Caltech Scientists Solve Decade-Long Mystery of Nanopillar Formations

Research paves way for new 3-D lithography method

Pasadena, Calif.—Scientists at the California Institute of Technology (Caltech) have uncovered the physical mechanism by which arrays of nanoscale (billionths-of-a-meter) pillars can be grown on polymer films with very high precision, in potentially limitless patterns.

This nanofluidic process—developed by Sandra Troian, professor of applied physics, aeronautics, and mechanical engineering at Caltech, and described in a recent article in the journal Physical Review Letters—could someday replace conventional lithographic patterning techniques now used to build three-dimensional nano- and microscale structures for use in optical, photonic, and biofluidic devices.

The fabrication of high-resolution, large-area nanoarrays relies heavily on conventional photolithographic patterning techniques, which involve treatments using ultraviolet light and harsh chemicals that alternately dissolve and etch silicon wafers and other materials. Photolithography is used to fabricate integrated circuits and microelectromechanical devices, for example.

However, the repeated cycles of dissolution and etching cause a significant amount of surface roughness in the nanostructures, ultimately limiting their performance.

"This process is also inherently two-dimensional, and thus three-dimensional structures must be patterned layer by layer," says Troian.

In an effort to reduce cost, processing time, and roughness, researchers have been exploring alternative techniques whereby molten films can be patterned and solidified in situ, and in a single step.

About a decade ago, groups in Germany, China, and the United States encountered a bizarre phenomenon while using techniques involving thermal gradients. When molten polymer nanofilms were inserted within a slender gap separating two silicon wafers that were held at different temperatures, arrays of nanoscale pillars spontaneously developed.

These protrusions grew until they reached the top wafer; the resulting pillars were typically several hundred nanometers high and several microns apart.

These pillars sometimes merged, forming patterns that looked like bicycle chains when viewed from above; in other films, the pillars grew in evenly spaced, honeycomb-like arrays. Once the system was brought back down to room temperature, the structures solidified in place to produce self-organized features.

In 2002, researchers in Germany who had observed this phenomenon hypothesized that the pillars arise from infinitesimal—but very real—pressure fluctuations along the surface of an otherwise quiescent flat film. They proposed that the differences in surface pressure were caused by equally tiny variations in the way individual packets (or quanta) of vibrational energy, known as phonons, reflect from the film interfaces.

"In their model, the difference in acoustic impedance between the air and polymer is believed to generate an imbalance in phonon flux that causes a radiation pressure that destabilizes the film, allowing pillar formation," says Troian. "Their mechanism is the acoustic analogue of the Casimir force, which is quite familiar to physicists working at the nanoscale."

But Troian, who was familiar with thermal effects at small scales—and knew that the propagation of these phonons is actually unlikely in amorphous polymer melts, which lack internal periodic structure—immediately recognized that another mechanism might be lurking in this system.

To determine the actual cause of nanopillar formation, she and Caltech postdoctoral scholar Mathias Dietzel developed a fluid-dynamical model of the same type of thin, molten nanofilm in a thermal gradient.

Their model, Troian says, "exhibited a self-organizing instability that was able to reproduce the strange formations," and showed that nanopillars, in fact, form not via pressure fluctuations but through a simple physical process known as thermocapillary flow.

In capillary flow—or capillary action—the attractive force, or cohesion, between molecules of the same liquid (say, water) produces surface tension, the compressive force that is responsible for holding together a droplet of water. Since surface tension tends to minimize the surface area of a liquid, it often acts as a stabilizing mechanism against deformation caused by other forces. Differences in temperature along a liquid interface, however, generate differences in surface tension. In most liquids, cooler regions will have a higher surface tension than warmer ones—and this imbalance can cause the liquid to flow from warmer- to cooler-temperature regions, a process known as thermocapillary flow.

Previously, Troian has used such forces for microfluidic applications, to move droplets from one point to another.

"You can see this effect very nicely if you move an ice cube in a figure eight beneath a metal sheet coated with a liquid like glycerol," she says. "The liquid wells up above the cube as it traces out the figure. You can draw your name in this way, and, presto! You have got yourself a new form of thermocapillary lithography!"

In their Physical Review Letters paper, Troian and Dietzel showed how this effect can theoretically dominate all other forces at nanoscale dimensions, and also showed that the phenomenon is not peculiar to polymer films.

In the thermal-gradient experiments, they say, the tips of the tiny protrusions in the polymer film experience a slightly colder temperature than the surrounding liquid, because of their proximity to the cooler wafer.

"The surface tension at an evolving tip is just a little bit greater, and this sets up a very strong force oriented parallel to the air/polymer interface, which bootstraps the fluid toward the cooler wafer. The closer the tip gets to the wafer, the colder it becomes, leading to a self-reinforcing instability," Troian explains.

 

Upper: Schematic showing typical experimental setup. Lower: AFM image of 260 nm high nanopillars spaced 3.4 microns apart which formed in a polymer film.
Credit: Upper: Dietzel and Troian/Caltech; PRL. Lower: Chou and Zhuang, J. Vac. Sci. Technol. B 17, 3197 (1999).

Ultimately, she says, "you can end up with very long columnar structures. The only limit to the height of the column, or nanopillar, is the separation distance of the wafers."

In computer models, the researchers were able to use targeted variations in the temperature of the cooler substrate to control precisely the pattern replicated in the nanofilm. In one such model, they created a three-dimensional "nanorelief" of the Caltech logo.

Troian and her colleagues are now beginning experiments in the laboratory in which they hope to fabricate a diverse array of nanoscale optical and photonic elements. "We are shooting for nanostructures with specularly smooth surfaces—as smooth as you could ever make them—and 3-D shapes that are not easily attainable using conventional lithography," Troian says.

"This is an example of how basic understanding of the principles of physics and mechanics can lead to unexpected discoveries which may have far-reaching, practical implications," says Ares Rosakis, chair of the Division of Engineering and Applied Science (EAS) and Theodore von Kármán Professor of Aeronautics and Mechanical Engineering at Caltech. "This is the real strength of the EAS division."

The work in the paper, "Formation of Nanopillar Arrays in Ultrathin Viscous Films: The Critical Role of Thermocapillary Stresses," was funded by the Engineering Directorate of the National Science Foundation.

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Kathy Svitil
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Two International Leaders Receive Caltech Aerospace Award

Former president of India and France's top space administrator recognized for their achievements in the field

Pasadena, Calif., Sept. 15, 2009- Two distinguished aerospace leaders are the recipients of the 25th annual International von Kármán Wings Award. Receiving the honor this year are Abdul Kalam, the 11th president of India and distinguished professor at the Indian Institute of Technology, and Yannick d'Escatha, chairman and chief executive officer of the Centre National d'Etudes Spatiales (CNES), the agency responsible for shaping France's space policy.

"Along with their tremendous accomplishments in aerospace, this year's honorees are leaders in international collaboration, climate monitoring, and energy harvesting," says Ares J. Rosakis, chair of the Aerospace Historical Society, chair of the Division of Engineering and Applied Science, and Theodore von Kármán Professor of Aeronautics and Mechanical Engineering at Caltech.

This is the 25th year that the International von Kármán Wings Award has been given by the Aerospace Historical Society (AHS), which is now apart of the Graduate Aerospace Laboratories at Caltech (GALCIT). The award has a rich heritage in the preservation of world aerospace history and the recognition of renowned aerospace pioneers and luminaries.

"GALCIT is proud and privileged to be the home of the Aerospace Historical Society," says G. Ravi Ravichandran, director of the Graduate Aerospace Laboratories and the John E. Goode, Jr., Professor of Aeronautics and Mechanical Engineering. "It is an honor to give this award, named after the founding director of GALCIT and the founder of the Jet Propulsion Laboratory, to Abdul Kalam and Yannick d'Escatha."

Rosakis described Adbul Kalam as an "international leader and humanitarian who is honored and admired by the next generation" and Yannick d'Escatha as a "visionary who is using space and technology to bring about collaboration and peace."  

One example of the honorees' collaborative efforts is the Megha-Tropiques weather satellite, a joint project of the Indian Space Research Organization (ISRO) and the French Centre National d'Etudes Spatiales (CNES).

The von Kármán Wings Awards will be handed out September 15 at a banquet on the Caltech campus and will be presented by Rosakis.

Previous recipients of the Wings Award include last year's winner, Northrop Grumman's chief technology officer Alexis Livanos; director of the NASA Jet Propulsion Laboratory Charles Elachi; Kent Kresa, chairman of Caltech's Board of Trustees; TRW cofounder Simon Ramo; aerospace engineer Burt Rutan; and astronaut Buzz Aldrin.

About Abdul Kalam:

Abdul Kalam, the 11th president of India, was born in 1931 in Rameswaram, in the Indian state of Tamil Nadu. He attended the Madras Institute of Technology, specializing in aeronautical engineering. Dr. Kalam was the project director of India's first indigenous Satellite Launch Vehicle (SLV-III), which successfully propelled the Rohini satellite into near-Earth orbit in July 1980 and made India a member of the exclusive "space club."

After working for two decades in the Indian Space Research Organization (ISRO) and mastering launch vehicle technologies, Dr. Kalam took up the responsibility of developing indigenous guided missiles at the Defense Research and Development Organization as the chief executive of the Integrated Guided Missile Development Program. He was responsible for the development and operations of AGNI and PRITHVI missiles and for building indigenous capability in critical technologies through networking with multiple institutions. Dr. Kalam was the scientific advisor to India's defense minister and secretary and boosted the country's self-reliance in defense systems by advancing multiple mission projects, such as the Light Combat Aircraft.

Dr. Kalam became the 11th president of India in July 2002 and served for five years. He led the country in arriving at Technology Vision 2020, giving a road map for transforming India from its present developing status to a developed nation.

Dr. Kalam is a distinguished professor at the Indian Institute of Technology and has also authored a number of books, including Wings of Fire, India 2020:A Vision for the New Millennium and Ignited Minds: Unleashing the Power Within India. These books have been translated into many Indian and foreign languages.

Being one of the most distinguished scientists of India, Dr. Kalam has received honorary doctorates from 36 universities and institutions in India and abroad. The Royal Society of the United Kingdom has awarded to him the King Charles II Medal for Science and Technology.

About Yannick d'Escatha:

Yannick d'Escatha was born in 1948 in Paris, France. He graduated from École des Mines and École Polytechnique, where he became a professor and was the chairman of the board of trustees. He was internationally recognized for his research in solid, structural, and fracture mechanics.

In 1973, he became an expert advisor to the minister of industry on nuclear regulatory and research issues. D'Escatha was the administrator general of the French Atomic Energy Commission (CEA) and chairman of the CEA Industrie Group. At CEA, he promoted astrophysics and global-change research and concentrated on the spin-off potential of the R&D activities.

Ares Rosakis (left) and Abdul Kalam
Credit: Bob Paz

He served as chief operating officer and vice president of Électricité de France from 2000 to 2003.

In 2003, d'Escatha was appointed chairman and chief executive officer of the CNES. He conducted an ambitious policy to restructure the French space agency. In addition, he secured the Ariane 5 launcher system and recently issued a report on future launch systems to the French prime minister. At CNES, he developed research and applications dedicated to global change. He led the CNES Automated Transfer Vehicle (ATV) Control Center to successfully dock the European ATV with the International Space Station.

D'Escatha is an advocate for international space cooperation. He is responsible for creating large partnerships with the top spacefaring nations in Europe and with other international partners. In Europe, he provides leadership for the new European Space Policy by encouraging strong partnership between ESA and European Union institutions. He also provided the road map for the European Space Council during the French presidency of the European Union (2008).

D'Escatha received two distinguished awards from the Académie des Sciences. He is a member of the Académie des Technologies and served on a variety of French Applied Science and Technology Councils. The French Republic awarded him both the Commandeur de l'Ordre National du Mérite and the Officier de la Légion d'Honneur decorations.

# # #

About GALCIT:

The research at the Graduate Aerospace Laboratories of the California Institute of Technology (GALCIT) has evolved over the past three-quarters of a century to include aerospace and biosystems engineering. However, the tradition of integrating basic experiments, theory, and simulations over a broad range of spatial and temporal scales continues to characterize its approach.

GALCIT faculty are highly visible in their fields and continue to garner numerous awards. GALCIT contains unparalleled experimental facilities in solids, fluids, biomechanics, propulsion, combustion, and materials, as well as unique large-scale computational capabilities.

Its educational emphasis is on the fundamentals and advanced diagnostics, with a view toward the future, of biomechanics, biopropulsion, micro and nanomechanics, space science, and space technology. GALCIT takes an interdisciplinary view of mechanics-fluids, solids, and materials-and its graduate training reflects this focus.

About Caltech:

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

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

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Caltech and IBM Scientists Use Self-Assembled DNA Scaffolding to Build Tiny Circuit Boards

Nanotechnology advance could lead to smaller, faster, more energy-efficient computer chips

Pasadena, Calif.--Scientists at the California Institute of Technology (Caltech) and IBM's Almaden Research Center have developed a new technique to orient and position self-assembled DNA shapes and patterns--or "DNA origami"--on surfaces that are compatible with today's semiconductor manufacturing equipment. These precisely positioned DNA nanostructures, each no more than one one-thousandth the width of a human hair, can serve as scaffolds or miniature circuit boards for the precise assembly of computer-chip components.

The advance, described in the current issue of the journal Nature Nanotechnology, could allow the semiconductor industry to pack more power and speed into tiny computer chips, while making them more energy efficient and less expensive to manufacture than is possible today.  

DNA origami structures have been heralded as a potential breakthrough for the creation of nanoscale circuits and devices. In a process created by Caltech senior research associate Paul W. K. Rothemund and his colleagues, DNA molecules self-assemble in solution via a reaction between a long single strand of viral DNA and a mixture of different short synthetic DNA strands. These short segments act as staples that effectively fold the viral DNA into desired two-dimensional shapes through complementary base-pair binding.

In this way, DNA nanostructures such as squares, triangles, and stars can be prepared that measure 100 to 150 nanometers on an edge and are as thick as the DNA double helix is wide. 

One roadblock to the use of DNA origami, however, is that the structures are made in saltwater solution--whereas electronic circuits are created on surfaces, like a silicon wafer, so they can be integrated with other technologies.

DNA origami structures also adhere randomly to surfaces, which means that "if you just pour DNA origami over a surface to which they stick, they attach everywhere," explains Rothemund, who jointly led the project with IBM. "It's a little like taking a deck of playing cards and throwing it on the floor; they are scattered willy-nilly all over the place. Such random arrangements of DNA origami are not very useful. If they carry electronic circuits, for example, they are difficult to find and wire up into larger circuits."  

To eliminate these problems, Rothemund and his colleagues at the Almaden Research Center developed a way to precisely position DNA origami nanostructures on a surface, "to line them up like little ducks in a row," Rothemund says. "This knocks down one of the major roadblocks for the use of DNA origami in technology," he adds.

In a process developed by IBM scientists, electron-beam lithography and oxygen plasma etching, conventional semiconductor techniques, are used to make patterns on silicon wafers, creating lithographic templates of the proper size and shape to match those of individual triangular DNA origami structures created by Rothemund. The etched patches are negatively charged, as are DNA origami structures, and are therefore "sticky."

To connect the origami to the templates, magnesium ions are added to the saltwater solution containing the origami. The positively charged magnesium ions can stick to both the DNA origami and the negatively charged patches on the template. Thus, when the solution is poured over the template, a negative-positive-negative "sandwich" is formed, with the magnesium atoms acting as a glue to hold the origami to the sticky patches.

"The triangles bind strongly to the sticky patches, but also they can wiggle a bit, so they line up with the outline of the sticky patch. So not only can we put origami where we want them, but they can be oriented in the direction we want them," Rothemund says.

The positioned DNA nanostructures can then serve as scaffolds or miniature circuit boards for the precise assembly of components such as carbon nanotubes, nanowires, and nanoparticles at dimensions significantly smaller than possible with conventional semiconductor fabrication techniques. This opens up the possibility of creating functional devices that can be integrated into larger structures as well as enabling studies of arrays of nanostructures with known coordinates.

"The spacing between the components can be 6 nanometers, so the resolution of the process is roughly 10 times higher than the process we currently use to make computer chips," Rothemund says. "Then, if you want to design a really small electronic device, say, you just design DNA strands to create the pattern you want, attach little chemical 'fastening posts' to those DNA strands, assemble the pattern, and then assemble the components onto the pattern," he explains.

The process isn't limited to organizing things that are of interest to physical scientists and engineers, like electronic components, Rothemund adds. For example, he says, "Biologists studying how proteins interact can place them in patterns on top of DNA origami. This may be useful in the case of motor proteins, the little machines that power our muscles. They work in gangs, with multiple motors pulling together. To study how different configurations of motors cooperate, scientists may use DNA origami to organize the gangs."

"Rothemund and his colleagues have removed a key barrier to the improvement and advancement of computer chips. They accomplished this through the revolutionary approach of combining the building blocks for life with the building blocks for computing," says Ares Rosakis, Theodore von Kármán Professor of Aeronautics and Mechanical Engineering and chair of Caltech's Division of Engineering and Applied Science.

The paper, "Placement and orientation of individual DNA shapes on lithographically patterned surfaces," was published in the August 16 issue of Nature Nanotechnology. The work was supported by the National Science Foundation and the Focus Center Research Program.

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Kathy Svitil
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Shang-Li and Betty Huang create endowment to support Caltech graduate students

PASADENA, Calif.—Alumnus Shang-Li "S.L." Huang (MS '69, PhD '76) and his wife have pledged $1 million to Caltech to endow the Shang-Li and Betty Huang Endowed Graduate Fellowship Fund in Mechanical Engineering. The gift will be matched by an additional $500,000 from the Gordon and Betty Moore Matching Program.

"This generous gift will assist us in continuing to attract engineering graduate students with exceptional talent and ability. S.L. and Betty have changed lives far into the future with their support," says Ares Rosakis, chair of Caltech's Division of Engineering and Applied Science and the Theodore von Kármán Professor of Aeronautics and Mechanical Engineering.

"S.L. was my graduate student and did an outstanding PhD thesis back in the 1970s—a thesis whose results are still widely used in the rocket-engine design business," adds Chris Brennen, the Richard L. and Dorothy M. Hayman Professor of Mechanical Engineering at Caltech. "He and Betty are deeply interested in education—in particular, graduate education. They have been instrumental in rallying support for mechanical engineering at Caltech. We are most grateful for their generous help and advice."

For his part, S.L. believes that graduate school at Caltech shaped his life, laying a foundation for his success in areas beyond engineering and research.  Some of his deepest friendships began when he and a few other students and professors would surface from their labs to share meals, establishing a club they called the Meat and Wine Society. More than three decades later, they still meet. S.L. has also stayed involved with Caltech in more formal ways. He and Betty became life members of the Caltech Associates, have hosted Associates gatherings in Orange County, and have participated in Associates events ranging from dinners with faculty to a tour of Paris, Burgundy, and Lyon. S.L. serves on the board of the Associates and is a life member of the Caltech Alumni Association. He also serves on a special advisory committee for mechanical engineering at Caltech.

The Huangs' gift will make it possible for hundreds of future graduate students to study and live at Caltech, and, perhaps, to form meat and wine societies of their own.

"Betty and I are so glad to have this opportunity to endow this graduate fellowship fund," says S.L. "We have always believed that education is one of the most important tools that we can provide to our next generation. We have only done a small part, and we sincerely hope others will come forward and participate in creating additional endowments to benefit Caltech undergraduates and graduate students—especially in these difficult times."
 
###

 About Caltech:
Caltech is recognized for its highly select student body of 900 undergraduates and 1,200 graduate students, and for its outstanding faculty. Since 1923, Caltech faculty and alumni have garnered 32 Nobel Prizes and five Crafoord Prizes. In addition to its prestigious on-campus research programs, Caltech operates the Jet Propulsion Laboratory (JPL), the W. M. Keck Observatory in Mauna Kea, the Palomar Observatory, and the Laser Interferometer Gravitational-Wave Observatory (LIGO). Caltech is a private university in Pasadena, California. For more information, visit http://www.caltech.edu.

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Jon Weiner
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Caltech Researchers Link Tiny Sea Creatures to Large-scale Ocean Mixing

Mechanism, which focuses on impact of drag and water viscosity, shows how swimming animals affect the ocean environment

PASADENA, Calif. —Using a combination of theoretical modeling, energy calculations, and field observations, researchers from the California Institute of Technology (Caltech) have for the first time described a mechanism that explains how some of the ocean's tiniest swimming animals can have a huge impact on large-scale ocean mixing.

Their findings are being published in the July 30 issue of the journal Nature.

"We've been studying swimming animals for quite some time," says John Dabiri, a Caltech assistant professor of aeronautics and bioengineering who, along with Caltech graduate student Kakani Katija, discovered the new mechanism. "The perspective we usually take is that of how the ocean—by its currents, temperature, and chemistry—is affecting the animals. But there have been increasing suggestions that the inverse is also important-how the animals themselves, via swimming, might impact the ocean environment."

Specifically, Dabiri says, scientists have increasingly been thinking about how and whether the animals in the ocean might play a role in larger-scale ocean mixing, the process by which various layers of water interact with one another to distribute heat, nutrients, and gasses throughout the oceans.

Dabiri notes that oceanographers have previously dismissed the idea that animals might have a significant effect on ocean mixing, saying that the viscosity of water would damp out any turbulence created, especially by small planktonic animals. "They said that there was no mechanism by which these animals could impact large-scale ocean mixing," he notes.

But Dabiri and Katija thought there might be a mechanism that had been overlooked—a mechanism they call Darwinian mixing, because it was first discovered and described by Charles Darwin. (No, not that Darwin; his grandson.)

"Darwin's grandson discovered a mechanism for mixing similar in principle to the idea of drafting in aerodynamics," Dabiri explains. "In this mechanism, an individual organism literally drags the surrounding water with it as it goes."

Using this idea as their basis, Dabiri and Katija did some mathematical simulations of what might happen if you had many small animals all moving at more or less the same time, in the same direction. After all, each day, billions of tiny krill and copepods migrate hundreds of meters from the depths of the ocean toward the surface. Darwin's mechanism would suggest that they drag some of the colder, heavier bottom water up with them toward the warmer, lighter water at the top. This would create instability, and eventually, the water would flip, mixing itself as it went.

What the Caltech researchers also found was that the water's viscosity enhances Darwin's mechanism and that the effects are magnified when you're dealing with such minuscule creatures as krill and copepods. "It's like a human swimming through honey," Dabiri explains. "What happens is that even more fluid ends up being carried up with a copepod, relatively speaking, than would be carried up by a whale."

"This research is truly reflective of the type of exciting, without-boundaries research at which Caltech engineering professors excel—in this case a deep analysis of the movement of fluid surrounding tiny ocean creatures leading to completely revelatory insights on possible mechanisms of global ocean mixing," says Ares Rosakis, chair of the Division of Engineering and Applied Science at Caltech.

To verify the findings from their simulations, Katija and collaborators Monty Graham (from the Dauphin Island Sea Laboratory), Jack Costello (from Providence College), and Mike Dawson (from the University of California, Merced) traveled to the island of Palau, where they studied this animal-led transport of water—otherwise known as induced drift—among jellyfish, which are the focus of much of Dabiri's work.

"From a fluid mechanics perspective, this study had less to do with the fact that they're jellyfish, and more to do with the fact that they're solid objects moving through water," Dabiri explains.

Katija's jellyfish experiments involved putting fluorescent dye in the water in front of the sea creatures, and then watching what happened to that dye—or, to be more specific, to the water that took up the dye—as the jellyfish swam. And, indeed, rather than being left behind the jellyfish—or being dissipated in turbulent eddies—the dye travelled right along with the swimming creatures, following them for long distances.

These findings verified that, yes, swimming animals are capable of carrying bottom water with them as they migrate upward, and that movement indeed creates an inversion that results in ocean mixing. But what the findings didn't address was just how much of an impact this type of ocean mixing—performed by impossibly tiny sea creatures—could have on a large scale.

After a series of calculations, Dabiri and Katija were able to estimate the impact of this so-called biogenic ocean mixing. And, Dabiri says, it's quite a significant impact.

"There are enough of these animals in the ocean," he notes, "that, on the whole, the global power input from this process is as much as a trillion watts of energy—comparable to that of wind forcing and tidal forcing."

In other words, the amount of power that copepods and krill put into ocean mixing is on the same scale as that of winds and tides, and thus their impact is expected to be on a similar scale as well.

And while these numbers are just estimates, Dabiri says, they are likely to be conservative estimates, having been "based on the fluid transport induced by individual animals swimming in isolation." In the ocean, these individual contributions to fluid transport may actually interact with one another, and amplify how far the ocean waters can be pulled upward.

In addition, says Dabiri, they have yet to consider the effects of such things as fecal pellets and marine snow (falling organic debris), which no doubt pull surface water with them as they drift downward. "This may have an impact on carbon sequestration on the ocean floor," says Dabiri. "It's something we need to look at in the future."

Dabiri says the next major question to answer is how these effects can be incorporated into computer models of the global ocean circulation. Such models are important for simulations of global climate-change scenarios.

The work described in the Nature paper, "A viscosity-enhanced mechanism for biogenic ocean mixing," was supported by grants from the National Science Foundation's Biological Oceanography, Ocean Technology, Fluid Dynamics, and Energy for Sustainability programs, and by the Office of Naval Research, the Department of Defense's National Science and Engineering Graduate Fellowship, and the Charles Lee Powell Foundation.

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Lori Oliwenstein
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Hans W. Liepmann, 94

Hans Wolfgang Liepmann, a pioneering researcher and passionate educator in fluid mechanics, passed away at the age of 94. Liepmann, the Theodore von Kármán Professor of Aeronautics, Emeritus, at the California Institute of Technology (Caltech), passed away June 24 at his home in La Canada Flintridge. Widely honored for his contributions to aeronautics, Liepmann came to Caltech in 1939 and was the third director of Caltech's Graduate Aeronautical Laboratories (GALCIT), from 1972 to1985.

Liepmann, known for his sharp wit and distinctive accent, was a noted teacher who mentored more than 60 PhD students and hundreds of undergraduates during his career at GALCIT. Through his students and colleagues at GALCIT, Liepmann was highly influential in spreading the fundamental research approach and rigorous curriculum of GALCIT. His students became leaders in the aerospace industry as well as universities around the world.

Liepmann was born in Berlin in 1914 and grew up surrounded by the political turmoil and liberal Berlin society of the 1920s.  His father—a well-known physician and hospital director—had a passion for the humanities and an abhorrence of mathematics.  Insisting that Hans have a classical education despite his interest in physics, his father nearly ended his son's scientific career before it began.  Looking back, Liepmann once observed, "of my 10 years in high school I can remember no more than maybe three teachers who were more than drillmasters." Those experiences likely contributed to Liepmann's passion for teaching at Caltech.

Liepmann's early years in Berlin came to a close shortly after graduating from high school and a stint in the Siemens factory as an apprentice. His father decided to emigrate following the rise of the Nazi government and the infamous Reichstag fire in 1933. Liepmann joined his family in Turkey in 1934 after his father was invited to be head of the gynecology department at the University of Istanbul. He enrolled in the university to study physics, mathematics, astrophysics, and mechanics. The classes were taught in a mix of German, French, and Turkish, under the numerous German expatriates that found Turkey more welcoming than Germany under Hitler. 

After a year in Istanbul and an unproductive term in Prague, Liepmann traveled to Switzerland and found academic success in the physics department at the University of Zürich. His talent as an experimenter was immediately recognized, leading to an invitation to pursue his doctoral studies on low-temperature physics under Richard Bar.  Liepmann's scientific temperament was strongly influenced by the exciting physics scene of 1930s Zürich and the teaching style of Gregor Wentzel, a student of Arnold Sommerfeld, whom many consider the father of modern physics. Throughout his life, Liepmann maintained the perspective of a physicist and emphasized to his students the importance of a scientific approach.

Liepmann came to the U.S. in 1939 after impulsively expressing an interest in "hydrodynamics" during a drinking party at the successful conclusion of his PhD defense. An offer from Theodore von Kármán led to a research position in experimental fluid mechanics at GALCIT, where Kármán was the first director. Liepmann's first experiments, on boundary layer instability and transition to turbulence, were followed by investigations of various turbulent flows that are relevant to engineering application—a recurring theme throughout his career. With the entry of the U.S. into World War II, he began research on problems associated with high-speed flight, including transonic flight phenomena and interaction of shock waves with boundary layers on aerodynamic surfaces. This also marked the beginning of a longtime interaction with the Southern California aircraft industry. With Allen Puckett, he organized short wartime courses on high-speed aerodynamics for working engineers, resulting in their pioneering textbook, "The Aerodynamics of Compressible Flow." It was followed in 1956 by "Elements of Gasdynamics," with Anatol Roshko, which impacted a broader, mainly graduate-student following, and was translated into Russian, Spanish, and Japanese.

In the rapid expansion of scientific and applied research that followed World War II, Liepmann emerged as a respected and influential contributor to the aeronautics scene and to physics of fluid flow. By 1949 he had advanced to professor of aeronautics at Caltech and had developed a vigorous program of research around his group of PhD students and visiting postdoctoral fellows as well as senior scientists, many of them seeking a change from their work in postwar Europe. Believing strongly that experimental research must relate to theoretical foundations and questions, Liepmann sought association for his group with applied mathematicians, visitors as well as Caltech faculty. An outgrowth of this was the establishment, in 1967, of the applied mathematics option at Caltech. As if tying up loose ends, he was also instrumental, along with Caltech's Amnon Yariv, Martin and Eileen Summerfield Professor of Applied Physics and professor of electrical engineering, and Roy W. Gould, Simon Ramo Professor of Engineering, Emeritus, in the establishment of the applied physics option, in 1974.

The work of his group was distinguished by its innovation in experimental apparatus and instrumentation, often designed for the specific needs of particular problems. Pioneering contributions were made to a wide range of topics that frequently anticipated future technology. These include flow instability and transition, turbulent shear flow, transonic flow, shock wave-boundary layer interaction, turbulent skin friction at supersonic speeds, aircraft buffeting, rarefied gas flow, magnetohydrodynamics, plasma physics, fluid mechanics of liquid helium, chemistry of turbulent mixing, and flow control.

Another strongly held principle was that teaching is vital, even in a research-oriented institution like Caltech.  Throughout his career, up to retirement, Liepmann was devoted to teaching both graduate and undergraduate courses. The enthusiasm, clarity, and teaching effectiveness of his lectures are legendary.
 
In recognition of his accomplishments, Liepmann was elected a member of the National Academy of Engineering and the National Academy of Sciences.  He was a recipient of the National Medal of Technology and the Ludwig Prandtl Ring—the highest honor conferred by the German Society for Aeronautics and Astronautics. In 1986, President Ronald Reagan awarded Liepmann the National Medal of Science.

Liepmann leaves behind sons Dorian, Till, Christopher, and Paul, and two grandchildren. His wife, Dietlind, passed away in 1990.

A memorial service for Liepmann will be announced at a later date.

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Jon Weiner
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Kent and Joyce Kresa Endow Professorial Chair at Caltech

Professorship creates permanent support for a leading investigator

Kent Kresa, interim chairman of General Motors, and his wife have pledged $2 million to the California Institute of Technology (Caltech) to endow the Joyce and Kent Kresa Professorship in Engineering and Applied Science. Kresa is Chairman of the Caltech Board of Trustees. The Kresa gift is matched with an additional $1 million provided by the Gordon and Betty Moore Matching Program.


"Our very good friends the Kresas have shared their time, vision, and resources with Caltech for many years," said Caltech president Jean-Lou Chameau. "They know our approach: attract leading thinkers, give them the best resources we can, and create an atmosphere in which they can collaborate easily. Kent and Joyce's generous gift will support one great engineer after another and will lead to great discoveries and inventions."


The chair will support and recognize a faculty member in engineering and applied science, with a preference for faculty in aeronautics and aerospace engineering, fields Kresa has helped shape, most notably in 28 years with Northrop Grumman that included 13 as the company's CEO and chairman. Ares Rosakis, the Theodore von Kármán Professor of Aeronautics and Mechanical Engineering and chair of the Division of Engineering and Applied Science, will work with Provost Edward Stolper to select the inaugural Joyce and Kent Kresa Professor.


"Endowed chairs offer our faculty the ultimate freedom to pursue the research thrusts they are most passionate about—and this is truly invaluable to Caltech and our continued vitality," said Rosakis.


Kresa has been a director of GM since 2003, when he retired from Northrop Grumman Corporation. Kresa joined that company in 1975 and was elected president in 1987, CEO in January 1990, and chairman in September 1990. Kent and Joyce Kresa are President's Circle members of the Caltech Associates.


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 About Caltech:
Caltech is recognized for its highly select student body of 900 undergraduates and 1,200 graduate students, and for its outstanding faculty. Since 1923, Caltech faculty and alumni have garnered 32 Nobel Prizes and five Crafoord Prizes. In addition to its prestigious on-campus research programs, Caltech operates the Jet Propulsion Laboratory (JPL), the W. M. Keck Observatory in Mauna Kea, the Palomar Observatory, and the Laser Interferometer Gravitational-Wave Observatory (LIGO). Caltech is a private university in Pasadena, California. For more information, visit http://www.caltech.edu

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Jon Weiner
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Caltech Scientists Use High-Pressure "Alchemy" to Create Nonexpanding Metals

PASADENA, Calif.—By squeezing a typical metal alloy at pressures hundreds of thousands of times greater than normal atmospheric pressure, scientists at the California Institute of Technology (Caltech) have created a material that does not expand when heated, as does nearly every normal metal, and acts like a metal with an entirely different chemical composition.

The discovery, described in a paper in Physical Review Letters (PRL), offers insight into the exotic behavior of materials existing at high pressures—which represent some 90 percent of the matter in our solar system.

Zero-expanding metal alloys were discovered in 1896 by Swiss physicist Charles Édouard Guillaume, who worked at the International Bureau of Weights and Measures in France. While attempting to develop an inexpensive international standard for the meter, the metric unit of length, Guillaume hit upon an inexpensive iron-nickel alloy that expands very little when heated. He dubbed the material an "Invar" alloy—because the metals are "invariant" when heated, such that the length of a piece of Invar metal does not change as its temperature is increased, as do normal metals. Since Guillaume's discovery—which, in 1920, earned him the Nobel Prize in Physics (besting Albert Einstein, who was awarded the prize in 1921)—other nonexpanding alloys have been identified.

It has long been known that Invar behavior is caused by unusual changes in the magnetic properties of the alloys that somehow cancel out the thermal expansion of the material. (Normally, heat increases the vibrations of the atoms that make up a material, and the atoms prefer to move apart a little, causing expansion.)

"Recent computer simulations indicate that electrons in Invar alloys take on a special energy configuration," says Caltech graduate student Michael Winterrose, the first author of the PRL paper. "This energy state is at the borderline between two types of magnetic behavior, and is very sensitive to the precise ratio of elements that make up the alloy. If you move away from the Invar chemical composition by only a couple of percent, the energy configuration will disappear," he says.

Because of their unresponsiveness to temperature change, Invar alloys have been used in devices ranging from watches, toasters, light bulbs, and engine parts to computer and television screens, satellites, lasers, and scientific instruments. "In our day-to-day lives, we are surrounded by items that make essential use of Invar alloys," Winterrose says.

The Caltech scientists did not set out to study Invar behavior—and, in fact, were hoping to avoid it. "We intentionally picked chemical compositions that do not show Invar behavior because I thought it would confuse our interpretations," says Brent Fultz, a professor of materials science and applied physics at Caltech, and a coauthor of the PRL paper.

Instead, Winterrose, Fultz, and their colleagues were examining the effect of pressure on the alloy of palladium (Pd) and iron (Fe) called Pd3Fe, where three of every four atoms are palladium, and one is an iron atom. (In the similarly named but chemically distinct PdFe3—which is a traditional Invar alloy—three of every four atoms are iron, and one is palladium).

 "The Fe and Pd atoms [in the alloy] have very different sizes, and we expected to see some interesting effects from this size difference when we put Pd3Fe under pressure and measured its volume," Winterrose explains. To test this, the scientists squeezed a small sample of the material between two diamond anvils, generating pressures inside the sample that were 326,000 times greater than standard atmospheric pressure.

"Our initial results from these studies showed that the alloy stiffened under pressure, but far more than we expected," he says. To figure out the cause, the scientists simulated the quantum mechanical behavior of the electrons in the alloy under pressure. "The simulations showed that under pressure, the electrons found the special energy levels between strong and weak magnetism that are associated with normal Invar behavior. Up to this point we had been quite unaware of the possibility for Invar behavior in our material," Winterrose says.

Subsequent experiments at the Advanced Photon Source at Argonne National Laboratory in Chicago and the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory in New York confirmed that the intense pressure had indeed suppressed thermal expansion in Pd3Fe, much like tuning the chemical composition.

The scientists had performed a kind of high-pressure "alchemy" on the alloy, where pressure makes the electrons act as if they are around atoms of a different chemical element, Winterrose says.

The research helps unify our understanding of Invar behavior, which is one of the oldest and most-studied unresolved problems in materials research. In addition, using pressure to force electrons into new states can point to directions in materials chemistry where new properties can be found, at least for magnetism.

"Today, materials physics has some excellent computational tools for predicting the structure and properties of materials, although there are suspicions about how well they work for magnetic materials," says Fultz. "It is satisfying that these computational tools worked so well for showing how pressure changed the material into an Invar alloy. Invar behavior is pretty subtle, requiring a very special condition for the electrons in the metal that is usually tuned by precise control of chemical composition. Pressure can make the electrons behave as if they are in a material of different chemical composition, so I really like Mike's use of the word 'alchemy'."

The paper, "Pressure-Induced Invar Behavior in Pd3Fe," was published in the June 12 issue of PRL. In addition to Winterrose and Fultz, the coauthors are Matthew S. Lucas, Alan F. Yue, Itzhak Halevy, Lisa Mauger, and Jorge Munoz (from Caltech); Jingzhu Hu, from the University of Chicago; and Michael Lerche, from the Carnegie Institution for Science.

The work was supported by the Carnegie—Department of Energy (DOE) Alliance Center, funded by the DOE through the Stewardship Sciences Academic Alliance of the National Nuclear Security Administration, and by the DOE's Office of Science, Office of Basic Energy Sciences; by the National Science Foundation and its Consortium for Materials Properties Research in Earth Sciences (COMPRES); and by the W. M. Keck Foundation.

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Caltech Announces Initiative for a New $90 Million Sustainability Institute

Initial $30 million in gifts revealed at Institute's commencement ceremony

Pasadena, Calif.—As the U.S. Secretary of Energy and hundreds of graduates and their families looked on, California Institute of Technology (Caltech) president Jean-Lou Chameau began today's commencement ceremony by announcing $30 million in gifts as the first phase of a proposed $90 million initiative for a new institute.

The funds will go towards the creation of the Resnick Sustainability Institute at Caltech. The initial gift of $20 million was made by Stewart and Lynda Resnick, and an additional $10 million came from the Gordon and Betty Moore Matching Program. The plans include a second phase of funding to be initiated next year as part of a challenge grant. Ultimately, the endowment for the new institute will exceed $90 million.

The vision of the Resnick Sustainability Institute is to provide a path to sustainability by focusing on innovative science and engineering developments required for groundbreaking energy technologies.  Such technologies may one day help solve our global energy and climate challenges. With the support of the Resnick Sustainability Institute, some of the brightest minds in the world will apply Caltech's unique approach to interdisciplinary research toward high-risk, high-return energy science and technology. 

"I have enjoyed many conversations with Stewart and Lynda on exciting developments in science and technology and their potential for addressing many of our environmental and economic challenges," says Chameau. "This generous gift from the Resnicks reflects their extraordinary desire and courage to make a difference. With their support, we are poised to launch an initiative at Caltech that will herald a new era in energy research."

"We are passionately committed to finding alternative and sustainable energy solutions," say Stewart and Lynda Resnick.  "We're making this investment because Caltech is truly one of America's greatest research universities, and we are confident that this new institute will develop the breakthrough technologies we need to address the daunting challenges of energy security, rapidly accelerating energy demand, and climate change."

The new institute will leverage prior grants from the Gordon and Betty Moore Foundation and work being done by Caltech researchers such as Harry Atwater, the Howard Hughes Professor and professor of applied physics and materials science, who leads Caltech's Energy Frontier Research Center, recently funded by the Department of Energy; and Harry Gray, the Arnold O. Beckman Professor of Chemistry, and Nate Lewis, the George L. Argyros Professor and professor of chemistry, who lead Caltech's Center for Chemical Innovation, funded by the National Science Foundation.

The Resnicks' existing relationship with Caltech includes Stewart Resnick's role as a member of the Board of Trustees. He is also chairman and, with his wife, Lynda, owner of Roll International Corporation, a private holding company he founded in 1962. The company has diverse interests including Paramount Citrus, Paramount Farming, and Paramount Farms, growers and processors of citrus, almonds, and pistachios; POM Wonderful, the largest grower of pomegranates and makers of POM Wonderful pomegranate juice; Teleflora, the floral-by-wire service; FIJI Water, a leading premium bottled-water brand; and Suterra, one of the largest biorational pest control providers in the world.

The Resnicks have a long history of giving to Los Angeles institutions, including a 2008 pledge of $55 million to the Los Angeles County Museum of Art.

The announcement of the gift was made during Chameau's opening remarks at Caltech's 115th annual commencement ceremony. This year's keynote speaker, Department of Energy Secretary Steven Chu, remarked that the timing of the gift announcement was especially appropriate, as it involved energy science and sustainability, two of his programmatic efforts at the energy department.

# # #

About the Resnicks:

Stewart Resnick is chairman and Lynda Resnick is vice chairman of Roll International Corporation, a privately held Los Angeles-based holding company.  Among their holdings are Paramount Citrus, Paramount Farming, and Paramount Farms, growers and processors of citrus, almonds, and pistachios; POM Wonderful, the largest grower of pomegranates and makers of the all-natural POM Wonderful pomegranate juice; Teleflora, the floral-by-wire service; FIJI Water, a leading premium bottled-water brand; and Suterra, one of the largest biorational pest control providers in the world.

Stewart Resnick is a member of the executive board of the UCLA Medical Sciences; a member of the board of trustees of Bard College, New York; a member of the board of trustees of the J. Paul Getty Trust; a member of the board of Conservation International; and a member of the advisory board of the Anderson School of Management at UCLA.

Lynda Resnick is vice chairman of the Los Angeles County Museum of Art's board of trustees, as well as the chair of the museum's acquisitions committee and executive committees. She is on the executive board of the Aspen Institute; the executive board for the UCLA Medical Sciences; Prostate Cancer Foundation; and the Milken Family Foundation. She is also a trustee of the Philadelphia Museum of Art. Lynda is also the best-selling author of the recently published book Rubies in the Orchards: How to Uncover the Hidden Gems in Your Business.

 

About Gordon and Betty Moore:

Gordon and Betty Moore are the cofounders and directors of the Gordon and Betty Moore Foundation. They have been contributing to science, technology, education, and conservation projects for decades. A California native,  Moore earned a PhD in chemistry from Caltech in 1954. In 1968 he cofounded Intel, and became president and CEO in 1975. He was elected chairman and CEO in 1979. In 1987, he relinquished the CEO title, he was named chairman emeritus in 1997. Moore is also a member of the National Academy of Engineering and an IEEE Fellow. He received the National Medal of Technology in 1990 and was named a Caltech Distinguished Alumni in 1975. Moore served as chairman of the Caltech Board of Trustees from 1995 until the beginning of 2001 and continues as a senior trustee. In 2001, the Moores announced two gifts to Caltech totaling $600 million--the largest donation in history to an institution of higher education.

About Caltech:

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

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

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Jon Weiner
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Caltech Scientists Create Nanoscale Zipper Cavity that Responds to Single Photons of Light

Device could be used for highly sensitive force detection, optical communications, and more

PASADENA, Calif.-Physicists at the California Institute of Technology (Caltech) have developed a nanoscale device that can be used for force detection, optical communication, and more. The device exploits the mechanical properties of light to create an optomechanical cavity in which interactions between light and motion are greatly strengthened and enhanced. These interactions, notes Oskar Painter, associate professor of applied physics at Caltech, and the principal investigator on the research, are the largest demonstrated to date.

The device and the work that led to it are described in a recent issue of the journal Nature.

The fact that photons of light, despite having no mass, nonetheless carry momentum and can interact with mechanical objects is an idea that dates back to Kepler and Newton. The mechanical properties of light are also known to limit the precision with which one can measure an object's position, since simply by using light to do the measurement, you apply a force and disturb the object.

It was important to consider these so-called back-action effects in the design of devices to measure weak, classical forces. Such considerations were part of the development of gravity-wave detectors like the Laser Interferometer Gravitational-Wave Observatory (LIGO). These sorts of interferometer-based detectors have also been used at much smaller scales, in scanning probe instruments used to detect or image atomic surfaces or even single electron spins.

To get an idea of how these systems work, consider a mirror attached to a floppy cantilever, or spring. The cantilever is designed to respond to a particular force-say, a magnetic field. Light shining down on the mirror will be deflected when the force is detected-i.e., when the cantilever moves-resulting in a variation in the light beam's intensity that can then be detected and recorded.

"LIGO is a huge multikilometer-scale interferometer," notes Painter. "What we did was to take that and scale it all the way down to the size of the wavelength of light itself, creating a nanoscale device."

They did this, he explains, because as these interferometer-based detectors are scaled down, the mechanical properties of light become more pronounced, and interesting interactions between light and mechanics can be explored.

"To this end, we made our cantilevers many, many times smaller, and made the optical interaction many, many times larger," explains Painter.

They call this nanoscale device a zipper cavity because of the way its dual cantilevers-or nanobeams, as Painter calls them-move together and apart when the device is in use. "If you look at it, it actually looks like a zipper," Painter notes.

"Zipper structures break new ground on coupling photonics with micromechanics, and can impact the way we measure motion, even into the quantum realm," adds Kerry Vahala, Caltech's Ted and Ginger Jenkins Professor of Information Science and Technology and professor of applied physics, and one of the paper's authors. "The method embodied in the zipper design also suggests new microfabrication design pathways that can speed advances in the subject of cavity optomechanics as a whole."

To create their zipper cavity device, the researchers made two nanobeams from a silicon chip, poking holes through the beams to form an effective optical mirror. Instead of training a light down onto the nanobeams, the researchers used optical fibers to send the light "in plane down the length of the beams," says Painter. The holes in the nanobeams intercept some of the photons, circulating them through the cavity between the beams rather than allowing them to travel straight through the device.

Or, to be more precise, the circulating photons actually create the cavity between the beams. As Painter puts it: "The mechanical rigidity of the structure and the changes in its optical response are predominantly governed by the internal light field itself."

Such an interaction is possible, he adds, because the structure is precisely designed to maximize the transfer of momentum from the input laser's photons to the mechanical nanobeams. Indeed, a single photon of laser light zipping through this structure produces a force equivalent to 10 times that of Earth's gravity. With the addition of several thousand photons to the cavity, the nanobeams are effectively suspended by the laser light.

Changes in the intensity and other properties of the light as it moves along the beams to the far end of the chip can be detected and recorded, just as with any large-scale interferometer.

The potential uses for this sort of optomechanical zipper cavity are myriad. It could be used as a sensor in biology by coating it with a solution that would bind to, say, a specific protein molecule that might be found in a sample. The binding of the protein molecule to the device would add mass to the nanobeams, and thus change the properties of the light traveling through them, signaling that such a molecule had been detected. Similarly, it could be used to detect other ultrasmall physical forces, Painter adds.

Zipper cavities could also be used in optical communications, where circuits route information via optical beams of different colors, i.e., wavelengths. "You could control and manipulate what the optical beams of light are doing," notes Painter. "As the optical signals moved around in a circuit, their direction or color could be manipulated via other control light fields." This would create tunable photonics, "optical circuits that can be tuned with light."

Additionally, the zipper cavity could lead to applications in RF-over-optical communications and microwave photonics as well, where a laser source is modulated at microwave frequencies, allowing the signals to travel for kilometers along optical fibers. In such systems, the high-frequency mechanical vibrations of the zipper cavity could be used to filter and recover the RF or microwave signal riding on the optical wave.

Other authors on the Nature paper, "A picogram- and nanometre-scale photonic-crystal optomechanical cavity," include graduate students Matt Eichenfield (the paper's first author) and Jasper Chan, and postdoctoral scholar Ryan Camacho.

Their research was supported by a Defense Advanced Research Projects Agency seeding effort, and an Emerging Models and Technologies grant from the National Science Foundation.

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