Wednesday, October 21, 2015
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Tuesday, October 21, 2014
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Engaging Students Beyond Their Field

Thursday, October 22, 2015

Confessions of a Converted Lecturer: TeachWeek Keynote by Eric Mazur

Monday, October 26, 2015
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Atomic Fractals in Metallic Glasses

Metallic glasses are very strong and elastic materials that appear with the naked eye to be identical to stainless steel. But metallic glasses differ from ordinary metals in that they are amorphous, lacking an orderly, crystalline atomic arrangement. This random distribution of atoms, which is the primary characteristic of all glass materials (such as windowpanes and tableware), gives metallic glasses unique mechanical properties but unpredictable internal structure. Researchers in the Caltech lab of Julia Greer, professor of materials science and mechanics in the Division of Engineering and Applied Science, have shown that metallic glasses do have an atomic-level structure—if you zoom in closely enough—although it differs from the periodic lattices that characterize crystalline metals.

If you looked at a metallic glass on a scale larger than a few atomic diameters, you would see tightly packed, jumbled clusters of atoms. A new study from the Greer group—published in the September 18, 2015 issue of the journal Science—shows that inside each of these clusters, on a scale of about two to three atomic diameters, atoms have a predictable arrangement called a fractal.

Fractals are patterns that are self-similar on different scales, and they can occur quite naturally.

"Take for example a piece of paper crumpled into a ball. If you look at the folds of the paper when it is flattened back after crumpling, it will look qualitatively the same if you zoom in on a smaller portion of the same paper. The scale that you use to examine the paper more or less does not change the way it looks," says David Chen, a fourth-year graduate student in the Greer lab and first author on this new paper.

The group did simulations and experiments to probe the atomic structure of metallic glass alloys of copper, zirconium, and aluminum. In crystalline solids like diamond or gold, atoms or molecules are arranged in an orderly lattice pattern. As a result, the local neighborhood around an atom in a crystalline material is identical to everywhere else in the material. In amorphous metals, every location within the material looks different—except, Greer and her colleagues found, when you zoom in to look at the distribution of atoms at the scale of two to three atomic diameters—about one nanometer. At this level, the same fractal pattern is present, regardless of location within the material. "Within the clusters of atoms that make up a metallic glass, atoms are arranged in a particular kind of fractal pattern called percolation," Chen says.

Other scientists have previously hypothesized that the atoms in metallic glasses are distributed fractally. However, this creates an apparent paradox: When atoms are distributed fractally, there should be empty space between them. However, metallic glasses—just like regular metals—are fully dense, meaning that they lack significant pockets of empty space.

"Our group has solved this paradox by showing that atoms are only arranged fractally up to a certain scale," Greer says. "Larger than that scale, clusters of atoms are packed randomly and tightly, making a fully dense material, just like a regular metal. So we can have something that is both fractal and fully dense."

The discovery was made with metallic glasses, but the group's conclusions about fractally arranged atomic structures can be applied to essentially any rigid amorphous material, like the glass in a windowpane or a frozen piece of chewing gum. "Amorphous metals can exhibit unique properties, like unusual strength and elasticity," Chen says. "Now that we know the structure of these materials, we can start studying how their atomic-level arrangement affects their large-scale properties."

In addition to applications within materials science, studies of naturally occurring fractal distributions are of high interest within the fields of mathematics, physics, and computer science. Fractals have been studied for centuries by mathematicians and physicists. Showing how they emerge in a metallic alloy provides a physical foundation for something that has only been studied theoretically.

Other Caltech co-authors on the paper, titled "Fractal atomic-level percolation in metallic glasses," include Qi An, a theoretical and computational materials scientist, and Professor William Goddard, the Charles and Mary Ferkel Professor of Chemistry, Materials Science, and Applied Physics.

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Lori Dajose
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Partnership with Heritage Medical Research Institute Will Augment Translational Medicine Research

A new partnership will support translational sciences and health technology at Caltech thanks to a three-year commitment from Heritage Medical Research Institute (HMRI), a nonprofit founded and led by Caltech trustee Richard N. Merkin.

With this gift, the Institute and HMRI have created the Heritage Research Institute for the Advancement of Medicine and Science at Caltech. Eight Caltech faculty members from three academic divisions have been selected for the inaugural cohort of Heritage researchers, with a ninth yet to be named. These scientists and engineers—who will hold the title of Heritage Principal Investigators—will receive salary and research support as well as opportunities to learn from and collaborate with each other and with practicing physicians in the local community.

"Dick Merkin's insights into the changing landscape of modern medicine, his devotion to supporting young talent, and his exceptional generosity have come together to create an innovative program to advance translational research," says President Thomas F. Rosenbaum, holder of the Sonja and William Davidow Presidential Chair and professor of physics. "The generous support of HMRI, through Dick's vision, will provide the freedom and resources for faculty from across the divisions to tackle difficult science and engineering problems for the betterment of the human condition."

As a physician and a healthcare executive, Merkin has witnessed the rapid evolution of medicine and patient care in recent decades—and says he sees monumental changes on the horizon.

"I think some of the greatest breakthroughs this century will occur in biology, and I think Caltech is particularly positioned to be a leader in this area," Merkin says. "Our biggest problems are our biggest opportunities, and Caltech is gifted in looking at the world not as it is, but as it could be."

Caltech is uniquely suited to accelerating progress due to its highly collaborative environment, Merkin adds. The convergence of multidisciplinary science and technology, he says, is driving innovation at an exponential rate, particularly in the areas of implantable sensors and precision medicine.

Many of Caltech's new Heritage Principal Investigators have already deepened our understanding of how the human body works—from the microbes in our gut to the chemicals in our brain—and are advancing the study of diseases such as diabetes, autism, and cancer. As a trustee and benefactor, Merkin has been energized by the potential impact of their investigations.

"The most imaginative scientists on the globe are concentrated at Caltech," Merkin says. "They are dedicated to understanding the world around us. Just being able to interact with so many passionate, hardworking, and brilliant people is inspiring. I'm very grateful to be part of the Institute." 

Adds Stephen Mayo, the William K. Bowes Jr. Leadership Chair of the Division of Biology and Biological Engineering and Bren Professor of Biology and Chemistry: "As a valued friend of the Institute and a physician, Richard Merkin knows that the treatments of tomorrow begin in the lab today. This gift will embolden the Heritage Principal Investigators—some of whom are in the early stages of their careers—to pursue their most promising ideas and, in turn, quicken the pace of discovery in the biosciences."

A graduate of the University of Miami, Merkin began his career as a physician before creating what is now known as Heritage Provider Network (HPN) in 1979. Merkin serves as HPN's president and chief executive officer and has overseen its growth into one of California's largest healthcare provider networks. In 2012, Fast Company magazine named HPN one of the most innovative healthcare companies for embracing techniques such as data mining and predictive modeling to better the well-being of patients and improve the nation's healthcare system.

Merkin's philanthropy focuses on medical research, the arts, and children, with a special emphasis on the people of Southern California. He has served on the Caltech Board of Trustees since 2007 and also sits on the boards of the Los Angeles County Museum of Art and United Friends of the Children, as well as educational institutions, including the Keck School of Medicine of USC and Alliance College-Ready Public Schools. The latter runs 27 charter schools in the greater Los Angeles area, including one site named after him—the Richard Merkin Middle School. 

In 2003, Merkin founded HMRI, a nonprofit that also has supported the Dana Farber Cancer Institute and the Prostate Cancer Foundation. In deciding where to direct HMRI's research funds, making a pledge to Caltech made sense for Merkin.

"Watching the Institute's stewardship of resources as a trustee makes me very comfortable investing as a benefactor," Merkin says. "Supporting Caltech and its faculty and students is a much broader investment in a better future—not just for the local community, not just for the United States, but, really, for the world."

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Marisa Demers
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Deformation of 3D Hierarchical Nanolattices

In a new paper published in the Proceedings of the National Academy of Science (PNAS), researchers in the laboratory of Julia R. Greer, professor of Materials Science, Mechanical Engineering and Medical Engineering, have designed a new kind of hierarchical nanostructure that is stronger than previous lattice structures and bounces back with less damage after compression. Hierarchical engineered structures, like the Eiffel Tower, are designed like fractals, repeating patterns that are the same, or self-similar, at every level of magnification.

"It's basically a lattice of beams made of a lattice of beams, with the smallest dimension of about 10,000th of your hair diameter," says Lucas Meza, a fourth-year graduate student in the Greer lab and the first author on the paper. "That's called a second-order lattice. The more lattices of beams made of lattices of beams, the higher order the hierarchy gets."

Meza and his colleagues experimented with several versions of this architecture, with the results of these trials shown in these videos. The nanolattice made of hollow ceramic beams showed the greatest "recoverability," or bounce-back, even after being grossly deformed. This happens through a process known as shell buckling, in which the ceramic can crumple like a piece of paper under stress and then recover when the stress is removed. On scales larger than nanometers, ceramic fails often from cracks and defects (a dropped coffee cup will demonstrate this effect). But at thicknesses of around 20 nanometers—the thickness of the shell of a hollow beam—ceramic approaches its theoretical material strength because the probability of finding a flaw or a crack in something that thin is significantly reduced. This represents the "true" amount of force a material can withstand if it has no defects. Although the lattice in the video is 99 percent air, its strength is comparable to that of foam structures that are more dense by two orders of magnitude.

In addition to Greer and Meza, other Caltech coauthors include assistant professor of aerospace Dennis Kochmann, and graduate students Alex Zelhofer and Arturo Mateos.

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New, Ultrathin Optical Devices Shape Light in Exotic Ways

Caltech engineers have created flat devices capable of manipulating light in ways that are very difficult or impossible to achieve with conventional optical components.

The new devices are not made of glass, but rather of silicon nanopillars that are precisely arranged into a honeycomb pattern to create a "metasurface" that can control the paths and properties of passing light waves.

These metasurface devices, described in a paper published online on August 31, 2015, in the journal Nature Nanotechnology, could lead to ultracompact optical systems such as advanced microscopes, displays, sensors, and cameras that can be mass-produced using the same photolithography techniques used to manufacture computer microchips.

"Currently, optical systems are made one component at a time, and the components are often manually assembled," says Andrei Faraon (BS '04), an assistant professor of applied physics and materials science, and the study's principal investigator. "But this new technology is very similar to the one used to print semiconductor chips onto silicon wafers, so you could conceivably manufacture millions of systems such as microscopes or cameras at a time."

Seen under a scanning electron microscope, the new metasurfaces that the team created resemble a cut forest where only the stumps remain. Each silicon stump, or pillar, has an elliptical cross section, and by carefully varying the diameters of each pillar and rotating them around their axes, the scientists were able to simultaneously manipulate the phase and polarization of passing light. Light is an electromagnetic field, and the field of single-color, or monochromatic, light oscillates at all points in space with the same frequency but varying relative delays, or phases.

Manipulating this relative delay, or phase, influences the degree to which a light ray bends, which in turn influences whether an image is in or out of focus.

Polarization refers to the trajectory of the oscillations of the electromagnetic field at each point in space. Manipulating the polarization of light is essential for the operation of advanced microscopes, cameras, and displays; the control of polarization also enables simple gadgets such as 3-D glasses and polarized sunglasses.

"Using our metasurfaces, we have complete control of the polarization and phase of light," says study first author Amir Arbabi, a senior researcher at Caltech. "We can take any incoming light and shape its phase and polarization profiles arbitrarily and with very high efficiency."

While the same goal can be achieved using an arrangement of multiple conventional optical components such as glass lenses, prisms, spatial light modulators, polarizers, and wave plates, these many components lead to much bulkier systems. "If you think of a modern microscope, it has multiple components that have to be carefully assembled inside," Faraon says. "But with our platform, we can actually make each of these optical components and stack them atop one another very easily using an automated process. Each component is just a millionth of a meter thick, or less than a hundredth of the thickness of a human hair. "

In addition to being compact, a metasurface device could manipulate light in novel ways that are very hard and sometimes impossible to do using current setups. For example, the Caltech team showed that one of their metasurfaces can project one image when illuminated by a horizontally polarized beam of light, and a different image when illuminated by a vertically polarized beam. "The two images will appear overlapped under illumination with light polarized at 45 degrees," Faraon says.

In another experiment, the team was able to use a metasurface to create a beam with radial polarization, that is, a beam whose polarization is pointing toward the beam axis. Such beams have doughnut-shaped intensity profiles and have applications in superresolution microscopy, laser cutting, and particle acceleration. "You generally would need a large optical setup, consisting of multiple components, to create this effect using conventional instruments," Arbabi says. "With our setup, we can compress all of the optical components into one device and generate these beams with higher efficiency and more purity."

The team is currently working with industrial partners to create metasurfaces for use in commercial devices such as miniature cameras and spectrometers, but a limited number have already been produced for use in optical experiments by collaborating scientists in other disciplines.

In addition, the Faraon lab current is investigating ways to combine different metasurfaces to create functioning optical systems and to correct for color distortions and other optical aberrations. "Like any optical system, you get distortions," Faraon said. "That's why expensive cameras have multiple lenses inside. Right now, we are experimenting with stacking different metasurfaces to correct for these aberrations and achieve novel functionalities."

The paper is entitled "Dielectric metasurfaces for complete control of phase and polarization with subwavelength spatial resolution and high transmission." In addition to Faraon and Arbabi, other Caltech coauthors include graduate student Yu Horie and Mahmood Bagheri, a microdevices engineer at JPL. The work was supported by the Caltech/JPL President's and Director's Fund and the Defense Advanced Research Projects Agency. Yu Horie was supported by the Department of Energy's Energy Frontier Research Center program and a Japan Student Services Organization fellowship. The device nanofabrication was performed in the Kavli Nanoscience Institute at Caltech.

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Seeing Quantum Motion

Consider the pendulum of a grandfather clock. If you forget to wind it, you will eventually find the pendulum at rest, unmoving. However, this simple observation is only valid at the level of classical physics—the laws and principles that appear to explain the physics of relatively large objects at human scale. However, quantum mechanics, the underlying physical rules that govern the fundamental behavior of matter and light at the atomic scale, state that nothing can quite be completely at rest.

For the first time, a team of Caltech researchers and collaborators has found a way to observe—and control—this quantum motion of an object that is large enough to see. Their results are published in the August 27 online issue of the journal Science.

Researchers have known for years that in classical physics, physical objects indeed can be motionless. Drop a ball into a bowl, and it will roll back and forth a few times. Eventually, however, this motion will be overcome by other forces (such as gravity and friction), and the ball will come to a stop at the bottom of the bowl.

"In the past couple of years, my group and a couple of other groups around the world have learned how to cool the motion of a small micrometer-scale object to produce this state at the bottom, or the quantum ground state," says Keith Schwab, a Caltech professor of applied physics, who led the study. "But we know that even at the quantum ground state, at zero-temperature, very small amplitude fluctuations—or noise—remain."

Because this quantum motion, or noise, is theoretically an intrinsic part of the motion of all objects, Schwab and his colleagues designed a device that would allow them to observe this noise and then manipulate it.

The micrometer-scale device consists of a flexible aluminum plate that sits atop a silicon substrate. The plate is coupled to a superconducting electrical circuit as the plate vibrates at a rate of 3.5 million times per second. According to the laws of classical mechanics, the vibrating structures eventually will come to a complete rest if cooled to the ground state.

But that is not what Schwab and his colleagues observed when they actually cooled the spring to the ground state in their experiments. Instead, the residual energy—quantum noise—remained.

"This energy is part of the quantum description of nature—you just can't get it out," says Schwab. "We all know quantum mechanics explains precisely why electrons behave weirdly. Here, we're applying quantum physics to something that is relatively big, a device that you can see under an optical microscope, and we're seeing the quantum effects in a trillion atoms instead of just one."

Because this noisy quantum motion is always present and cannot be removed, it places a fundamental limit on how precisely one can measure the position of an object.

But that limit, Schwab and his colleagues discovered, is not insurmountable. The researchers and collaborators developed a technique to manipulate the inherent quantum noise and found that it is possible to reduce it periodically. Coauthors Aashish Clerk from McGill University and Florian Marquardt from the Max Planck Institute for the Science of Light proposed a novel method to control the quantum noise, which was expected to reduce it periodically. This technique was then implemented on a micron-scale mechanical device in Schwab's low-temperature laboratory at Caltech.

"There are two main variables that describe the noise or movement," Schwab explains. "We showed that we can actually make the fluctuations of one of the variables smaller—at the expense of making the quantum fluctuations of the other variable larger. That is what's called a quantum squeezed state; we squeezed the noise down in one place, but because of the squeezing, the noise has to squirt out in other places. But as long as those more noisy places aren't where you're obtaining a measurement, it doesn't matter."

The ability to control quantum noise could one day be used to improve the precision of very sensitive measurements, such as those obtained by LIGO, the Laser Interferometry Gravitational-wave Observatory, a Caltech-and-MIT-led project searching for signs of gravitational waves, ripples in the fabric of space-time.

"We've been thinking a lot about using these methods to detect gravitational waves from pulsars—incredibly dense stars that are the mass of our sun compressed into a 10 km radius and spin at 10 to 100 times a second," Schwab says. "In the 1970s, Kip Thorne [Caltech's Richard P. Feynman Professor of Theoretical Physics, Emeritus] and others wrote papers saying that these pulsars should be emitting gravity waves that are nearly perfectly periodic, so we're thinking hard about how to use these techniques on a gram-scale object to reduce quantum noise in detectors, thus increasing the sensitivity to pick up on those gravity waves," Schwab says.

In order to do that, the current device would have to be scaled up. "Our work aims to detect quantum mechanics at bigger and bigger scales, and one day, our hope is that this will eventually start touching on something as big as gravitational waves," he says.

These results were published in an article titled, "Quantum squeezing of motion in a mechanical resonator." In addition to Schwab, Clerk, and Marquardt, other coauthors include former graduate student Emma E. Wollman (PhD '15); graduate students Chan U. Lei and Ari J. Weinstein; former postdoctoral scholar Junho Suh; and Andreas Kronwald of Friedrich-Alexander-Universität in Erlangen, Germany. The work was funded by the National Science Foundation (NSF), the Defense Advanced Research Projects Agency, and the Institute for Quantum Information and Matter, an NSF Physics Frontiers Center that also has support from the Gordon and Betty Moore Foundation.

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Gharib Addresses Youths at U.S. Citizenship Ceremony

On Wednesday, August 26, 2015, Caltech Vice Provost Mory Gharib welcomed 80 mostly high school-age young people who were at Caltech for a U.S. citizenship ceremony. The ceremony, which was organized by the U.S. Citizenship and Immigration Services and hosted by Caltech, recognized young people who obtained citizenship through their parents.

Gharib, the Hans W. Liepmann Professor of Aeronautics and Professor of Bio-Inspired Engineering, shared with the audience his own story of becoming a naturalized U.S. Citizen. He also talked about the importance of having people from different backgrounds who can bring their own unique perspectives and ideas to a place like Caltech.

"Like this country itself, Caltech is a place where people have come together from many different parts of the globe—on any given day on this campus, you would find students, faculty, and staff from more than 80 countries," Gharib said during his remarks. "We all come together here to find ways to not only to better understand our world, but to make it better."

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