Researchers at the Pennsylvania State University in the US have developed “illusion coatings” that can hide things by making them appear like something else.
The so-called “illusion coatings” – flexible, light-weight materials – could help soldiers or spies hide antennae and sensors from view while still allowing the devices to scan the outside world.
“Previous attempts at cloaking using a single metasurface layer were restricted to very small-sized objects,” said Zhi Hao Jiang, postdoctoral fellow in electrical engineering, Penn State. “Also, the act of cloaking would prevent an enclosed antenna or sensor from communicating with the outside world.”
“The demonstrated illusion/cloaking coating is a light-weight two-dimensional metasurface, not a bulky three-dimensional metamaterial,” said Douglas Werner from the Pennsylvania State University.
Jiang and Douglas H. Werner, John L. and Genevieve H. McCain Chair Professor of Electrical Engineering, developed a metamaterial coating with a negligible thickness that allows coated objects to function normally while appearing as something other than what they really are, or even completely disappearing.
The coating is made of thin sheets of a composite material composed of glass fibres and Teflon. These were covered with patterns of copper stripes that interacted with the composite material to scatter radio waves in a very precise way. The stripes are one third the thickness of a human hair.
Depending on the copper patterns used, the coating can make a copper antenna or sensor appear to be silicon or Teflon.
Another application of this material would be to protect objects from other emitting objects nearby while still allowing electromagnetic communication between them. This was not possible with the conventional transformation optics-based cloaking method because the cloaking mechanism electromagnetically blocked the cloaked object from the outside, but this new coating allows the object surrounded to continue working while being protected. In an array of antennae, for example, interference from the nearby antennas can be suppressed.
The metasurface coating consists of a series of copper, geometric patterns placed on a flexible substrate using standard lithographic methods currently used to create printed circuit boards. Each illusion coating must be designed for the specific application, but the designs are optimized mathematically. This method of manufacture is low cost and well established.
Another advantage of this method is that it works not only for direct hits by radio frequency waves incident normally on the coated object, but also continues to operate properly within a 20 degree field of view, making it a better angle-tolerant shield than previous attempts that employed bulky metamaterials. Currently, the metasurface coatings only work on narrow bands of the spectrum for any application, but can be adapted to work in other bands of the electromagnetic spectrum including the visible spectrum.
“We haven’t tried expanding the bandwidth yet,” said Werner. “But the theory suggests that it should be possible and it will probably require multiple layers with different patterns to do that.”
Illusion coatings could be used for things other than hiding. They could enhance the way radio frequency ID tags work or could redistribute energy in different, controlled patterns making things more visible rather than less visible. The materials shielding ability can also be used to protect any type of equipment from stray or intentional electromagnetic interference.
The researchers published their findings in the journal Advanced Functional Materials.
Jiang, Z. H. and Werner, D. H. (2014), Quasi-Three-Dimensional Angle-Tolerant Electromagnetic Illusion Using Ultrathin Metasurface Coatings. Adv. Funct. Mater., 24: 7728–7736. doi: 10.1002/adfm.201401561