Light as a Feather, Stiffer Than a Board

Caltech Researchers Help Develop World’s Lightest Solid Material

When you pick up the newest material in Julia Greer's office, it takes a second for your mind to adjust. Despite its looks, the little brick of metal weighs next to nothing. 

Greer, assistant professor of materials science and mechanics, is part of a team of researchers from Caltech, HRL Laboratories, LLC, and the University of California, Irvine, who have developed the world's lightest solid material, with a density of just 0.9 milligrams per cubic centimeter, or approximately 100 times lighter than Styrofoam™. Though the material is ultra-low in density, it has incredible strength and absorbs energy well, making it potentially useful for applications ranging from battery electrodes to protective shielding.

"We're entering a new era of materials science where material properties are determined not only by the microscopic makeup of the material but also by the architecture of the constituents," Greer says.

The new material, called a micro-lattice, relies, appropriately, on a lattice architecture: tiny hollow tubes made of nickel-phosphorous are angled to connect at nodes, forming repeating, asterisklike unit cells in three dimensions. Everything between the tubes is open air. In fact, the structure consists of 99.99% open volume. Tobias Schaedler, a research staff scientist at HRL Laboratories, LLC, and lead author on the report described it as "a lattice of interconnected hollow tubes with a wall thickness of 100 nanometers, 1,000 times thinner than a human hair."

The material takes advantage of a hierarchical design: the wall thickness can be measured in nanometers, the diameter of each tube can be measured in microns, each tube is millimeters in length, and the entire micro-lattice material can be measured in centimeters (but might one day be made meters in length). Just as with large-scale structures, such as the Eiffel Tower, where order and hierarchy can lead to more efficient use of materials and improved properties, the same can be achieved by ordering materials on a tiny scale. In addition to its ultra-low density, the micro-lattice's hierarchical architecture allows it to recover almost completely from loads that compress it by as much as 50 percent, making it excellent at absorbing energy.

"The emergence of the unique properties of these ultra-light micro-lattice structures is due, in part, to the different mechanical behavior that emerges in nano-sized solids, which is the focus of my research," Greer says. Her team uses a machine called a SEMentor, which is both an electron microscope and a nanoindenter, to visualize the deformation of nano-sized structures and to concurrently measure mechanical properties, such as how much force it takes to break a material, how much energy it can absorb, and how much it stretches. The extremely small wall thickness-to-diameter ratio in the micro-lattice material makes the individual tubes ductile (i.e. they do not fail catastrophically); at higher aspect ratios, the material simply collapses and cannot recover.

The research appears in the November 18 issue of Science. Additional coauthors on the report, "Ultralight Metallic Lattices," include Caltech postdoctoral scholar Jane Lian, as well as Alan Jacobsen, Adam Sorensen, and Bill Carter from HRL Laboratories, and Anna Torrents and Lorenzo Valdevit from the University of California, Irvine. The research was funded by the Defense Advanced Research Projects Agency. 

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Auctions, Traffic, Selfishness, and Data Privacy—It All Comes Down to Math

Every time you run a Google search, a split-second automated auction takes place to determine which of many competing companies will get to fill the ad space in your browsing window. The program controlling that auction is designed to fulfill a specific set of goals that probably differ from the goals of the individual companies. Similarly, your motivation during your morning commute is unlikely to be maintaining the overall flow of freeway traffic, and when you give a hospital your personal information, you're probably not trying to improve their data analysis.

These types of problems—where a conflict or tension exists between individual incentives and more global objectives—are Katrina Ligett's bread and butter. They fall into a category known as algorithmic game theory, and Ligett, a new assistant professor of economics and computer science at Caltech, uses ideas from computer science and mathematics to approach them.

"I'm interested in new algorithms, in understanding how difficult it is to solve problems," Ligett says. "Sometimes this comes from a particular application—maybe it's inspired by auctions that Google actually needs to run, for example."

Ligett says the field of algorithmic game theory has emerged in the last decade or so at the interface between computer science and economics. It got its start when computer scientists began to realize that they had something to offer to existing ideas in economics and game theory. For example, much of traditional game theory looks at equilibria, such as the Nash Equilibrium (developed by Nobel Prize–winning mathematician John Forbes Nash, Jr., and made famous on the big screen by A Beautiful Mind). That equilibrium describes a set of decisions that create a scenario in which no "players" want to change their decision, given how everyone else is making their decisions. "There is a huge amount of beautiful work in economics on such equilibria," Ligett says. "But there is relatively little work looking at things like how difficult it is to actually compute an equilibrium, and computer scientists have excellent tools to address such problems."

At first, computer scientists made up most of the field. But today, researchers are coming to algorithmic game theory from both directions—from computer science and from economics. Ligett says she was thrilled to come to Caltech in part because the Institute was looking for researchers to work specifically at this juncture. "There aren't that many places where computer scientists and economists actually talk to each other," Ligett says. "At Caltech, people are really interested in and committed to investigating at this intersection, and that's very appealing. 

It wasn't always clear that Ligett would become a computer scientist. In high school and for a while afterward, she worked in an Army Corps of Engineers research lab in New Hampshire focused on studying the Arctic and Antarctic regions of the world. She got to see real scientists doing research and working in the field. She even got to travel to Barrow, Alaska, to study patterns of seasonal ice melt.

So when Ligett went to college at Brown University, she thought she'd go into a lab science—perhaps chemistry or physics. But that all changed when she took her first computer-science class.  "I would get so wrapped up in problems that I was just really excited every week to work on my homework and projects for the class," she says. "So I thought, maybe I'll do a little bit more of this." Eventually, she switched over to computer science and mathematics, and went on to earn a PhD in computer science at Carnegie Mellon University.

Ligett's main research interests lie in trying to find better ways of thinking about and describing selfish behavior and in modeling alternatives to equilibrium states.  "Some of my work says, 'What makes you think that people are going to end up at an equilibrium?'" she says. "Let's talk about where people might end up or what the whole system might look like if people act in a less prescribed way and just act selfishly. " She analyzes systems that never reach equilibrium and devises alternative models to try to account for situations where, for example, individuals might try to "game the system" and alter outcomes in unexpected ways.

In the area of data privacy, she's looking at the tension between privacy and the usefulness of data, trying to come up with new theorems to describe the relationship. Take, for example, a medical database, where Ligett might examine whether it is possible to mathematically transform the data in such a way that the database would provide researchers with meaningful data while still protecting the privacy of individuals.

Although the problems Ligett deals with involve complex, dynamic situations, most of what she does is good old paper-and-pencil math. "I might write down a mathematical description of people acting a certain way, using equations to establish the types of decisions people make. Once I have a mathematical model, I study the properties and consequences of the model," she says. So those equations on the white board in Ligett's new office? They might represent online auctions, the morning commute, or issues of data privacy. "I'm interested in the fundamental mathematics and in solving problems that are as general as possible," she says. "I hope that can be applied in a bunch of different ways."

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An Incredible Shrinking Material

Caltech Engineers Reveal How Scandium Trifluoride Contracts with Heat

PASADENA, Calif.—They shrink when you heat 'em. Most materials expand when heated, but a few contract. Now engineers at the California Institute of Technology (Caltech) have figured out how one of these curious materials, scandium trifluoride (ScF3), does the trick—a finding, they say, that will lead to a deeper understanding of all kinds of materials. 

The researchers, led by graduate student Chen Li, published their results in the November 4 issue of Physical Review Letters (PRL).

Materials that don't expand under heat aren't just an oddity. They're useful in a variety of applications—in mechanical machines such as clocks, for example, that have to be extremely precise. Materials that contract could counteract the expansion of more conventional ones, helping devices remain stable even when the heat is on.

"When you heat a solid, most of the heat goes into the vibrations of the atoms," explains Brent Fultz, professor of materials science and applied physics and a coauthor of the paper. In normal materials, this vibration causes atoms to move apart and the material to expand. A few of the known shrinking materials, however, have unique crystal structures that cause them to contract when heated, a property called negative thermal expansion. But because these crystal structures are complicated, scientists have not been able to clearly see how heat—in the form of atomic vibrations—could lead to contraction.

But in 2010 researchers discovered negative thermal expansion in ScF3, a powdery substance with a relatively simple crystal structure. To figure out how its atoms vibrated under heat, Li, Fultz, and their colleagues used a computer to simulate each atom's quantum behavior. The team also probed the material's properties by blasting it with neutrons at the Spallation Neutron Source at Oak Ridge National Laboratory (ORNL) in Tennessee; by measuring the angles and speeds with which the neutrons scattered off the atoms in the crystal lattice, the team could study the atoms' vibrations. The more the material is heated the more it contracts, so by doing this scattering experiment at increasing temperatures, the team learned how the vibrations changed as the material shrank.

The results paint a clear picture of how the material shrinks, the researchers say. You can imagine the bound scandium and fluorine atoms as balls attached to one another with springs. The lighter fluorine atom is linked to two heavier scandium atoms on opposite sides. As the temperature is cranked up, all the atoms jiggle in many directions. But because of the linear arrangement of the fluorine and two scandiums, the fluorine vibrates more in directions perpendicular to the springs. With every shake, the fluorine pulls the scandium atoms toward each other. Since this happens throughout the material, the entire structure shrinks.

The surprise, the researchers say, was that in the large fluorine vibrations, the energy in the springs is proportional to the atom's displacement—how far the atom moves while shaking—raised to the fourth power, a behavior known as a quartic oscillation. Most materials are dominated by quadratic (or harmonic) oscillations—characteristic of the typical back-and-forth motion of springs and pendulums—in which the stored energy is proportional to the square of the displacement.

"A nearly pure quantum quartic oscillator has never been seen in atom vibrations in crystals," Fultz says. Many materials have a little bit of quartic behavior, he explains, but their quartic tendencies are pretty small. In the case of ScF3, however, the team observed the quartic behavior very clearly. "A pure quartic oscillator is a lot of fun," he says. "Now that we've found a case that's very pure, I think we know where to look for it in many other materials." Understanding quartic oscillator behavior will help engineers design materials with unusual thermal properties. "In my opinion," Fultz says, "that will be the biggest long-term impact of this work."

The other authors of the PRL paper, "The structural relationship between negative thermal expansion and quartic anharmonicity of cubic ScF3," are former Caltech postdoctoral scholars Xiaoli Tang and J. Brandon Keith; Caltech graduate students Jorge Muñoz and Sally Tracy; and Doug Abernathy of ORNL. The research was supported by the Department of Energy.

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Early Years of GALCIT Featured in New Display

The Guggenheim Aeronautical Laboratory at the California Institute of Technology (GALCIT) was a key component in the success of the Southern California aeronautic industry, which took flight in the 1920s. Working Together, Learning to Fly is a historical look at the research and industry collaborations at Caltech during the early years of GALCIT and is currently on display at Parsons-Gates Hall of Administration.

The exhibit, a collaboration of the Caltech Library, the Caltech Archives, GALCIT, and the Division of Engineering and Applied Science, features four cases full of documents, photographs, and images that highlight pioneers in the field, the history of Caltech's legendary wind tunnel, industry partnerships, and the important contributions that GALCIT graduates made to aviation. 

The exhibit is open Monday through Friday from 8:30 a.m. to 4:30 p.m. on the 2nd floor of Parsons-Gates. For more information, visit the exhibit's website.

 

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Caltech Retains Top Spot in Engineering and Technology Rankings

In addition to their recent ranking of Caltech as first among world universities, for the second consecutive year the Times Higher Education has also ranked the Institute first for its engineering and technology programs.

"Once again, Caltech has been recognized for its contributions to academia and society," says Ares Rosakis, chair of the Division of Engineering and Applied Science (EAS) at Caltech. "We know that rankings are often imperfect and a number one position may come and go. In contrast, our sustained impact is in the creation of new schools of thought which are the true indicators of our combined achievements and excellence in both research and education."

This year, Caltech shares the lead position with MIT; Princeton, UC Berkeley, and Stanford University round out the top five. The engineering and technology ranking is one of six subsections of the Times Higher Education World University Rankings 2011-2012. It covers a wide range of subjects—from aerospace engineering to sustainable energy research—making it "one of the most diverse of the subject tables in terms of national representation." The list is compiled data supplied by Thomson Reuters.

For a full list of the world's top 50 engineering and technology schools and all of the performance indicators, go to the Times Higher Education website.

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Caltech Engineer Receives Popular Mechanics Award

Each year, Popular Mechanics magazine honors inventors, engineers, and researchers with Breakthrough Innovator Awards for their significant advances in medicine, technology, entertainment, and more. At a ceremony in New York City on October 10, Caltech engineer Joel Burdick was among the recipients of a Breakthrough Award for his work that helped a paralyzed man stand. The awards are in recognition of "innovators whose inventions will make the world smarter, safer, and more efficient in the years to come."

Burdick, a professor of mechanical engineering and bioengineering, was part of a team that made headlines earlier this year for implanting a stimulating electrode array near the spine of a paralyzed man to help him regain movement in his legs. The group—which also includes V. Reggie Edgerton and Yury Gerasimenko from UCLA, Susan Harkema from the University of Louisville, and patient Rob Summers—received a 2011 Breakthrough Innovator Award for their "bold experiment" that resulted in "unprecedented voluntary movement" in Summers' legs. The team was one of 11 groups or individuals to receive the award

As a robotics expert, Burdick developed robotically guided physical therapy equipment (seen in the image at right) used by animal models in early studies of the electrode array. He also introduced the concept of using high-density epidural spinal stimulation to treat patients with spinal cord injuries, and is currently building physical therapy equipment for human patients with the spinal implant.

"Our Breakthrough Award winners not only capture the imagination, but hold the potential to improve and save lives," said James B. Meigs, editor-in-chief of Popular Mechanics, in a press release. The winners are selected by the editors of Popular Mechanics after soliciting recommendations from a wide range of experts and past Breakthrough Award winners in fields ranging from aerospace and robotics to medicine and energy.

To read more about the awards, go to the Popular Mechanics website. Full descriptions of the winners are also available in the November issue of Popular Mechanics, on newsstands today.  

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Caltech Named World's Top University in New Times Higher Education Global Ranking

PASADENA, Calif.—The California Institute of Technology (Caltech) has been rated the world's number one university in the 2011–2012 Times Higher Education global ranking of the top 200 universities, displacing Harvard University from the top spot for the first time in the survey's eight-year history.

Caltech was number two in the 2010–2011 ranking; Harvard and Stanford University share the second spot in the 2011–2012 survey, while the University of Oxford and Princeton University round out the top five.

"It's gratifying to be recognized for the work we do here and the impact it has—both on our students and on the global community," says Caltech president Jean-Lou Chameau. "Today's announcement reinforces Caltech's legacy of innovation, and our unwavering dedication to giving our extraordinary people the environment and resources with which to pursue their best ideas. It's also truly gratifying to see three California schools—including my alma mater, Stanford—in the top ten."

Thirteen performance indicators representing research (worth 30% of a school's overall ranking score), teaching (30%), citations (30%), international outlook (which includes the total numbers of international students and faculty and the ratio of scholarly papers with international collaborators; 7.5%), and industry income (a measure of innovation; 2.5%) are included in the data. Among the measures included are a reputation survey of 17,500 academics; institutional, industry, and faculty research income; and an analysis of 50 million scholarly papers to determine the average number of citations per scholarly paper, a measure of research impact.

"We know that innovation is the driver of the global economy, and is especially important during times of economic volatility," says Kent Kresa, chairman of the Caltech Board of Trustees. "I am pleased that Caltech is being recognized for its leadership and impact; this just confirms what many of us have known for a long time about this extraordinary place."

"Caltech has been one of California's best-kept secrets for a long time," says Caltech trustee Narendra Gupta. "But I think the secret is out!"

Times Higher Education, which compiled the listing using data supplied by Thomson Reuters, reports that this year's methodology was refined to ensure that universities with particular strength in the arts, humanities, and social sciences are placed on a more equal footing with those with a specialty in science subjects. Caltech—described in a Times Higher Education press release as "much younger, smaller, and specialised" than Harvard—was nevertheless ranked the highest based on their metrics.

According to Phil Baty, editor of the Times Higher Education World University Rankings, "the differences at the top of the university rankings are miniscule, but Caltech just pips Harvard with marginally better scores for 'research—volume, income, and reputation,' research influence, and the income it attracts from industry. With differentials so slight, a simple factor plays a decisive role in determining rank order: money."

"Harvard reported funding increases similar in proportion to other institutions, whereas Caltech reported a steep rise (16%) in research funding and an increase in total institutional income," Baty says.

Data for the Times Higher Education's World University Rankings was provided by Thomson Reuters from its Global Institutional Profiles Project (http://science.thomsonreuters.com/globalprofilesproject/), an ongoing, multistage process to collect and validate factual data about academic institutional performance across a variety of aspects and multiple disciplines.

For a full list of the world's top 200 schools and all of the performance indicators, go to http://www.timeshighereducation.co.uk/world-university-rankings/.

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The California Institute of Technology (Caltech) is a small, private university in Pasadena that conducts instruction and research in science and engineering, with a student body of about 900 undergraduates and 1,200 graduate students. Recognized for its outstanding faculty, including several Nobel laureates, and such renowned off-campus facilities as the Jet Propulsion Laboratory, the W. M. Keck Observatory, and the Palomar Observatory, Caltech is one of the world's preeminent research centers.

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Caltech Team Uses Laser Light to Cool Object to Quantum Ground State

PASADENA, Calif.—For the first time, researchers at the California Institute of Technology (Caltech), in collaboration with a team from the University of Vienna, have managed to cool a miniature mechanical object to its lowest possible energy state using laser light. The achievement paves the way for the development of exquisitely sensitive detectors as well as for quantum experiments that scientists have long dreamed of conducting.

"We've taken a solid mechanical system—one made up of billions of atoms—and used optical light to put it into a state in which it behaves according to the laws of quantum mechanics. In the past, this has only been achieved with trapped single atoms or ions," says Oskar Painter, professor of applied physics and executive officer for applied physics and materials science at Caltech and the principal investigator on a paper describing the work that appears in the October 6 issue of the journal Nature

As described in the paper, Painter and his colleagues have engineered a nanoscale object—a tiny mechanical silicon beam—such that laser light of a carefully selected frequency can enter the system and, once reflected, can carry thermal energy away, cooling the system.

By carefully designing each element of the beam as well as a patterned silicon shield that isolates it from the environment, Painter and colleagues were able to use the laser cooling technique to bring the system down to the quantum ground state, where mechanical vibrations are at an absolute minimum. Such a cold mechanical object could help detect very small forces or masses, whose presence would normally be masked by the noisy thermal vibrations of the sensor.

"In many ways, the experiment we've done provides a starting point for the really interesting quantum-mechanical experiments one wants to do," Painter says. For example, scientists would like to show that a mechanical system could be coaxed into a quantum superposition—a bizarre quantum state in which a physical system can exist in more than one position at once. But they need a system at the quantum ground state to begin such experiments.

To reach the ground state, Painter's group had to cool its mechanical beam to a temperature below 100 millikelvin (-273.15°C). That's because the beam is designed to vibrate at gigahertz frequencies (corresponding to a billion cycles per second)—a range where a large number of phonons are present at room temperature. Phonons are the most basic units of vibration just as the most basic units or packets of light are called photons. All of the phonons in a system have to be removed to cool it to the ground state. 

Conventional means of cryogenically cooling to such temperatures exist but require expensive and, in some cases, impractical equipment. There's also the problem of figuring out how to measure such a cold mechanical system. To solve both problems, the Caltech team used a different cooling strategy. 

"What we've done is used the photons—the light field—to extract phonons from the system," says Jasper Chan, lead author of the new paper and a graduate student in Painter's group. To do so, the researchers drilled tiny holes at precise locations in their mechanical beam so that when they directed laser light of a particular frequency down the length of the beam, the holes acted as mirrors, trapping the light in a cavity and causing it to interact strongly with the mechanical vibrations of the beam. 

Because a shift in the frequency of the light is directly related to the thermal motion of the mechanical object, the light—when it eventually escapes from the cavity—also carries with it information about the mechanical system, such as the motion and temperature of the beam. Thus, the researchers have created an efficient optical interface to a mechanical element—or an optomechanical transducer—that can convert information from the mechanical system into photons of light.

Importantly, since optical light, unlike microwaves or electrons, can be transmitted over large, kilometer-length distances without attenuation, such an optomechanical transducer could be useful for linking different quantum systems—a microwave system with an optical system, for example. While Painter's system involves an optical interface to a mechanical element, other teams have been developing systems that link a microwave interface to a mechanical element. What if those two mechanical elements were the same? "Then," says Painter, "I could imagine connecting the microwave world to the optical world via this mechanical conduit one photon at a time." 

The Caltech team isn't the first to cool a nanomechanical object to the quantum ground state; a group led by former Caltech postdoctoral scholar Andrew Cleland, now at the University of California, Santa Barbara, accomplished this in 2010 using more conventional refrigeration techniques, and, earlier this year, a group from the National Institute of Standards and Technology in Boulder, Colorado, cooled an object to the ground state using microwave radiation. The new work, however, is the first in which a nanomechanical object has been put into the ground state using optical light.

"This is an exciting development because there are so many established techniques for manipulating and measuring the quantum properties of systems using optics," Painter says.

The other cooling techniques used starting temperatures of approximately 20 millikelvin—more than a factor of 10,000 times cooler than room temperature. Ideally, to simplify designs, scientists would like to initiate these experiments at room temperature. Using laser cooling, Painter and his colleagues were able to perform their experiment at a much higher temperature—only about 10 times lower than room temperature.

Along with Painter and Chan, additional coauthors of the paper, "Laser cooling of a nanomechanical oscillator into its quantum ground state," include Caltech postdoctoral scholar T.P. Mayer Alegre and graduate students Amir Safavi-Naeini, Jeff Hill, and Alex Krause, along with postdoctoral scholar Simon Gröblacher and Markus Aspelmeyer of the Vienna Center for Quantum Science and Technology. The work was supported by Caltech's Kavli Nanoscience Institute; the Defense Advanced Research Projects Agency's Microsystems Technology Office through a grant from the Air Force Office of Scientific Research; the European Commission; the European Research Council; and the Austrian Science Fund.

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Caltech Engineers Build Smart Petri Dish

Device can be used for medical diagnostics, to image cell growth continuously

PASADENA, Calif.—The cameras in our cell phones have dramatically changed the way we share the special moments in our lives, making photographs instantly available to friends and family. Now, the imaging sensor chips that form the heart of these built-in cameras are helping engineers at the California Institute of Technology (Caltech) transform the way cell cultures are imaged by serving as the platform for a "smart" petri dish.

Dubbed ePetri, the device is described in a paper that appears online this week in the Proceedings of the National Academy of Sciences (PNAS).

Since the late 1800s, biologists have used petri dishes primarily to grow cells. In the medical field, they are used to identify bacterial infections, such as tuberculosis. Conventional use of a petri dish requires that the cells being cultured be placed in an incubator to grow. As the sample grows, it is removed—often numerous times—from the incubator to be studied under a microscope.

Not so with the ePetri, whose platform does away with the need for bulky microscopes and significantly reduces human labor time, while improving the way in which the culture growth can be recorded.

"Our ePetri dish is a compact, small, lens-free microscopy imaging platform. We can directly track the cell culture or bacteria culture within the incubator," explains Guoan Zheng, lead author of the study and a graduate student in electrical engineering at Caltech. "The data from the ePetri dish automatically transfers to a computer outside the incubator by a cable connection. Therefore, this technology can significantly streamline and improve cell culture experiments by cutting down on human labor and contamination risks."

The team built the platform prototype using a Google smart phone, a commercially available cell-phone image sensor, and Lego building blocks. The culture is placed on the image-sensor chip, while the phone's LED screen is used as a scanning light source. The device is placed in an incubator with a wire running from the chip to a laptop outside the incubator. As the image sensor takes pictures of the culture, that information is sent out to the laptop, enabling the researchers to acquire and save images of the cells as they are growing in real time. The technology is particularly adept at imaging confluent cells—those that grow very close to one another and typically cover the entire petri dish.

"Until now, imaging of confluent cell cultures has been a highly labor-intensive process in which the traditional microscope has to serve as an expensive and suboptimal workhorse," says Changhuei Yang, senior author of the study and professor of electrical engineering and bioengineering at Caltech. "What this technology allows us to do is create a system in which you can do wide field-of-view microscopy imaging of confluent cell samples. It capitalizes on the use of readily available image-sensor technology, which is found in all cell-phone cameras."

In addition to simplifying medical diagnostic tests, the ePetri platform may be useful in various other areas, such as drug screening and the detection of toxic compounds. It has also proved to be practical for use in basic research.

Caltech biologist Michael Elowitz, a coauthor on the study, has put the ePetri system to the test, using it to observe embryonic stem cells. Stem cells in different parts of a petri dish often behave differently, changing into various types of other, more specialized cells. Using a conventional microscope with its lens's limitations, a researcher effectively wears blinders and is only able to focus on one region of the petri dish at a time, says Elowitz. But by using the ePetri platform, Elowitz was able to follow the stem-cell changes over the entire surface of the device.

"It radically reconceives the whole idea of what a light microscope is," says Elowitz, a professor of biology and bioengineering at Caltech and a Howard Hughes Medical Institute investigator. "Instead of a large, heavy instrument full of delicate lenses, Yang and his team have invented a compact lightweight microscope with no lens at all, yet one that can still produce high-resolution images of living cells. Not only that, it can do so dynamically, following events over time in live cells, and across a wide range of spatial scales from the subcellular to the macroscopic."

Elowitz says the technology can capture things that would otherwise be difficult or impossible—even with state-of-the-art light microscopes that are both much more complicated and much more expensive.

"With ePetri, you can survey the entire field at once, but still maintain the ability to 'zoom in' to any cells of interest," he says. "In this regard, perhaps it's a bit like an episode of CSI where they zoom in on what would otherwise be unresolvable details in a photograph."  

Yang and his team believe the ePetri system is likely to open up a whole range of new approaches to many other biological systems as well. Since it is a platform technology, it can be applied to other devices. For example, ePetri could provide microscopy-imaging capabilities for other portable diagnostic lab-on-a-chip tools. The team is also working to build a self-contained system that would include its own small incubator. This advance would make the system more useful as a desktop diagnostic tool that could be housed in a doctor's office, reducing the need to send bacteria samples out to a lab for testing.

Seung Ah Lee, a graduate student in electrical engineering, and Yaron Antebi, a postdoctoral scholar in biology—both from Caltech—were also coauthors on the study, which is titled "The ePetri dish, an on-chip cell imaging platform based on subpixel perspective sweeping microscopy (SPSM)." Funding support was provided by the Coulter Foundation.

Written by Katie Neith

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Caltech-Led Engineers Solve Longstanding Problem in Photonic Chip Technology

Findings Help Pave Way for Next Generation of Computer Chips

PASADENA, Calif.—Stretching for thousands of miles beneath oceans, optical fibers now connect every continent except for Antarctica. With less data loss and higher bandwidth, optical-fiber technology allows information to zip around the world, bringing pictures, video, and other data from every corner of the globe to your computer in a split second. But although optical fibers are increasingly replacing copper wires, carrying information via photons instead of electrons, today's computer technology still relies on electronic chips.

Now, researchers led by engineers at the California Institute of Technology (Caltech) are paving the way for the next generation of computer-chip technology: photonic chips. With integrated circuits that use light instead of electricity, photonic chips will allow for faster computers and less data loss when connected to the global fiber-optic network.

"We want to take everything on an electronic chip and reproduce it on a photonic chip," says Liang Feng, a postdoctoral scholar in electrical engineering and the lead author on a paper to be published in the August 5 issue of the journal Science. Feng is part of Caltech's nanofabrication group, led by Axel Scherer, Bernard A. Neches Professor of Electrical Engineering, Applied Physics, and Physics, and co-director of the Kavli Nanoscience Institute at Caltech.

In that paper, the researchers describe a new technique to isolate light signals on a silicon chip, solving a longstanding problem in engineering photonic chips.

An isolated light signal can only travel in one direction. If light weren't isolated, signals sent and received between different components on a photonic circuit could interfere with one another, causing the chip to become unstable. In an electrical circuit, a device called a diode isolates electrical signals by allowing current to travel in one direction but not the other. The goal, then, is to create the photonic analog of a diode, a device called an optical isolator. "This is something scientists have been pursuing for 20 years," Feng says.

Normally, a light beam has exactly the same properties when it moves forward as when it's reflected backward. "If you can see me, then I can see you," he says. In order to isolate light, its properties need to somehow change when going in the opposite direction. An optical isolator can then block light that has these changed properties, which allows light signals to travel only in one direction between devices on a chip.

"We want to build something where you can see me, but I can't see you," Feng explains. "That means there's no signal from your side to me. The device on my side is isolated; it won't be affected by my surroundings, so the functionality of my device will be stable."

To isolate light, Feng and his colleagues designed a new type of optical waveguide, a 0.8-micron-wide silicon device that channels light. The waveguide allows light to go in one direction but changes the mode of the light when it travels in the opposite direction.

A light wave's mode corresponds to the pattern of the electromagnetic field lines that make up the wave. In the researchers' new waveguide, the light travels in a symmetric mode in one direction, but changes to an asymmetric mode in the other. Because different light modes can't interact with one another, the two beams of light thus pass through each other.

Previously, there were two main ways to achieve this kind of optical isolation. The first way—developed almost a century ago—is to use a magnetic field. The magnetic field changes the polarization of light—the orientation of the light's electric-field lines—when it travels in the opposite direction, so that the light going one way can't interfere with the light going the other way. "The problem is, you can't put a large magnetic field next to a computer," Feng says. "It's not healthy."

The second conventional method requires so-called nonlinear optical materials, which change light's frequency rather than its polarization. This technique was developed about 50 years ago, but is problematic because silicon, the material that's the basis for the integrated circuit, is a linear material. If computers were to use optical isolators made out of nonlinear materials, silicon would have to be replaced, which would require revamping all of computer technology. But with their new silicon waveguides, the researchers have become the first to isolate light with a linear material.

Although this work is just a proof-of-principle experiment, the researchers are already building an optical isolator that can be integrated onto a silicon chip. An optical isolator is essential for building the integrated, nanoscale photonic devices and components that will enable future integrated information systems on a chip. Current, state-of-the-art photonic chips operate at 10 gigabits per second (Gbps)—hundreds of times the data-transfer rates of today's personal computers—with the next generation expected to soon hit 40 Gbps. But without built-in optical isolators, those chips are much simpler than their electronic counterparts and are not yet ready for the market. Optical isolators like those based on the researchers' designs will therefore be crucial for commercially viable photonic chips.

In addition to Feng and Scherer, the other authors on the Science paper, "Non-reciprocal light propagation in a silicon photonic circuit," are Jingqing Huang, a Caltech graduate student; Maurice Ayache of UC San Diego and Yeshaiahu Fainman, Cymer Professor in Advanced Optical Technologies at UC San Diego; and Ye-Long Xu, Ming-Hui Lu, and Yan-Feng Chen of the Nanjing National Laboratory of Microstructures in China. This research was done as part of the Center for Integrated Access Networks (CIAN), one of the National Science Foundation's Engineering Research Centers. Fainman is also the deputy director of CIAN. Funding was provided by the National Science Foundation, and the Defense Advanced Research Projects Agency.

Writer: 
Marcus Woo
Writer: 

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