August 15, 2011
Physics News Highlights of the American Institute of Physics (AIP) contains summaries of interesting research from the AIP journals, notices of upcoming meetings, and other information from the AIP Member Societies. Copies of papers are available to journalists upon request.
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_______________________________________________________1. Acoustic cloaking device echoes advances in optical cloaking
Optical cloaking devices that enable light to gracefully slip around a solid object were once strictly in the realm of science fiction. Today they have emerged as an exciting area of study, at least on microscopic scales. A new twist on this intriguing technology can now be “seen” in the field of acoustics. A team of researchers from the Universitat Politecnica de Valencia and the Universidad de Valencia have created a prototype of an acoustic cloak by using a 2-D mathematical model. Unlike sound-canceling technologies that eliminate noise by creating the exact-but-opposite waveform, an acoustic cloak would enable sound waves to travel around an object without changing their shape or direction. The proposed sound cloak, as described in the AIP’s journal Applied Physics Letters, would consist of 120 cylinders, each 15 millimeters in diameter. By carefully arranging them around an object 22.5 centimeters across, the researchers experimentally demonstrated that sound waves of a specific frequency (3061 Hertz, with about a 100-Hertz bandwidth) maintain their original wave-front pattern as they pass around and beyond the object. According to the researchers, the narrow operating band of the cloak can be overcome by increasing the number of cylinders used to create the cloak. If such a technique could be applied in real-world designs, it could enable better soundscapes in urban environments, better acoustics in performance halls, and quieter helmets that protect the ears from extreme noises, the researchers speculate.
Article: “Acoustic cloak for airborne sound by inverse design” is published in Applied Physics Letters.
(1) Grupo de Fenomenos Ondulatorios, Universitat Politecnica de Valencia
2. New device exposes explosive vapors
Decades after the bullets have stopped flying, wars can leave behind a lingering danger: landmines that maim civilians and render land unusable for agriculture. Minefields are a humanitarian disaster throughout the world, and now researchers in Scotland have designed a new device that could more reliably sense explosives, helping workers to identify and deactivate unexploded mines. The prototype sensor features a thin film of polymer whose many electrons jump into higher energy levels when exposed to light. If left alone, the electrons would eventually fall back down, re-emitting light. When the ‘excited’ polymer is exposed to the electron-deficient molecules that are common to many explosives, however, the molecules steal some of the polymer’s electrons, and so quench the light emission. Other devices have used the change in a fluorescent polymer’s light-emitting power to detect explosive vapors, but the Scottish team’s prototype, described in the AIP’s new journal AIP Advances, is the first to use a compact silicon-based micro-system to measure the change in the length of time an electron stays in the ‘excited’ higher energy state. This measurement is less affected by environmental factors, such as stray light, which should make the device more reliable. It is also an example of how the complementary properties of an organic semiconductor (the polymer) and an inorganic semiconductor (the silicon) can be combined to make novel devices, the researchers write. The team’s current prototype is not yet ready for commercialization, but future work may soon see it helping to reclaim landmine-littered land.
Article: “Ultra-portable explosives sensor based on a CMOS florescence lifetime analysis micro-system” is published in AIP Advances.
Authors: Yue Wang (1), Bruce R. Rae (2), Robert K. Henderson (2), Zheng Gong (3), Jonathan Mckendry (3), Erdan Gu (3), Martin D. Dawson (3), Graham A. Turnbull (1), and Ifor D.W. Samuel (1).
(1) Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of St. Andrews
3. Strain and spin may enable ultra-low-energy computing
By combining two frontier technologies, spintronics and straintronics, a team of researchers from Virginia Commonwealth University has devised perhaps the world’s most miserly integrated circuit. Their proposed design runs on so little energy that batteries are not even necessary; it could run merely by tapping the ambient energy from the environment. Rather than the traditional charge-based electronic switches that encode the basic 0s and 1s of computer lingo, spintronics harnesses the natural spin – either up or down – of electrons to store bits of data. Spin one way and you get a 0; switch the spin the other way – typically by applying a magnetic field or by a spin-polarized current pulse – and you get a 1. During switching, spintronics uses considerably less energy than charge-based electronics. However, when ramped up to usable processing speeds, much of that energy savings is lost in the mechanism through which the energy from the outside world is transferred to the magnet. The solution, as proposed in the AIP’s journal Applied Physics Letters, is to use a special class of composite structure called multiferroics. These composite structures consist of a layer of piezoelectric material with intimate contact to a magnetostrictive nanomagnet (one that changes shape in response to strain). When a tiny voltage is applied across the structure, it generates strain in the piezoelectric layer, which is then transferred to the magnetostrictive layer. This strain rotates the direction of magnetism, achieving the flip. With the proper choice of materials, the energy dissipated can be as low as 0.4 attojoules, or about a billionth of a billionth of a joule. This proposed design would create an extremely low-power, yet high-density, non-volatile magnetic logic and memory system. The processors would be well suited for implantable medical devices and could run on energy harvested from the patient’s body motion. They also could be incorporated into buoy-mounted computers that would harvest energy from sea waves, among other intriguing possibilities.
Article: “Hybrid spintronics and straintronics: A magnetic technology for ultra-low-energy computing signal processing” is published in Applied Physics Letters.
Authors: Kuntal Roy (1), Supriyo Bandyopadhyay (1), and Jayasimha Atulasimha (2).
(1) Department of Electrical and Computer Engineering, Virginia Commonwealth University
4. Bending light with better precision
Physicists from the University of California at San Diego (UCSD) have demonstrated a new technique to control the speed and direction of light using memory metamaterials whose properties can be repeatedly changed. A metamaterial is a structure engineered from a variety of substances that, when put together, yield optical properties that do not exist in nature. In this experiment, the metamaterial in use is a hybrid device made of split ring resonators (SRRs) – gold rings with a chunk taken out of one side – over a thin layer of vanadium dioxide (VO2). By applying a pulse of electricity to this SRR-VO2 hybrid, the physicists can create a temperature gradient along the device that selectively changes the way the material interacts with light – changing the light’s speed and direction, for example, or how much light is reflected or absorbed at each point along the device. The material even “remembers” these changes after the voltage is removed. In a paper published in the AIP’s Applied Physics Letters, the UCSD team – in collaboration with researchers from Duke University in Durham, N.C., and the Electronics and Telecommunications Research Institute (ETRI) in South Korea – applied this gradient-producing principle to show that it’s possible to modify the way that light interacts with a metamaterial on the order of a single wavelength for 1-terahertz-frequency radiation. Being able to tune metamaterial devices at this level of precision – repeatedly, as required, and after the metamaterial has been fabricated – opens the door to new techniques, including the ability to manufacture Gradient Index of Refraction (GRIN) devices, that can be used for a variety of imaging and communication technologies.
Article: “Reconfigurable Gradient Index Using VO2 Memory Metamaterials” is published in Applied Physics Letters.
Authors: M.D. Goldflam (1), T. Driscoll (1, 2), B. Chapler (1), O. Khatib (1), N. Marie Jokerst (2), S. Palit (2), D.R. Smith (2), Bong-jun Kim (3), Gi-wan Seo (4), Hyun-Tak Kim (3, 4), M. Di Ventra (1), and D.N. Basov (1).
(1) University of California, San Diego
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