MIT Reports to the President 1997-98
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 1997 are mentioned. The interested reader can obtain further information from RLE Progress Report No. 140, which describes research activities during calendar year 1997.
ELECTRONICS AND OPTICS
MATERIALS AND FABRICATION
Professor Jesús del Alamo built a dynamic model for the kink effect in indium-aluminum arsenide/indium-gallium arsenide high-electron mobility transistors on indium phosphide. The kink effect refers to prominent distortion of the output characteristics in these transistors, which seriously affects high-frequency circuit operation. Professor del Alamo found that impact ionization generation of holes and subsequent storage is the origin of the kink. Hole storage takes place at the source and at the surface of the device. With this new understanding, the kink can now be modeled by only three parameters, which provides a useful framework for practical circuit design.
Professor Marc Kastner studied the Kondo effect in a quantum dot. The theory for this effect first appeared in 1963, and it seeks to explain why the resistance of some metals begins to increase as the metal is cooled below a certain temperature. Kondo theorized that the local moments of magnetic impurity atoms have an antiferromagnetic coupling to the spins of the conduction electrons. This coupling becomes stronger, and increasingly impedes the flow of current, as the temperature decreases. The variation of conductance with gate voltage, where temperature is a parameter, shows peaks that are clustered in pairs. A pair of peaks corresponds to the addition of an electron pair in the same spatial state; one electron with spin up and the other electron with spin down. The next electron that is added goes to the next spatial state. Additionally, in the region between the paired peaks, the quantum dot (or artificial atom) has an odd number of electrons. The non-zero conductance between the paired peaks arises because the quantum dot has an unpaired electron, which is free to form a singlet with the electrons in the leads. Thus, as theoretically predicted, the Kondo interaction accounts for conductance in a region where none is ordinarily expected.
Professor Henry Smith developed a new scheme for spatial phase-locked electron-beam lithography that is based on an efficient, patternable scintillator. This technique is widely recognized as the most promising approach to achieve nanometer-level accuracy in electron-beam patterning. Professor Smith also made the first definitive study of the resolution limits of x-ray lithography, and has demonstrated that this is the only technique with reliable replication of sub-100-nanometer features. He has also carried out the first lithography using an array of Fresnel zone plates. This is a promising approach to future maskless lithography in the sub-100-nanometer domain. In collaboration with Professor Caroline Ross of the Department of Materials Science and Engineering, he has fabricated large arrays of discrete magnetic posts that measure 50-nanometers in diameter on 100-nanometer centers. These arrays provide the highest density of magnetic information storage yet attained.
OPTICS AND DEVICES
Professor James Fujimoto demonstrated how optical coherence tomography could be combined with fiberoptic delivery systems to create catheters and endoscopes for imaging internal organ systems. He performed the first demonstration of in vivo optical coherence tomography imaging by using such a catheter/endoscope system. He also investigated the development of new nonlinear optical materials for short-pulse generation in the near-infrared spectrum. This will enable the study of ultrafast processes in physics, chemistry, and biology.
Professor Hermann Haus studied the characteristics of optical waveguide structures with high index contrast, such as silicon and silicon dioxide. Using computer simulation, he demonstrated right-angle bends and waveguide crossings with excellent performance. He also carried out simulations on pulse propagation along dispersion-managed fibers. He found a form of nonlinear pulse propagation that suppresses the Gordon-Haus effect, which refers to the pulse timing jitter that is produced by amplified spontaneous emission-induced frequency shifts of the pulses. Finally, he demonstrated shot-noise reduction as much as 5 decibels, which was produced by propagating pulses along a nonlinear birefringent fiber and subsequent superposition using a beam splitter. This is a new method of "squeezing" that is particularly robust against environmental perturbations.
Professor Erich Ippen observed coherent terahertz acoustic oscillations, which were excited by femtosecond pulses, in lead telluride quantum dots in a glass matrix. These oscillations modulate optical transmission and reveal information about dot sizes and distributions. Professor Ippen also demonstrated a new ultrashort-pulse glass waveguide laser that promises a more compact and higher repetition-rate source for fiber communications. In collaboration with Dr. Katherine Hall of Lincoln Laboratory, he demonstrated the operation of a novel optical storage loop that buffers kilobits of optical data at 5 gigabits per second for more than 150 milliseconds.
Professor Qing Hu seeks to develop sensors and sources for millimeter-wave, terahertz, and infrared frequencies. He observed narrow-linewidth intersubband emission (less than 0.5 terahertz) at the designed frequency of 4 terahertz. This research contributes to the design of a future laser source that can operate in this frequency range.
SURFACES AND INTERFACES
Professor John Joannopoulos studied the difference in energy barriers between adsorption and desorption of the hydrogen molecule in relation to the silicon (111)-(7x7) surface. Based on first-principals density functional theory calculations, he verified the experimentally observed difference in these energy barriers. These results demonstrate a novel effect, whereby the localized dangling bonds on the semiconductor surface "bend toward" the incoming molecule in order to facilitate its dissociation.
Professor Simon Mochrie studied the morphology of stepped surfaces of silicon (113) and silicon (112), each of which may develop a periodic grooved superstructure under appropriate conditions. Recently, he showed how steps on a silicon (113) surface agglomerate after a rapid change in temperature at which single steps are stable to a temperature at which step bunches are stable.
CIRCUITS AND SYSTEMS
Professor Jacob White uses computational prototyping and fast numerical algorithms to characterize integrated circuit interconnect and packaging, micromachined devices, and offshore structures. He developed the next generation of fast algorithms by using the precorrected-fast Fourier transform (FFT) method. This method is faster than fast multipole algorithms and can be used with any Green's function. In contrast, fast multipole methods are only used for 1/R Green's functions. Combined with new integral formulations, these new algorithms enable the accurate determination of capacitances and inductances in the presence of materials with high permitivity or permeability.
Professor John Wyatt continues to develop an ocular prosthesis for blind patients with outer retinal degeneration, such as macular degeneration or retinitis pigmentosa. Recently, he briefly implanted a thin, flexible, and nondestructive electrode array in the eye of a blind subject under local anesthesia. Currents were applied to the electrodes and the patient reported his perceptions, which, unfortunately, were inconclusive. Nevertheless, this initial surgery was successfully performed with no harm to the subject, and much was learned about improvements for further human testing.
LANGUAGE, SPEECH, HEARING, AND SENSORY COMMUNICATION
Professor Kenneth Stevens continues to develop models of speech sound generation and human word recognition, with special attention paid to the variability that occurs in speech sounds across speakers and their modes of speaking. He refined his model of human word recognition and developed procedures that account for some of the rule-generated modifications of speech sounds that occur in casual speech. His research on the acoustic and articulatory manifestations of certain speech disorders has led to the development of measures that provide quantitative assessments of the severity of these disorders. These assessments have potential for future clinical use.
Dr. Donald Eddington investigates hearing produced by electrical stimulation of the human auditory system. He has focused on loudness growth in cochlear implant subjects by using commercial sound-processing schemes. By measuring the electric loudness growth function of each implanted electrode, he computed a level mapping function for each processing channel that restores normal loudness growth for tones. Initial speech reception tests show that some subjects score significantly better when using sound processors based on the level mapping functions that are designed to restore normal loudness growth.
Professor Dennis Freeman continues to develop video methods to measure nanometer motions of micrometer-sized objects. He has applied this system to measure sound-induced motions of inner-ear structures, as well as microfabricated accelerometers and gyroscopes. There is considerable interest in developing this technology as a research tool and as an aid to fabricate microelectromechanical systems. He has also begun to apply this technology to microfabricated optics, such as those used in fiberoptic communication networks.
ATOMIC, MOLECULAR, AND OPTICAL PHYSICS
Professor Wolfgang Ketterle continues to study the properties of Bose-Einstein condensates and to further develop the atom laser--an intense, coherent beam of atoms. Recently, he observed the formation process of a condensate in situ. This provided evidence for bosonic stimulation or coherent matter-wave amplification, which is crucial to the concept of the atom laser. He also realized the all-optical confinement of a Bose-Einstein condensate, thus allowing the study of condensates in arbitrary magnetic fields and with arbitrary spin orientation. In addition, he observed that the forces between Bose-condensed atoms could be altered significantly through Feshbach resonances. Such resonances were observed by varying an external magnetic field, and this opened the possibilities to study and manipulate Bose-Einstein condensates.
Professor David Pritchard recently developed theories and techniques for longitudinal atom interferometry. One of his theories explains how short regions of oscillating fields can create and recombine longitudinal momentum coherences in an atomic beam. Thus, these fields serve as atomic beam splitters. Professor Pritchard also used his techniques to carry out the first search for longitudinal coherences that emanate from a supersonic atomic oven, which is a topic of considerable historical controversy. With enough sensitivity to detect signals 20 decibels weaker than 100 percent modulation, he found no modulation between 5 hertz and 150 kilohertz, which is the anticipated frequency range of such modulation (if it exists). This means the quantum density matrix has finally been determined for an atom source.
Dr. Philip Rosenkranz built an improved microwave temperature sounder for use in high-altitude aircraft. New algorithms that infer profiles of temperature, water vapor, and cloud liquid water from the measurements have enabled the improved detection of clouds and precipitation.
DIGITAL SIGNAL PROCESSING
Professor Gregory Wornell developed a new class of highly robust and efficient information-embedding techniques for digital watermarking of media that includes audio, video, and various types of imagery and graphics. These techniques can be used for copyright protection and authentication. He also developed a new class of ultralow-complexity error-correction coding strategies that can be used for reliable transmission over unreliable channels with feedback. These new methods were adapted from familiar, computationally efficient source-coding algorithms for channel coding applications. For unknown channels, the methods are universal and can achieve the performance of the best possible codes, were the channel known.
Professor Jin Au Kong developed new theories for inverse scattering and forward models to invert important geophysical parameters, such as a biomass of forest and sea ice thickness. In addition to his theoretical development, he participated in data interpretation of the active spaceborne imaging radar and passive airborne radiometric experiments for remote sensing of the earth and its oceans.
Professor Jeffrey Shapiro continues to explore issues that relate to the measurement of eigenkets for continuous-time photodetection in its various basic modes of operation. His results are being applied to unify and extend the theory for quantum-wave propagation in a single-mode fiber. He also developed new techniques for laser radar-range imaging with model-based object recognition using posterior marginal pose estimation. This is the first principled end-to-end system for object recognition based on laser radar-range images.
Dr. Ngai Chuen Wong continues to study optical parametric oscillator technology for applications in precision measurements and quantum optics. This work will improve understanding of the optical parametric oscillator's phase noise characteristics, which are essential in ultraprecise optical frequency metrology.
Professor Donald Troxel developed a tool to assess the reliability of metal interconnect in a VLSI device from its mask layout information. This tool is being used to understand and model the crystal structure of copper, which is of increasing importance for large integrated circuits. In addition, he continues to develop MEMStation, a workstation used to design microelectromechanical systems.
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 the following URL: http://rleweb.mit.edu/
MIT Reports to the President 1997-98