Invisibility: Just One Facet of the Wide, Wild World of Optical Illusions

Cat's eyes look creepy because, at certain sudden angles, they reflect light directly back at you, the viewer. The ever-popular question of invisibility is wrapped up in that ability to reflect light at certain directions, commonly known as the refractive index. This week, two physicists from the UK and the Czech Republic published their method for designing optical singularities — in other words, refractive indices of either zero or infinity that could produce the wildest optical illusions we've ever seen. And that's just one of the ideas presented in the latest open-access issue of the New Journal of Physics on cloaking and transformation optics.

You may remember the metamaterials that will make you invisible or the metamaterials that can act as cloaking devices; both of those findings are part of a great body of research on creating substances that transform the behavior of light. Citing Arthur C. Clarke, who said that "any sufficiently advanced technology is indistinguishable from magic," the New Journal of Physics explains the theory and the practice behind what they call "pure and applied magic":

Transformation optics gathers an unusual mix of scientists, ranging from practically-minded engineers to imaginative theoretical physicists and mathematicians or hybrids of all three. The engineers have been developing new materials with extraordinary electromagnetic properties, from materials for microwaves, to be used in radar or wireless technology, to materials for terahertz radiation and visible light. These materials typically are composites—they consist of artificial structures much smaller than the wavelength that act like man-made atoms, apart being much larger in size. The properties of these artificial atoms depend on their shapes and sizes and so they are tunable, in contrast to most real atoms or molecules. This degree of control is what makes these materials—called metamaterials—so interesting. Such new-won freedom invites the other side of the spectrum of scientists, the theorists, to dream. Just imagine there are no practical limits on electromagnetic materials—what could we do with them? One exciting application of metamaterials has been Veselago's idea of negative refraction, dating back to the 1960s. Metamaterials have breathed life into Veselago's idea, culminating in recent optical demonstrations. Another application is cloaking, developing ideas and first experimental demonstrations for invisibility devices. It turns out that both negative refraction and cloaking are examples where materials seem to transform the geometry of space.

Scientists Tomáš Tyc and Ulf Leonhardt — of Masaryk University, the University of St. Andrews, and the National University of Singapore — recognized the difficulty of constructing an actual device that contained an optical singularity. Achieving a refractive index of zero or infinity is possible in theoretical thought experiments, but understanding the visual reality of such a singularity is far more difficult. They found a way to mathematically transmute the singularity equations to make them more practical for actual optical devices; soon, it might be possible to understand and use the effects of these unconventional refractive indices.

In the same issue, Sergei A. Tretyakov, Igor S. Nefedov, and Pekka Alitalo of the Helsinki University of Technology introduce their field-transforming metamaterials — substances that can transform any electromagnetic fields surrounding them in a specified way. This has applications not only for invisibility cloaks, they wrote, but also for the creation of perfect lenses or artificial black holes. Yup, you read that right. This issue is full of research on the engineered direction of electromagnetic waves (i.e. not just light); when it comes to future possibilities for applying this research, all of the authors seem to be saying "you name it!"

We really aren't limited to invisibility capes, or even Create-Your-Own-Black-Hole (just add water). J.B. Pendry and Jensen Li, working at Imperial College of London and the University of California in Berkeley, applied metamaterial research to acoustics. Their contribution to the New Journal of Physics focuses on fashioning what they call a "broadband acoustic cloak" — they claim that with the right materials, sound waves "can be controlled and directed almost at will." And so visual invisibility meets acoustic invisibility: Once these scientists get it all right, they might just throw in the towel and become superheroes before they ever get around to explaining their research to us.

Cat's eye image from Wikimedia Commons.

Using invisibility to increase visibility [via EurekAlert]
Focus on cloaking and transformation optics [New Journal of Physics]