Real invisibility threads would be fit for an emperor


FORGET the imaginary filaments used to weave the clothes that fooled the fabled emperor, can we make real invisible threads instead? Combining techniques used to produce light-bending metamaterials with those used to make optical fibres might just do the trick.

Alessandro Tuniz at the University of Sydney's Institute of Photonics and Optical Science in Australia is one of many physicists interested in the optical metamaterials that are being fashioned into "invisibility cloaks" in labs across the world. These metamaterials incorporate components much smaller than the wavelength of light, which allows them to control the light waves and gives them optical properties beyond those of conventional materials.
However, as physicists have discovered, fabricating metamaterials using components small enough to manipulate the sub-micrometre wavelengths of visible light is no mean feat. To avoid that problem, Tuniz's colleagues Boris Kuhlmey, Simon Fleming and Maryanne Large have suggested an elegant way to shrink a larger metamaterial-like structure down to a size capable of controlling visible light: assemble standard glass rods and metal tubes into a cylinder, heat the assembly until it softens, and draw it into a long thin fibre. The process preserves the shapes of internal structures, but shrinks them down to the nanoscale needed to control visible light, and the resultant metamaterial is in the form of a thread that is thin enough to be flexible, like an optical fibre. So far, Tuniz and colleagues have produced 10-micrometre-thick threads.
Now, the researchers have used a computer model to design an invisible version of their thread. To achieve that, the thread must be just 1 micrometre thick - the metamaterial absorbs some light and so would appear dark if it was any thicker. Their calculations suggest that the thread would be invisible if seen from the side - rather than end on - in polarised light. Fortunately for worried emperors, the model shows that, like other optical metamaterials, the fibre's optical properties depend strongly on wavelength. An emperor might wear threads that make him appear naked in red light, says Tuniz, but "if you shine green light at him, you see the threads completely" (Optics Express, vol 18, p 18095).

"The theory is sound," say Asger Mortensen of the Technical University of Denmark in Lyngby and Min Yan of the Royal Institute of Technology in Stockholm, Sweden, who have stretched metamaterials into optical fibres which can transport infrared light. Precise control over the threads' invisibility will be tricky, they warn. Tuniz is now going to try to make the 1-micrometre threads.