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EETE FEB 2014

Magnetic ‘lens’ aids wireless power transfer By Peter Clarke Researchers from Pratt School of Engineering at Duke University (Durham, North Carolina) have demonstrated a “magnetic lens” that supports the transfer of wireless power over distances much larger than the traditional limit, which was roughly equivalent to the size of the transmitter and receiver. A team of researchers in Duke’s Pratt School of Engineering worked with Toyota Research Institute of North America to create an array of hollow cubes to act as a lens for the transfer of energy using low-frequency magnetic fields. The walls of these cells are etched with copper spirals and the geometry of these coils and their repeating nature form a metamaterial that interacts with magnetic fields in such a way that the fields are transmitted and confined into a narrow cone in which the power intensity is much higher than the conventional pattern. The resulting lens, in some ways analogous to an optical Fresnel lens, focuses a magnetic field emanating from one power coil onto its twin nearly a foot away, inducing an electric current in the receiving coil. “For the first time we have demonstrated that the efficiency of magneto-inductive wireless power transfer can be enhanced over distances many times larger than the size of the receiver and transmitter,” said Yaroslav Urzhumov, assistant research professor of electrical and computer engineering at Duke University, in a statement. “If your electromagnet is one inch in diameter, you get almost no power just three inches away,” said Urzhumov. “You only get about 0.1 percent of what’s inside the coil.” But with the superlens in place, he explained, the magnetic field is focused nearly a foot away with enough strength to induce noticeable electric current in an identically sized receiver coil. Urzhumov said that in future experiments researchers would investigate a dynamically tunable lens that could perform beamsteering. This would allow mobile devices to be charged as they move around a room. “The true functionality that consumers want and expect from a useful wireless power system is the ability to charge a device wherever it is – not simply to charge it without a cable,” said Urzhumov. Quantum dots from Warsaw By Nick Flaherty researchers at the Faculty of Physics at the University of Warsaw have used cobalt for the first time surrounded by cadmium telluride (CdTe) and say they may be able to improve the performance by a factor of 10. In quantum dots where tellurium is replaced by the lighter selenium, researchers observed that the duration for which information was remembered increased by an order of magnitude. This finding suggests that using lighter elements should prolong the time quantum dots containing single magnetic ions store information, perhaps even by several orders of magnitude. “We have demonstrated that two quantum systems that were believed not to be viable in fact worked very effectively. This opens up a broad field in our search for other, previously rejected combinations of materials for quantum dots and magnetic ions,” said Dr Wojciech Pacuski at the Institute of Experimental Physics at the University of Warsaw (FUW). Researchers are able to control the behaviour of individual atoms by situating them within special semiconductor structures – this is the method used to form quantum dots that contain single magnetic ions. Until recently, only two variants of such structures were known. However, physicists from the have successfully created and studied two completely new types of the structures. “Quantum dots are semiconductor crystals on a nanometre scale. They are so tiny that the electrons within them exist only in states with specific energies. As such, quantum dots exhibit similar characteristics to atoms, and – just like atoms – they can be stimulated with light to reach higher energy levels. Conversely, this means they emit light as they return to states with lower energy levels,” says Prof. Piotr Kossacki at FUW. The University laboratory creates quantum dots using molecular beam epitaxy and by carefully selecting materials and experimental conditions, the atoms assemble into quantum dots. “Atoms with magnetic properties disrupt the energy levels of electrons in a quantum dot, which affects how they interact with light. As a result, the quantum dot becomes a detector of such an atom’s state. The relationship also works the other way: by changing energy states of electrons in quantum dots, we can affect the respective magnetic atoms,” said Michał Papaj, a student at the UW Faculty of Physics. The most powerful magnetic properties are observed in manganese atoms stripped of two electrons (Mn2+). In experiments conducted thus far, the ions have been mounted in quantum dots made of cadmium telluride (CdTe) or indium arsenide (InAs). “It was commonly believed that other magnetic ions, such as cobalt (Co2+), cannot be used in quantum dots. We decided to verify this, and nature gave us a pleasant surprise: the presence of a new magnetic ion turned out not to destroy the properties of the quantum dot,” says Jakub Kobak, doctoral student at the University of Warsaw. 8 Electronic Engineering Times Europe February 2014 www.electronics-eetimes.com


EETE FEB 2014
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