Scientists make ultra-thin invisibility cloak fit for Harry Potter

Scientists at the US Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley have devised an ultra-thin invisibility ‘skin’ cloak that can conform to the shape of an object and conceal it from detection with visible light. Although this cloak is only microscopic in size, the principles behind the technology should enable it to be scaled-up to conceal macroscopic items as well.

Working with brick-like blocks of gold nanoantennas, the Berkeley researchers fashioned a ‘skin cloak’ barely 80nm in thickness that can be wrapped around a three-dimensional object about the size of a few biological cells. By shaping this skin cloak with multiple bumps and dents, it can be meta-engineered to reroute reflected light waves so that the object is rendered invisible to optical detection when the cloak is activated.

"This is the first time a 3D object of arbitrary shape has been cloaked from visible light," says Xiang Zhang, director of Berkeley Lab's Materials Sciences Division and a world authority on metamaterials, which are artificial nanostructures engineered with electromagnetic properties not found in nature. "Our ultra-thin cloak now looks like a coat. It is easy to design and implement, and is potentially scalable for hiding macroscopic objects."

Zhang, who also holds the Ernest S. Kuh Endowed Chair at UC Berkeley and is a member of the Kavli Energy NanoSciences Institute, is the corresponding author of a paper describing this research in Science.

It is the scattering of light – be it visible, infrared, X-ray, etc – by its interaction with matter that enables us to detect and observe objects. The rules that govern these interactions in natural materials can be circumvented in metamaterials, whose optical properties arise from their physical structure rather than their chemical composition. For the past ten years, Zhang and his research group have been pushing the boundaries of how light interacts with metamaterials, managing to curve the path of light or bend it backwards, phenomena not seen in natural materials, and to render objects optically undetectable. In the past, their metamaterial-based optical carpet cloaks were bulky and hard to scale-up, and entailed a phase difference between the cloaked region and the surrounding background that made the cloak itself detectable – even though what it concealed was not.

"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," explains co-lead author Xingjie Ni, a former member of Zhang's research group who is now an assistant professor at Penn State University. "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."

In the study, the scientists shone red light onto an arbitrarily-shaped 3D sample object with an area of approximately 1300µm² that was conformally wrapped in the gold nanoantenna skin cloak. The light reflected off the surface of the skin cloak in exactly the same manner as off a flat mirror, making the object underneath it invisible even by phase-sensitive detection. The cloak can be turned ‘on’ or ‘off’ simply by switching the polarization of the gold nanoantennas.

"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," says co-lead author Zi Jing Wong, also a member of Zhang's research group.

The ability to manipulate the interactions between light and metamaterials offers tantalizing future prospects for technologies such as high resolution optical microscopes and superfast optical computers. Invisibility skin cloaks on the microscopic scale might prove valuable for hiding the detailed layout of microelectronic components or for security encryption purposes. At larger scales, invisibility cloaks could prove useful for 3D displays, and possibly even for hiding teenage wizards.

This story is adapted from material from the Lawrence Berkeley National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.