CNT INVISIBILITY CLOAK

Metamaterials are no longer the only option for invisibility. For years that technology has been the source of hope for some future invisibility cloak, but so far only certain frequencies of light can be affected. Carbon nanotubes, on the other hand, can be used to bend all frequencies of visible light right now. Researchers at the University of Texas have used a sheet of carbon nanotubes to artificially create the mirage effect.

The mirage effect can often be seen on hot days when the sun heats paved roads. Sometimes you can see a puddle of water far down a road, but you know it is just an illusion. How does it happen? The air in contact with the pavement is much hotter than surrounding air. This temperature difference bends light so that instead of seeing the road, you see a warped image of the sky (which closely resembles reflecting water). Wherever there is an extreme temperature difference, there will be this mirage effect. The metallic bodies of cars can effectively heat the air around them, but carbon nanotubes (CNTs) are better. When an electric current passes through a sheet of CNTs, heat is quickly produced and dissipated. In the demonstration video, water was used instead of air. Like flicking a switch, the mirage effect can be activated and deactivated at will. This is a very convenient quality of the technology.

The CNTs seem a preferable alternative to metamaterials. The main drawbacks are excessive heat and possibly limited angles of cloaking. On the positive side, CNTs are much more durable than metamaterials and can bend all colors at once. Metamaterials have difficulty bending more than one color of light at a time. Indeed, it is hard to make a metamaterial that works with visible light at all. Rather than a small sheet, imagine a blanket of CNTs. With the press of a button that blanket becomes a personal wall of invisibility. With some advanced insulation on the inner side, the blanket might be able to hide a person, small car, or other item of interest. The main obstacles to this dream are the high cost of producing CNTs and supplying the energy necessary for what is essentially a high-tech electric heater.

THE SCIENCE OF BEING UNSEEN

Have you ever seen a photograph of DNA? Most likely not. The biological blueprint molecule is so small that no optical lens can possibly provide enough magnification for our eyes to see it. In the future, however, metamaterials might change that. Generally speaking, metamaterials are composite structures that interact with waves in ways that are dependent on their structure. For example imagine a sheet of fiberglass circuit board imprinted with an array of tiny copper rings. The rings are at most a few millimeters in diameter. Such an array would bend electromagnetic waves that go through it with a certain wavelength. The wavelength is dependent on the size of the rings. Smaller rings affect radiation with smaller wavelengths (higher frequencies). When such a structure is arranged in concentric circles, it can be used to bend light around the center. As light passes through the layers of the metamaterial it is redirected around the center like a river around a boulder. An observer outside the metamaterial would not see the space in its center or any object placed there. For certain wavelengths of light an object would be invisible. At first this “cloaking” technology only worked for microwaves, but, since then, considerable effort has been devoted to developing metamaterials that work at smaller wavelengths. To have an invisibility effect in the visible light range would require much smaller structures, such as the copper rings, to the point where nanoscale fabrication techniques are required. Nanotechnology is neither cheap nor easy, but it is within our means. Barring the cliché mention of certain wizards, there is a definite possibility that someday we may be able to hide objects from plain sight, thanks to mind-boggling technology.

What does that have to do with DNA? Well, metamaterials can be designed to manipulate light in many ways. Conventional lenses have fundamental limits to their magnification capabilities. Metamaterials effectively break conventional rules and extend the limits of optical magnification to the point where molecules can be observed. A microscope equipped with a “superlens” would allow you to view DNA molecules and other similarly sized objects.

Metamaterials can also greatly improve the functionality of radio antennas, making them smaller and more efficient. With sufficient ability to manipulate light, metamaterial structures could one day replace the circuit boards we know so well. Replacing electric signals with light pulses, computers would work faster and produce less heat. There are claims that metamaterials could even function as artificial noses for computers to sniff out bombs.

As I said earlier, generally, metamaterials manipulate waves. Meaning, this field of technology is not limited to electromagnetic radiation; it can also apply to vibrations in matter. Think of the invisibility idea applied to sound; you could have a perfectly soundproof room. Sound waves can be redirected like light waves. How about water waves? Perhaps a very large metamaterial structure could “cloak” coastal areas from rough sea storms. Or ground waves? In the future, maybe buildings will be able to safely redirect seismic vibrations around them. It would seem the only limitations on metamaterials are our imaginations and our small-scale engineering abilities.