If there was one superpower that we were allowed to possess, it would definitely be invisibility. From Star Trek to Harry Porter, invisibility cloaks have largely remained confined to the world of science fiction and fantasy. Not anymore, though. A team of researchers has developed the technology to create super-thin invisibility skin cloaks which, when wrapped around three-dimensional objects, can render them completely invisible to optical detection.
Recently published in the Science journal, the project is the result of a collaboration between the University of California, Berkeley, and the Lawrence Berkeley National Laboratory (Berkeley Lab) of the United States Department of Energy (DOE). In this research, the scientists used minute blocks of gold antennas to construct a “skin cloak”, around 80 nanometers in thickness. It was then wrapped around a microscopic three-dimensional object, with dents and bumps. The specially-engineered surface of the coat allowed the researchers to reroute the light waves, reflected from the object, thus making it undetectable to the eye. Speaking about the project, Xiang Zhang, the head of the Materials Sciences Division at the Berkeley Lab and a professor at the UC Berkeley, said:
This is the first time a 3D object of arbitrary shape has been cloaked from visible light. Our ultra-thin cloak now looks like a coat. It is easy to design and implement, and is potentially scalable for hiding macroscopic objects.
For the last several years, Zhang and his team have been studying how light interacts with metamaterials, i.e. artificially-crafted materials with properties that are not found in nature. Visibility of an object basically depends on the scattering of light rays, from its surface to our eyes. The unique optical features of the metamaterial skin cloak allows it to redirect the incoming waves, rendering the substance completely invisible. Although based on the same principle, optical carpet cloaks are often too bulky to scale-up, and also induce phase difference between the cloaked material and the surrounding area, thus becoming detectable themselves. Xingjie Ni, a professor at Penn State University and the lead author of the study, explained:
Creating a carpet cloak that works in air was so difficult we had to embed it in a dielectric prism that introduced an additional phase in the reflected light, which made the cloak visible by phase-sensitive detection. Recent developments in metasurfaces, however, allow us to manipulate the phase of a propagating wave directly through the use of subwavelength-sized elements that locally tailor the electromagnetic response at the nanoscale, a response that is accompanied by dramatic light confinement
For the research, the scientists chose an undulating three-dimensional object, measuring around 1,300-square-microns in area. When wrapped around the object, the gold nanoantenna cloak caused the reflected light to look as if it were bouncing right off a flat mirror, thereby making the underlying object invisible to our eyes. According to the team, the technology can be easily scaled-up to conceal normal sized objects, although it might be a few years before that is possible. Zi Jing Wong, another member of Zhang’s team said:
A phase shift provided by each individual nanoantenna fully restores both the wavefront and the phase of the scattered light so that the object remains perfectly hidden.
The technology could potentially be used by the military to make aircraft, vehicles and even individual soldiers invisible. Ni believes that skin cloak could also have some unusual uses, for instance cloaking masks for the face. He said:
All the pimples and wrinkles will no longer be visible… [It] can be made to hide one’s belly.