Two Caltech Scientists Receive 2010 NIH Director's Pioneer Awards

Michael Roukes, Pamela Bjorkman recognized for their "highly innovative approaches" to biomedical research

PASADENA, Calif.—Two scientists from the California Institute of Technology (Caltech) have been recognized by the National Institutes of Health (NIH) for their innovative and high-impact biomedical research programs.

Michael Roukes, professor of physics, applied physics, and bioengineering, and co-director of the Kavli Nanoscience Institute, and

Pamela Bjorkman, Caltech's Max Delbrück Professor of Biology and a Howard Hughes Medical Institute investigator, now join the 81 Pioneers—including Caltech researchers Rob Phillips and Bruce Hay—who have been selected since the program's inception in 2004.

"NIH is pleased to be supporting scientists from across the country who are taking considered risks in a wide range of areas in order to accelerate research," said NIH Director Francis S. Collins in announcing the awards. "We look forward to the result of their work."

According to its website, the program provides each investigator chosen with up to $500,000 in direct costs each year for five years to pursue what the NIH refers to as "high-risk research," and was created to "support individual scientists of exceptional creativity who propose pioneering—and possibly transforming—approaches to major challenges in biomedical and behavioral research."

For Roukes, that means using "nanoscale tools to push biomedical frontiers." Specifically, he plans to leverage advances in nanosystems technology, "an approach that coordinates vast numbers of individual nanodevices into a coherent whole," he explains.

The goal? To create tiny "chips" that can be used to rapidly identify which specific bacteria are plaguing an individual patient—quickly, at the patient's bedside, and without the need for culturing. Similar chips, he says, will be capable of "obtaining physiological 'fingerprints' from exhaled breath" for use in disease diagnostics.

Roukes says the chips will also provide new approaches to cancer research through the analysis of cell mechanics and motility, and will provide less-costly ways to screen libraries of therapeutic drug candidates. Roukes's highly collaborative efforts are aimed at jump-starting what he calls a "nanobiotech incubator" at Caltech.

Roukes received his PhD in physics in 1985 from Cornell University. He has been at Caltech since 1992, and was named founding director of the Kavli Nanoscience Institute in 2004.

Bjorkman's Pioneer project will focus on ways to improve the human immune response to HIV. "HIV/AIDS remains one of the most important current threats to global public health," she says. "Although humans can mount effective immune responses using antibodies against many other viruses, the antibody response to HIV in infected individuals is generally ineffective."

This, she believes, is the result of the "unusually low number and low density of spikes" on the surface membrane of the virus. Antibodies have two identical "arms" with which to attach to a virus or bacterium. In most cases, the density of spikes on a pathogen's surface is high enough that these arms can simultaneously attach to neighboring spikes. Not so with HIV; because its spikes are so few and far between, antibodies tend to bind with only one arm attaching to a single spike. Such binding is weak, says Bjorkman, "much like if you were hanging from a bar with only one arm," and is easily eliminated by viral mutations.

That is why Bjorkman is proposing "a new methodology, designed to screen for and produce novel anti-HIV binding proteins that can bind simultaneously to all three monomers in an HIV spike trimer." A trimer is a protein made of three identical macromolecules; if an antibody can bind to all three proteins at one time, it will "interact very tightly and render the low spike density of HIV and its high mutation rate irrelevant to effective neutralization," Bjorkman explains.

Bjorkman received her PhD in biochemistry and molecular biology in 1984 from Harvard University. She has been at Caltech since 1989, and was named the Delbrück Professor in 2004.

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Lori Oliwenstein
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Schooling Fish Offer New Ideas for Wind Farming

The quest to derive energy from wind may soon be getting some help from California Institute of Technology (Caltech) fluid-dynamics expert John Dabiri-and a school of fish.

As head of Caltech's Biological Propulsion Laboratory, Dabiri studies water- and wind-energy concepts that share the theme of bioinspiration: that is, identifying energy-related processes in biological systems that may provide insight into new approaches to-in this case-wind energy.

"I became inspired by observations of schooling fish, and the suggestion that there is constructive hydrodynamic interference between the wakes of neighboring fish," says Dabiri, associate professor of aeronautics and bioengineering at Caltech. "It turns out that many of the same physical principles can be applied to the interaction of vertical-axis wind turbines."

The biggest challenge with current wind farms is lack of space. The horizontal-axis wind turbines most commonly seen-those with large propellers-require a substantial amount of land to perform properly. "Propeller-style wind turbines suffer in performance as they come in proximity to one another," says Dabiri.

In the Los Angeles basin, the challenge of finding suitable space for such large wind farms has prevented further progress in the use of wind energy. But with help from the principles supplied by schooling fish, and the use of vertical-axis turbines, that may change.

Vertical turbines-which are relatively new additions to the wind-energy landscape-have no propellers; instead, they use a vertical rotor. Because of this, the devices can be placed on smaller plots of land in a denser pattern. Caltech graduate students Robert Whittlesey and Sebastian Liska researched the use of vertical-axis turbines on small plots during a class research project supervised by Dabiri. Their results suggest that there may be substantial benefits to placing vertical-axis turbines in a strategic array, and that some configurations may allow the turbines to work more efficiently as a result of their relationship to others around them-a concept first triggered by examining schools of fish.

In current wind farms, all of the turbines rotate in the same direction. But while studying the vortices left behind by fish swimming in a school, Dabiri noticed that some vortices rotated clockwise, while others rotated counter-clockwise. Dabiri therefore wants to examine whether alternating the rotation of vertical-axis turbines in close proximity will help improve efficiency. The second observation he made studying fish-and seen in Whittlesey and Liska's simulation-was that the vortices formed a "staircase" pattern, which contrasts with current wind farms that place turbines neatly in rows.

Whittlesey and Liska's computer models predicted that the wind energy extracted from a parcel of land using this staggered placement approach would be several times that of conventional wind farms using horizontal-axis turbines. Once they've identified the optimal placement, Dabiri believes it may be possible to produce more than 10 times the amount of energy currently provided by a farm of horizontal turbines. The results are sufficiently compelling that the Caltech group is pursuing a field demonstration of the idea.

Dabiri has purchased two acres of land north of Los Angeles, where he is establishing the Caltech Field Laboratory for Optimized Wind Energy (FLOWE). The pilot program at the site will feature six vertical turbines on mobile platforms.

Dabiri and his team will systematically move the turbines around, testing various configurations to find the most efficient patterns.

"Our goal is to demonstrate a new technology that enables us to extract significantly more wind energy from a given parcel of land than is currently possible using existing methods," says Dabiri. "We want to take advantage of constructive aerodynamic interference between closely spaced vertical-axis wind turbines. Our results can potentially make better use of existing wind farms, allow for wind farms to be located closer to urban centers-reducing power transmission costs-and reduce the size of offshore installations."

Three of Dabiri's turbines are being provided in partnership with Windspire Energy. In exchange for the use of the turbines, Dabiri will share his research results with the company. Each Windspire turbine stands approximately 30 feet tall and 4 feet wide, and can generate up to 1.2 kW of power.

"This leading-edge project is a great example of how thinking differently can drive meaningful innovation," says Windspire Energy President and CEO Walt Borland. "We are very excited to be able to work with Dr. Dabiri and Caltech to better leverage the unique attributes of vertical-axis technology in harvesting wind energy."   

Three turbines from another manufacturer have been purchased; the six turbines give the pilot facility a total power capacity of 15 kW, enough to power several homes.

"This project is unique in that we are conducting these experiments in real-world conditions, as opposed to on the computer or in a laboratory wind tunnel," says Dabiri. "We have intentionally focused on a field demonstration because this can more easily facilitate a future expansion of the project from basic science research into a power-generating facility. Our ability to make that transition will depend on the results of the pilot program."

The initial phase of the study will attempt to demonstrate which configuration of units will improve power output and performance relative to a horizontal-axis wind turbine farm with a similar sized plot of land.

"In the future, we hope to transition to power-generation experiments in which the generated power can be put to use either locally or via a grid connection," Dabiri says.

The American Recovery and Reinvestment Act provided partial funding for this project.

For more information on FLOWE, visit: http://dabiri.caltech.edu/research/wind-energy.html.

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Jon Weiner
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Spiders at the Nanoscale: Molecules that Behave Like Robots

PASADENA, Calif.—A team of scientists from Columbia University, Arizona State University, the University of Michigan, and the California Institute of Technology (Caltech) have programmed an autonomous molecular "robot" made out of DNA to start, move, turn, and stop while following a DNA track.

The development could ultimately lead to molecular systems that might one day be used for medical therapeutic devices and molecular-scale reconfigurable robots—robots made of many simple units that can reposition or even rebuild themselves to accomplish different tasks.

A paper describing the work appears in the current issue of the journal Nature.

The traditional view of a robot is that it is "a machine that senses its environment, makes a decision, and then does something—it acts," says Erik Winfree, associate professor of computer science, computation and neural systems, and bioengineering at Caltech.

Milan N. Stojanovic, a faculty member in the Division of Experimental Therapeutics at Columbia University, led the project and teamed up with Winfree and Hao Yan, professor of chemistry and biochemistry at Arizona State University and an expert in DNA nanotechnology, and with Nils G. Walter, professor of chemistry and director of the Single Molecule Analysis in Real-Time (SMART) Center at the University of Michigan in Ann Arbor, for what became a modern-day self-assembly of like-minded scientists with the complementary areas of expertise needed to tackle a tough problem.

Shrinking robots down to the molecular scale would provide, for molecular processes, the same kinds of benefits that classical robotics and automation provide at the macroscopic scale. Molecular robots, in theory, could be programmed to sense their environment (say, the presence of disease markers on a cell), make a decision (that the cell is cancerous and needs to be neutralized), and act on that decision (deliver a cargo of cancer-killing drugs).

Or, like the robots in a modern-day factory, they could be programmed to assemble complex molecular products.  The power of robotics lies in the fact that once programmed, the robots can carry out their tasks autonomously, without further human intervention.

With that promise, however, comes a practical problem: how do you program a molecule to perform complex behaviors?

"In normal robotics, the robot itself contains the knowledge about the commands, but with individual molecules, you can't store that amount of information, so the idea instead is to store information on the commands on the outside," says Walter. And you do that, says Stojanovic, "by imbuing the molecule's environment with informational cues."

"We were able to create such a programmed or 'prescribed' environment using DNA origami," explains Yan. DNA origami, an invention by Caltech Senior Research Associate Paul W. K. Rothemund, is a type of self-assembled structure made from DNA that can be programmed to form nearly limitless shapes and patterns (such as smiley faces or maps of the Western Hemisphere or even electrical diagrams). Exploiting the sequence-recognition properties of DNA base pairing, DNA origami are created from a long single strand of DNA and a mixture of different short synthetic DNA strands that bind to and "staple" the long DNA into the desired shape. The origami used in the Nature study was a rectangle that was 2 nanometers (nm) thick and roughly 100 nm on each side.

The researchers constructed a trail of molecular "bread crumbs" on the DNA origami track by stringing additional single-stranded DNA molecules, or oligonucleotides, off the ends of the staples. These represent the cues that tell the molecular robots what to do—start, walk, turn left, turn right, or stop, for example—akin to the commands given to traditional robots. 

The molecular robot the researchers chose to use—dubbed a "spider"—was invented by Stojanovic several years ago, at which time it was shown to be capable of extended, but undirected, random walks on two-dimensional surfaces, eating through a field of bread crumbs.

To build the 4-nm-diameter molecular robot, the researchers started with a common protein called streptavidin, which has four symmetrically placed binding pockets for a chemical moiety called biotin. Each robot leg is a short biotin-labeled strand of DNA, "so this way we can bind up to four legs to the body of our robot," Walter says. "It's a four-legged spider," quips Stojanovic. Three of the legs are made of enzymatic DNA, which is DNA that binds to and cuts a particular sequence of DNA. The spider also is outfitted with a "start strand"—the fourth leg—that tethers the spider to the start site (one particular oligonucleotide on the DNA origami track). "After the robot is released from its start site by a trigger strand, it follows the track by binding to and then cutting the DNA strands extending off of the staple strands on the molecular track," Stojanovic explains.

"Once it cleaves," adds Yan, "the product will dissociate, and the leg will start searching for the next substrate." In this way, the spider is guided down the path laid out by the researchers. Finally, explains Yan, "the robot stops when it encounters a patch of DNA that it can bind to but that it cannot cut," which acts as a sort of flypaper.

Although other DNA walkers have been developed before, they've never ventured farther than about three steps. "This one," says Yan, "can walk up to about 100 nanometers. That's roughly 50 steps."

"This in itself wasn't a surprise," adds Winfree, "since Milan's original work suggested that spiders can take hundreds if not thousands of processive steps. What's exciting here is that not only can we directly confirm the spiders' multistep movement, but we can direct the spiders to follow a specific path, and they do it all by themselves—autonomously."

In fact, using atomic force microscopy and single-molecule fluorescence microscopy, the researchers were able to watch directly spiders crawling over the origami, showing that they were able to guide their molecular robots to follow four different paths.

"Monitoring this at a single molecule level is very challenging," says Walter. "This is why we have an interdisciplinary, multi-institute operation. We have people constructing the spider, characterizing the basic spider. We have the capability to assemble the track, and analyze the system with single-molecule imaging. That's the technical challenge." The scientific challenges for the future, Yan says, "are how to make the spider walk faster and how to make it more programmable, so it can follow many commands on the track and make more decisions, implementing logical behavior."

"In the current system," says Stojanovic, "interactions are restricted to the walker and the environment. Our next step is to add a second walker, so the walkers can communicate with each other directly and via the environment. The spiders will work together to accomplish a goal." Adds Winfree, "The key is how to learn to program higher-level behaviors through lower-level interactions." 

Such collaboration ultimately could be the basis for developing molecular-scale reconfigurable robots—complicated machines that are made of many simple units that can reorganize themselves into any shape—to accomplish different tasks, or fix themselves if they break.  For example, it may be possible to use the robots for medical applications. "The idea is to have molecular robots build a structure or repair damaged tissues," says Stojanovic.

"You could imagine the spider carrying a drug and bonding to a two-dimensional surface like a cell membrane, finding the receptors and, depending on the local environment," adds Yan, "triggering the activation of this drug."

Such applications, while intriguing, are decades or more away. "This may be 100 years in the future," Stojanovic says. "We're so far from that right now." 

"But," Walter adds, "just as researchers self-assemble today to solve a tough problem, molecular nanorobots may do so in the future."

The other coauthors on the paper, "Molecular robots guided by prescriptive landscapes," are Kyle Lund and Jeanette Nangreave from Arizona State University; Anthony J. Manzo, Alexander Johnson-Buck, and Nicole Michelotti from the University of Michigan; Nadine Dabby from Caltech; and Steven Taylor and Renjun Pei from Columbia University. The work was supported by the National Science Foundation, the Army Research Office, the Office of Naval Research, the National Institutes of Health, the Department of Energy, the Searle Foundation, the Lymphoma and Leukemia Society, the Juvenile Diabetes Research Foundation, and a Sloan Research Fellowship.

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Kathy Svitil
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Caltech Biologists Link Gut Microbial Equilibrium to Inflammatory Bowel Disease

PASADENA, Calif.—We are not alone—even in our own bodies. The human gut is home to 100 trillion bacteria, which, for millions of years, have co-evolved along with our digestive and immune systems. Most people view bacteria as harmful pathogens that cause infections and disease. Other, more agreeable, microbes (known as symbionts) have taken a different evolutionary path, and have established beneficial relationships with their hosts. Still other microbes may be perched somewhere in between, according to research by biologists at the California Institute of Technology (Caltech) that offers new insight into the causes of inflammatory bowel disease (IBD) and colon cancer.

A paper about their work appears in the April 22 issue of the journal Cell Host & Microbe

 

"It has been proposed that the coupled equilibrium between potentially harmful and potentially beneficial bacteria in the gut mediates health versus disease," says Sarkis K. Mazmanian, assistant professor of biology at Caltech. "If the balance is altered," say, by changes in diet, the effects of stress, or the use of antibiotics, "then the immune response in the intestines is also changed." This altered host–microbe relationship, called dysbiosis, has been linked to IBD and colon cancer as well as to obesity and diabetes.

Close to a thousand different species of bacteria reside in the gut, which makes understanding the consequences of dysbiosis a challenge. One way of studying the effects of a balanced host–microbe relationship, and how it arises in the first place, is to change experimentally the relative population size of the microbe. That's exactly what Mazmanian and graduate student Janet Chow accomplished in a bacterium called Helicobacter hepaticus.

Helicobacter hepaticus has an unusual modus operandi. It is not an opportunistic pathogen like the bacteria that cause diseases such as tuberculosis or strep throat, nor is it a beneficial symbiont. While H. hepaticus can persist for a lifetime in the gut of a healthy organism without causing any ill effects, it causes syndromes similar to IBD in immunocompromised mice—animals with artificially depressed or inactive immune systems. "Perhaps this organism is somewhere within the evolutionary spectrum between pathogen and symbiont," says Mazmanian. The authors have coined the term "pathobiont" to describe the unique lifestyle of H. hepaticus and the relationship it establishes with its host.

Mazmanian and Chow suspected that the effect of the bug's presence—whether it lives in quiet coexistence with its host or causes disease—may be determined by its ability to communicate with and, more importantly, to modify the immune system of its host.

To examine this possibility, Chow genetically altered the bacterium to inactivate its "secretion system." The secretion system is a collection of proteins the microbe uses to send chemical messages to its host; Mazmanian says it represents a biological "needle and syringe" that delivers bacterial molecules directly into eukaryotic cells. Although the specific functions and identities of these chemicals are unknown, they appear to establish a truce between the bug and the host's immune system.

When Chow genetically disrupted the secretion system—shutting off this communication—she saw two unexpected and intriguing effects. First, the size of the H. hepaticus population expanded dramatically, leading to dysbiosis. In turn, the host immune system ramped up its activity. This manifested in inflammation—the body's response to infection or injury.

"The bacteria appear to have struck a deal with their host," Mazmanian says. They keep their own numbers low so they don't overwhelm the immune system, and in return, the immune system leaves them alone. "The bacteria need the secretion system to put the host in 'don't attack' mode." In return, the presence of the bacteria does not induce inflammation, as would be the case with a pathogen that has not evolved a similar "agreement."

"There has to be communication. It could be peaceful—as is the case for symbionts—or it could be an argument—as is the case for pathogens. But when this molecular dialogue breaks down, it's probably harmful to both microbe and man," Mazmanian says.

Disrupt that communication, and the balance gets thrown out of whack. "Inflammation leads to cancer, and this bacterium has been associated with inflammation and colon cancer in animals," he says. Understanding if dysbiosis causes disease in humans could lead to therapies based on restoring the healthy microbial balance in the gut.

The work in the paper, "A Pathobiont of the Microbiota Balances Host Colonization and Intestinal Inflammation," was supported by funding from the Emerald Foundation and the Crohn's & Colitis Foundation of America. The inception of the project was supported by a Damon Runyon-Rachleff Innovation Award from the Damon Runyon Cancer Research Foundation.

 

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Kathy Svitil
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R. David Middlebrook, 80

Pasadena, Calif.-R. David Middlebrook, emeritus professor of electrical engineering at the California Institute of Technology (Caltech), passed away on April 16. He was 80 years old.

Middlebrook died at his home with family by his side. Born in 1929, he was raised in Newcastle, England, and came to the United States in 1952 on the Queen Mary.

Middlebrook wrote a pioneering transistor textbook that included mathematical models to help engineers use transistors in their circuit designs; a later book focused on differential amplifiers. In 1970, he founded the Caltech Power Electronics Group. The group graduated 36 PhD students, many of whom are now leaders in the power electronics field.

A distinguished international lecturer, Middlebrook was particularly noted for presenting complex material in a simple, interesting, effective, and entertaining manner. He was especially interested in design-oriented circuit analysis and measurement techniques, and his Structured Analog Design course was attended by design engineers and managers from the United States, Canada, and Europe. 

Middlebrook also taught inhouse analog-design courses for more than 20 years, working with companies such as AT&T, Boeing, Ericsson, Hewlett Packard, Hughes Aircraft, IBM, Motorola, Philips, Tektronix, TRW, and many others.

He is well known for his Extra Element Theorem, which gives simple formulas for the effects of adding a single element to a circuit. This theorem and its variations are widely used in circuit design and measurements.  

Middlebrook received his BA and MA degrees from the University of Cambridge, and his MS and PhD degrees from Stanford University. He joined Caltech as an assistant professor in 1955; he was named associate professor in 1958, and professor in 1965. He became emeritus in 1998.

In 1996, the Caltech student body recognized him as an outstanding educator with its Feynman Prize for Excellence in Teaching.

"For more than 40 years, Dr. Middlebrook taught his students a way of thinking, not just a body of knowledge," the award's citation noted. "[H]e demonstrated to thousands of delighted students how to simplify complex subjects and how to marry theory and experiment. He also taught them a lesson in scientific modesty, as he constantly adopted the best solutions generated by his students."

Middlebrook was a Life Fellow of the IEEE and a Fellow of the IEE (UK). In addition to the Feynman Prize, he was the recipient of the Franklin Institute's Edward Longstreth Medal, the IEEE's Millennium and Centennial medals and its William E. Newell Power Electronics Award, and the Award for Excellence in Teaching, presented by the Board of Directors of the Associated Students of the California Institute of Technology.

He leaves behind a wife, Val, sons John Garrison and Joe Middler, daughter Trudy Wolsky, and grandchildren Chad and Teagan.

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Jon Weiner
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Caltech-Led Team Designs Novel Negative-Index Metamaterial that Responds to Visible Light

Uniquely versatile material could be used for more efficient light collection in solar cells

PASADENA, Calif.—A group of scientists led by researchers from the California Institute of Technology (Caltech) has engineered a type of artificial optical material—a metamaterial—with a particular three-dimensional structure such that light exhibits a negative index of refraction upon entering the material. In other words, this material bends light in the "wrong" direction from what normally would be expected, irrespective of the angle of the approaching light.

This new type of negative-index metamaterial (NIM), described in an advance online publication in the journal Nature Materials, is simpler than previous NIMs—requiring only a single functional layer—and yet more versatile, in that it can handle light with any polarization over a broad range of incident angles. And it can do all of this in the blue part of the visible spectrum, making it "the first negative index metamaterial to operate at visible frequencies," says graduate student Stanley Burgos, a researcher at the Light-Material Interactions in Energy Conversion Energy Frontier Research Center at Caltech and the paper's first author.

"By engineering a metamaterial with such properties, we are opening the door to such unusual—but potentially useful—phenomena as superlensing (high-resolution imaging past the diffraction limit), invisibility cloaking, and the synthesis of materials index-matched to air, for potential enhancement of light collection in solar cells," says Harry Atwater, Howard Hughes Professor and professor of applied physics and materials science, director of Caltech's Resnick Institute, founding member of the Kavli Nanoscience Institute, and leader of the research team

What makes this NIM unique, says Burgos, is its engineering.

"The source of the negative-index response is fundamentally different from that of previous NIM designs," he explains. Those previous efforts used multiple layers of "resonant elements" to refract the light in this unusual way, while this version is composed of a single layer of silver permeated with "coupled plasmonic waveguide elements."

Surface plasmons are light waves coupled to waves of electrons at the interface between a metal and a dielectric (a non-conducting material like air). Plasmonic waveguide elements route these coupled waves through the material. Not only is this material more feasible to fabricate than those previously used, Burgos says, it also allows for simple "tuning" of the negative-index response; by changing the materials used, or the geometry of the waveguide, the NIM can be tuned to respond to a different wavelength of light coming from nearly any angle with any polarization. "By carefully engineering the coupling between such waveguide elements, it was possible to develop a material with a nearly isotopic refractive index tuned to operate at visible frequencies."

This sort of functional flexibility is critical if the material is to be used in a wide variety of ways, says Atwater. "For practical applications, it is very important for a material's response to be insensitive to both incidence angle and polarization," he says. "Take eyeglasses, for example. In order for them to properly focus light reflected off an object on the back of your eye, they must be able to accept and focus light coming from a broad range of angles, independent of polarization. Said another way, their response must be nearly isotropic. Our metamaterial has the same capabilities in terms of its response to incident light."

This means the new metamaterial is particularly well suited to use in solar cells, Atwater adds. "The fact that our NIM design is tunable means we could potentially tune its index response to better match the solar spectrum, allowing for the development of broadband wide-angle metamaterials that could enhance light collection in solar cells," he explains. "And the fact that the metamaterial has a wide-angle response is important because it means that it can 'accept' light from a broad range of angles. In the case of solar cells, this means more light collection and less reflected or 'wasted' light."

"This work stands out because, through careful engineering, greater simplicity has been achieved," says Ares Rosakis, chair of the Division of Engineering and Applied Science at Caltech and Theodore von Kármán Professor of Aeronautics and Mechanical Engineering.

In addition to Burgos and Atwater, the other authors on the Nature Materials paper, "A single-layer wide-angle negative index metamaterial at visible frequencies," are Rene de Waele and Albert Polman from the Foundation for Fundamental Research on Matter Institute for Atomic and Molecular Physics in Amsterdam. Their work was supported by the Energy Frontier Research Centers program of the Office of Science of the Department of Energy, the National Science Foundation, the Nederlandse Organisatie voor Wetenschappelijk Onderzoek, and "NanoNed," a nanotechnology program funded by the Dutch Ministry of Economic Affairs.

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Lori Oliwenstein

Caltech Researchers Create "Sound Bullets"

Pulses could produce superior acoustic images; be used as sonic scalpels; probe for damage in opaque materials

PASADENA, Calif.—Taking inspiration from a popular executive toy ("Newton's cradle"), researchers at the California Institute of Technology (Caltech) have built a device—called a nonlinear acoustic lens—that produces highly focused, high-amplitude acoustic signals dubbed "sound bullets."

The acoustic lens and its sound bullets (which can exist in fluids—like air and water—as well as in solids) have "the potential to revolutionize applications from medical imaging and therapy to the nondestructive evaluation of materials and engineering systems," says Chiara Daraio, assistant professor of aeronautics and applied physics at Caltech and corresponding author of a recent paper in the Proceedings of the National Academy of Sciences (PNAS) describing the development.

Daraio and postdoctoral scholar Alessandro Spadoni, first author of the paper, crafted their acoustic lens by assembling 21 parallel chains of stainless steel spheres into an array. Each of the 21 chains was strung with 21 9.5-millimeter-wide spheres. (Daraio says particles composed of other elastic materials and/or with different shapes also could be used.)

The device is akin to the Newton's cradle toy, which consists of a line of identical balls suspended from a frame by wires in such a way that they only move in one plane, and just barely touch one another. When one of the end balls is pulled back and released, it strikes the next ball in line and the ball at the opposite end of the cradle flies out; the balls in the middle appear to remain stationary (but really are not, because of the nonlinear dynamics triggered in the system).

The chains of particles in Daraio's and Spadoni's acoustic lens are like a longer version of a Newton's cradle. In the lens, a pulse is excited at one end by an impact with a striker, and nonlinear waves are generated within each chain. These chains, Daraio says, "are the simplest representation of highly nonlinear acoustic waveguides, which exploit the properties of particle contacts to tune the shapes of the traveling acoustic signals and their speed of propagation, creating compact acoustic pulses known as solitary waves."

Solitary waves—unlike the rippling waves produced by dropping a pebble into a pond—can exist in isolation, neither preceded nor followed by other waves.

"The solitary waves always maintain the same spatial wavelength in a given system," she adds, "and can have very high amplitude without undergoing any distortion within the lens, unlike the signals produced by currently available technology."

The chains are squeezed closer together—or "precompressed"—using fishing line. By changing the amount of precompression, Daraio and Spadoni were able to vary the speed of the solitary wave. When a series of those waves exit the array, they coalesce at a particular location—a focal point—in a target material (which can be a gas, like air; a liquid; or a solid). This superposition of solitary waves at the focal point forms the sound bullet—a highly compact, large-amplitude acoustic wave. Varying the parameters of the system can also produce a rapid-fire barrage of sound bullets, all trained on the same spot.

In the current design, the spheres are assembled in a two-dimensional arrangement, with each row independent of its neighbors. "Three-dimensional arrangements will be just as easy to create and will allow 3-D control of the sound bullets' appearance and travel path," Spadoni says.

"Our lens introduces the ability to generate compact, high-amplitude signals in a linear medium, and also allows us to dynamically control the location of the focal point," Daraio says. That means it isn't necessary to change any of the geometric components of the lens to change the location of the focal point.

"All we do is adjust the precompression for each chain of spheres," she says.

This simple adjustment should make the sound bullets easy to adapt to a variety of applications. "Anybody who has had an ultrasound exam has noted that the operator switches the probes according to the characteristics and location within the body of what is being imaged," Daraio says. "The acoustic lens we propose would not require replacement of its components, but rather simple adjustments of the precompression on each chain."

The acoustic lens created by Daraio and Spadoni was intended to be a proof of concept, and is probably many years away from being used in commercial applications. "For practical uses," Daraio says, "an improved design for controlling the application of static precompression on each chain would be required-based, for example, on electronics rather than on mechanical impacts as is currently done in our lab."

Still, the instrument has the potential to surpass the clarity and safety of conventional medical ultrasound imaging. The pulses produced by the acoustic lens—which are an order of magnitude more focused and have amplitudes that are orders of magnitude greater than can be created with conventional acoustic devices—"reduce the detrimental effects of noise, producing a clearer image of the target." They also "can travel farther"—deeper within the body—"than low-amplitude pulses," Daraio says.

More intriguingly, the device could enable the development of a non-invasive scalpel that could home in on and destroy cancerous tissues located deep within the body.

"Medical procedures such as hyperthermia therapy seek to act on human tissues by locally increasing the temperature. This is often done by focusing high-energy acoustic signals onto a small area, requiring significant control of the focal region" so that healthy tissue is not also heated and damaged, Daraio explains. "Our lens produces a very compact focal region which could aid further development of hyperthermia techniques."

Furthermore, sound bullets could offer a nondestructive way to probe and analyze the interior of nontransparent objects like bridges, ship hulls, and airplane wings, looking for cracks or other defects.

"Today the performance of acoustic devices is decreased by their linear operational range, which limits the accuracy of the focusing and the amplitude achievable at the focal point," Daraio says. "The new nonlinear acoustic lens proposed with this work leverages nonlinear effects to generate compact acoustic pulses with energies much higher than are currently achievable, with the added benefit of providing great control of the focal position."

The paper, "Generation and control of sound bullets with a nonlinear acoustic lens," was funded by the Army Research Office and the National Science Foundation.

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Kathy Svitil
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The Light and Sound Fantastic

Producing coherent light on a microchip is old hat—LED lasers underpin our high-tech world, appearing in gadgets ranging from DVD players and supermarket checkout scanners to digital data lines. Now a chip-compatible component developed in the laboratory of Caltech Associate Professor of Applied Physics Oskar Painter (MS '95, PhD '01) can produce coherent sound as well, and even interconvert the two.

"What is new here is the ability to manipulate sound in a circuit with the same level of control, and in almost the same way, that we manipulate light or electrons," says collaborator Kerry Vahala (BS '80, MS '81, PhD '85), the Jenkins Professor of Information Science and Technology and professor of applied physics. "It helps to level the playing field for these three different particles—electrons, photons, and phonons."

This light-to-sound and sound-to-light translator, known in the trade as an optomechanical crystal, looks like the rope bridge in an Indiana Jones movie. The bridge traps photons of light and phonons—with an "n"—of sound in the gaps between its floorboards, causing the energy to resonate and accumulate in the acoustic equivalent of a laser. And, like a laser, a beam of sonic energy can carry information.

 

The photon trap looks like a rope bridge in an Indiana Jones movie. Photons can only run along the bridge for as long as the floorboards match their stride--that is, their wavelength. Light leaks from the thin, gray optical taper in the background into the bridge, and there it gets stuck.
With this technology, engineers will be able to design circuits to manipulate sound in almost the same way that we now use light or electrons, switching from one to another as best suits the application. For example, inserting so-called acoustical delays into fiber-optic systems will vastly increase the number of channels that can be sent through the same hardware.

The technology will also find use as a new tool for the physical sciences. Biolabs on a chip could measure subtle changes in a bridge's vibrations to detect and identify individual protein molecules. And the bridges are so insubstantial that they can behave quantum mechanically, allowing physicists to manipulate a nanoscale object that is literally neither here nor there. Farther down the road, this window on the mechanical quantum world could provide the input-output interface to quantum computers, which would exploit such quantum weirdnesses to solve problems uncrackable by ordinary machines.

Pairs of bridges span essentially bottomless pits on the microchip. Each bridge is about 30 millionths of a meter long and one millionth of a meter wide. The red and blue bands represent the trapped light.

Eventually, sonic "lasers" are likely to change our lives in ways we can't begin to imagine. Says postdoc Matthew Eichenfield (PhD '10), "When Charles Townes [PhD '39; Nobel laureate in physics, 1964] invented the maser, which eventually gave birth to the laser, he didn't envision CDs, or supermarket checkout scanners, or using them to write your name on the diamond in an engagement ring. We're in the ruby-laser era with these. In a few years, we'll be putting out phonons on a level with commercial lasers."

"This field of research is blossoming at Caltech," says Painter, "because we have a number of groups bringing together expertise in areas as diverse as nanofabrication and quantum optics. I think the next few years are going to be tremendously productive. I find it fascinating that we can work in areas that touch upon fundamental quantum physics and at the same time have a real impact on engineering and technology." 

Read the full story as a PDF or Zmag.

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Douglas Smith
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Caltech Receives More than $33 Million from American Recovery and Reinvestment Act

Neuroeconomics and the fundamentals of jet noise just some of the many projects supported

PASADENA, Calif.-Research in genomic sciences, astronomy, seismology, and neuroeconomics are some of the many projects being funded at the California Institute of Technology (Caltech) by the American Recovery and Reinvestment Act (ARRA).

As part of the federal government program of stimulating the economy, ARRA is providing approximately $21 billion for research and development. The goal is for the funding to lead to new scientific discoveries and to support jobs.

ARRA provides the funds to federal research agencies such as the National Institutes of Health, the National Science Foundation, and the Department of Energy, which then support proposals submitted by universities and other research institutions from across the country.

Caltech has received 82 awards to date, totaling more than $33 million. Spending from the grants began in the spring of 2009 and thus far has led to the support of 93 jobs at the Institute.

"This funding will help lead to substantive and important work here at Caltech," says Caltech president Jean-Lou Chameau. "We're grateful to have this opportunity to advance research designed to benefit the entire country."

For biologist Paul Sternberg, the Thomas Hunt Morgan Professor of Biology at Caltech and a Howard Hughes Medical Institute investigator, the ARRA funds mean an opportunity to improve upon WormBase, an ongoing multi-institutional effort to make genetic information on the experimental animal C. elegans freely available to the world.

"All biological and biomedical researchers rely on publicly available databases of genetic information," says Sternberg. "But it has been expensive and difficult to extract information from scientific research articles. We have developed some tools to make it less expensive and less tedious to get the job done, for WormBase and many other groups."

Sternberg's ARRA funds-$989,492-will go towards developing a more efficient approach to extracting key facts from published biological-science papers.

Among the other diverse Caltech projects receiving ARRA funds are:

  • a catalog of jellyfish DNA;
  • improving the speed of data collection at Caltech's Center of Excellence in Genomic Science;
  • studies into the fundamentals of particle physics;
  • the California High School Cosmic Ray Observatory (CHICOS) program, which provides high school students access to cosmic ray research;
  • the search for new astronomical objects such as flare stars and gamma-ray bursts, and the means to make those discoveries accessible to the public; and
  • a $1 million upgrade of the Southern California Seismic Network.

Caltech Professor of Mechanical Engineering Tim Colonius received ARRA funds for research into better understanding how noise is created by turbulence in the exhaust of turbofan aircraft engines and what might be done to mitigate it. Jet noise is an environmental problem subject to increasingly severe regulation throughout the world.

"To meet the ambitious noise-reduction goals under discussion, a greatly enhanced understanding of the basic physics is needed," says Colonius. "Very large-scale computer simulations and follow-up analyses will bring us much closer to the goal of discovering the subtle physical mechanisms responsible for the radiation of jet noise and allow us to develop methods for suppressing it."

Colonius received $987,032 in ARRA funds from the National Science Foundation.

Colin Camerer, the Robert Kirby Professor of Behavioral Economics, received his ARRA funds to explore the application of neurotechnologies to solving real-life economic problems.

"Our project, with my Caltech colleague Antonio Rangel, will explore the psychological and neural correlates of value and decision-making and their use in improving the efficiency of social allocations," says Camerer.

Camerer and his colleagues previously found that they could use information obtained through functional magnetic resonance imaging measurements to develop solutions to economic challenges.

Rangel, an associate professor of economics, has a second ARRA-funded project to analyze the neuroeconomics of self-control in dieting populations.

"Funding of this nature is critical to much of the work we do here at Caltech," adds Chameau. "And with ARRA support, dramatic discoveries may be just around the corner."

For a complete list of ARRA projects, visit: http://www.recovery.gov

# # #

About Caltech:

Caltech is recognized for its highly select student body of 900 undergraduates and 1,200 graduate students, and for its outstanding faculty. Since 1923, Caltech faculty and alumni have garnered 32 Nobel Prizes and five Crafoord Prizes.

In addition to its prestigious on-campus research programs, Caltech operates the Jet Propulsion Laboratory (JPL), the W. M. Keck Observatory in Mauna Kea, the Palomar Observatory, and the Laser Interferometer Gravitational-Wave Observatory (LIGO). Caltech is a private university in Pasadena, California. For more information, visit http://www.caltech.edu.

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Jon Weiner
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Caltech Researchers Create Highly Absorbing, Flexible Solar Cells with Silicon Wire Arrays

PASADENA, Calif.—Using arrays of long, thin silicon wires embedded in a polymer substrate, a team of scientists from the California Institute of Technology (Caltech) has created a new type of flexible solar cell that enhances the absorption of sunlight and efficiently converts its photons into electrons. The solar cell does all this using only a fraction of the expensive semiconductor materials required by conventional solar cells.

"These solar cells have, for the first time, surpassed the conventional light-trapping limit for absorbing materials," says Harry Atwater, Howard Hughes Professor, professor of applied physics and materials science, and director of Caltech's Resnick Institute, which focuses on sustainability research.

The light-trapping limit of a material refers to how much sunlight it is able to absorb. The silicon-wire arrays absorb up to 96 percent of incident sunlight at a single wavelength and 85 percent of total collectible sunlight. "We've surpassed previous optical microstructures developed to trap light," he says. 

Atwater and his colleagues—including Nathan Lewis, the George L. Argyros Professor and professor of chemistry at Caltech, and graduate student Michael Kelzenberg—assessed the performance of these arrays in a paper appearing in the February 14 advance online edition of the journal Nature Materials.

Atwater notes that the solar cells' enhanced absorption is "useful absorption."

"Many materials can absorb light quite well but not generate electricity—like, for instance, black paint," he explains. "What's most important in a solar cell is whether that absorption leads to the creation of charge carriers."

The silicon wire arrays created by Atwater and his colleagues are able to convert between 90 and 100 percent of the photons they absorb into electrons—in technical terms, the wires have a near-perfect internal quantum efficiency. "High absorption plus good conversion makes for a high-quality solar cell," says Atwater. "It's an important advance."

The key to the success of these solar cells is their silicon wires, each of which, says Atwater, "is independently a high-efficiency, high-quality solar cell." When brought together in an array, however, they're even more effective, because they interact to increase the cell's ability to absorb light.

"Light comes into each wire, and a portion is absorbed and another portion scatters. The collective scattering interactions between the wires make the array very absorbing," he says.

This is a photomicrograph of a silicon wire array embedded within a transparent, flexible polymer film.
Credit: Caltech/Michael Kelzenberg

This effect occurs despite the sparseness of the wires in the array—they cover only between 2 and 10 percent of the cell's surface area.

"When we first considered silicon wire-array solar cells, we assumed that sunlight would be wasted on the space between wires," explains Kelzenberg. "So our initial plan was to grow the wires as close together as possible. But when we started quantifying their absorption, we realized that more light could be absorbed than predicted by the wire-packing fraction alone. By developing light-trapping techniques for relatively sparse wire arrays, not only did we achieve suitable absorption, we also demonstrated effective optical concentration—an exciting prospect for further enhancing the efficiency of silicon-wire-array solar cells."

Each wire measures between 30 and 100 microns in length and only 1 micron in diameter. “The entire thickness of the array is the length of the wire,” notes Atwater. “But in terms of area or volume, just 2 percent of it is silicon, and 98 percent is polymer.”

In other words, while these arrays have the thickness of a conventional crystalline solar cell, their volume is equivalent to that of a two-micron-thick film.

Since the silicon material is an expensive component of a conventional solar cell, a cell that requires just one-fiftieth of the amount of this semiconductor will be much cheaper to produce.

The composite nature of these solar cells, Atwater adds, means that they are also flexible. "Having these be complete flexible sheets of material ends up being important," he says, "because flexible thin films can be manufactured in a roll-to-roll process, an inherently lower-cost process than one that involves brittle wafers, like those used to make conventional solar cells."

Atwater, Lewis, and their colleagues had earlier demonstrated that it was possible to create these innovative solar cells. "They were visually striking," says Atwater. "But it wasn't until now that we could show that they are both highly efficient at carrier collection and highly absorbing."

The next steps, Atwater says, are to increase the operating voltage and the overall size of the solar cell. "The structures we've made are square centimeters in size," he explains. "We're now scaling up to make cells that will be hundreds of square centimeters—the size of a normal cell."

Atwater says that the team is already "on its way" to showing that large-area cells work just as well as these smaller versions.

In addition to Atwater, Lewis, and Kelzenberg, the all-Caltech coauthors on the Nature Materials paper, "Enhanced absorption and carrier collection in Si wire arrays for photovoltaic applications," are postdoctoral scholars Shannon Boettcher and Joshua Spurgeon; undergraduate student Jan Petykiewicz; and graduate students Daniel Turner-Evans, Morgan Putnam, Emily Warren, and Ryan Briggs.

Their research was supported by BP and the Energy Frontier Research Center program of the Department of Energy, and made use of facilities supported by the Center for Science and Engineering of Materials, a National Science Foundation Materials Research Science and Engineering Center at Caltech. In addition, Boettcher received fellowship support from the Kavli Nanoscience Institute at Caltech.

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Lori Oliwenstein
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