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A thermal invisibility cloak that actively redirects heat

Active thermal cloak hides a circular object in conductive heat flow by “pumping” heat from hot end to cold end (credit: Xu & Zhang/NTU)

A new thermal cloak that can render an object thermally invisible by actively redirecting incident heat has been developed by scientists at the Nanyang Technological University (NTU) in Singapore. It’s similar to how optical invisibility cloaks can bend and diffract light to shield an object from sight and specially fabricated acoustic metamaterials can hide an object from sound waves.

The system has the potential to fine-tune temperature distribution and heat flow in electronic and semiconductor systems for applications that require efficient heat dissipation (cooling) and homogenous (even) thermal expansion, such as high-power engines, magnetic resonance imaging (MRI) instruments, thermal sensors, and clothing, said Prof. Baile Zhang of NTU.

Zhang and colleagues previously designed a metamaterial thermal cloak that passively guided conductive heat around a hidden object, with no way to control heat flow and direction. The researchers decided to look into controlling thermal cloaking electrically by actively “pumping” heat from one side of the hidden object to the other side, using thermoelectric modules, as described in an open-access paper and on the cover of Applied Physics Letters, from AIP Publishing.

Building the thermal cloak

Design of active thermal cloak. (a) Multiple thermoelectic components are arranged around the air hole with equal distance on the Carbon Steel plate. Blue components absorb incident thermal flux while red ones release heat back to the plate. (b) The side view of the marked region in (a), illustrating the working mechanism of TE components when functioning as heat absorber/emitter. An applied voltage causes a directional motion of charge carriers in positive/negative blocks, resulting in a heat flux in the designed direction. The bottom/top orange arrow indicates the absorption/release of heat by TE components. The dashed arrow indicates the heat transfer through a constant-temperature heat “reservoir” which is a large copper bulk in the experiment. (credit: Dang Minh Nguyen et al./Applied Physics Letters)

To construct their active thermal cloak, the researchers deployed 24 small thermoelectric modules, which are semiconductor heat pumps controlled by an external input voltage, around a 62-millimeter diameter air hole in a carbon steel plate just 5 mm thick. The modules operate via the Peltier effect, in which a current running through the junction between two conductors can remove or generate heat.

When many modules are attached in series, they can redirect heat flow. The researchers attached the bottom and top ends of the modules to hot and cold surfaces at 60° C and 0° C respectively to generate diffusive heat flux.

When the researchers applied a variety of specific voltages to each of the 24 modules, the heat falling on the hot-surface side of the air hole was absorbed and delivered to a constant-temperature copper heat reservoir attached to the modules. The modules on the cold-surface side released the same amount of heat from the reservoir into the steel plate. This prevented heat from diffusing through the air hole, a technique, the researchers say, that can be used to shield sensitive electronic components from heat dissipation.

The researchers found that their active thermal cloaking was not limited by the shape of the object being hidden.

Zhang and his colleagues plan to apply the thermal cloaks in electronic systems, improve the efficiency of heat transfer, and develop an intelligent control system for the cloak.

Abstract of Active thermal cloak

Thermal cloaking, as an ultimate thermal “illusion” phenomenon, is the result of advanced heat manipulation with thermal metamaterials—heat can be guided around a hidden object smoothly without disturbing the ambient thermal environment. However, all previous thermal metamaterial cloaks were passive devices, lacking the functionality of switching on/off and the flexibility of changing geometries. In this letter, we report an active thermal cloaking device that is controllable. Different from previous thermal cloaking approaches, this thermal cloak adopts active thermoelectric components to “pump” heat from one side to the other side of the hidden object, in a process controlled by input electric voltages. Our work not only incorporates active components in thermal cloaking but also provides controllable functionality in thermal metamaterials that can be used to construct more flexible thermal devices.

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Massive clash of black holes raises astronomers’ hopes of witnessing gravitational waves

Artist’s conception of converging supermassive black holes in the Virgo constellation (credit: P. Marenfeld/NOAO/AURA/NSF)

Circling like prizefighters in a ring, a pair of supermassive black holes is heading toward an epic collision. One so powerful it would send a burst of gravitational waves surging through and distorting the very fabric of space-time.

Already, the intensity of the encounter is causing mysterious rhythmic flashes of light coming from quasar PG 1302-102 — 3.5 billion light-years away in the Virgo constellation.

“This is the closest we’ve come to observing two black holes on their way to a massive collision,” said Columbia University astronomer Zoltan Haiman in a new study in the journal Nature.

“Watching this process reach its culmination [confirming the existence of a binary black hole in the relativistic regime by measuring optical and UV brightness] can tell us whether black holes and galaxies grow at the same rate, and ultimately test* a fundamental property of space-time: its ability to carry vibrations called gravitational waves, produced in the last, most violent, stage of the merger.”

The ultimate crash

They estimated the combined and relative mass of PG 1302-102’s black holes, allowing them to narrow down the pair’s predicted crash time: about 100,000 years.

Meanwhile, a recent uptick in the number of black hole binary discoveries has made astronomers hopeful they may in fact witness an actual collision in the next decade and therefore detect gravitational waves, said study coauthor David Schiminovich, also an astronomer at Columbia.

Such a detection would let them “probe the secrets of gravity and test Einstein’s theory in the most extreme environment in our universe — black holes,” said the study’s lead author, Daniel D’Orazio, a graduate student at Columbia. “Getting there is a holy grail of our field.”

* The hope of doing such a test has energized astronomers. Previously, a team led by Matthew Graham, a computational astronomer at the California Institute of Technology, designed an algorithm to pick out repeating light signals from 247,000 quasars monitored by telescopes in Arizona and Australia. Of the 20 pairs of black hole candidates discovered, they focused on the bright quasar. In a January study in Nature, they showed that PG 1302-102 appeared to brighten by 14 percent every five years, indicating the pair was less than a tenth of a light-year apart.

Intrigued, Haiman and his colleagues wondered if they could build a theoretical model to explain the repeating signal. If the black holes were as close as predicted, one had to be circling a much larger counterpart at nearly a tenth of the speed of light, they hypothesized. At that speed, the smaller black hole would appear to brighten as it approached Earth’s line of sight under the relativistic Doppler beaming effect.

If correct, they predicted they would find a five-year cycle in the quasar’s ultraviolet emissions. Analyzing UV observations collected by NASA’s Hubble and GALEX space telescopes they found exactly that.

Abstract of Relativistic boost as the cause of periodicity in a massive black-hole binary candidate

Because most large galaxies contain a central black hole, and galaxies often merge, black-hole binaries are expected to be common in galactic nuclei. Although they cannot be imaged, periodicities in the light curves of quasars have been interpreted as evidence for binaries, most recently in PG 1302-102, which has a short rest-frame optical period of four years. If the orbital period of the black-hole binary matches this value, then for the range of estimated black-hole masses, the components would be separated by 0.007–0.017 parsecs, implying relativistic orbital speeds. There has been much debate over whether black-hole orbits could be smaller than one parsec. Here we report that the amplitude and the sinusoid-like shape of the variability of the light curve of PG 1302-102 can be fitted by relativistic Doppler boosting of emission from a compact, steadily accreting, unequal-mass binary. We predict that brightness variations in the ultraviolet light curve track those in the optical, but with a two to three times larger amplitude. This prediction is relatively insensitive to the details of the emission process, and is consistent with archival ultraviolet data. Follow-up ultraviolet and optical observations in the next few years can further test this prediction and confirm the existence of a binary black hole in the relativistic regime.

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First known magnetic wormhole created

(Left) 3-D diagram of the magnetic wormhole shows how the magnetic field lines (in red) leaving a magnet on the right side of the sphere pass through the wormhole. (Right) As seen by a magnet, the magnetic field seems to disappear on the right side of the sphere only to reappear on the left in the form of a magnetic monopole. (credit: Jordi Prat-Camps/Universitat Autònoma de Barcelona)

A wormhole* that can connect two regions of space magnetically has been created in the laboratory and experimentally demonstrated by physicists at Universitat Autònoma de Barcelona in Spain.

This is not a wormhole in space, as in the movie Interstellar. It’s a special design that transfers a magnetic field from one location in space to another in such a way that the process is magnetically undetectable and invisible (only visible by light). This is explained in the diagrams above.

The researchers used metamaterials and metasurfaces to build the tunnel experimentally. The magnetic field from a source, such as a magnet or an electromagnet, appears at the other end of the wormhole as an isolated magnetic monopole (a magnet with only one pole, whether north or south, which does not exist in nature).

That causes the magnetic field to appear to magically travel from one location to another through a dimension that appears to (or actually?) lies outside the conventional three dimensions.

How to build a wormhole

The magnetic wormhole device is composed of (from left to right) an outer spherical ferromagnetic metasurface, an inner spherical superconducting layer, and inside that, an inner spirally wound ferromagnetic sheet (credit: (credit: Jordi Prat-Camps/Universitat Autònoma de Barcelona)

To create the wormhole used in this experiment, the researchers designed a sphere made of three layers: an external layer with a ferromagnetic (as in a standard iron magnet) surface, a second inner layer made of superconducting material, and a ferromagnetic sheet rolled into a cylinder that crosses the sphere from one end to the other and conducts the magnetic field.

The magnetic wormhole is analogous to a gravitational wormhole — it “changes the topology of space, as if the inner region has been magnetically erased from space,” explains Àlvar Sánchez, the lead researcher.

Practical applications

The study was published in an open-access paper in Scientific Reports, but the researchers first published the fundamental concept in a paper in Physical Review Letters, where they described it as a “magnetic hose.”

Experimental magnetic wormhole (credit: Jordi Prat-Camps/Universitat Autònoma de Barcelona)

The researchers built a magnetic “hose” capable of channeling a magnetic field from a source to a distance more than 10 centimeters away. (However, that version was detectable magnetically.)

As Steven Anlage, a professor of Physics at the University of Maryland, explains in this open-access article in the journal Physics, channeling magnetic fields could improve the spatial resolution of magnetic-field images, for example.

“Another application would be magnetic resonance imaging, in which the patient and superconducting magnet required to generate the magnetic field could be physically separated, and the intense magnetic field could be applied locally to the patient” (instead of having to enter an MRI machine).

Or it could allow MRI images of different parts of the body to be obtained simultaneously.

There may be other possible uses. Any ideas? Please comment below.

* Wormholes are cosmic tunnels that can connect two distant regions of the universe, popularized by science fiction like Stargate, Star Trek or, more recently, Interstellar. Using present-day technology, it would be impossible to create a gravitational wormhole because the field would have to be manipulated with huge amounts of gravitational energy, which no one yet knows how to generate. In electromagnetism, however, advances in metamaterials and invisibility have allowed researchers to put forward several designs to achieve this.

Abstract of A Magnetic Wormhole

Wormholes are fascinating cosmological objects that can connect two distant regions of the universe. Because of their intriguing nature, constructing a wormhole in a lab seems a formidable task. A theoretical proposal by Greenleaf et al. presented a strategy to build a wormhole for electromagnetic waves. Based on metamaterials, it could allow electromagnetic wave propagation between two points in space through an invisible tunnel. However, an actual realization has not been possible until now. Here we construct and experimentally demonstrate a magnetostatic wormhole. Using magnetic metamaterials and metasurfaces, our wormhole transfers the magnetic field from one point in space to another through a path that is magnetically undetectable. We experimentally show that the magnetic field from a source at one end of the wormhole appears at the other end as an isolated magnetic monopolar field, creating the illusion of a magnetic field propagating through a tunnel outside the 3D space. Practical applications of the results can be envisaged, including medical techniques based on magnetism.

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Hawking offers new solution to ‘black hole information paradox’

Nobel physics laureate Gerard ‘t Hooft of Utrecht University, the Netherlands, confers with Stephen Hawking at a week-long conference at KTH Royal Institute of Technology on the information loss paradox (photo credit: Håkan Lindgren)

Addressing a current controversy in physics about information in black holes, “I propose that the information is stored not in the interior of the black hole as one might expect, but on its boundary, the event horizon.”

The event horizon is a boundary around a black hole beyond which events cannot affect an outside observer, also known as “the point of no return” — where gravitational pull becomes so great as to make escape impossible.

Hawking is now suggesting that the information about any incoming particles passing through this event horizon is translated into a 2D hologram. “The idea is the super-translations are a hologram of the ingoing particles,” he said. “Thus they contain all the information that would otherwise be lost.”

That provides a new solution for the “black hole information paradox“: what happens to the information about the physical state of things that are swallowed up by black holes? Is it destroyed, as our understanding of general relativity would predict? If so, that would violate the laws of quantum mechanics.


KTH Royal Institute of Technology | Hawking presents new idea on how information could escape black holes

Hawking said that also offers hope (at least for the information that represents you) if you happen to have fallen into a black hole — supporting the premise of the movie Interstellar. If the hole was large and rotating, “it might have a passage to another universe” via Hawking radiation.

“But you couldn’t come back to our universe. So although I’m keen on space flight, I’m not going to try that.”

Model of a black hole for the movie Interstellar (credit: Warner Bros. Pictures International )

The conference is co-sponsored by Nordita, the University of North Carolina (UNC), and the Julian Schwinger Foundation. UNC physicist Laura Mersini-Houghton was instrumental in assembling 32 of the world’s leading physicists to tackle the problem, which stems from contradictions between quantum mechanics and general relativity.

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MIT designs small, modular, efficient fusion power plant

A cutaway view of the proposed ARC reactor (credit: MIT ARC team)

MIT plans to create a new compact version of a tokamak fusion reactor with the goal of producing practical fusion power, which could offer a nearly inexhaustible energy resource in as little as a decade.

Fusion, the nuclear reaction that powers the sun, involves fusing pairs of hydrogen atoms together to form helium, accompanied by enormous releases of energy.

The new fusion reactor, called ARC, would take advantage of new, commercially available superconductors — rare-earth barium copper oxide (REBCO) superconducting tapes (the dark brown areas in the illustration above) — to produce stronger magnetic field coils, according to Dennis Whyte, a professor of Nuclear Science and Engineering and director of MIT’s Plasma Science and Fusion Center.

The stronger magnetic field makes it possible to produce the required magnetic confinement of the superhot plasma — that is, the working material of a fusion reaction — but in a much smaller device than those previously envisioned. The reduction in size, in turn, makes the whole system less expensive and faster to build, and also allows for some ingenious new features in the power plant design.

The proposed reactor is described in a paper in the journal Fusion Engineering and Design, co-authored by Whyte, PhD candidate Brandon Sorbom, and 11 others at MIT.

Power plant prototype

The new reactor is designed for basic research on fusion and also as a potential prototype power plant that could produce 270MW of electrical power. The basic reactor concept and its associated elements are based on well-tested and proven principles developed over decades of research at MIT and around the world, the team says. An experimental tokamak was built at Princeton Plasma Physics Laboratory circa 1980.

The hard part has been confining the superhot plasma — an electrically charged gas — while heating it to temperatures hotter than the cores of stars. This is where the magnetic fields are so important — they effectively trap the heat and particles in the hot center of the device.

While most characteristics of a system tend to vary in proportion to changes in dimensions, the effect of changes in the magnetic field on fusion reactions is much more extreme: The achievable fusion power increases according to the fourth power of the increase in the magnetic field.

Tenfold boost in power

The new superconductors are strong enough to increase fusion power by about a factor of 10 compared to standard superconducting technology, Sorbom says. This dramatic improvement leads to a cascade of potential improvements in reactor design.

ITER — the world’s largest tokamak — is expected to be completed in 2019, with deuterium-tritium operations in 2027 and 2000–4000MW of fusion power onto the grid in 2040 (credit: ITER Organization)

The world’s most powerful planned fusion reactor, a huge device called ITER that is under construction in France, is expected to cost around $40 billion. Sorbom and the MIT team estimate that the new design, about half the diameter of ITER (which was designed before the new superconductors became available), would produce about the same power at a fraction of the cost, in a shorter construction time, and with the same physics.

Another key advance in the new design is a method for removing the fusion power core from the donut-shaped reactor without having to dismantle the entire device. That makes it especially well-suited for research aimed at further improving the system by using different materials or designs to fine-tune the performance.

In addition, as with ITER, the new superconducting magnets would enable the reactor to operate in a sustained way, producing a steady power output, unlike today’s experimental reactors that can only operate for a few seconds at a time without overheating of copper coils.

Liquid protection

Another key advantage is that most of the solid blanket materials used to surround the fusion chamber in such reactors are replaced by a liquid material that can easily be circulated and replaced, eliminating the need for costly replacement procedures as the materials degrade over time.

Right now, as designed, the reactor should be capable of producing about three times as much electricity as is needed to keep it running, but the design could probably be improved to increase that proportion to about five or six times, Sorbom says. So far, no fusion reactor has produced as much energy as it consumes, so this kind of net energy production would be a major breakthrough in fusion technology, the team says.

The design could produce a reactor that would provide electricity to about 100,000 people, they say. Devices of a similar complexity and size have been built within about five years, they say.

“Fusion energy is certain to be the most important source of electricity on earth in the 22nd century, but we need it much sooner than that to avoid catastrophic global warming,” says David Kingham, CEO of Tokamak Energy Ltd. in the UK, who was not connected with this research. “This paper shows a good way to make quicker progress,” he says.

The MIT research, Kingham says, “shows that going to higher magnetic fields, an MIT specialty, can lead to much smaller (and hence cheaper and quicker-to-build) devices.” The work is of “exceptional quality,” he says; “the next step … would be to refine the design and work out more of the engineering details, but already the work should be catching the attention of policy makers, philanthropists and private investors.”

The research was supported by the U.S. Department of Energy and the National Science Foundation.

Abstract of ARC: A compact, high-field, fusion nuclear science facility and demonstration power plant with demountable magnets

The affordable, robust, compact (ARC) reactor is the product of a conceptual design study aimed at reducing the size, cost, and complexity of a combined fusion nuclear science facility (FNSF) and demonstration fusion Pilot power plant. ARC is a ∼200–250 MWe tokamak reactor with a major radius of 3.3 m, a minor radius of 1.1 m, and an on-axis magnetic field of 9.2 T. ARC has rare earth barium copper oxide (REBCO) superconducting toroidal field coils, which have joints to enable disassembly. This allows the vacuum vessel to be replaced quickly, mitigating first wall survivability concerns, and permits a single device to test many vacuum vessel designs and divertor materials. The design point has a plasma fusion gain of Qp ≈ 13.6, yet is fully non-inductive, with a modest bootstrap fraction of only ∼63%. Thus ARC offers a high power gain with relatively large external control of the current profile. This highly attractive combination is enabled by the ∼23 T peak field on coil achievable with newly available REBCO superconductor technology. External current drive is provided by two innovative inboard RF launchers using 25 MW of lower hybrid and 13.6 MW of ion cyclotron fast wave power. The resulting efficient current drive provides a robust, steady state core plasma far from disruptive limits. ARC uses an all-liquid blanket, consisting of low pressure, slowly flowing fluorine lithium beryllium (FLiBe) molten salt. The liquid blanket is low-risk technology and provides effective neutron moderation and shielding, excellent heat removal, and a tritium breeding ratio ≥ 1.1. The large temperature range over which FLiBe is liquid permits an output blanket temperature of 900 K, single phase fluid cooling, and a high efficiency helium Brayton cycle, which allows for net electricity generation when operating ARC as a Pilot power plant.

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Russian billionaire, Hawking announce $100 million search for ET

Green Bank Telescope (credit: Geremia/Wikimedia Commons)

Russian billionaire Yuri Milner, Stephen Hawking, Martin Rees, Frank Drake and others announced at The Royal Society today $100 million funding for Breakthrough Listen — the “most powerful, comprehensive, and intensive scientific search ever undertaken for signs of intelligent life beyond Earth.”

They also announced $1 million prize funding for Breakthrough Message, a competition to generate messages representing humanity and planet Earth.

“It’s time to commit to finding the answer to search for life beyond Earth,” said Hawking. “We are live, we are intelligent, we must know … if we are alone in the dark.”

The search will be done at two of the largest radio telescopes, the 100 Meter Robert C. Byrd Green Bank Telescope in West Virginia, the world’s largest steerable radio telescope; and the 64-metre diameter Parkes Telescope in New South Wales, Australia.  And the Automated Planet Finder Telescope at Lick Observatory in California will undertake the world’s deepest and broadest search for optical laser transmissions.

More sensitive, faster, wider spectrum, more sky coverage

The Breakthrough Listen initiative will be 50 times more sensitive than previous programs dedicated to SETI research, the scientists say. It will cover ten times more of the sky than previous programs and will scan at least five times more of the radio spectrum, and 100 times faster.  It will survey the one million closest stars to Earth and the 100 closest galaxies.

The program will generate what may be the largest amount of scientific data ever made available to the public, at tens of gigaHertz bandwidth, the scientists said, and all data and software will be open-source and available to the public. The initiative will also be joining and supporting SETI@home, UC Berkeley’s distributed computing platform, with 9 million volunteers donating their spare computing power to search astronomical data for signs of life.

The second initiative, Breakthrough Message, will be an international competition to create digital messages that represent humanity and planet Earth, with prizes totaling $1,000,000. It will not be a commitment to send messages.

Other leaders for the two initiatives are astronomer Pete Worden, Chairman, Breakthrough Prize Foundation and former Director, NASA Ames Research Center; professor of astronomy Geoff Marcy, UC Berkeley; writer/producer Ann Druyan, Creative Director of the Interstellar Message, NASA Voyager; SETI@home project chief scientist Dan Werthimer; and Andrew Siemion, Director, Berkeley SETI Research Center.

More information: Breakthrough Initiatives.


Breakthrough Life In The Universe Initiatives Press Conference

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AI algorithm learns to ‘see’ features in galaxy images

Hubble Space Telescope image of the cluster of galaxies MACS0416.1-2403, one of the Hubble “Frontier Fields” images. Bright yellow “elliptical” galaxies can be seen, surrounded by numerous blue spiral and amorphous (star-forming) galaxies. This image forms the test data that the machine learning algorithm is applied to, having not previously “seen” the image. (credit: NASA/ESA/J. Geach/A. Hocking)

A team of astronomers and computer scientists at the University of Hertfordshire have taught a machine to “see” astronomical images, using data from the Hubble Space Telescope Frontier Fields set of images of distant clusters of galaxies that contain several different types of galaxies.

The technique, which uses a form of AI called unsupervised machine learning, allows galaxies to be automatically classified at high speed, something previously done by thousands of human volunteers in projects like Galaxy Zoo.

Image highlighting parts of the MACS0416.1-2403 cluster image that the algorithm has identified as “star-forming” galaxies (credit: NASA/ESA/J. Geach/A. Hocking)

“We have not told the machine what to look for in the images, but instead taught it how to ‘see,’” said graduate student Alex Hocking.

“Our aim is to deploy this tool on the next generation of giant imaging surveys where no human, or even group of humans, could closely inspect every piece of data. But this algorithm has a huge number of applications far beyond astronomy, and investigating these applications will be our next step,” said University of Hertfordshire Royal Society University Research Fellow James Geach, PhD.

The scientists are now looking for collaborators to make use of the technique in applications like medicine, where it could for example help doctors to spot tumors, and in security, to find suspicious items in airport scans.

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Super-resolution electron microscopy of soft materials like biomaterials

CLAIRE image of Al nanostructures with an inset that shows a cluster of six Al nanostructures (credit: Lawrence Berkeley National Laboratory)

Soft matter encompasses a broad swath of materials, including liquids, polymers, gels, foam and — most importantly — biomolecules. At the heart of soft materials, governing their overall properties and capabilities, are the interactions of nano-sized components.

Observing the dynamics behind these interactions is critical to understanding key biological processes, such as protein crystallization and metabolism, and could help accelerate the development of important new technologies, such as artificial photosynthesis or high-efficiency photovoltaic cells.

Observing these dynamics at sufficient resolution has been a major challenge, but this challenge is now being met with a new non-invasive nanoscale imaging technique that goes by the acronym of CLAIRE.

CLAIRE stands for “cathodoluminescence activated imaging by resonant energy transfer.” Invented by researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley, CLAIRE extends the extremely high resolution of electron microscopy to the dynamic imaging of soft matter.

“Traditional electron microscopy damages soft materials and has therefore mainly been used to provide topographical or compositional information about robust inorganic solids or fixed sections of biological specimens,” says chemist Naomi Ginsberg, who leads CLAIRE’s development and holds appointments with Berkeley Lab’s Physical Biosciences Division and its Materials Sciences Division, as well as UC Berkeley’s departments of chemistry and physics.

“CLAIRE allows us to convert electron microscopy into a new non-invasive imaging modality for studying soft materials and providing spectrally specific information about them on the nanoscale.”

Ginsberg is also a member of the Kavli Energy NanoScience Institute (Kavli-ENSI) at Berkeley. She and her research group recently demonstrated CLAIRE’s imaging capabilities by applying the technique to aluminum nanostructures and polymer films that could not have been directly imaged with electron microscopy.

“What microscopic defects in molecular solids give rise to their functional optical and electronic properties? By what potentially controllable process do such solids form from their individual microscopic components, initially in the solution phase? The answers require observing the dynamics of electronic excitations or of molecules themselves as they explore spatially heterogeneous landscapes in condensed phase systems,” Ginsberg says.

“In our demonstration, we obtained optical images of aluminum nanostructures with 46 nanometer resolution, then validated the non-invasiveness of CLAIRE by imaging a conjugated polymer film. The high resolution, speed and non-invasiveness we demonstrated with CLAIRE positions us to transform our current understanding of key biomolecular interactions.”

How to avoid destroying soft matter with electron beams

CLAIRE works by essentially combining the best attributes of optical and scanning electron microscopy into a single imaging platform.

Scanning electron microscopes use beams of electrons rather than light for illumination and magnification. With much shorter wavelengths than photons of visible light, electron beams can be used to observe objects hundreds of times smaller than those that can be resolved with an optical microscope. However, these electron beams destroy most forms of soft matter and are incapable of spectrally specific molecular excitation.

Ginsberg and her colleagues get around these problems by employing a process called “cathodoluminescence,” in which an ultrathin scintillating film, about 20 nanometers thick, composed of cerium-doped yttrium aluminum perovskite, is inserted between the electron beam and the sample.

When the scintillating film is excited by a low-energy electron beam (about 1 KeV), it emits energy that is transferred to the sample, causing the sample to radiate. This luminescence is recorded and correlated to the electron beam position to form an image that is not restricted by the optical diffraction limit (which limits optical microscopy).

The CLAIRE imaging demonstration was carried out at the Molecular Foundry, a DOE Office of Science User Facility.

Observing biomolecular interactions, solar cells, and LEDs

While there is still more work to do to make CLAIRE widely accessible, Ginsberg and her group are moving forward with further refinements for several specific applications.

“We’re interested in non-invasively imaging soft functional materials like the active layers in solar cells and light-emitting devices,” she says. “It is especially true in organics and organic/inorganic hybrids that the morphology of these materials is complex and requires nanoscale resolution to correlate morphological features to functions.”

Ginsberg and her group are also working on the creation of liquid cells for observing biomolecular interactions under physiological conditions. Since electron microscopes can only operate in a high vacuum, as molecules in the air disrupt the electron beam, and since liquids evaporate in high vacuum, aqueous samples must either be freeze-dried or hermetically sealed in special cells.

“We need liquid cells for CLAIRE to study the dynamic organization of light-harvesting proteins in photosynthetic membranes,” Ginsberg says. “We should also be able to perform other studies in membrane biophysics to see how molecules diffuse in complex environments, and we’d like to be able to study molecular recognition at the single molecule level.”

In addition, Ginsberg and her group will be using CLAIRE to study the dynamics of nanoscale systems for soft materials in general. “We would love to be able to observe crystallization processes or to watch a material made of nanoscale components anneal or undergo a phase transition,” she says. “We would also love to be able to watch the electric double layer at a charged surface as it evolves, as this phenomenon is crucial to battery science.”

A paper describing the most recent work on CLAIRE has been published in the journal Nano Letters. This research was primarily supported by the DOE Office of Science and by the National Science Foundation.

Abstract of Cathodoluminescence-Activated Nanoimaging: Noninvasive Near-Field Optical Microscopy in an Electron Microscope

We demonstrate a new nanoimaging platform in which optical excitations generated by a low-energy electron beam in an ultrathin scintillator are used as a noninvasive, near-field optical scanning probe of an underlying sample. We obtain optical images of Al nanostructures with 46 nm resolution and validate the noninvasiveness of this approach by imaging a conjugated polymer film otherwise incompatible with electron microscopy due to electron-induced damage. The high resolution, speed, and noninvasiveness of this “cathodoluminescence-activated” platform also show promise for super-resolution bioimaging.