Department of Energy’s Office of Science through its Basic Energy Sciences program and by the U.S. This research was funded in part by the U.S.
“Our next goal is to realize a cloak for all three dimensions, extending the transformation optics into potential applications.” “In this experiment, we have demonstrated a proof of concept for optical cloaking that works well in two dimensions” says Zhang. He is also optimistic that with more precise fabrication this all dielectric approach to cloaking should yield a material that operates for visible light - in other words, true invisibility to the naked eye. However, because of its all dielectric composition and design, Zhang says the cloak is relatively easy to fabricate and should be upwardly scalable. Right now the cloak operates for light between 1,400 and 1,800 nanometers in wavelength, which is the near-infrared portion of the electromagnetic spectrum, just slightly longer than light that can be seen with the human eye. It demonstrated invisibility at variable angles of light incident. In the experiments reported in Nature Materials, the cloak was used to cover an area that measured about 3.8 microns by 400 nanometers. A carefully designed pattern of holes - each 110 nanometers in diameter - perforates the silicon, transforming the slab into a metamaterial that forces light to bend like water flowing around a rock.
INVISIBLE CLOAK BERKELEY FREE
The cloak was demonstrated in a rectangular slab of silicon (250 nanometers thick) that serves as an optical waveguide in which light is confined in the vertical dimension but free to propagate in the other two dimensions. The new cloak created by Zhang and his team is made exclusively from dielectric materials, which are often transparent at optical frequencies. Says Zhang, “Even with the advances that have been made in optical metamaterials, scaling sub-wavelength metallic elements and placing them in an arbitrarily designed spatial manner remains a challenge at optical frequencies.” While metallic metamaterials have been successfully used to achieve invisibility cloaking at microwave frequencies, until now cloaking at optical frequencies, a key step towards achieving actual invisibility, has not been successful because the metal elements absorb too much light. With these metallic metamaterials, Zhang and his group demonstrated that light can be bent backwards, a property unprecedented in nature. They constructed one material out of an elaborate fishnet of alternating layers of silver and magnesium fluoride, and another out of silver nanowires grown inside porous aluminum oxide. Previous work by Zhang and his group with invisibility devices involved complex metamaterials - composites of metals and dielectrics whose extraordinary optical properties arise from their unique structure rather than their composition. “Our optical cloak not only suggests that true invisibility materials are within reach, it also represents a major step towards transformation optics, opening the door to manipulating light at will for the creation of powerful new microscopes and faster computers.” “We have come up with a new solution to the problem of invisibility based on the use of dielectric (nonconducting) materials,” says Zhang. Shining a beam of light on the bulge shows a reflection identical to that of a beam reflected from a flat surface, meaning the object itself has essentially been rendered invisible.
While the carpet itself can still be seen, the bulge of the object underneath it disappears from view. A team led by Xiang Zhang, a principal investigator with Berkeley Lab’s Materials Sciences Division and director of UC Berkeley’s Nano-scale Science and Engineering Center, has created a “carpet cloak” from nanostructured silicon that conceals the presence of objects placed under it from optical detection.
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