Scientists at the Berkeley Lab and UC Berkeley have created an invisibility cloak fit for Harry Potter – if the boy wizard were smaller than a grain of sand.
They 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.
Unlike the fictional character, the ultrathin cloak is real. The achievement is described in a study published in the journal Science. The paper is titled “An Ultra-Thin Invisibility Skin Cloak for Visible Light.”
“This is the first time a 3D object of arbitrary shape has been cloaked from visible light,” said Xiang Zhang, a UC Berkeley professor. “Our ultra-thin cloak now looks like a coat. It is easy to design and implement, and is potentially scalable for hiding macroscopic objects.”
The scientists used brick-like blocks of gold nanoantennas to form the 80-nanometer-thick cloak, which conformed to the arbitrary bumps and dents in the 1,300-square-micrometer sample object. The cloak, a metamaterial engineered to bend light in ways not seen in nature, was able to reflect red light as if it were bouncing off a flat mirror.
The surface of the skin cloak was meta-engineered to reroute reflected light waves so that the object was rendered invisible to optical detection when the cloak is activated.
The principles used to conceal the microscopic object should be possible to scale up to work with macroscopic items.
It is the scattering of light – be it visible, infrared, X-ray, etc., – from 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, the 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 – 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,” says co-lead author Xingjie Ni, 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 Berkeley study, when red light struck an arbitrarily shaped 3D sample object measuring approximately 1,300 square microns in area that was conformally wrapped in the gold nanoantenna skin cloak, the light reflected off the surface of the skin cloak was identical to light reflected 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 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.
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 the macroscale, among other applications, invisibility cloaks could prove useful for 3D displays.
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Last year, “Surrey NanoSystems,” a British Company has produced a strange material so black that it absorbs all but 0.035 per cent of visual light, setting a new world record. This “super black” material is named as “vantablack” by combining first letters of the term “Vertically Aligned NanoTube Arrays”
It is so dark that the human eye cannot understand what it is seeing. Shapes and contours are lost. If it was used to make black dresses, the wearer’s head and limbs might appear to float around a dress-shaped hole.
It can be used for enabling astronomical cameras, telescopes and infrared scanning systems to function more effectively.
