Berkeley (CA) – The secret that allows objects to become completely invisible to the human eye has been one of the big quests of science, but remains a mystery can only be achieved through special effects in science-fiction movies. U.S. researchers now were able to actually build and define the first 3D cloaking device that can, in a limited way, achieve 3D cloaking ability.
The foundation of cloaking research may sound relatively simple, but is only enabled by a capability to manufacture smaller and smaller structures of certain material. It has been known for some time that successful cloaking is based on metamaterials with negative refraction: Such materials, composite materials with extraordinary capabilities to bend electromagnetic waves, need to be built with repeating nano-structures that are smaller than the wavelength of light they are exposed to.
Using a simple example, negative refraction applied to a fish swimming in the water would make the fish appear to be “flying” above the water, rendering the fish in the water invisible. The problem is to find the “secret water” that manipulates the electromagnetic wavelength being used.
In the past, manipulation of wavelengths has been successful, however, only in the 2D space and in the longer microwave band of 1 mm to 30 cm, which compares to a wavelength of light visible to the human eye of 400 nm (violet and purple light) to 700 nm (deep red light). Infrared light wavelengths are longer, measuring from about 750 nm to 1 mm. (A human hair is about 100,000 nanometers in diameter.)
In a paper published in Nature, the UC Berkeley researchers described an approach that uses stacked alternating layers of silver and non-conducting magnesium fluoride. The scientists cut nanoscale-sized fishnet patterns into the layers to create a bulk optical metamaterial and found that at wavelengths as short as 1500 nanometers, the near-infrared light range, a negative index of refraction. In another paper, to be published in Science, researchers claim to have observed a negative refraction from red light wavelengths as short as 660 nanometers. If in fact true, this observation is the first demonstration of bulk media bending visible light backwards.
The metamaterial described in the Science paper is composed of silver nanowires grown inside porous aluminum oxide. Although the structure is about 10 times thinner than a piece of paper - a wayward sneeze could blow it away - it is considered a bulk metamaterial because it is more than 10 times the size of a wavelength of light. "The geometry of the vertical nanowires, which were equidistant and parallel to each other, were designed to only respond to the electrical field in light waves," said Jie Yao, a student in UC Berkeley's Graduate Program in Applied Science and Technology and co-lead author of the study in Science. "The magnetic field, which oscillates at a perpendicular angle to the electrical field in a light wave, is essentially blind to the upright nanowires, a feature which significantly reduces energy loss."
Jason Valentine, UC Berkeley graduate student and co-lead author of the Nature paper, explained that each pair of conducting and non-conducting layers forms a circuit, or current loop. Stacking the alternating layers together creates a series of circuits that respond together in opposition to that of the magnetic field from the incoming light. "Natural materials do not respond to the magnetic field of light, but the metamaterial we created here does," Valentine said. "It is the first bulk material that can be described as having optical magnetism, so both the electrical and magnetic fields in a light wave move backward in the material."
No matter how you look at the results of both research efforts, they are breathtaking. However, James Bond-like applications are not in reach yet. The metamaterials described in both papers are made of metal and are fragile, which means that you should not count on that cloaking suit to become a reality anytime soon. Developing a way to manufacture these materials on a large scale will also be a challenge, the researchers said.
Nevertheless, the researchers said achieving negative refraction in an optical wavelength with bulk metamaterials is an important milestone in the quest for such devices. We have no doubt that these results are likely to attract much more interest in the future and that supporters of the research, which includes the National Science Foundation, the U.S. Army Research Office and the U.S. Air Force Office of Scientific Research are happy with what has been achieved so far.