside an atom interferometer caused loss of quantum coherence (i.e., wave behavior) in accord with general theories of decoherence. This was the first experiment to make such comparisons with no adjustable parameters.
Professor Abraham Bers is studying laser-plasma interactions, ionospheric modeling, and heating in magnetically-confined plasmas. Recent highlights from his research include unique experiments, done in collaboration with Los Alamos National Laboratory, designed to characterize the interaction of a single laser hot spot with a pre-formed laser-produced plasma. Stimulated Raman scattering and its coupling to Langmuir decay instability cascades were observed unequivocally for the first time.
Professor Bruno Coppi is working on a broad range of problems associated with high-energy plasma physics, including the design and study of advanced experiments for magnetically-confined fusion ignition, the development of new areas in computational plasma physics, and the analysis of astrophysical accretion disks. His analysis of the optimal regimes for fusion ignition in a high magnetic field has been confirmed by results from the FTU experiment in Italy.
Remote Sensing And Estimation
Professor David Staelin has developed the iterative order and noise (ION) estimation algorithm, which is capable of retrieving multivariate signal information from noisy data, and successfully applied it to detect tachycardia 140 seconds in advance from 31-state patient-state data obtained during anesthesia. In the area of atmospheric remote sensing, he has developed improved methods for estimating precipitation rate from microwave satellite data, and shown that these retrieval methods respond strongly to snowfall, a parameter that is extremely difficult to measure remotely with accuracy.
Dr. Philiip Rosenkranz is carrying out research on the inversion of microwave radiometer measurements made from aircraft or satellites to obtain profiles of temperature and moisture in the atmosphere. The microwave components of his algorithms for NASA's Aqua spacecraft were successfully tested with data from the NOAA-15 weather satellite.
Digital Signal Processing
Professor Alan Oppenheim is addressing the development and application of new paradigms and algorithms for signal processing. Highlights from last year include development of a generalized frequency modulation, quantum signal processing for matched-filter detection of multiple signatures in multi-user detection systems. Patent applications have been filed on several of these advances.
Professor Gregory Wornell is doing research on fundamental principles and algorithms for wireless and broadband communication networks, and multimedia signal processing. His work on cooperative diversity techniques, which forms virtual antenna arrays by coordinating the transmissions from handheld devices, both combats multipath fading and dramatically extends battery life. Professor Wornell has developed a suite of interference suppression algorithms for wireless communications that, with linear complexity, asymptotically achieve the best possible performance of any technique. He has also developed a new generation of digital watermarking algorithms that are compelling candidates for next-generation copyright notification and enforcement.
Advanced Television And Signal Processing
Professor Jae Lim's work concerns the development of signal processing theories and their applications to real-world problems in video, audio, and digital communications. During the past year, research has continued on migration approaches for higher-resolution digital television. Preliminary results, which rely on the use of video enhancement bits, have been encouraging, making these techniques potentially the basis for a future standard. The approaches that are being developed may also be used for the development of scalable video systems.
Professor Jin Kong is working on electromagnetic wave theory and its applications. During the past year, his research has focused on polarimetric passive remote sensing, synthetic aperture radar (SAR) simulation, wide-band antenna applications, scattering by large objects, electromagnetic waves in multilayer media, and negative isotropic materials. His polarimetric remote sensing work has been used as a basis in the WindSat satellite for measuring ocean wind directions.
Professor Jeffrey Shapiro is the leader of a Multidisciplinary University Research Initiative on quantum communication involving researchers from MIT and Northwestern University. As part of this effort, Professor Seth Lloyd and Dr. Selim Shahriar have conceived a trapped-atom quantum memory that, together with an ultrabright source of polarization-entangled photons (conceived by Professor Shapiro and Dr. Franco Wong) forms the basis for an architecture for long-distance quantum communication. Experimental work is now underway to develop both the memory and the entanglement source. Development of this technology will permit quantum computers to be networked and open new approaches to secure communications.
Professor Seth Lloyd has shown that quantum effects can be used, in principle, to dramatically improve the accuracy of radar and position-sensing measurements. Essentially any method for determining distance or time that operates by sending pulses of light from one place to another can be improved by his method.
Professor Donald Troxel has been developing CAD tools for reliability analysis of 3-D integrated circuits. As part of this effort, he has developed a methodology and supporting CAD software for designing 3-D ICs. Prior to this work, layouts for such circuits were always done by hand.
Professor Jeffrey Shapiro succeeded the late Prof. Jonathan Allen as the Director of RLE.
Drs. Gale Petrich and Selim Shahriar were promoted to Principal Research Scientist.
Professor James Fujimoto was elected to the National Academy of Engineering.
Professor Erich Ippen received the James R. Killian, Jr., Faculty Achievement Award.
Professor Seth Lloyd received the Harold E. Edgerton Faculty Achievement Award.
Professors Vladimir Bulovic and Rahul Sarpeshkar received National Science Foundation Faculty Early Career Development (CAREER) Awards.
Dr. Andrew Oxenham received the R. Bruce Lindsey Award of the Acoustical Society of America.
Ms. Mary E. Young, and Mr. David W. Foss received 2001 MIT Infinite Miles Awards.
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: maintaining our high standards for recruitment procedures that include sending job postings to minority colleges and organizations; 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 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. The one research staff appointment for this year was made to a woman.
More information about the Research Laboratory of Electronics can be found online at http://rleweb.mit.edu/.