The concept of transformation optics, used in the design of so-called "invisibility cloaks," has now been exploited to make something more visible. A team from the National University of Singapore, Masaryk University (Brno, Czech Republic), and the University of St. Andrews (St. Andrews, Scotland) came up with a singular solution: the perfect cat's eye.
Sometimes it is important to be seen, in particular in traffic at night. Cat's eyes are lined up to mark roads and cyclists wear coats that shine when they are caught in the light of cars. Cat's eyes work just like their namesakes, appearing to glow in the dark: at the back of a cat's eye sits a mirror (or, for a cat, a reflective retina) that sends light back from where it came from. This works well for light from some angles, but not for all.
Physicists have wondered whether it would be possible to design a cat's eye that back-reflects light from all angles. As it turned out, this cannot be done with mirrors, but could be with singularities, if one could make them.
From impossible to possible
In a transformation-optics device, singularities are points at which the optics of the material need to have properties that cannot realistically exist. For example, an optical material like glass changes the speed of light and, in turn, may change its direction; at a singularity the speed of light should appear to become infinite or zero, which is impossible. However, according to ideas by Tomas Tyc from Masaryk University and Ulf Leonhardt from the University of St. Andrews, singularities can be faked.
The two physicists used concepts from transformation optics to transmute the singularity into something more workable. Transformation optics is the key to making invisibility devices, as Sir John Pendry from Imperial College and Leonhardt put forward in 2006. So far, several prototypes of cloaking devices have been developed.
Ma Yun Gui and Ong Chong Kim from the National University of Singapore, closely collaborating with Tyc and Leonhardt, built a perfect retroreflector for microwave radiation.1 (As those familiar with the design of invisibility cloaks know, the use of microwaves rather than visible light allows prototyping of transformation optics on a macroscopic scale.) The team started work on this project in September 2008 when Leonhardt began a half-year term as visiting professor in Singapore. The device was finished and tested in January 2009.
The resulting retroreflectors can be applied for tracking and signaling with microwaves; perhaps someday (as is the dream for large-scale invisibility cloaks), they could also be made to work at visible wavelengths.
1. Yun Gui Ma et al., Nature Materials, published online: 28 June 2009, doi:10.1038/nmat2489