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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Watson Lecture: Creating Laboratory Earthquakes

What They Can Teach Us

The recent 7.0-magnitude Haiti earthquake on January 12 caused catastrophic damage to the island nation—once again reminding us that these natural disasters are ever-present and that their force can be devastating.  And locally, leading scientists have concluded that a 7.8-magnitude earthquake could occur on the San Andreas Fault, which would cause major damage to the infrastructure, crippling California and the West Coast.

Now imagine an extremely fast or "intersonic" earthquake that produces a shock-wave pattern similar to a jet fighter breaking the sound barrier. Imagine the effect on buildings and structures of the shock wave or Mach cone, as it is called, produced by such an earthquake. The propagating waves generated by such special ruptures could produce potentially catastrophic ground shaking—equivalent to a sonic boom traveling through the ground—with unexpected implications to seismic hazard analysis.

Scientists at the California Institute of Technology (Caltech) have demonstrated that these high-speed intersonic ruptures, which propagate at speeds between three to six kilometers per second, do exist and could occur during the next major earthquake.  Utilizing an innovative methodology, Caltech researchers now have the ability to create laboratory earthquakes of varying force and magnitudes that mimic actual quakes.  By triggering laboratory earthquakes, researchers can utilize ultrahigh-speed imaging tools to study the behavior of quakes, the force of wave propagation, and intersonic rupture impact, allowing better measurement of the potential force and destructiveness of earthquakes—without a real quake actually occurring.

The scientists have also developed complex simulations of the effects of these shock waves on buildings and the likelihood of Southern California buildings and structures to withstand this type of impact.  These high-performance simulations can determine ground motion, rupture propagation, and structural response, to help ultimately identify remedial measures to prevent the collapse of buildings.

In his Earnest C. Watson Lecture on February 17, at 8 p.m., Ares Rosakis, the Theodore von Karman Professor of Aeronautics and professor of mechanical engineering; chair, Division of Engineering and Applied Science, will present a talk entitled "Intersonic Earthquakes: What Laboratory Earthquakes Teach Us About Real Ones."  He will explain how his team of collaborators has demonstrated the existence of intersonic earthquakes. He will discuss his research for triggering laboratory-generated earthquakes, and will also discuss simulation methodologies for determining the probability of building collapse and structural damage and for estimating loss. His colleagues on these findings include professors Hiroo Kanamori, Swaminathan Krishnan, and Nadia Lapusta; graduate student Michael Mello; and postdoctoral scholar Harsha Bhat.

Studying earthquakes inherently presents a host of insurmountable difficulties—for example, our inability to trigger a "real" quake or control the magnitude or speed of rupture propagation.  Laboratory-generated quakes give us an opportunity to measure and obtain valuable information that can better prepare us for the mitigation of potentially destructive hazards.

The lecture will take place in Beckman Auditorium, 332 S. Michigan Avenue, south of Del Mar Boulevard, on the Caltech campus in Pasadena.  Seating is available on a free, first-come, first-served basis.

For over 85 years, Caltech has presented the Earnest C. Watson Lecture Series.  It was conceived by the late Caltech physicist Earnest Watson as a way to provide scientific insight for the general public and local community.

Upcoming lectures in the 2009-2010 series include:

  • April 7, "The Ancient California River and How It Carved the Grand Canyon in the Age of T. Rex," by Brian P. Wernicke, Chandler Family Professor of Geology;
  • May 5, "From Newton's Cradle to New Materials," by Chiara Daraio, assistant professor of aeronautics and applied physics;
  • May 19, "Neuronal Mechanisms of Memory Formation," by Thanos Siapas, associate professor of computation and neural systems and Bren Scholar.
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Caltech Researchers Develop Nanoscale Structures with Superior Mechanical Properties

PASADENA, Calif.—Researchers at the California Institute of Technology (Caltech) have developed a way to make some notoriously brittle materials ductile—yet stronger than ever—simply by reducing their size. 

The work, by Dongchan Jang, senior postdoctoral scholar, and Julia R. Greer, assistant professor of materials science and mechanics at Caltech, could eventually lead to the development of innovative, superstrong, yet light and damage-tolerant materials. These new materials could be used as components in structural applications, such as in lightweight aerospace vehicles that last longer under extreme environmental conditions and in naval vessels that are resistant to corrosion and wear. 

A paper about the work appears in the February 7 advance online edition of the journal Nature Materials.

"Historically," says Greer, "structural materials have always had to rely on their processing conditions, and thereby have been 'slaves' to their properties." For example, ceramics are very strong, which makes them great for structural applications. At the same time, these materials are very heavy, which is problematic for many applications, and they are extremely brittle, which is less than ideal for supporting heavy loads. In fact, says Greer, "they fail catastrophically under mechanical loads." Metals and alloys, on the other hand, are ductile, and therefore unlikely to shatter, but they lack the strength of ceramics. 

Materials scientists have developed an intriguing class of materials called glassy metallic alloys, which are amorphous and lack the crystalline structure of traditional metals. The materials, also known as metallic glasses, are composed of random arrangements of metallic elements like zirconium, titanium, copper, and nickel. They are lightweight—a "huge advantage" for their incorporation into new types of devices, Greer says—and yet are comparable in strength to ceramics. Unfortunately, their random structure makes metallic glasses quite brittle. "They also fail catastrophically under tensile loads," she says. 

But now Greer and Jang, the first author on the Nature Materials paper, have developed a strategy to overcome these obstacles—by making metallic glasses that are almost vanishingly small.

The scientists devised a process to make zirconium-rich metallic glass pillars that are just 100 nanometers in diameter—roughly 400 times narrower than the width of a human hair. At this size, Greer says, "the metallic glasses become not only even stronger, but also ductile, which means they can be deformed to a certain elongation without breaking. Strength plus ductility," she says, represents "a very lucrative combination for structural applications." 

As yet, there are no immediate applications for the new materials, although it may be possible to combine the nanopillars into arrays, which could then form the building blocks of larger hierarchical structures with the strength and ductility of the smaller objects. 

The work, however, "convincingly shows that 'size' can be successfully used as a design parameter," Greer says. "We are entering a new era in materials science, where structural materials can be created not only by utilizing monolith structures, like ceramics and metals, but also by introducing 'architectural' features into them."

For example, Greer is working toward fabricating a "brick-and-mortar" architecture using tiny plates of a metallic glass and ultrafine-grained ductile metal with nanoscale dimensions that could then be used to fabricate new engineering composites with amplified strength and ductility.

To use this architecture-driven approach to create structural materials with enhanced properties—that are, for example, superstrong, yet light and ductile—researchers must understand how each constituent part deforms during use and under stress.

"Our findings," she says, "provide a powerful foundation for utilizing nanoscale components, which are capable of sustaining very high loads without exhibiting catastrophic failure, in bulk-scale structural applications specifically by incorporating architectural and microstructural control."

Adds Greer: "The particularly cool aspect of the experiment is that it is nearly impossible to do! Dongchan, my amazing postdoc, was able to make individual 100-nanometer-diameter tensile metallic glass nanopillar samples, which no one had ever done before, and then used our custom-built in situ mechanical deformation instrument, SEMentor, to perform the experiments. He fabricated the samples, tested them, and analyzed the data. Together we were able to interpret the results and to formulate the phenomenological theory, but the credit goes all to him."

The work in the Nature Materials paper, "Transition from a strong-yet-brittle to a stronger-and-ductile state by size reduction of metallic glasses," was funded by the National Science Foundation and the Office of Naval Research, and utilized the fabrication and characterization facilities of the Kavli Nanoscience Institute at Caltech. 

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Kathy Svitil
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Caltech Physicists Propose Quantum Entanglement for Motion of Microscopic Objects

PASADENA, Calif.—Researchers at the California Institute of Technology (Caltech) have proposed a new paradigm that should allow scientists to observe quantum behavior in small mechanical systems. 

Their ideas, described in the early online issue of the Proceedings of the National Academy of Sciences, offer a new means of addressing one of the most fascinating issues in quantum mechanics: the nature of quantum superposition and entanglement in progressively larger and more complex systems. 

A quantum superposition is a state in which a particle, such as a photon or atom, exists simultaneously in two locations. Entanglement, which Albert Einstein called "spooky action at a distance," allows particles to share information even if they are physically separated.

A key challenge in observing quantum behavior in a small mechanical system is suppressing interactions between the system and its noisy environment—i.e., the surrounding material supporting the system or any other external contact. The random thermal vibrations of the system's surroundings, for example, can be transferred to the mechanical object and destroy its fragile quantum properties. To address this issue, a number of groups worldwide have begun to use cryogenic setups in which the immediate environment is cooled down to a very low temperature to reduce the magnitude of these random vibrations.

The Caltech team suggests a fundamentally different approach: using the forces imparted by intense beams of light to "levitate" the entire mechanical object, thereby freeing it from external contact and material supports. This approach, the researchers show, can dramatically reduce environmental noise, to the point where diverse manifestations of quantum behavior should be observable even when the environment is at room temperature. 

Among the scientists involved in the work are Darrick Chang, a postdoctoral scholar at Caltech's Institute for Quantum Information; Oskar Painter, associate professor of applied physics; and H. Jeff Kimble, Caltech's William L. Valentine Professor and professor of physics.

The idea of using optical forces to trap or levitate small particles is actually well established. It was pioneered by Arthur Ashkin of Bell Laboratories in the 1970s and 1980s, and has since formed the basis for scientific advances such as the development of "optical tweezers"—which are frequently used to control the motion of small biological objects—and the use of lasers to cool atoms and trap them in space. These techniques provide an extremely versatile toolbox for manipulating atoms, and have been employed to demonstrate a variety of quantum phenomena at the atomic level. 

In the new work, Chang and his colleagues demonstrate theoretically that similar success can be achieved when an individual atom is replaced by a much more massive—but still nanoscale—mechanical system. A related scheme has been presented simultaneously by a group at the Max Planck Institute of Quantum Optics in Garching, Germany [http://arxiv.org/abs/0909.1469]. 

The system proposed by the Caltech team consists of a small sphere made out of a highly transparent material such as fused silica. When the sphere comes into contact with a laser beam, optical forces naturally push the sphere toward the point where the intensity of light is greatest, trapping the sphere at that point. The sphere typically spans about 100 nm in diameter, or roughly a thousandth the width of a human hair.  Because of its small size, the sphere's remaining interactions with the environment—any that don't involve direct contact with another material, because the sphere is levitating—are sufficiently weak that quantum behavior should easily emerge.

For such behavior to appear, however, the sphere must also be placed inside an optical cavity, which is formed by two mirrors located on either side of the trapped sphere. The light that bounces back and forth between the mirrors both senses the motion of the sphere and is used to manipulate that motion at a quantum-mechanical level.

The researchers describe how this interaction can be used to remove energy from, or cool, the mechanical motion until it reaches its quantum ground state—the lowest energy allowable by quantum mechanics. A fundamental limit to this process is set by the relative strengths of the optical cooling and the rate at which the environment tends to heat (return energy to) the motion, bringing it back to the ambient temperature. 

In principle, the motion of the well-isolated sphere can be cooled starting from room temperature down to a final temperature that is ten million times lower; in that super-cooled state, the center of mass of the sphere moves by only the minimum possible amount set by intrinsic quantum fluctuations. 

The researchers also propose a scheme to observe a feature known as entanglement, which lies at the heart of quantum mechanics. Two remotely located systems that are quantum entangled share correlations between them that are stronger than classically allowed. In certain circumstances, entanglement can be a very valuable resource; it forms the basis for proposals to realize improved metrology and more powerful (quantum) computers.

The proposed scheme consists of sending a pair of initially entangled beams of light —the production of which was first accomplished by Kimble's group at Caltech in 1992—into two separate cavities, each containing a levitated sphere. Through a process known as quantum-state transfer, all of the properties of the light—in particular, the entanglement and its associated correlations—can be mapped onto the motion of the two spheres. 

While the sizes of these nanomechanical objects are still very far from those we associate with everyday experience, the Caltech researchers believe that their proposal presents an exciting opportunity to realize and control quantum phenomena at unprecedented scales—in this case, for objects containing approximately 10 million atoms.

Other researchers involved in this work include graduate student Dalziel Wilson and postdoctoral scholars Cindy Regal and Scott Papp at Caltech; Jun Ye, a fellow at JILA, a joint institute of the University of Colorado at Boulder and the National Institute of Standards and Technology; and Peter Zoller, a professor at the University of Innsbruck. The work was initiated while Ye and Zoller were visiting as Gordon and Betty Moore Distinguished Scholars at Caltech.

The work in the PNAS paper, "Cavity optomechanics using an optically levitated nanosphere," was supported by the Gordon and Betty Moore Foundation, the National Science Foundation, the Army Research Office, Northrop Grumman Space Technology, the Austrian Science Fund, and European Union Projects.

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Kathy Svitil
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