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Terahertz spectroscopy, which uses the band of electromagnetic radiation between microwaves and infrared light, is a promising security technology because it can extract the spectroscopic “fingerprints” of a wide range of materials, including chemicals used in explosives. But traditional terahertz spectroscopy requires a radiation source that’s heavy and about the size of a large suitcase, and it takes 15 to 30 minutes to analyze a single sample, rendering it impractical for most applications. In the latest issue of the journal Optica, researchers from MIT’s Research Laboratory of Electronics and their colleagues present a new terahertz spectroscopy system that uses a quantum cascade laser, a source of terahertz radiation that’s the size of a computer chip. The system can extract a material’s spectroscopic signature in just 100 microseconds. The device is so efficient because it emits terahertz radiation in what’s known as a “frequency comb,” meaning a range of frequencies that are perfectly evenly spaced. “With this work, we answer the question, ‘What is the real application of quantum-cascade laser frequency combs?’” says Yang Yang, a graduate student in electrical engineering and computer science and first author on the new paper. “Terahertz is such a unique region that spectroscopy is probably the best application. And QCL-based frequency combs are a great candidate for spectroscopy.” Different materials absorb different frequencies of terahertz radiation to different degrees, giving each of them a unique terahertz-absorption profile. Traditionally, however, terahertz spectroscopy has required measuring a material’s response to each frequency separately, a process that involves mechanically readjusting the spectroscopic apparatus. That’s why the method has been so time consuming. Because the frequencies in a frequency comb are evenly spaced, however, it’s possible to mathematically reconstruct a material’s absorption fingerprint from just a few measurements, without any mechanical adjustments. The trick is evening out the spacing in the comb. Quantum cascade lasers, like all electrically powered lasers, bounce electromagnetic radiation back and forth through a “gain medium” until the radiation has enough energy to escape. They emit radiation at multiple frequencies that are determined by the length of the gain medium. But those frequencies are also dependent on the medium’s refractive index, which describes the speed at which electromagnetic radiation passes through it. And the refractive index varies for different frequencies, so the gaps between frequencies in the comb vary, too. To even out their lasers’ frequencies, the MIT researchers and their colleagues use an oddly shaped gain medium, with regular, symmetrical indentations in its sides that alter the medium’s refractive index and restore uniformity to the distribution of the emitted frequencies. Yang; his advisor, Qing Hu, the Distinguished Professor in Electrical Engineering and Computer Science; and first author David Burghoff, who received his PhD in electrical engineering and computer science from MIT in 2014 and is now a research scientist in Hu’s group, reported this design in Nature Photonics in 2014. But while their first prototype demonstrated the design’s feasibility, it in fact emitted two frequency combs, clustered around two different central frequencies, with a gap between them, which made it less than ideal for spectroscopy. In the new work, Yang and Burghoff, who are joint first authors; Hu; Darren Hayton and Jian-Rong Gao of the Netherlands Institute for Space Research; and John Reno of Sandia National Laboratories developed a new gain medium that produces a single, unbroken frequency comb. Like the previous gain medium, the new one consists of hundreds of alternating layers of gallium arsenide and aluminum gallium arsenide, with different but precisely calibrated thicknesses. As a proof of concept, the researchers used their system to measure the spectral signature of not a chemical sample but an optical device called an etalon, made from a wafer of gallium arsenide, whose spectral properties could be calculated theoretically in advance, providing a clear standard of comparison. The new system’s measurements were a very good fit for the etalon’s terahertz-transmission profile, suggesting that it could be useful for detecting chemicals. Although terahertz quantum cascade lasers are of chip scale, they need to be cooled to very low temperatures, so they require refrigerated housings that can be inconveniently bulky. Hu’s group continues to work on the design of increasingly high-temperature quantum cascade lasers, but in the new paper, Yang and his colleagues demonstrated that they could extract a reliable spectroscopic signature from a target using only very short bursts of terahertz radiation. That could make terahertz spectroscopy practical even at low temperatures. “We used to consume 10 watts, but my laser turns on only 1 percent of the time, which significantly reduces the refrigeration constraints,” Yang explains. “So we can use compact-sized cooling.” “This paper is a breakthrough, because these kinds of sources were not available in terahertz,” says Gerard Wysocki, an assistant professor of electrical engineering at Princeton University. “Qing Hu is the first to actually present terahertz frequency combs that are semiconductor devices, all integrated, which promise very compact broadband terahertz spectrometers.” “Because they used these very inventive phase correction techniques, they have demonstrated that even with pulsed sources you can extract data that is reasonably high resolution already,” Wysocki continues. “That’s a technique that they are pioneering, and this is a great first step toward chemical sensing in the terahertz region.”

Home > Press > Polymer nanowires that assemble in perpendicular layers could offer route to tinier chip components Abstract: Since the 1960s, computer chips have been built using a process called photolithography. But in the past five years, chip features have gotten smaller than the wavelength of light, which has required some ingenious modifications of photolithographic processes. Keeping up the rate of circuit miniaturization that we’ve come to expect — as predicted by Moore’s Law — will eventually require new manufacturing techniques. Block copolymers, molecules that spontaneously self-assemble into useful shapes, are one promising alternative to photolithography. In a new paper in the journal Nature Communications, MIT researchers describe the first technique for stacking layers of block-copolymer wires such that the wires in one layer naturally orient themselves perpendicularly to those in the layer below. The ability to easily produce such “mesh structures” could make self-assembly a much more practical way to manufacture memory, optical chips, and even future generations of computer processors. “There is previous work on fabricating a mesh structure — for example our work,” says Amir Tavakkoli, a postdoc in MIT’s Research Laboratory of Electronics and one of three first authors on the new paper. “We used posts that we had fabricated by electron-beam lithography, which is time consuming. But here, we don’t use the electron-beam lithography. We use the first layer of block copolymer as a template to self-assemble another layer of block copolymer on top of it.” Tavakkoli’s co-first-authors on the paper are Sam Nicaise, a graduate student in electrical engineering, and Karim Gadelrab, a graduate student in materials science and engineering. The senior authors are Alfredo Alexander-Katz, the Walter Henry Gale Associate Professor of Materials Science and Engineering; Caroline Ross, the Toyota Professor of Materials Science and Engineering; and Karl Berggren, a professor of electrical engineering. Unhappy couples Polymers are long molecules made from basic molecular units strung into chains. Plastics are polymers, and so are biological molecules like DNA and proteins. A copolymer is a polymer made by joining two different polymers. In a block copolymer, the constituent polymers are chosen so that they’re chemically incompatible with each other. It’s their attempts to push away from each other — both within a single polymer chain and within a polymer film — that causes them to self-organize. In the MIT researchers’ case, one of the constituent polymers is carbon-based, the other silicon-based. In their efforts to escape the carbon-based polymer, the silicon-based polymers fold in on themselves, forming cylinders with loops of silicon-based polymer on the inside and the other polymer bristling on the outside. When the cylinders are exposed to an oxygen plasma, the carbon-based polymer burns away and the silicon oxidizes, leaving glass-like cylinders attached to a base. To assemble a second layer of cylinders, the researchers simply repeat the process, albeit using copolymers with slightly different chain lengths. The cylinders in the new layer naturally orient themselves perpendicularly to those in the first. Chemically treating the surface on which the first group of cylinders are formed will cause them to line up in parallel rows. In that case, the second layer of cylinders will also form parallel rows, perpendicular to those in the first. But if the cylinders in the bottom layer are allowed to form haphazardly, snaking out into elaborate, looping patterns, the cylinders in the second layer will maintain their relative orientation, creating their own elaborate, but perpendicular, patterns. The orderly mesh structure is the one that has the most obvious applications, but the disorderly one is perhaps the more impressive technical feat. “That’s the one the materials scientists are excited about,” Nicaise says. Whys and wherefores Glass-like wires are not directly useful for electronic applications, but it might be possible to seed them with other types of molecules, which would make them electronically active, or to use them as a template for depositing other materials. The researchers hope that they can reproduce their results with more functional polymers. To that end, they had to theoretically characterize the process that yielded their results. “We use computer simulations to understand the key parameters controlling the polymer orientation,” Gadelrab says. What they found was that the geometry of the cylinders in the bottom layer limited the possible orientations of the cylinders in the upper layer. If the walls of the lower cylinders are too steep to permit the upper cylinders from fitting in comfortably, the upper cylinders will try to find a different orientation. It’s also important that the upper and lower layers have only weak chemical interactions. Otherwise, the upper cylinders will try to stack themselves on top of the lower ones like logs on a pile. Both of these properties — cylinder geometry and chemical interaction — can be predicted from the physics of polymer molecules. So it should be possible to identify other polymers that will exhibit the same behavior. The research was funded by the National Science Foundation and the Taiwan Semiconductor Manufacturing Corporation. ### Written by Larry Hardesty, MIT News Office For more information, please click If you have a comment, please us. Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.

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How he found a way to work toward a clean energy future, as an electrical engineer, is precisely the insight that MIT professor Rajeev Ram shared with undergraduates, faculty, and graduate students who attended his Feb. 25 talk, as part of a series of undergraduate energy seminars sponsored by the MIT Energy Initiative (MITEI). MITEI academic coordinator Ann Greaney-Williams says the seminars “help students access information on energy studies and current research, and connect students with career professionals and faculty engaging on energy topics in their classroom and research.” Not only do the seminars bring MIT's energy studies to a large audience, but they also aim, in the long-term, to create an active community, connecting graduate and undergraduate students who care about a clean energy future and can see themselves contributing to change and progress. Ram is a professor of electrical engineering and the principal investigator for the Physical Optics and Electronics group at the Research Laboratory of Electronics. He was recently named a co-chair of the Energy Education Task Force and the Energy Minor Oversight Committee alongside Bradford Hager, providing institutional leadership for undergraduate and graduate education in energy studies at MIT. In this role, Ram helps guide the development of energy studies at MIT, and encourages students to engage with energy science, technology, and policy. Ram is eager to dispute a commonly held idea that policy alone — without research or engineering solutions — can solve energy issues. Because he spent three years in Washington working with Congress and the executive branch, Ram is well positioned to challenge this view, though he noteed how difficult it is to change the minds of elected officials. In addition, the number of new laws being passed by Congress is at an all-time low — 165 this past year (compared with 1,028 during the Ford administration) — and the chances of proposed energy legislation being passed are likewise lower than in the past. Electricity generation is a lead contributor to carbon dioxide (CO ) emissions, and was responsible for 41 percent of total emissions in 2008; this share is expected to grow by another 35 percent by 2035. Ram pointed to the concept of cap-and-trade policy as one example of how to address emissions from electricity generation and other sources: a government sets a limit (cap) on the total amount of greenhouse gases that can be emitted nationally. Companies would then buy or sell the limited permits to emit these gases, primarily CO . A cap-and-trade bill drafted by then-U.S. Representatives Edward Markey (now a U.S. Senator) and Henry Waxman, the American Clean Energy and Security Act of 2009, passed the House, but due to the predominance of debate around the Affordable Care Act, it never came up for a vote in the Senate. Rather than being discouraged by this, Ram said that engineers took up the challenge to examine technologies, such as carbon capture, and eventually made an economic case for capture within existing markets. Innovations in carbon capture technology have since brought down prices so that industry has an incentive to invest — at least with oil at $85 per barrel. Ram showed a slide concerning RTI International’s work in this domain, e.g., a sorbent-based CO capture process, which is being pilot tested at a cement plant in Norway. “This is an example of an engineering solution stepping into the vacuum created by not having a legislative solution,” Ram remarked. He pointed to and explained a number of technologies that have benefited from advanced research and have been picked up for additional work, such as a chemical looping process, which uses and transforms carbon fuels to pick up CO . Using one of his favorite topics — lighting — as an example, he explained how he has found a particularly important place for electrical engineering in the climate change landscape. Electricity generation is evenly divided between commercial, industrial, and residential uses — and 19 percent of commercial and residential electricity use is for lighting. Providing the legislative context, Ram pointed out that a law, the Energy Independence and Security Act (EISA), was passed in 2007. Among its many focuses were biofuels and automobile fuel economy, and it phased out the sale of incandescent lights. Yet its enforcement — after much wrangling — was eventually defunded. Unfazed, electrical engineers pushed ahead, achieving light-emitting diode (LED) efficiencies and price reductions that made the eventual defunding of EISA inconsequential — and there is much more that can be done. One of Ram’s graduate students discovered a unique aspect of the LED: No matter how small the voltage, some current will flow and it can generate light. What he found was that at low voltages the electrons harvest most of their energy as heat from the environment, not just from the battery. This is an amazing development, said Ram, making it possible to create 110 percent efficient LEDs. This additional source of energy means that the battery is no longer the only supply — hence the electrical-to-optical conversion efficiency can be greater than 100 percent. With discoveries of this kind, electrical engineers are in unique position to double-down on creating small, inexpensive and efficient devices to light the world. “There is still so much more for us to do in this space,” Ram concluded. Electrical engineering majors can look forward to learning more about power electronics, new materials for power transistors, and magnetic materials for power applications, along with the use of new materials such as silicon carbide and gallium nitride, and wider bandgap semiconductors — all areas of potential research and engagement for students. MIT Energy Initiative’s next Undergraduate Energy Seminar will be with Kristala Prather, associate professor in the department of chemical engineering. Prather will hold an interactive workshop with undergraduates, “Is it too late (or too early) for biofuels” on Monday, April 25 from  12 to 1:30 p.m. in Room E19-319.

News Article | August 18, 2016
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MIT biological engineers have devised a way to record complex histories in the DNA of human cells, allowing them to retrieve “memories” of past events, such as inflammation, by sequencing the DNA. This analog memory storage system — the first that can record the duration and/or intensity of events in human cells — could also help scientists study how cells differentiate into various tissues during embryonic development, how cells experience environmental conditions,  and how they undergo genetic changes that lead to disease. “To enable a deeper understanding of biology, we engineered human cells that are able to report on their own history based on genetically encoded recorders,” says Timothy Lu, an associate professor of electrical engineering and computer science, and of biological engineering. This technology should offer insights into how gene regulation and other events within cells contribute to disease and development, he adds. Lu, who is head of the Synthetic Biology Group at MIT’s Research Laboratory of Electronics, is the senior author of the new study, which appears in the Aug. 18 online edition of Science. The paper’s lead authors are Samuel Perli SM ’10, PhD ’15 and graduate student Cheryl Cui. Many scientists, including Lu, have devised ways to record digital information in living cells. Using enzymes called recombinases, they program cells to flip sections of their DNA when a particular event occurs, such as exposure to a particular chemical. However, that method reveals only whether the event occurred, not how much exposure there was or how long it lasted. Lu and other researchers have previously devised ways to record that kind of analog information in bacteria, but until now, no one has achieved it in human cells. The new MIT approach is based on the genome-editing system known as CRISPR, which consists of a DNA-cutting enzyme called Cas9 and a short RNA strand that guides the enzyme to a specific area of the genome, directing Cas9 where to make its cut. CRISPR is widely used for gene editing, but the MIT team decided to adapt it for memory storage. In bacteria, where CRISPR originally evolved, the system records past viral infections so that cells can recognize and fight off invading viruses. “We wanted to adapt the CRISPR system to store information in the human genome,” Perli says. When using CRISPR to edit genes, researchers create RNA guide strands that match a target sequence in the host organism’s genome. To encode memories, the MIT team took a different approach: They designed guide strands that recognize the DNA that encodes the very same guide strand, creating what they call “self-targeting guide RNA.” Led by this self-targeting guide RNA strand, Cas9 cuts the DNA encoding the guide strand, generating a mutation that becomes a permanent record of the event. That DNA sequence, once mutated, generates a new guide RNA strand that directs Cas9 to the newly mutated DNA, allowing further mutations to accumulate as long as Cas9 is active or the self-targeting guide RNA is expressed. By using sensors for specific biological events to regulate Cas9 or self-targeting guide RNA activity, this system enables progressive mutations that accumulate as a function of those biological inputs, thus providing genomically encoded memory. For example, the researchers engineered a gene circuit that only expresses Cas9 in the presence of a target molecule, such as TNF-alpha, which is produced by immune cells during inflammation. Whenever TNF- alpha is present, Cas9 cuts the DNA encoding the guide sequence, generating mutations. The longer the exposure to TNF-alpha or the greater the TNF-alpha concentration, the more mutations accumulate in the DNA sequence. By sequencing the DNA later on, researchers can determine how much exposure there was. “This is the rich analog behavior that we are looking for, where, as you increase the amount or duration of TNF-alpha, you get increases in the amount of mutations,” Perli says. “Moreover, we wanted to test our system in living animals. Being able to record and extract information from live cells in mice can help answer meaningful biological questions,” Cui says. The researchers showed that the system is capable of recording inflammation in mice. Most of the mutations result in deletion of part of the DNA sequence, so the researchers designed their RNA guide strands to be longer than the usual 20 nucleotides, so they won’t become too short to function. Sequences of 40 nucleotides are more than long enough to record for a month, and the researchers have also designed 70-nucleotide sequences that could be used to record biological signals for even longer. The researchers also showed that they could engineer cells to detect and record more than one input, by producing multiple self-targeting RNA guide strands in the same cell. Each RNA guide is linked to a specific input and is only produced when that input is present. In this study, the researchers showed that they could record the presence of both the antibiotic doxycycline and a molecule known as IPTG. Currently this method is most likely to be used for studies of human cells, tissues, or engineered organs, the researchers say. By programming cells to record multiple events, scientists could use this system to monitor inflammation or infection, or to monitor cancer progression. It could also be useful for tracing how cells specialize into different tissues during development of animals from embryos to adults. “With this technology you could have different memory registers that are recording exposures to different signals, and you could see that each of those signals was received by the cell for this duration of time or at that intensity,” Perli says. “That way you could get closer to understanding what’s happening in development.”

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The School of Engineering has announced that seven members of its faculty have been granted tenure by MIT. “These newly tenured colleagues have demonstrated a commitment to outstanding research and teaching,” said Ian A. Waitz, dean of the School of Engineering. “They have made a significant impact on MIT and their fields, and we look forward to the continuation of their remarkable work.” Steven Barrett, associate professor in the Department of Aeronautics and Astronautics, Finmeccanica Career Development Professor of Engineering, and director of the Laboratory for Aviation and the Environment. The main goal of his research is to advance understanding of the environmental impacts of aviation, and to develop strategies that mitigate these impacts. Mark Bathe ’98, SM ’01, PhD ’04, associate professor in the Department of Biological Engineering. His research focuses on quantitative physical approaches to understanding complex biological processes from a molecular perspective. He runs an interdisciplinary research group that draws together biologists, chemists, physicists, and engineers focused on this area. Paola Cappellaro PhD ’06, associate professor in the Department of Nuclear Science and Engineering and an Esther and Harold E. Edgerton Career Development Professor. She leads the Quantum Engineering Group in the Research Laboratory of Electronics, where her work focuses on improving both the experimental techniques and the coherent control theory of quantum bits and gaining a deeper knowledge of the mechanics of decoherence. Sangbae Kim, associate professor in the Department of Mechanical Engineering, an Esther and Harold E. Edgerton Professor, and leader of the Biomimetics Robotics Lab. He conducts research in biomimetics, using biological systems as models for the design and engineering of robots. His interests include biomechanics of locomotion and printable robotics. Jesse Kroll, associate professor in the Department of Civil and Environmental Engineering. His research involves the experimental study of the properties and chemical transformation of organic species in the Earth’s atmosphere. Particular interests include the development of new analytical tools for the measurement and characterizations of organics in both the gas and condensed phase, and the use of these tools in the lab and field to better constrain the amount, nature, and chemical evolution of atmospheric organics. Youssef Marzouk ’97, SM ’99, PhD ’04, associate professor in the Department of Aeronautics and Astronautics and director of the Aerospace Computational Design Laboratory. His research focuses on uncertainty quantification, inverse problems, statistical inference, and Bayesian computation for complex physical systems, and using these algorithms to address modeling challenges in energy conversion and environmental applications. Armando Solar-Lezama, associate professor in the Department of Electrical Engineering and Computer Science. He works with the Computer Assisted Programming Group to develop techniques that exploit automated reasoning and computing power to tackle challenging programming problems.

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