Turning science-fiction idea into fact
Keywords:metamaterial? invisibility cloak design? microwave range? David Schurig? R. Colin Johnson?
The invisibility cloak!it sounds like the stuff of comic-book superheroes. But invisibility cloaks for any type of electromagnetic radiation are something that Duke University postdoctoral fellow David Schurig believes are within grasp. Schurig and the professors directing his research!David Smith at Duke and Sir John Pendry at the Imperial College in London!maintain that by the end of 2007, metamaterials will enable an invisibility cloak that works in the microwave range. Further engineering effort will create such cloaks for other types of light, the researchers say. Metamaterials or engineered composites substitute macroscopic objects for atoms in a giant crystalline-like lattice, enabling the pitch of passive-component arrays to set the wavelengths affected. The design of these component arrays harks back to the first principles of electronics!simple resistor-capacitor-inductor circuits. The electromagnetic waves passing through arrays of tiny resistors, capacitors, inductors and other dielectric materials positioned in free space can be bent down any designer-specified path. In 2000, Smith, who was then at the University of California-San Diego, and his colleagues demonstrated a composite metamaterial that used embedded passive resonators to bend microwaves backward. The circuit elements that the team used were based on the theoretical analysis provided by Pendry and his colleagues in London. Schurig recently sat down with EE Times to describe how the cloaking device works and what electronic engineers (EEs) can do to turn this science-fiction idea into fact. EE Times: Where did the cloaking concept originate? So it was a decade!from 1996 to 2006!between Pendry's seminal paper and the cloaking device research that he collaborated on with you and professor David Smith at Duke? How should EEs imagine these transformations? Of course, the trick is to come up with the material properties that make our normal space, which doesn't have holes in it, behave as if it did. To do that, you take a mathematical description of how the distorted thread pattern differs from the normal weft and weave of the cloth. This is the coordinate transformation. And then you ask: Is there a set of material properties that will give the same form for Maxwell's equations as you find for them under this coordinate transformation? Those material properties make electromagnetic fields in our boring, flat, hole-free space behave as they do in the much more interesting, distorted space. Maxwell's four differential equations summarize all the properties of electromagnetic fields. So I guess you spent the last year learning how to transform them to handle odd-shaped spaces? And that all proved out. Since then, another professor here, Steve Cummer, has done full-wave simulations using Maxwell equation solvers. Those also worked without a hitch, which was very surprising to us. For the light to go around an object and still be in phase on the other side, you would think it would have to exceed the speed of light. I've read about that. The light has to travel a longer distance than usual around the outside of the cloaked region, and since by definition it is already traveling at the speed of light, the extra distance would mean it had to go even faster to stay in phase with the light in the surroundings. But it doesn't really transport energy faster than the speed of light, does it? Doesn't it really just use stored energy built up in the steady state to make one specific frequency's phase fronts exceed the speed of light? Don't these metamaterials work only for long wavelengths, such as microwaves? Are metamaterials available for the visible region today? I can imagine making a traditional lens-and-projection system to route visible light around an object. Why, in principle, do you need metamaterials at all? Is that where the negative index of refraction comes from? Permittivity and permeability are measures of a material's capacity to form electrical and magnetic fields inside it, respectively. Aren't both always positive for natural materials, meaning that the response of the material is always in phase with applied fields? But isn't free space defined as having an index of refraction equal to 1? Metamaterials research has already shown that negative indices are difficult, if not impossible to do with natural materials. But it's also true that indices of refraction that are positive but less than one are just as difficult to achieve in natural materials as negative values for the same reasons. Both cases require a resonance, and in traditional materials, the resonance is seldom where you want it to be. But with engineered composites or metamaterials, you can put the resonance where you want it. A resonance at the working frequency, here at the wavelength you are trying to cloak? The metamaterials I'm familiar with work on microwaves and use circuit boards with split-ring oscillators on them to provide resonance at the working frequency. Is that the setup you are proposing too? Are you going to build a real working cloak? I am guessing that your first real cloaking device will provide a smooth transition effect around a known volume at microwave frequencies. Is that the case? Even that's got real applications. For example, why couldn't you use one of those to cloak a spy satellite to keep radar from finding it? For your initial experiments in the microwave range, how big a volume are you going to try to cloak? So you will start with a volume that measures less than 1m on a side? When do you expect to finish it? What advice do you have for EEs who might be interested in experimenting with cloaking? Does this mark a resurgence of basic electronic-engineering skills, but applied at a different size scale? After perfecting these circuitry principles at the scale of circuit boards for microwaves, do you expect these structures to then migrate down to MEMS? The cloaking effect is always going to be limited in bandwidth-tied to a frequency range. But could there be some way to use three parallel systems like RGB to make a cloak work for the whole range of visible light? If EEs are interested in these cloaking devices, I suggest that they start learning about metamaterials today. That's the technology that is going to make the cloaking device a reality. - R. Colin Johnson
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