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. 

Kathy Svitil

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.

Kathy Svitil
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The Book of Bruck

The 2008 Winner of Caltech's Top Teaching Prize Talks About the Art and Science of Sharing Information

How would Jehoshua "Shuki" Bruck sum up his success as a teacher in just three words? "I love ignorance," declares the exuberant winner of Caltech's 2008 Feynman Prize for Excellence in Teaching.  

Shuki, as everyone calls him, is Caltech's Gordon and Betty Moore Professor of Computation and Neural Systems and Electrical Engineering. The Feynman Prize citation praised him for his "development of a new course, IST [Information Science and Technology] 4, 'Information and Logic,' as well as other teaching activities, including informal mentoring of undergraduates." An avalanche of recommendations commented on the breadth, humor, warmth, openness, and originality he brings to his teaching and on the special quality of his relationship with students. No one mentioned ignorance.

Shuki first taught IST4 in 2007, and he credits Caltech's interdisciplinary environment with sparking his interest in the subject not long after he arrived on campus in 1994. He played a leading role in conceiving and establishing Caltech's IST program, which culminated this October in the opening of the new Walter and Leonore Annenberg Center for Information Science and Technology.  Heidi Aspaturian of Caltech's communications office recently sat down with Shuki to get a firsthand idea of what it takes to be deemed a great teacher at Caltech.

Many people who nominated you for the Feynman Prize described IST4 as one of the best courses, if not the best, that they had taken at Caltech. What can you tell us about it?

SB: I first started thinking about this topic many years ago. Caltech turns out to be the perfect environment for thinking broadly about information science because we have so few disciplinary boundaries. Since I first came here, I have been very lucky to interact with people across all our divisions, from biology to physics to the humanities. And that helped me to realize that the topic of information systems encompasses far more than just computers. The cell, the brain, the Internet, and national and global economies are all information systems. What they all have in common is that information flows in and some manifestation of it flows out.

At the same time I realized that the biggest challenge in trying to understand all these different systems is that we don't have a good language—good mathematical tools, if you will—for dealing with them. We are collecting huge amounts of information, and we not only don't know what to do with all of it, we are also missing a good unifying framework for thinking about it. And yet that hasn't stopped us from being able to design, build, program, and use complex systems like tiny computer chips with one billion components to solve extremely difficult computations. How are we able to do it?

That question led me to a much larger one, namely, how did we get here from there?  How did we, as humans, get from having no conscious concept of numbers to a point where we are suddenly awash in all this advanced computational technology? What were the key milestones in this evolution? Classes about information science don't really deal with these questions. They focus on what's happened over the last 50 years or so, when that is really only a small part of a much bigger picture.

So I started reading and the more I read, the more questions I had. And you know, one of the best ways for a professor to learn more about a topic is to teach it. That's when I designed and started teaching this course. Three years later, I have to say, I still feel ignorant about the subject. But I love ignorance. It's something I truly love about my job—how to enjoy ignorance, and how to turn that feeling of ignorance into high energy and curiosity that I can then share and pass on to my students.

How do you go about relishing ignorance in this particular class?

SB: I begin with what you might call the puzzle of our innate sense for quantities. How is it, for example, that when we look at two piles of apples, we somehow know that one pile contains more fruit than the other?  Without knowing about the actual numbers themselves, we can still make this determination. There is a whole research area in cognitive science regarding number sense and the discovery that many animals—rats, crows, dogs, and so forth—have at least a rudimentary quantitative understanding. We now know that babies, long before they can talk, are able to relate to quantities and are pretty good at adding small numbers. So the first problem sets relate to these questions.

From there we look at early civilizations, starting with the Babylonians, who were amazingly sophisticated with numbers and numerical calculations. Among many other things, they invented base 60, which still serves as the basis for how we tell time. In computation, they had a wonderful representation that is actually a combination of base 60 and the binary system.

So we start with anthropology and ancient cultures, and even as we move into the Age of Enlightenment and the dawn of computational science in the 17th century, I try to retain this context. Eventually we reach the 20th century and the birth of modern computational approaches and computer technologies.

It is an evolving process, and I am always adding and experimenting with new material. Last year, I introduced material about language, specifically literacy versus illiteracy, and how that affects our thinking and our perceptions. We talked about some very interesting research related to how the ability to read and write shapes the way we perceive the world, and we tried to connect that aspect to the development of mathematics as our language for reasoning about the physical world. When I teach the class in '10, I am planning to bring in the whole notion of how creativity works. We'll talk about some famous innovators who developed ideas that are completely out of the intellectual mainstream and we'll explore how that occurred.

When your students encounter this rather unorthodox blend of hard science, mathematics, and intellectual history, how do they react?

SB: Some students really like it. Some just tolerate it. There are others who say, okay, just show us how we got computers and let us work on computational techniques—who cares about the Babylonians?  In some sense this relates to the whole philosophy-of-education question—whether it is more important to teach content or context. As I said earlier, I think we need to teach both. I will sometimes say to my students: This class is useless in the sense that you cannot take it and go get a job in a high-tech company and do something useful. This is not the idea. The idea is to teach you about the context in which ideas evolve, to amplify your curiosity, and to help you understand the dynamics of creativity and problem solving in a global sense over time. That's what I'm trying to convey.

 How would you describe your overall philosophy of teaching? Or do you have one?

SB: I'll tell you a story. When my son was in second grade, he asked me about something mathematical, I think involving addition. I was trying to explain to him the idea and afterward, he said, "Dad, when you were explaining this, you were smiling all the time. Why?" And I said, "It's because I like this stuff." And you see, that's a key thing that we need to teach. We are teaching "the smile."  We as teachers need to show the students that it's fun to understand the ideas that we present. This is how you amplify interest and curiosity. And you cannot cheat on that, right?  If it's not fun, who will want do it?

Feat of Clay: This tablet, dating back to at least 1,600 BCE, depicts Babylonian mathematical calculations in the civilization's cuneiform script.
Credit: Yale University Collection

But I think that the most important thing we do is actually shown on the Caltech logo, where you see the flame, the torch, and the two hands. The two hands aren't holding the torch together; one hand is passing it to the other. And this is something that is key to civilization. It is like DNA, but it is intellectual DNA—passing down the light of knowledge. To do that, it's not enough to teach and learn the mechanics of how to solve a quadratic equation. If you want to deepen understanding, you need to talk about the context in which the ideas you're teaching evolved.

I always try to emphasize that solving a problem or coming up with new ideas is a process, and that as students become accustomed to this approach, they should feel joyful in accomplishing it.  When I give them their early homework sets, I will tell them that certain problems will probably not be difficult—they are directly related to what we have studied in class. However, they will also find that other problems are not so straightforward, that they require more thinking. I tell them, "At first, you won't have any idea how to approach it. That's wonderful—that's the feeling you should have. Go and have fun. Go play music, go to the gym, go and see a movie. When you come back, take a look at it again, and the next morning you will have some idea, and you need to let yourself go through that."

Nearly every letter about you to the Feynman Prize Committee commented on the strong personal interest that you take in your students' welfare, both in and beyond the classroom. Do you think this attitude comes naturally to you, or did it develop out of years of teaching?

SB: What I think it comes down to is that I really like people. It's very hard to teach people unless you also have a sense of how they think and feel and where they are emotionally. I believe that classroom teaching is just a small part of our job. In the end what the students remember about the coursework is not the key. The key is that hopefully I am serving as a good model for them with regard to how to behave in a position like this—how to teach and how to relate to other people. And I hope that I can give them an example of what I believe is a good value system. Sometimes my students will tell me, "You helped me so much, and I don't know how I can thank you properly."  I tell them it's very simple: just do the same thing when you are in my position.

You spent almost two years, in 2006–2008, living with your family among students as a professor-in-residence at Avery House. What impact did that have on your teaching?

SB: I think it made me a better teacher, particularly of undergraduates. In my first years here, I taught mainly graduate classes, and my interaction with undergraduates was limited. I don't think I was able to "read" what they were saying all that well. Living in Avery, I began to feel that I could understand them much better. I suddenly became much more knowledgeable about the challenges of being an undergrad here.

What would you say are the top three challenges?

SB: The students who come here are gifted in so many ways—it's unbelievable. But for many of them, it's their first time in a community where everyone else is just as talented. It can be disconcerting to discover this, and there are different ways to handle it. One way is to be extremely happy that now you too are ordinary. Personally I feel like an average guy here at Caltech, and it's a great feeling, because this is how I know I am at the right place. For some people, however, it's hard to deal with. My experience with the students here is that the happy ones are those who accept who they are, regardless of where they are academically. Even if you are the best student at Caltech, you still need to learn how to manage that. Don't be arrogant. Do develop a social system that is not based on academic achievements but on interpersonal relationships and emotions. It doesn't matter what your class ranking is. You need to accept it and forget about it, and treat yourself as a whole person.

The second challenge is that when someone is the best student in their school, or their city, or maybe even in their state, then that is "their" movie. You know the saying that everyone lives in some kind of a movie?  That's their role—best student in New York. Then they come to Caltech. Now they are just one of the undergrads at Caltech. They are not even the best student in their student house. So suddenly it's a different movie and they need to adjust to their new role. That's a positive challenge, in my opinion.

The third challenge is that when these students are in high school, they are so talented that they can do very well without actually having to go into much depth about the material they are studying. It's not so easy to skate by that way here, and learning how to delve deeply into subjects or ideas doesn't just happen overnight. It's a matter partly of talent and partly of curiosity. And I think the main challenge for us as teachers is to make sure that curiosity flourishes.

If young colleagues just starting out asked you for advice on how to be a successful teacher, what would you tell them?

SB: Let me just say first that I am not sure I am a successful teacher. This award was a huge honor, but an even bigger surprise. Over the years I have started to feel more confident about teaching, but I still have a long way to go.

But I think that the key is really to love the material. It all starts with that and continues with a constant effort to discover new possibilities in the material and new ways to explain it, first to yourself, and then to others. That is how you pass on the light to the next generation. And you have to be willing to take risks. It's never clear when you set out to explain something whether the approach you've chosen is going to work. It might work well in your mind but then not so well in practice. It might work with one audience but fall flat with another. I have to say that I am naturally nervous about my teaching. I might not appear that way, but I put a huge amount of time and thought into preparing for my classes and I am always very anxious before a class. I have some story line that's all over the place in my brain and I'm trying to get it out in a lucid way. You have to be willing to experiment, to listen to feedback from your students, and to accept the fact that not everything you try is going to succeed.

What's most important, I think, is to reach into whatever your passion is and try to express it, explain it, and if it doesn't work well the first time, then try again.

And, again, I think the connection with the students is really why we do this job. Even if all that my students learn is how I treat them in class and how I respond to questions, that's a valuable lesson about information. It may be the most valuable one I can teach.

So that would be my suggestion to young people who aspire to teach: try to connect with your students because, in the end, whatever they get from you, you will get so much more from them. And I should also mention something that strikes me each year as I look at a new class for the first time: your students help keep you young. We get a bit older every year, but our undergraduates—they stay forever the same age.

Heidi Aspaturian
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Caltech Scientists Find Emotion-like Behaviors, Regulated by Dopamine, in Fruit Flies

Finding may provide new insights into the neurological basis of ADHD, learning deficits, and more

PASADENA, Calif.—Scientists at the California Institute of Technology (Caltech) have uncovered evidence of a primitive emotion-like behavior in the fruit fly, Drosophila melanogaster.

Their findings, which may be relevant to the relationship between the neurotransmitter dopamine and attention deficit hyperactivity disorder (ADHD), are described in the December issue of the journal Neuron.

The Drosophila brain contains only about 20,000 neurons and has long been considered a powerful system with which to study the genetic basis of behaviors such as learning and courtship, as well as memory and circadian rhythms. What hasn't been clear is whether the Drosophila brain also could be used to study the genetic basis of "emotional" behaviors.

"Such studies are important," says David Anderson, Caltech's Seymour Benzer Professor of Biology and a Howard Hughes Medical Institute investigator, "because it's believed that abnormalities in these types of behaviors may underlie many psychiatric disorders."

Most of the genes found in the fruit fly—more accurately referred to as the vinegar fly—are found in humans as well, including those neurons that produce brain chemicals like dopamine and serotonin, which have been implicated in psychiatric disorders.

In their Neuron paper, the Caltech team—led by postdoctoral fellow Tim Lebestky—found that a series of brief but brisk air puffs, delivered in rapid succession, caused flies to run around their test chamber in what Anderson calls a "frantic manner." This behavior persisted for several minutes after the last of the puffs.

"Even after the flies had 'calmed down,'" he adds, "they remained hypersensitive to a single air puff."

To quantify the flies' behavior, Anderson's group collaborated with Pietro Perona, the Allen E. Puckett Professor of Electrical Engineering at Caltech. Together with his students, Perona developed an automated machine-vision-based system to track the movement of the flies, and derived a simple mathematical model to fit the movement data and to extract metrics that described various aspects of the flies' responses under different conditions.

The researchers used this test to search for flies with an abnormally exaggerated hyperactivity response; genetic studies of these flies revealed that a mutation in a dopamine receptor (a mutation that eliminates the receptor) produced the aberrant behavior. Flies with this dopamine-receptor mutation were hypersensitive to the air puffs, and took much longer to calm down than did "normal" flies without the mutation.

What is surprising about this result, notes Lebestky, “is that previous studies in both flies and vertebrates had suggested that dopamine promotes activity, but our experiments uncovered a function of dopamine in the opposite direction."  Because removing the receptor causes hypersensitivity to the air puffs, these results “suggested that dopamine actively inhibits the hyperactivity response,” Lebestky says.

This observation suggested a possible link to ADHD, a behavioral disorder characterized by impulsivity, hyperactivity, and short attention span. Humans with the disorder often take drugs, such as Ritalin, that increase levels of brain dopamine in order to reduce hyperactivity.

The ways the mutant flies respond to the air puffs is, moreover, "reminiscent of the way in which individuals with ADHD display hypersensitivity to environmental stimuli and are more easily aroused by such influences," says Anderson. Importantly, ADHD has been genetically linked to abnormalities of the dopamine system in humans, further strengthening the analogy between the mutant flies and this psychiatric disorder.

There is also another possible link: some individuals with ADHD display learning disabilities. Similarly, researchers from Pennsylvania State University—who collaborated on the Neuron study—have shown previously that flies with the same dopamine receptor mutation are unable to learn to associate a particular odor with an electric shock, and do not avoid the odor when subsequently tested. (Flies without the mutation quickly learn to make the association.)

It is often assumed that because individuals with ADHD are hyperactive and easily distracted, they have difficulty learning. The Caltech group showed, however, that hyperactivity and learning disabilities are not causally related in flies bearing the dopamine receptor mutation, thereby disproving the theory—at least in flies.

"We could separately 'rescue' the hyperactivity and learning deficits in a completely independent manner," says Anderson, "by genetically restoring the dopamine receptor to different regions of the fly's brain."

Thus, in dopamine-receptor-mutant flies, hyperactivity does not seem to cause learning deficits. Instead, these two "symptoms" reflect independent effects of the mutation that manifest themselves in distinct brain regions. "Being able to observe and manipulate the different neural substrates for learning and arousal will hopefully give us a unique method for identifying new molecular pathways that could be investigated and validated in higher organisms," says Lebestky.

This finding in flies, notes Anderson, raises the possibility that hyperactivity and learning deficits also may not be causally linked in humans with ADHD. If so, he says, it ultimately may prove more effective to develop drugs to treat these two symptoms separately, rather than trying to cure them both with the broad-spectrum pharmaceuticals currently available, which have many undesirable side effects.

"The finding that flies exhibit emotion-like behaviors that are controlled by some of the same brain chemicals as in humans opens up the possibility of applying the powerful genetics of this 'model organism' to understanding how these chemicals influence behavior through their actions on specific brain circuits," says Anderson. "While the specific details of where and how this occurs are likely to be different in flies and in humans, the basic principles are likely to be evolutionarily conserved, and may aid in our understanding of what goes wrong in disorders such as ADHD."

In addition to Lebestky, Anderson, and Perona, other researchers on the Neuron paper, "Two Different Forms of Arousal in Drosophila are Oppositely Regulated by the Dopamine D1 Receptor Ortholog DopR via Distinct Neural Circuits," are Jung-Sook Chang, Heiko Dankert, and Lihi Zelnik from Caltech; Young-Cho Kim and Kyung-An Han from Pennsylvania State University; and Fred Wolf from the University of California, San Francisco. Their work was funded by grants from the National Science Foundation and the Howard Hughes Medical Institute.

Lori Oliwenstein

Caltech and Dow Chemical Team Up in Solar Materials Effort

Collaboration includes the creation of Dow Graduate Fellowship in Chemical Sciences and Engineering

PASADENA, Calif.—The California Institute of Technology (Caltech) and the Dow Chemical Company today announced a new solar-research collaboration aimed at developing the use of semiconductor materials that are less expensive and more abundant than those used in many of today's solar cells.

In addition, they announced the creation of the Dow Chemical Company Graduate Fellowship in Chemical Sciences and Engineering.

The fellowship will be granted to a second- or third-year doctoral student who shows excellence in research, leadership, and interpersonal effectiveness, and whose research program aligns with broad areas of interest to Dow, such as alternative energy sources, the development of novel specialty chemicals, and the investigation of new polymer systems. Dow's $500,000 gift will be matched by $250,000 in funds from the Gordon and Betty Moore Matching Program.

Each recipient will be selected by the chair of the Division of Chemistry and Chemical Engineering, and will hold the fellowship for up to two years.

"We are pleased that Dow and Caltech are building this relationship to support innovative research as a basis for new technologies," says Jacqueline K. Barton, the division's current chair.

The solar-research collaboration will be a four-year, $4.2 million effort to explore earth-abundant materials for solar-energy applications. The project is led by applied physicist Harry Atwater, Caltech's Howard Hughes Professor, and chemist Nate Lewis, Caltech's George L. Argyros Professor.

"In combining the R&D strengths of Dow and Caltech, we have created a powerful alliance for innovation in the field of photovoltaics," says Bill Banholzer, executive vice president and chief technology officer of Dow. "This alliance will allow the best scientists the opportunity to work together to achieve the kinds of breakthrough technologies that will be game-changing in solar-energy capture."

The new Dow/Caltech solar-research initiative is one of the company's largest externally funded research agreements, Banholzer notes.

As part of the agreement, Atwater, Lewis, and their team will develop new mineral-like electronic materials suitable for use in thin-film solar-energy-conversion devices. 

"Development of materials that are abundant in the earth's crust will enable solar-energy technologies to ultimately scale to large volumes at low cost without concern about the materials' availability," says Atwater.

Most solar cells today are made with silicon, which is itself an abundant material. Still, silicon solar technology has a relatively higher cost than that of current thin-film solar materials like cadmium telluride and copper indium diselenide. But these inexpensive semiconductors pose a problem of their own: they contain materials too scarce to ultimately meet the demands of full-scale solar-energy technologies.

That's why Atwater and Lewis are turning their attention to semiconductors found in the earth's crust.

"Use of earth-abundant materials can provide new technology options and could open new areas of design space," Lewis notes. "But it also brings new challenges. This project will develop the science and technology base for thin-film solar-energy conversion using these widely available materials."

"This is an example of industry stepping up to the plate with a long-term vision that acknowledges the importance of supporting research in its most fundamental forms," says Ares Rosakis, chair of the Division of Engineering and Applied Science at Caltech and Theodore von Kármán Professor of Aeronautics and Mechanical Engineering.

"Dow understands that high-quality research is occurring in both industrial and academic laboratories. We believe that partnerships like this one are crucial to our success in the development of efficient, affordable energy solutions," says Banholzer.

Lori Oliwenstein
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Caltech Scientists Develop DNA Origami Nanoscale Breadboards for Carbon Nanotube Circuits

PASADENA, Calif.—In work that someday may lead to the development of novel types of nanoscale electronic devices, an interdisciplinary team of researchers at the California Institute of Technology (Caltech) has combined DNA's talent for self-assembly with the remarkable electronic properties of carbon nanotubes, thereby suggesting a solution to the long-standing problem of organizing carbon nanotubes into nanoscale electronic circuits.

A paper about the work appeared November 8 in the early online edition of Nature Nanotechnology

"This project is one of those great 'Where else but at Caltech?' stories," says Erik Winfree, associate professor of computer science, computation and neural systems, and bioengineering at Caltech, and one of four faculty members supervising the project.

Both the initial idea for the project and its eventual execution came from three students: Hareem T. Maune, a graduate student studying carbon nanotube physics in the laboratory of Marc Bockrath (then Caltech assistant professor of applied physics, now at the University of California, Riverside); Si-ping Han, a theorist in materials science, investigating the interactions between carbon nanotubes and DNA in the Caltech laboratory of William A. Goddard III, Charles and Mary Ferkel Professor of Chemistry, Materials Science, and Applied Physics; and Robert D. Barish, an undergraduate majoring in computer science who was working on complex DNA self-assembly in Winfree's lab.

The project began in 2005, shortly after Paul W. K. Rothemund invented his revolutionary DNA origami technique. At the time, Rothemund was a postdoctoral scholar in Winfree's laboratory; today, he is a senior research associate in bioengineering, computer science, and computation and neural systems.

Rothemund's work gave Maune, Han, and Barish the idea to use DNA origami to build carbon nanotube circuits. 

DNA origami 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 viral DNA and a mixture of different short synthetic DNA strands that bind to and "staple" the viral DNA into the desired shape, typically about 100 nanometers (nm) on a side.

Single-wall carbon nanotubes are molecular tubes composed of rolled-up hexagonal mesh of carbon atoms. With diameters measuring less than 2 nm and yet with lengths of many microns, they have a reputation as some of the strongest, most heat-conductive, and most electronically interesting materials that are known. For years, researchers have been trying to harness their unique properties in nanoscale devices, but precisely arranging them into desirable geometric patterns has been a major stumbling block. 

"After hearing Paul's talk, Hareem got excited about the idea of putting nanotubes on origami," Winfree recalls. "Meanwhile, Rob had been talking to his friend Si-ping, and they independently had become excited about the same idea."

Underlying the students' excitement was the hope that DNA origami could be used as 100 nm by 100 nm molecular breadboards—construction bases for prototyping electronic circuits—on which researchers could build sophisticated devices simply by designing the sequences in the origami so that specific nanotubes would attach in preassigned positions. 

"Before talking with these students," Winfree continues, "I had zero interest in working with carbon nanotubes or applying our lab's DNA-engineering expertise toward such practical ends. But, seemingly out of nowhere, a team had self-assembled with a remarkable spectrum of skills and a lot of enthusiasm. Even Si-Ping, a consummate theorist, went into the lab to help make the idea become reality."

"This collaborative research project is evidence of how we at Caltech select the top students in science and engineering and place them in an environment where their creativity and imagination can thrive," says Ares Rosakis, chair of the Division of Engineering and Applied Science at Caltech and Theodore von Kármán Professor of Aeronautics and Professor of Mechanical Engineering.

Bringing the students' ideas to fruition wasn't easy. "Carbon nanotube chemistry is notoriously difficult and messy—the things are entirely carbon, after all, so it's extremely difficult to make a reaction happen at one chosen carbon atom and not at all the others," Winfree explains.

"This difficulty with chemically grabbing a nanotube at a well-defined 'handle' is the essence of the problem when you're trying to place nanotubes where you want them so you can build complex devices and circuits," he says.

The scientists' ingenious solution was to exploit the stickiness of single-stranded DNA to create those missing handles. It's this stickiness that unites the two strands that make up a DNA helix, through the pairing of DNA's nucleotide bases (A, T, C, and G) with those that have complementary sequences (A with T, C with G). 

"DNA is the perfect molecule for recognizing other strands of DNA, and single-stranded DNA also just happens to like sticking to carbon nanotubes," says Han, "so we mix bare nanotubes with DNA molecules in salt water, and they stick all over the nanotubes' surfaces. However, we make sure that a little bit of each DNA molecule is protected, so that that little portion doesn't stick to the nanotube, and we can use it to recognize DNA attached to the DNA origami instead."

The scientists created two batches of carbon nanotubes labeled by DNA with different sequences, which they called "red" and "blue."

"Metaphorically, we dipped one batch of nanotubes in red DNA paint, and dipped another batch of nanotubes in blue DNA paint," Winfree says. Remarkably, this DNA paint acts like color-specific Velcro.

"These DNA molecules served as handles because a pair of single-stranded DNA molecules with complementary sequences will wrap around each other to form a double helix. Thus," he says, "'red' can bind strongly to 'anti-red,' and 'blue' with 'anti-blue.'"

"Consequently," he adds, "if we draw a stripe of anti-red DNA on a surface, and pour the red-coated nanotubes over it, the nanotubes will stick on the line. But the blue-coated nanotubes won't stick, because they only stick to an anti-blue line."

To make nanometer-scale electronic circuits out of carbon nanotubes requires the ability to draw nanometer-scale stripes of DNA. Previously, this would have been an impossible task.

Rothemund's invention of DNA origami, however, made it possible.

"A standard DNA origami is a rectangle about 100 nm in size, with over 200 'pixel' positions where arbitrary DNA strands can be attached," Winfree says. To integrate the carbon nanotubes into this system, the scientists colored some of those pixels anti-red, and others anti-blue, effectively marking the positions where they wanted the color-matched nanotubes to stick. They then designed the origami so that the red-labeled nanotubes would cross perpendicular to the blue nanotubes, making what is known as a field-effect transistor (FET), one of the most basic devices for building semiconductor circuits.

Although their process is conceptually simple, the researchers had to work out many kinks, such as separating the bundles of carbon nanotubes into individual molecules and attaching the single-stranded DNA; finding the right protection for these DNA strands so they remained able to recognize their partners on the origami; and finding the right chemical conditions for self-assembly.

After about a year, the team had successfully placed crossed nanotubes on the origami; they were able to see the crossing via atomic force microscopy. These systems were removed from solution and placed on a surface, after which leads were attached to measure the device's electrical properties. When the team's simple device was wired up to electrodes, it indeed behaved like a field-effect transistor.  The "field effect" is useful because "the two components of the transistor, the channel and the gate, don't actually have to touch for there to be a switching effect," Rothemund explains. "One carbon nanotube can switch the conductivity of the other due only to the electric field that forms when a voltage is applied to it."

At this point, the researchers were confident that they had created a method that could construct a device from a mixture of nanotubes and origami.

"It worked," Winfree says. "I can't say perfectly—there's lots of room for improvement. But it was sufficient to demonstrate the controlled construction of a simple device, a cross-junction of a pair of carbon nanotubes."

"We expect that our approach can be improved and extended to reliably construct more complex circuits involving carbon nanotubes and perhaps other elements including electrodes and wiring," Goddard says, "which we anticipate will provide new ways to probe the behavior and properties of these remarkable molecules."

The real benefit of the approach, he points out, is that self-assembly doesn't just make one device at a time. "This is a scalable technology. That is, one can design the origami to construct complex logic units, and to do this for thousands or millions or billions of units that self-assemble in parallel."

The work in the paper, "Self-assembly of carbon nanotubes into two-dimensional geometries using DNA origami templates," was supported by the National Science Foundation, the Office of Naval Research, and the Center on Functional Engineered Nano Architectonics.

Kathy Svitil

James. K. Knowles, 78

James K. Knowles, William J. Keenan Jr. Professor of Applied Mechanics, Emeritus, at the California Institute of Technology (Caltech), passed away November 1. He was 78 years old.

Knowles made fundamental research contributions to the theory of nonlinear elasticity and the mathematical theories of materials and structures.

His work provided important insight into how various materials and structures behave and enabled him and others to develop predictive theories.

Born in Cleveland, Ohio, on April 14, 1931, Knowles grew up in Phoenix, Arizona. He entered the Massachusetts Institute of Technology (MIT) in the fall of 1948, earning his bachelor's and doctoral degrees, both in mathematics, in 1952 and 1957, respectively. He then stayed at MIT for an additional year, as an instructor in mathematics.

Knowles joined the faculty at Caltech in 1958 as assistant professor of applied mechanics; he was named associate professor in 1961, followed by full professor in 1965. He spent the remainder of his academic career at Caltech, becoming professor emeritus in 1997.

Considered a remarkable teacher and mentor, Knowles inspired and influenced generations of students and scholars through classes in mathematics and mechanics. A visionary thinker, he recruited and mentored a number of junior colleagues who took Caltech in new and fruitful research directions. He had a deep affection for Caltech and served in various administrative capacities.

"Jim was the greatest mentor I ever had. He held my hand when I first came to Caltech as an assistant professor. He also taught me how to teach," says Ares Rosakis, chair of the Division of Engineering and Applied Science, and Theodore von Kármán Professor of Aeronautics and Mechanical Engineering at Caltech. "He would look for the spark in people's eyes and help them make their dreams a reality. As we at Caltech seek to create the best mentoring opportunities for our young faculty, we should be guided by Jim's example."

Knowles' research was primarily focused on mathematical problems in structural mechanics, and in particular on linear and nonlinear elasticity. In 1960, he provided the first solution for a dynamical problem in finite elasticity and in 1966, he published what would turn out to be a seminal paper concerning the foundations of Saint-Venant's principle in linear elasticity theory.

His later papers on the influence of nonlinearity on point singularities, such as those found at the tip of a crack, demonstrated how they could lead to new phenomena.

In 1979, Knowles published a paper concerning the dissipation of mechanical energy during quasi-static motions of elastic bodies. This led to his later work on the evolution of metastable states of equilibrium, which had applications in phase transformations.

Knowles' contributions are described in more than one hundred journal publications. In 1998, he authored a textbook for graduate students entitled Linear Vector Spaces and Cartesian Tensors (Oxford University Press).

In 1991, he was made an honorary member of the Caltech Alumni Association in recognition of his distinguished service. That same year, the Journal of Elasticity dedicated an issue to Knowles on the occasion of his 60th birthday for "seminal contributions made to the field of elasticity."

"He set an example of scholarship and fundamental thought, both broad and deep, that challenged students as well as researchers," says Roger Fosdick, editor-in-chief of the Journal of Elasticity. "He was highly inquisitive, deeply thoughtful, masterfully insightful and always seeking an explanation. He made indelible marks of value during his life both personally and professionally, and he will most certainly be missed."

Knowles' contributions were also recognized by the Society of Engineering Science with the Eringen Medal, and by the American Society of Mechanical Engineers with the Koiter Medal.

Knowles was a fellow of the American Academy of Mechanics, the American Society of Mechanical Engineers, and the American Association for the Advancement of Science, and was associate editor for the Journal of Applied Mechanics. From 1985 to 1986, he served as president of the American Academy of Mechanics.

Knowles was known outside the classroom for his paintings and baritone voice.

He leaves behind a wife, Jacqueline, and sons John, Jeff, and James, and their families.

A graveside service is scheduled for Saturday, November 14, at 10:30 a.m. at Sierra Madre Pioneer Cemetery, 553 East Sierra Madre Boulevard (at the corner of Coburn Avenue and Sierra Madre Boulevard), in Sierra Madre.

The Division of Engineering and Applied Science has established a memorial fund in honor of Knowles. The fund will support the James K. Knowles Lecture in Solid Mechanics at Caltech, to be delivered annually by an internationally recognized scholar chosen by the faculty. The lecture will be followed by a workshop on solid mechanics, which will be a daylong event of talks by selected current Caltech graduate students and postdoctoral scholars in the area of solid mechanics. The Knowles lecture and workshop will commemorate Knowles' contributions to solid mechanics, his love for Caltech, and his encouragement of young researchers.

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Jon Weiner
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Two Caltech Researchers Receive DARPA Young Faculty Awards

PASADENA, Calif.—The Defense Advanced Research Projects Agency (DARPA) has selected two researchers from the California Institute of Technology (Caltech) to participate in its Young Faculty Award (YFA) program.

Julia R. Greer, assistant professor of materials science, and Doris Tsao, assistant professor of biology, are among the 33 "rising stars" from 24 U.S. universities who each will receive grants of approximately $300,000 to develop and validate their research ideas over the next 24 months.

Greer joined the Caltech faculty in the Division of Engineering and Applied Science (EAS) in 2007 after receiving her PhD from Stanford University in 2005. In 2008, Greer made Technology Review's list of the world's top innovators under the age of 35 for her work with materials at the nanoscale level. In 2008, she received a Faculty Early Career Development award from the National Science Foundation.

Greer's YFA project is aimed at understanding and subsequently mimicking the superior mechanical robustness and strength of naturally occurring protective layers—such as nacre, or mother of pearl, a composite produced by some mollusks to line their inner shell—to create strong, ductile, damage-tolerant materials that maintain a relatively low density.

"Drawing inspiration from hard biological systems will allow us to gain insight into new physical phenomena operating in these materials, and to subsequently create innovative material systems with greatly amplified mechanical properties dictated by the choice of individual components, specific geometries, and microstructure in a truly across-scales fashion," says Greer.

One key objective of the work will be to fabricate a "brick-and-mortar" architecture using tiny plates of a metallic glass and ultrafine-grained ductile metal with nanoscale dimensions; this hierarchical architecture could then be used to fabricate new engineering composites with amplified strength and ductility.

"Greer's nature-inspired work exemplifies the cutting-edge research being carried out in the division," says Ares Rosakis, chair of EAS and Theodore von Kármán Professor of Aeronautics and Mechanical Engineering.

Tsao received her PhD from Harvard University in 2002, and came to Caltech in 2009 from the University of Bremen in Germany. She was named on Technology Review's 2007 list of top young innovators; in 2009, she became a John Merck Scholar, a Searle Scholar, and a Klingenstein Scholar.

Tsao uses functional magnetic resonance imaging, electrical recordings from single neurons, anatomical measurements, and mathematical modeling to understand how the brain identifies objects and reconstructs the three-dimensional world. Specifically, her proposed work will attempt to decipher the neural machinery underlying spatial navigation. 

"Navigation, which is the purposeful movement through space guided by sensory feedback and memory, is a defining behavior in all animals," says Tsao. Understanding the brain mechanisms responsible for navigation, she says, "constitutes a critical step toward designing artificial systems capable of human-like autonomous navigation. Such systems may be used to explore dangerous terrain and to perform tasks, such as clearing land mines, that could risk the loss of human life."

The objective of the DARPA YFA program is to identify and engage rising research stars in junior faculty positions in academia.  The YFA program provides funding mentoring, and industry and Department of Defense (DOD) contacts to these faculty members early in their careers, so that they can develop their research ideas in the context of the needs of the DOD. DARPA's long-term goal for this program is to develop the next generation of academic scientists, engineers, and mathematicians in key disciplines who will focus a significant portion of their careers on DOD and National Security issues.

The YFA awardees were chosen though a competitive selection process. Applicants were required to be untenured faculty at U.S. institutions within six years of appointment to a tenure-track position. Nearly 300 proposals were reviewed for the 2009 awards.

Kathy Svitil
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Caltech Scientists First to Trap Light and Sound Vibrations Together in Nanocrystal

Optomechanical crystals could be used in information processing, as supersensitive biosensors, and more

PASADENA, Calif.—Researchers at the California Institute of Technology (Caltech) have created a nanoscale crystal device that, for the first time, allows scientists to confine both light and sound vibrations in the same tiny space.

"This is a whole new concept," notes Oskar Painter, associate professor of applied physics at Caltech. Painter is the principal investigator on the paper describing the work, which was published in the online edition of the journal Nature. "People have known how to manipulate light, and they've known how to manipulate sound. But they hadn't realized that we can manipulate both at the same time, and that the waves will interact very strongly within this single structure."

Indeed, Painter points out, the interactions between sound and light in this device—dubbed an optomechanical crystal—can result in mechanical vibrations with frequencies as high as tens of gigahertz, or 10 billion cycles per second. Being able to achieve such frequencies, he explains, gives these devices the ability to send large amounts of information, and opens up a wide array of potential applications—everything from lightwave communication systems to biosensors capable of detecting (or weighing) a single macromolecule. It could also, Painter says, be used as a research tool by scientists studying nanomechanics. "These structures would give a mass sensitivity that would rival conventional nanoelectromechanical systems because light in these structures is more sensitive to motion than a conventional electrical system is."

"And all of this," he adds, "can be done on a silicon microchip."

Optomechanical crystals focus on the most basic units—or quanta—of light and sound. (These are called photons and phonons, respectively.) As Painter notes, there has been a rich history of research into both photonic and phononic crystals, which use tiny energy traps called bandgaps to capture quanta of light or sound within their structures.

What hadn't been done before was to put those two types of crystals together and see what they are capable of doing. That is what the Caltech team has done.

"We now have the ability to manipulate sound and light in the same nanoplatform, and are able to interconvert energy between the two systems," says Painter. "And we can engineer these in nearly limitless ways."

The volume in which the light and sound are simultaneously confined is more than 100,000 times smaller than that of a human cell, notes Caltech graduate student Matt Eichenfield, the paper's first author. "This does two things," he says. "First, the interactions of the light and sound get stronger as the volume to which they are confined decreases. Second, the amount of mass that has to move to create the sound wave gets smaller as the volume decreases. We made the volume in which the light and sound live so small that the mass that vibrates to make the sound is about ten times less than a trillionth of a gram."

Eichenfield points out that, in addition to measuring high-frequency sound waves, the team demonstrated that it's actually possible to produce these waves using only light. "We can now convert light waves into microwave-frequency sound waves on the surface of a silicon microchip," he says.

These sound waves, he adds, are analogous to the light waves of a laser. "The way we have designed the system makes it possible to use these sound waves by routing them around on the chip, and making them interact with other on-chip systems. And, of course, we can then detect all these interactions again by using the light. Essentially, optomechanical crystals provide a whole new on-chip architecture in which light can generate, interact with, and detect high-frequency sound waves."

These optomechanical crystals were created as an offshoot of previous work done by Painter and colleagues on a nanoscale "zipper cavity," in which the mechanical properties of light and its interactions with motion were strengthened and enhanced. (That release can be found at http://media.caltech.edu/press_releases/13263.)

Like the zipper cavity, optomechanical crystals trap light; the difference is that the crystals trap—and intensify—sound waves, as well. Similarly, while the zipper cavities worked by funneling the light into the gap between two nanobeams—allowing the researchers to detect the beams' motion relative to one another—optomechanical crystals work on an even tinier scale, trapping both light and sound within a single nanobeam.

"Here we can actually see very small vibrations of sound trapped well inside a single 'string,' using the light trapped inside that string," says Eichenfield. "Importantly, although the method of sensing the motion is very different, we didn't lose the exquisite sensitivity to motion that the zipper had. We were able to keep the sensitivity to motion high while making another huge leap down in mass."

"As a technology, optomechanical crystals provide a platform on which to create planar circuits of sound and light," says Kerry Vahala, the Ted and Ginger Jenkins Professor of Information Science and Technology and professor of applied physics, and coauthor on the Nature paper. "These circuits can include an array of functions for generation, detection, and control. Moreover," he says, "optomechanical crystal structures are fabricated using materials and tools that are similar to those found in the semiconductor and photonics industries. Collectively, this means that phonons have joined photons and electrons as possible ways to manipulate and process information on a chip."

And these information-processing possibilities are well within reach, notes Painter. "It's not one plus one equals two, but one plus one equals ten in terms of what you can do with these things. All of these applications are much closer than they were before."

"This novel approach to bringing both light and sound together and letting them play off of each other exemplifies the forward-thinking work being done by the Engineering and Applied Science (EAS) division," says Ares Rosakis, chair of EAS and Theodore von Kármán Professor of Aeronautics and Mechanical Engineering at Caltech.

Other authors on the Nature paper, "Optomechanical crystals," include Caltech graduate student Jasper Chan and postdoctoral scholar Ryan Camacho. Funding for their work was provided by a Defense Advanced Research Projects Agency seed grant and by grants from the National Science Foundation.

Lori Oliwenstein


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