Nano-thin invisibility cloak makes 3D objects disappear (w/video)

Invisibility cloaks are a staple of science fiction and fantasy, from Star Trek to Harry Potter, but don't exist in real life, or do they? Scientists at the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley have devised an ultra-thin invisibility "skin" cloak that can conform to the shape of an object and conceal it from detection with visible light. Although this cloak is only microscopic in size, the principles behind the technology should enable it to be scaled-up to conceal macroscopic items as well. Invisibility Skin Cloak This image shows a 3-D illustration of a metasurface skin cloak made from an ultrathin layer of nanoantennas (gold blocks) covering an arbitrarily shaped object. Light reflects off the cloak (red arrows) as if it were reflecting off a flat mirror. (Image courtesy of Xiang Zhang group, Berkeley Lab/UC Berkeley) Working with brick-like blocks of gold nanoantennas, the Berkeley researchers fashioned a "skin cloak" barely 80 nanometers in thickness, that was wrapped around a three-dimensional object about the size of a few biological cells and arbitrarily shaped with multiple bumps and dents. The surface of the skin cloak was meta-engineered to reroute reflected light waves so that the object was rendered invisible to optical detection when the cloak is activated. "This is the first time a 3D object of arbitrary shape has been cloaked from visible light," said Xiang Zhang, director of Berkeley Lab's Materials Sciences Division and a world authority on metamaterials - artificial nanostructures engineered with electromagnetic properties not found in nature. "Our ultra-thin cloak now looks like a coat. It is easy to design and implement, and is potentially scalable for hiding macroscopic objects." Zhang, who holds the Ernest S. Kuh Endowed Chair at UC Berkeley and is a member of the Kavli Energy NanoSciences Institute at Berkeley (Kavli ENSI), is the corresponding author of a paper describing this research in ("An ultrathin invisibility skin cloak for visible light"). It is the scattering of light - be it visible, infrared, X-ray, etc., - from its interaction with matter that enables us to detect and observe objects. The rules that govern these interactions in natural materials can be circumvented in metamaterials whose optical properties arise from their physical structure rather than their chemical composition. For the past ten years, Zhang and his research group have been pushing the boundaries of how light interacts with metamaterials, managing to curve the path of light or bend it backwards, phenomena not seen in natural materials, and to render objects optically undetectable. In the past, their metamaterial-based optical carpet cloaks were bulky and hard to scale-up, and entailed a phase difference between the cloaked region and the surrounding background that made the cloak itself detectable - though what it concealed was not.

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"Creating a carpet cloak that works in air was so difficult we had to embed it in a dielectric prism that introduced an additional phase in the reflected light, which made the cloak visible by phase-sensitive detection," says co-lead author Xingjie Ni, a recent member of Zhang's research group who is now an assistant professor at Penn State University. "Recent developments in metasurfaces, however, allow us to manipulate the phase of a propagating wave directly through the use of subwavelength-sized elements that locally tailor the electromagnetic response at the nanoscale, a response that is accompanied by dramatic light confinement." In the Berkeley study, when red light struck an arbitrarily shaped 3D sample object measuring approximately 1,300 square microns in area that was conformally wrapped in the gold nanoantenna skin cloak, the light reflected off the surface of the skin cloak was identical to light reflected off a flat mirror, making the object underneath it invisible even by phase-sensitive detection. The cloak can be turned "on" or "off" simply by switching the polarization of the nanoantennas. "A phase shift provided by each individual nanoantenna fully restores both the wavefront and the phase of the scattered light so that the object remains perfectly hidden," says co-lead author Zi Jing Wong, also a member of Zhang's research group. The ability to manipulate the interactions between light and metamaterials offers tantalizing future prospects for technologies such as high resolution optical microscopes and superfast optical computers. Invisibility skin cloaks on the microscopic scale might prove valuable for hiding the detailed layout of microelectronic components or for security encryption purposes. At the macroscale, among other applications, invisibility cloaks could prove useful for 3D displays.
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New diamond structures produce bright luminescence for quantum cryotography and biomarkers applications

Germanium defects in a diamond crystal lattice act as a reliable source for single photons, new research shows. The results are reported in ("Germanium-Vacancy Single Color Centers in Diamond") and provide a promising new route to building components for quantum cryptography and biomarkers. Pure diamonds are naturally colorless, but gaps in the crystal structure or impurities of other elements can create colors and even emit fluorescence. Recently, researchers have shown that the fluorescent lattice defects could be useful as single photon sources for quantum cryptography and as bright luminescent makers in living cells. Now, Takayuki Iwasaki and co-workers at Tokyo Institute of Technology (Tokyo Tech), together with scientists across Japan and Germany, have demonstrated a new type of diamond crystal defect that fluoresces to produce single photons in a narrow, high energy wavelength band. The defects, which have been named germanium-vacancy (GeV) centres, are relatively easy to fabricate in a reliable, reproducible way. Single GeV center in diamond Single GeV center in diamond. (left) Fluorescence mapping and (right) atomic structure model. Iwasaki and co-workers were inspired by recent work that demonstrated fluorescence from nitrogen-vacancy (NV) and silicon-vacancy (SiV) defects in diamond. They used an ion implantation method to insert germanium atoms into diamond films, before heating the films at 800°C. The resulting samples showed fluorescence only after heating, which induces diffusion of vacancies in the diamond lattice. The researchers therefore concluded that the fluorescence was produced by combined defects, each comprising a germanium atom side-by-side with a vacancy. The GeV centres produced single-photon bursts of fluorescence centred at a wavelength of around 602 nm, representing a higher energy fluorescence than SiV centres. Moreover, the researchers were also able to create the films through the less destructive method of chemical vapor deposition, producing films with narrower and more stable emission peaks of ensemble GeV centres which are useful for biomarkers. Overall, the work opens up a promising new avenue for developing sources of single photons, which are essential for quantum cryptography. Iwasaki and co-workers are also hopeful that they could incorporate GeV centres in nanodiamonds for use as biological markers. Background Quantum computing and cryptography Our current digital computers encode information in bits, which can have values of either 0 or 1. In quantum computers, data will instead be stored in ‘qubits’, which can take on values of not only 0 or 1, but also a superposition of the two states. This small difference represents a huge change in functionality, and allows information and data to be encrypted in ways that are impossible to decode using only classical methods-this is known as quantum cryptography. Indeed, quantum-encoded data cannot be copied or read without changing its state, meaning that it is impossible for third parties to eavesdrop on communications without being discovered. Single photon generation To achieve the secure data transmission by quantum cryptography, individual photons of known wavelengths must be used but are difficult to generate. Herein lies the motivation behind the work of Iwasaki and co-workers. Defects in diamond have been shown to produce fluorescence – emitting photons of fixed-wavelength light when illuminated by higher energy light – but these are often unreliable or difficult to fabricate. The search is on for new defect structures that not only produce strong, consistent fluorescence, but can also be made in a reproducible way. Biomarker To monitor individual proteins and the interior of living cells, nanometer sized markers such as fluorescent proteins and quantum dots are used. Due to the high biological compatibility of diamond, fluorescent defects in diamond nanostructures are stable biomarkers without optical bleaching. The bright emission from the GeV centres could be suitable for such biological applications. Methodology Iwasaki and co-workers began with an ion implantation method, which involved firing germanium atoms at high speed into pure diamond surfaces. They then heated the samples at 800°C to induce diffusion of vacancies – gaps in the diamond crystal lattice where a carbon atom is missing. By using Raman spectroscopy and confocal microscopy they observed fluorescent light emerging from the samples at a wavelength of around 602 nm. The team used theoretical calculations to deduce that this fluorescence resulted from combined defects, each comprising a germanium atom next to a lattice vacancy. The biggest step forward in the work was when Iwasaki and co-workers managed to create the same types of defects through a different method, microwave plasma chemical vapor deposition (MPCVD). MPCVD involves reactions of volatile chemicals on a substrate, and is often used to make synthetic diamonds. The defects in the sample prepared using MPCVD gave off more consistent fluorescence with a narrower and more stable peak. Moreover, MPCVD provides closer control over the fabrication process, and is less likely to produce unwanted damage to the samples than ion implantation. Future work Further work is needed to refine the fabrication process so that diamond films with germanium defects could be incorporated into devices for reliable single photon generation on demand.
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Carbyne research may boost nanoelectronics

The smallest of electronics could one day have the ability to turn on and off at an atomic scale. Lawrence Livermore National Laboratory scientists have investigated a way to create linear chains of carbon atoms from laser-melted graphite ("Carbyne Fiber Synthesis within Evaporating Metallic Liquid Carbon"). The material, called carbyne, could have a number of novel properties, including the ability to adjust the amount of electrical current traveling through a circuit, depending on the user’s needs. Carbyne is the subject of intense research because of its presence in astrophysical bodies, as well as its potential use in nanoelectronic devices and superhard materials. Its linear shape gives it unique electrical properties that are sensitive to stretching and bending, and it is 40 times stiffer than diamond. It also was found in the Murchison and Allende meteorites and could be an ingredient of interstellar dust. A carbyne strand forms in laser-melted graphite A carbyne strand forms in laser-melted graphite. Carbyne is found in astrophysical bodies and has the potential to be used in nanoelectronic devices and superhard materials. (Image by Liam Krauss/LLNL) Using computer simulations, LLNL scientist Nir Goldman and colleague Christopher Cannella, an undergraduate summer researcher from Caltech, initially intended to study the properties of liquid carbon as it evaporates, after being formed by shining a laser beam on the surface of graphite. The laser can heat the graphite surface to a few thousands of degrees, which then forms a fairly volatile droplet. To their surprise, as the liquid droplet evaporated and cooled in their simulations, it formed bundles of linear chains of carbon atoms. “There’s been a lot of speculation about how to make carbyne and how stable it is,” Goldman said. “We showed that laser melting of graphite is one viable avenue for its synthesis. If you regulate carbyne synthesis in a controlled way, it could have applications as a new material for a number of different research areas, including as a tunable semiconductor or even for hydrogen storage. “Our method shows that carbyne can be formed easily in the laboratory or otherwise. The process also could occur in astrophysical bodies or in the interstellar medium, where carbon-containing material can be exposed to relatively high temperatures and carbon can liquefy.” Goldman’s study and computational models allow for direct comparison with experiments and can help determine parameters for synthesis of carbon-based materials with potentially exotic properties. “Our simulations indicate a possible mechanism for carbyne fiber synthesis that confirms previous experimental observation of its formation,” Goldman said. “These results help determine one set of thermodynamic conditions for its synthesis and could account for its detection in meteorites resulting from high-pressure conditions due to impact.”
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