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July 10 2015


Ash Beckham: When to take a stand -- and when to let it go

Ash Beckham recently found herself in a situation that made her ask: who am I? She felt pulled between two roles — as an aunt and as an advocate. Each of us feels this struggle sometimes, she says -- and offers bold suggestions for how to stand up for your moral integrity when it isn’t convenient.

A graphene microphone and loudspeaker that operate at up to 500 kilohertz

Construction of graphene electrostatic wideband receiver (microphone). The graphene membrane is suspended across the supporting frame (A). The membrane is electrically contacted with gold wires, and spacers are added (B) to control the distance from the membrane to the gold-coated stationary electrodes (C). (credit: Qin Zhou et al./PNAS)

University of California, Berkeley, physicists have used graphene to build lightweight ultrasonic loudspeakers and microphones, enabling people to mimic bats or dolphins’ ability to use sound to communicate and gauge the distance and speed of objects around them.

More practically, the wireless ultrasound devices complement standard radio transmission using electromagnetic waves in areas where radio is impractical, such as underwater, but with far greater fidelity than current ultrasound or sonar devices. They can also be used to communicate through objects, such as steel, that electromagnetic waves can’t penetrate.

“Sea mammals and bats use high-frequency sound for echolocation and communication, but humans just haven’t fully exploited that before, in my opinion, because the technology has not been there,” said UC Berkeley physicist Alex Zettl. “Until now, we have not had good wideband ultrasound transmitters or receivers. These new devices are a technology opportunity.”

The diaphragms in the new devices are graphene sheets a mere one atom thick that have the right combination of stiffness, strength and light weight to respond to frequencies ranging from subsonic (below 20 hertz) to ultrasonic (above 20 kilohertz). Humans can hear from 20 hertz up to 20,000 hertz, whereas bats hear only in the kilohertz range, from 9 to 200 kilohertz. The graphene loudspeakers and microphones operate from well below 20 hertz to over 500 kilohertz.

Practical graphene uses

“There’s a lot of talk about using graphene in electronics and small nanoscale devices, but they’re all a ways away,” said Zettl, who is a senior scientist at Lawrence Berkeley National Laboratory and a member of the Kavli Energy NanoSciences Institute, operated jointly by UC Berkeley and Berkeley Lab. “The microphone and loudspeaker are some of the closest devices to commercial viability, because we’ve worked out how to make the graphene and mount it, and it’s easy to scale up.”

Zettl, UC Berkeley postdoctoral fellow Qin Zhou and colleagues describe their graphene microphone and ultrasonic radio in a paper appearing online this week in the Proceedings of the National Academy of Sciences.

One big advantage of graphene is that the atom-thick sheet is so lightweight that it responds well to the different frequencies of an electronic pulse, unlike today’s piezoelectric microphones and speakers. This comes in handy when using ultrasonic transmitters and receivers to transmit large amounts of information through many different frequency channels simultaneously, or to measure distance, as in sonar applications.

“Because our membrane is so light, it has an extremely wide frequency response and is able to generate sharp pulses and measure distance much more accurately than traditional methods,” Zhou said.

Graphene membranes are also more efficient, converting over 99 percent of the energy driving the device into sound, whereas today’s conventional loudspeakers and headphones convert only 8 percent into sound. Zettl anticipates that in the future, communications devices like cellphones will utilize not only electromagnetic waves – radio – but also acoustic or ultrasonic sound, which can be highly directional and long-range.

Bat chirps

A recording of the pipistrelle bat’s ultrasonic chirps, slowed to one-eighth normal speed (credit: Qin Zhou/UC Berkeley)

Bat expert Michael Yartsev, a newly hired UC Berkeley assistant professor of bioengineering and member of the Helen Wills Neuroscience Institute, said, “These new microphones will be incredibly valuable for studying auditory signals at high frequencies, such as the ones used by bats.

The use of graphene allows the authors to obtain very flat frequency responses in a wide range of frequencies, including ultrasound, and will permit a detailed study of the auditory pulses that are used by bats.”

Zettl noted that audiophiles would also appreciate the graphene loudspeakers and headphones, which have a flat response across the entire audible frequency range.

The work was supported by the U.S. Department of Energy, the Office of Naval Research and the National Science Foundation. Other co-authors were Zheng, Michael Crommie, a UC Berkeley professor of physics, and Seita Onishi.

Abstract of Graphene electrostatic microphone and ultrasonic radio

We present a graphene-based wideband microphone and a related ultrasonic radio that can be used for wireless communication. It is shown that graphene-based acoustic transmitters and receivers have a wide bandwidth, from the audible region (20∼20 kHz) to the ultrasonic region (20 kHz to at least 0.5 MHz). Using the graphene-based components, we demonstrate efficient high-fidelity information transmission using an ultrasonic band centered at 0.3 MHz. The graphene-based microphone is also shown to be capable of directly receiving ultrasound signals generated by bats in the field, and the ultrasonic radio, coupled to electromagnetic (EM) radio, is shown to function as a high-accuracy rangefinder. The ultrasonic radio could serve as a useful addition to wireless communication technology where the propagation of EM waves is difficult.


Crowdsourcing neurofeedback data

In front of an audience, the collective neurofeedback of 20 participants were projected on the 360° surface of the semi-transparent dome as artistic video animations with soundscapes generated based on a pre-recorded sound library and improvisations from live musicians (credit: Natasha Kovacevic et al./PLoS ONE/Photo: David Pisarek)

In a large-scale art-science installation called My Virtual Dream in Toronto in 2013, more than 500 adults wearing a Muse wireless electroencephalography (EEG) headband inside a 60-foot geodesic dom participated in an unusual neuroscience experiment.

As they played a collective neurofeedback computer game where they were required to manipulate their mental states of relaxation and concentration, the group’s collective EEG signals triggered a catalog of related artistic imagery displayed on the dome’s 360-degree interior, along with spontaneous musical interpretation by live musicians on stage.

“What we’ve done is taken the lab to the public. We collaborated with multimedia artists, made this experiment incredibly engaging, attracted highly motivated subjects, which is not easy to do in the traditional lab setting, and collected useful scientific data from their experience.”

Collective neurofeedback: a new kind of neuroscience research

Participant instructions (credit: Natasha Kovacevic et al./PLoS ONE)

Results from the experiment demonstrated the scientific viability of collective neurofeedback as a potential new avenue of neuroscience research that takes into account individuality, complexity and sociability of the human mind. They also yielded new evidence that neurofeedback learning can have an effect on the brain almost immediately the researchers say.

Studying brains in a social and multi-sensory environment is closer to real life and may help scientists to approach questions of complex real-life social cognition that otherwise are not accessible in traditional labs that study one person’s cognitive functions at a time.

“In traditional lab settings, the environment is so controlled that you can lose some of the fine points of real-time brain activity that occur in a social life setting,” said Natasha Kovacevic, creative producer of My Virtual Dream and program manager of the Centre for Integrative Brain Dynamics at Baycrest’s Rotman Research Institute.

The massive amount of EEG data collected in one night yielded an interesting and statistically relevant finding: that subtle brain activity changes were taking place within approximately one minute of the neurofeedback learning exercise — unprecedented speed of learning changes that have not been demonstrated before.

Building the world’s first virtual brain

“These results really open up a whole new domain of neuroscience study that actively engages the public to advance our understanding of the brain,” said Randy McIntosh, director of the Rotman Research Institute and vice-president of Research at Baycrest. He is a senior author on the paper.

The idea for the Nuit Blanche art-science experiment was inspired by Baycrest’s ongoing international project to build the world’s first functional, virtual brain — a research and diagnostic tool that could one day revolutionize brain healthcare.

Baycrest cognitive neuroscientists collaborated with artists and gaming and wearable technology industry partners for over a year to create the My Virtual Dream installation. Partners included the University of Toronto, Scotiabank Nuit Blanche, Muse, and Uken Games.

Plans are underway to travel My Virtual Dream to other cities around the world.

Abstract of ‘My Virtual Dream’: Collective Neurofeedback in an Immersive Art Environment

While human brains are specialized for complex and variable real world tasks, most neuroscience studies reduce environmental complexity, which limits the range of behaviours that can be explored. Motivated to overcome this limitation, we conducted a large-scale experiment with electroencephalography (EEG) based brain-computer interface (BCI) technology as part of an immersive multi-media science-art installation. Data from 523 participants were collected in a single night. The exploratory experiment was designed as a collective computer game where players manipulated mental states of relaxation and concentration with neurofeedback targeting modulation of relative spectral power in alpha and beta frequency ranges. Besides validating robust time-of-night effects, gender differences and distinct spectral power patterns for the two mental states, our results also show differences in neurofeedback learning outcome. The unusually large sample size allowed us to detect unprecedented speed of learning changes in the power spectrum (~ 1 min). Moreover, we found that participants’ baseline brain activity predicted subsequent neurofeedback beta training, indicating state-dependent learning. Besides revealing these training effects, which are relevant for BCI applications, our results validate a novel platform engaging art and science and fostering the understanding of brains under natural conditions.

July 09 2015


A graphene-based molecule sensor

Shining infrared light on a graphene surface makes surface electrons oscillate in different ways that identify the specific molecule attached to the surface (EPFL/Miguel Spuch /Daniel Rodrigo )

European scientists have harnessed graphene’s unique optical and electronic properties to develop a highly sensitive sensor to detect molecules such as proteins and drugs — one of the first such applications of graphene.

The results are described in an article appearing in the latest edition of the journal Science.

The researchers at EPFL’s Bionanophotonic Systems Laboratory (BIOS) and the Institute of Photonic Sciences (ICFO, Spain) used graphene to improve on a molecule-detection method called infrared absorption spectroscopy, which uses infrared light is used to excite the molecules. Each type of molecule absorbs differently across the spectrum, creating a signature that can be recognized.

This method is not effective, however, in detecting molecules that are under 10 nanometers in size (such as proteins), because the size of the mid-infrared wavelengths used are huge in comparison — 2 to 6 micrometers (2,000 to 6,000 nanometers).

Conceptual view of the graphene biosensor. An infrared beam excites a plasmon resonance across the graphene nanoribbons. Protein sensing is achieved by changing the voltage applied to the graphene and detecting a plasmon resonance spectral shift accompanied by narrow dips corresponding to the molecular vibration bands of the protein. (credit: Daniel Rodrigo et al./Science)

Resonant vibrations

With the new graphene method, the target proteins to be analyzed are attached to the graphene surface. “We pattern nanostructures on the graphene surface by bombarding it with electron beams and etching it with oxygen ions,” said Daniel Rodrigo, co-author of the publication. “When the light arrives, the electrons in graphene nanostructures begin to oscillate. This phenomenon, known as ‘localized surface plasmon resonance,’ serves to concentrate light into tiny spots, which are comparable with the [tiny] dimensions of the target molecules. It is then possible to detect nanometric structures.”

This process can also reveal the nature of the bonds connecting the atoms that the molecule is composed of. When a molecule vibrates, it does so in a range of frequencies, which are generated by the bonds connecting the different atoms. To detect these frequencies,  the researchers “tuned” the graphene to different frequencies by applying voltage, which is not possible with current sensors. Making graphene’s electrons oscillate in different ways makes it possible to “read” all the vibrations of the molecule on its surface. “It gave us a full picture of the molecule,” said co-author Hatice Altug.

According to the researchers, this simple method shows that it is possible to conduct a complex analysis using only one device, while it normally requires many different ones, and without stressing or modifying the biological sample. “The method should also work for polymers, and many other substances,” she added.

Abstract of Mid-infrared plasmonic biosensing with graphene

Infrared spectroscopy is the technique of choice for chemical identification of biomolecules through their vibrational fingerprints. However, infrared light interacts poorly with nanometric-size molecules. We exploit the unique electro-optical properties of graphene to demonstrate a high-sensitivity tunable plasmonic biosensor for chemically specific label-free detection of protein monolayers. The plasmon resonance of nanostructured graphene is dynamically tuned to selectively probe the protein at different frequencies and extract its complex refractive index. Additionally, the extreme spatial light confinement in graphene—up to two orders of magnitude higher than in metals—produces an unprecedentedly high overlap with nanometric biomolecules, enabling superior sensitivity in the detection of their refractive index and vibrational fingerprints. The combination of tunable spectral selectivity and enhanced sensitivity of graphene opens exciting prospects for biosensing.


Russia’s War on Ukraine’s Economy

Ukraine’s primary economic problems are not homegrown but the result of Russian aggression. The international community, with the EU in the lead, must provide substantial humanitarian assistance and help stabilize the country's economy.


Johann Hari: Everything you think you know about addiction is wrong

What really causes addiction -- to everything from cocaine to smart-phones? And how can we overcome it? Johann Hari has seen our current methods fail firsthand, as he has watched loved ones struggle to manage their addictions. He started to wonder why we treat addicts the way we do -- and if there might be a better way. As he shares in this deeply personal talk, his questions took him around the world, and unearthed some surprising and hopeful ways of thinking about an age-old problem.

The Paris Climate-Change Spectacular

The UN Climate Change Conference in Paris in December will feature all the tightly choreographed production values of a Hollywood blockbuster. But while the plot calls for the planet to be saved, the script is based mostly on fiction.


A Strategic Rallying Cry for Europe

The EU’s move to create a new security strategy could not be more important. Not only will it help Europe to address the security challenges that it faces; it also has the potential to give the EU the sense of purpose that it so badly needs.


Europe’s Greek Failure

Narratives matter, especially when they are intertwined with hard interests. As Greece and its creditors court catastrophe, we are getting a clear picture of how conflicting narratives can lead to a lose-lose result.


Omnidirectional wireless charging up to half a meter away from a power source

Omnidirectional wireless-charging system can charge multiple numbers of mobile devices simultaneously in a one-cubic-meter range. Above: charging transmitter; below: a Samsung Galaxy Note with embedded receiver. (credit: KAIST)

A group of researchers at KAIST in Korea has developed a wireless-power transfer (WPT) technology that allows mobile devices in the “Wi-Power” zone (within 0.5 meters from the power source) to be charged at any location and in any direction and orientation, tether-free.

The WPT system is capable of charging 30 smartphones with a power capacity of one watt each or 5 laptops with 2.4 watts.

The research team used its Dipole Coil Resonance System (DCRS) to induce magnetic fields, composed of two (transmitting and receiving) magnetic dipole coils, placed in parallel. Each coil has a ferrite core and is connected with a resonant capacitor.

Current wireless-power technologies require close contact with a charging pad and are limited to a fixed position.

The research was published in the June 2015 on-line issue of IEEE Transactions on Power Electronics.

KAIST | KAIST Omnidirectional Wireless Smartphone Charger at 1m

Abstract of Six Degrees of Freedom Mobile Inductive Power Transfer by Crossed Dipole Tx and Rx Coils

Crossed dipole coils for the wide-range 3-D omnidirectional inductive power transfer (IPT) are proposed. Free positioning of a plane receiving (Rx) coil is obtained for an arbitrary direction within 1m from a plane transmission (Tx) coil. Both the Tx and Rx coils consist of crossed dipole coils with an orthogonal phase difference; hence, a rotating magnetic field is generated from the Tx, which enables the Rx to receive power vertically or horizontally. Thus, the 3-D omnidirectional IPT is first realized for both the plate type Tx and Rx coils, which is crucial for practical applications where volumetric coil structure is highly prohibited. This optimized configuration of coils has been obtained through a general classification of power transfer and searching for mathematical constraints on multi-D omnidirectional IPT. Conventional loop coils are thoroughly analyzed and verified to be inadequate for the plate-type omnidirectional IPT in this paper. Simulation-based design of the proposed crossed dipole coils for a uniform magnetic field distribution is provided, and the 3-D omnidirectional IPT is experimentally verified by prototype Rx coils for a wireless power zone of 1 m3 with a prototype Tx coil of 1 m2 at an operating frequency of 280 kHz, meeting the Power Matters Alliance (PMA). The maximum overall efficiency was 33.6% when the input power was 100 W.


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.


Could black phosphorus be the next silicon?

Schematic of the “puckered honeycomb” crystal structure of black phosphorus (credit: Vahid Tayari/McGill University)

A two-dimensional material called “black phosphorus” could emerge as a strong candidate for future energy-efficient transistors, new research from McGill University and Université de Montréal suggests. The material is a form of phosphorus that is similar to graphite (also known as pencil lead and the source of graphene), so it can be exfoliated (separated) easily into single atomic layers known as phosphorene.

Unlike graphene, which acts like a metal, black phosphorus (bP) is a natural semiconductor: it can be switched on and off.

“To lower the operating voltage of transistors, and thereby reduce the heat they generate, we have to get closer and closer to designing the transistor at the atomic level,” says Thomas Szkopek, an associate professor in McGill’s Department of Electrical and Computer Engineering and senior author of an open-access paper published Tuesday (July 7) in Nature Communications.

“Transistors work more efficiently when they are thin, with electrons moving in only two dimensions,” he says. “Nothing gets thinner than a single layer of atoms.”

Left: Optical image of an encapsulated black phosphorus FET (field effect transistor). scale bar, 10 μm. (Right) Atomic force microscope image of the same device with encapsulating layer removed. The black phosphorus thickness is 43±2 nm (82±4 atomic layers). Scale bar, 10 μm. (credit: V. Tayari et al./ Nature Communications)

That’s the ideal, but so far, they have only managed to fabricate field effect transistors (FETs) with exfoliated bP layers ranging in thickness from 6nm to 47nm (about 11 to 90 atomic layers). That’s because bP “suffers from photo-oxidation in a reaction that proceeds faster as atomic film thickness is approached,” the authors explain. “The deleterious effects of photo-oxidation are mitigated by using bP layers thicker than a few atomic layers, by encapsulating the bP in a polymer superstrate, and by minimizing exposure to oxygen, water and visible light.”

“There is a great emerging interest around the world in black phosphorus,” Szkopek says. “We are still a long way from seeing atomic layer transistors in a commercial product, but we have now moved one step closer.”

Black arsenic phosphorous

Meanwhile, researchers at USC Viterbi School of Engineering, in collaboration with Technische Universität München, Germany, Universität Regensburg, Germany, and Yale University, have developed a new version of a related 2D material — black arsenic phosphorous (b-AsP) — using a new low-pressure fabrication method that demands less energy, is cheaper, and creates materials have some new electronic and optical properties not available with other 2D materials.

For example, the new b-AsP material can sense long wavelength infrared (LWIR), which is important for LIDAR (light radar) systems used in autonomous (and other new) vehicles, for infrared thermal imaging technology, flexible night vision glasses, and environmental sensing. The paper appeared in Advanced Materials on June 25, 2015.

Abstract of Two-dimensional magnetotransport in a black phosphorus naked quantum well

Black phosphorus (bP) is the second known elemental allotrope with a layered crystal structure that can be mechanically exfoliated to atomic layer thickness. Unlike metallic graphite and semi-metallic graphene, bP is a semiconductor in both bulk and few-layer form. Here we fabricate bP-naked quantum wells in a back-gated field effect transistor geometry with bP thicknesses ranging from 6±1 nm to 47±1 nm. Using a polymer encapsulant, we suppress bP oxidation and observe field effect mobilities up to 900 cm2 V−1 s−1 and on/off current ratios exceeding 105. Shubnikov-de Haas oscillations observed in magnetic fields up to 35 T reveal a 2D hole gas with Schrödinger fermion character in a surface accumulation layer. Our work demonstrates that 2D electronic structure and 2D atomic structure are independent. 2D carrier confinement can be achieved without approaching atomic layer thickness, advantageous for materials that become increasingly reactive in the few-layer limit such as bP.

Abstract of Black Arsenic–Phosphorus: Layered Anisotropic Infrared Semiconductors with Highly Tunable Compositions and Properties

New layered anisotropic infrared semiconductors, black arsenic–phosphorus (b-AsP), with highly tunable chemical compositions and electronic and optical properties are introduced. Transport and infrared absorption studies demonstrate the semiconducting nature of b-AsP with tunable band gaps, ranging from 0.3 to 0.15 eV. These band gaps fall into long-wavelength infrared regime and cannot be readily reached by other layered materials.


July 08 2015

Two Cheers for the Middle Ages!
Gaudí’s Great Temple

Modernizing the IMF

If the IMF is to remain relevant at a time of rapid economic transformation, it must adapt. By adding the Chinese renminbi to the basket of currencies that determines the value of its reserve asset, the Special Drawing Right, the IMF would demonstrate its willingness and ability to do just that.


Good People, Bad Judgments

Despite being remarkably progressive, the 1814 Norwegian constitution contained a clause barring Jews from entering the country, based on the judgment that Jewish beliefs and customs were incompatible with enlightened Western values. This is the same flawed logic being used today to persecute Muslims.


Syriza’s German Fixation?

The Greek people have spoken, decisively rejecting their creditors’ latest bailout terms in the recent referendum. With anti-German discourse on the rise in Greece, including among some elements of the ruling Syriza party, it is difficult to imagine how Greece’s government will reach a “dignified compromise” with its creditors.

July 07 2015


Engineers give invisibility cloaks a slimmer, stealthier design

An extremely thin cloaking device is designed using dielectric materials. The cloak is a thin Teflon sheet (light blue) embedded with many small, cylindrical ceramic particles (dark blue). (credit: Li-Yi Hsu/UC San Diego)

Electrical engineers at the University of California, San Diego have developed a new design for a cloaking device that overcomes some of the limitations of existing “invisibility cloaks”: it’s both thin and does not alter the brightness of light around a hidden object.

The technology behind this cloak will have more applications than just invisibility, such as concentrating solar energy and increasing signal speed in optical communications.

“Invisibility may seem like magic at first, but its underlying concepts are familiar to everyone. All it requires is a clever manipulation of our perception,” said Boubacar Kanté, a professor in the Department of Electrical and Computer Engineering at the UC San Diego Jacobs School of Engineering and the senior author of the study. “Full invisibility still seems beyond reach today, but it might become a reality in the near future thanks to recent progress in cloaking devices.”

The idea behind cloaking is to change the scattering of electromagnetic waves — such as light and radar — off an object to make it less detectable to specific frequencies.

One of the drawbacks of cloaking devices is that they are typically bulky.

The reflection pattern from an uncloaked object on a flat surface (top) is compared to the reflection pattern of the same object covered with the cloaking device (bottom), which effectively mimics the reflection from a completely flat surface (credit: Li-Yi Hsu/UC San Diego)

“Previous cloaking studies needed many layers of materials to hide an object, [so] the cloak ended up being much thicker than the size of the object being covered,” said Li-Yi Hsu, electrical engineering Ph.D. student at UC San Diego and the first author of the study, which was recently published in an open-access paper in the journal Progress In Electromagnetics Research. “In this study, we show that we can use a thin single-layer sheet for cloaking.”

The researchers say that their cloak also overcomes another fundamental drawback of existing cloaking devices: being “lossy” they loose energy). Cloaks that are lossy reflect light at a lower intensity than what hits their surface.

“Imagine if you saw a sharp drop in brightness around the hidden object, it would be an obvious telltale. This is what happens when you use a lossy cloaking device,” said Kanté. “What we have achieved in this study is a ‘lossless’ cloak. It won’t lose any intensity of the light that it reflects.”

Many cloaks are lossy because they are made with metal particles, which absorb light. The researchers report that one of the keys to their new cloak’s design is the use of non-conductive materials called dielectrics, which, unlike metals, do not absorb light. This cloak includes two dielectrics, a proprietary ceramic and Teflon, which are structurally tailored on a very fine scale to change the way light waves reflect off of the cloak.

Schematic of the cloaking metasurface, discretized with 25 cylinders, and coordinate system. (Inset) unit cell of the metasurface. The bottom black line is the ground plane, the light blue substrate is Teflon, and the dark blue cylinder is ceramic. The incident wave is polarized along the y axis. (credit: Li Yi Hsu et al./Progress In Electromagnetics Research)

A carpet cloak

In their experiments, the researchers specifically designed a “carpet” cloak, which works by cloaking an object sitting on top of a flat surface. The cloak makes the whole system — object and surface — appear flat by mimicking the reflection of light off the flat surface. Any object reflects light differently from a flat surface, but when the object is covered by the cloak, light from different points is reflected out of sync, effectively cancelling the overall revealing distortion of light caused by the object’s shape.

“This cloaking device basically fools the observer into thinking that there’s a flat surface,” said Kanté.

The researchers used Computer-Aided Design software with electromagnetic simulation to design and optimize the cloak. The cloak was modeled as a thin matrix of Teflon in which many small cylindrical ceramic particles were embedded, each with a different height depending on its position on the cloak.

“By changing the height of each dielectric particle, we were able to control the reflection of light at each point on the cloak,” explained Hsu. “Our computer simulations show how our cloaking device would behave in reality. We were able to demonstrate that a thin cloak designed with cylinder-shaped dielectric particles can help us significantly reduce the object’s shadow.”

“Doing whatever we want with light waves is really exciting,” said Kanté. “Using this technology, we can do more than make things invisible. We can change the way light waves are being reflected at will and ultimately focus a large area of sunlight onto a solar power tower, like what a solar concentrator does. We also expect this technology to have applications in optics, interior design and art.”

This particular cloak was designed for a 4.15GHz microwave frequency (close to WiFi), but the approach can be extended to higher frequencies up to the visible, the researchers say in the paper.

This work was supported by a grant from the Calit2 Strategic Research Opportunities (CSRO) program at the Qualcomm Institute at UC San Diego.

Abstract of Extremely thin dielectric metasurface for carpet cloaking

We demonstrate a novel and simple geometrical approach to cloaking a scatterer on a ground plane. We use an extremely thin dielectric metasurface to reshape the wavefronts distorted by a scatterer in order to mimic the reflection pattern of a flat ground plane. To achieve such carpet cloaking, the reflection angle has to be equal to the incident angle everywhere on the scatterer. We use a graded metasurface and calculate the required phase gradient to achieve cloaking. Our metasurface locally provides additional phase to the wavefronts to compensate for the phase difference amongst light paths induced by the geometrical distortion. We design our metasurface in the microwave range using highly sub-wavelength dielectric resonators. We verify our design by full-wave time-domain simulations using micro-structured resonators and show that results match theory very well. This approach can be applied to hide any scatterer under a metasurface of class C1 (first derivative continuous) on a ground plane not only in the microwave regime, but also at higher frequencies up to the visible.


Imaging electric fields of molecules or atoms at sub-nanoscale

Illustration of the measuring principle: depending on the local electric potential (voltage) field of a nanostructure on the surface of a sample molecule or atom (bottom), a single electron (red) in a sensor molecule attached to the tip of an atomic force microscope (AFM) jumps from the tip of the microscope to the sensor molecule or not (credit: Forschungszentrum Jülich)

Using a single molecule attached to an atomic force microscope (AFM) as a more sensitive sensor, scientists in Forschungszentrum Jülich in Germany have used a new “scanning quantum dot microscopy” method to image electric potential fields (voltages) of electron shells of single molecules and even atoms with high precision for the first time, providing contact-free information on the distribution of charges.

The breakthrough technique is relevant for diverse scientific fields including investigations into biomolecules and semiconductor materials.

“Our method is the first to image electric fields near the surface of a sample quantitatively with atomic precision on the sub-nanometer scale,” says Ruslan Temirov from Forschungszentrum Jülich. Such electric fields surround all nanostructures like an aura. Their properties provide information, for instance, on the distribution of charges in atoms or molecules.

For their measurements, the Jülich researchers used an atomic force microscope. This device functions a bit like a record player: a tip moves across the sample and pieces together an image of the properties of the surface. Its limitation is that the large size difference between the tip and the sample causes resolution difficulties — imagine that a single atom is the size as a head of a pin; the tip of the microscope would then be as large as the Empire State Building.

A single silver atom (green-yellow) on a silver substrate under the scanning quantum dot microscope (credit: Forschungszentrum Jülich)

Single quantum dot molecule functions as sensor

To improve resolution and sensitivity of the AFM, the scientists attached a single molecule as a “quantum dot” to the tip of the microscope. Quantum dots are tiny nanocrystal structures, measuring no more than a few nanometers across, which due to quantum confinement can only assume certain, discrete states comparable to the energy level of a single atom, making them ideal as a sensor.

The molecule at the tip of the microscope functions like a beam balance (measuring scale) that tilts to one side or the other. A shift in direction corresponds to the presence or absence of an additional electron, which either jumps from the tip to the molecule or does not. (The balance does not compare weights; it is affected by two electric fields that act on the mobile electron of the molecular sensor: the first is the field of a nanostructure being measured, and the second is a field surrounding the tip of the microscope, which carries a voltage.)

“The  [bias] voltage at the tip is varied until equilibrium is achieved. If we know what voltage has been applied, we can determine the [electric potential] field of the sample at the position of the molecule,” explains Dr. Christian Wagner, a member of Temirov’s Young Investigators group at Jülich’s Peter Grünberg Institute (PGI-3). “Because the whole molecular balance is so small, comprising only 38 atoms, we can create a very sharp image of the electric field of the sample. It’s a bit like a camera with very small pixels.”

A patent is pending for the method, which is particularly suitable for measuring rough surfaces, such as in semiconductor structures for electronic devices or folded biomolecules.

“In contrast to many other forms of scanning probe microscopy, scanning quantum dot microscopy can even work at a distance of several nanometers. In the nanoworld, this is quite a considerable distance,” says Wagner. The technique has so far only been applied in high vacuum and at low temperatures, which are required to attach the single molecule to the tip of the microscope. “In principle, variations that would work at room temperature are conceivable,” suggests Wagner.

He says other forms of quantum dots could also be used as a sensor in place of the current molecule, using semiconductor material, such as quantum dots made of nanocrystals like those already being used in fundamental research.

The research results have been published in an open-access paper in the current issue of Physical Review Letters.

Abstract of Scanning Quantum Dot Microscopy

We introduce a scanning probe technique that enables three-dimensional imaging of local electrostatic potential fields with subnanometer resolution. Registering single electron charging events of a molecular quantum dot attached to the tip of an atomic force microscope operated at 5 K, equipped with a qPlus tuning fork, we image the quadrupole field of a single molecule. To demonstrate quantitative measurements, we investigate the dipole field of a single metal adatom adsorbed on a metal surface. We show that because of its high sensitivity the technique can probe electrostatic potentials at large distances from their sources, which should allow for the imaging of samples with increased surface roughness.

Jello Submarine
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