Fishnet invisibility cloak

It is what fans of science fiction and technologists have been waiting for since HG Wells' Invisible Man first came into view - or not, as the case may be. Scientists at the University of California, Berkeley, have engineered three-dimensional metamaterials that can reverse the natural direction of visible and near-infrared light, which could one day lead to an invisibility device.

Metamaterials, are composite materials with the peculiar capacity to bend electromagnetic waves. In 2006, David Smith at Duke University's Pratt School of Engineering and colleagues developed a metamaterial that becomes invisible to microwaves. The metamaterial was comprised of concentric copper rings and wires patterned on to a sheet of fibreglass. Relatively long wavelength microwaves are rerouted around the metamaterial and meet again around the other side. The effect is akin to water flowing around boulders in a stream Anyone downstream of the boulders would not know that the water had ever deviated from its course. A sensor or eye that saw only in microwave light would not see any object "cloaked" by the metamaterial.

Now, two reports appearing in the journals Nature and Science outline developments that could allow a cloaking device to operate at the much shorter electromagnetic wavelengths of near-infrared and visible light. The emerging technology not only appeals to sci-fi fans, but could form the basis for higher resolution optical imaging, electronic circuits built at the nanoscale as opposed to microscale for future high-powered computers and telecommunications.

The common feature of metamaterials in development is that unlike conventional materials, such as glass or water, they have a negative index of refraction, rather than a positive index. Refractive index determines how much electromagnetic waves deviate from their original path as they change velocity when crossing the interface between two materials. The phenomenon results in the classic children's observation of the "bent" fishing pole in the clear pond.

Metamaterials too cause electromagnetic waves to deviate, but in the opposite direction to all known "natural" materials, the fishing pole would appear to jut out of the pond if water had a negative refractive index. Similarly, a fish swimming underwater would appear to be gliding through the air above the pond if this were the case.

"What we have done is take two very different approaches to the challenge of creating bulk metamaterials that can exhibit negative refraction in optical frequencies," explains Xiang Zhang, of UC Berkeley's Nanoscale Science and Engineering Center and head of the research teams that developed the two new metamaterials. "Both bring us a major step closer to the development of practical applications for metamaterials."

For a metamaterial to achieve negative refraction, the spacing between elements of its structural array must be smaller than the wavelength of the electromagnetic being investigated. Microwaves have a wavelength between 1 and 300 millimetres so creating metamaterials for these materials was more straightforward than it would be for microscopic wavelengths.

In the Nature paper, the UC Berkeley researchers stacked together alternating layers of silver and non-conducting magnesium fluoride, and cut nanoscale-sized fishnet patterns into the layers to create a bulk optical metamaterial. At wavelengths as short as 1.5 micrometres (near-infrared), the researchers observed a negative refractive index.

Jason Valentine, co-lead author on the Nature paper, explains 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. He also points out that both materials achieve negative refraction without significant energy losses. In the case of the fishnet material, the strongly interacting nanocircuits allow NIR to pass through the material while expending less energy moving through the metal layers.

"This is the first bulk material that can be described as having optical magnetism," says Valentine, "so both the electrical and magnetic fields in a light wave move backward in the material."

The metamaterial described in Science takes a different approach. It is composed of silver nanowires grown inside porous aluminium oxide. The researchers observed negative refraction at red light wavelengths as short as 660 nm with this material.

"The geometry of the vertical nanowires, which were equidistant and parallel to each other, were designed to only respond to the electric field in light waves," explains UC Berkeley's Jie Yao. "The magnetic field, which oscillates at a perpendicular angle to the electrical field, is essentially blind to the upright nanowires, a feature which significantly reduces energy loss." This nanowire material bends light backwards but does not technically achieve a negative index of refraction.

For most applications of metamaterials, such as nanoscale optical imaging or cloaking devices, both the nanowire and fishnet metamaterials could play a role, the researchers claim. However, they caution that we are not quite ready for the Invisible Man, cloaked Star Trek spaceships or the invisibility cloak of the Harry Potter stories. The metamaterials described here are not only very fragile but manufacturing them on a large scale represents a significant materials challenge.

One very important future application of these invisibility materials is in imaging. "It could be especially useful in the bio-imaging area because traditional high-resolution imaging methods, such as electron microscopy, cause damage to living cells," explains graduent student Jie Yao, "A high-resolution optical imaging method is a better choice since it is usually non-invasive. Metamaterials could help us to do optical imaging beyond the diffraction limit. Moreover, a thin slab of such a metamaterial could serve as a lens to image tiny objects; thick, convex- or concave lenses would not be needed."