Cloaking devices are the stuff of science fiction, and while the reports of revolutionary new invisibility cloaks have a grain of truth to them, these are still just very basic prototypes. However, researchers at the University of Cambridge may have made a breakthrough that will allow us to begin building materials that could one day form the basis for a Star Trek-style cloaking device. The technique relies on lasers to thread chains of nanoparticles together like a needle so we can take advantage of their unique properties on the large scale.
Light really only does one of two things when it hits an object — it can be reflected or absorbed. When you look at a surface, the color you’re seeing is the light that is reflected off its surface. To make something “invisible” all you really need to do is wrap it in something that makes light behave in a different way . There are a variety of promising nanoparticles that may be able to refract light in unusual directions, making them essentially invisible, but assembling those into a metamaterial has been virtually impossible until now.
Metamaterials might be the way to develop cloaking technology, but it has potential in a wide variety of more practical uses. For example, some metamaterials could absorb certain very narrow wavelengths of the electromagnetic spectrum while reflecting others, making them ideal for use in sensors and scientific equipment.
Whatever you’re going to use a metamaterial for, you first have to build one, and the nanoscale construction method developed by the University of Cambridge team has the potential to make that possible. The lasers being used are unfocused and produce billions of tiny needles of light that line up gold nanoparticles in long strings. The strings can then be stacked on top of each other to make larger pieces, eventually leading to a macroscopic scrap of the desired metamaterial.
It’s not as easy as flipping on the laser and watching the nanoparticles string themselves together, though. The particles have to be electrically connected to join up into a proper metamaterial strand. To get everything lined up, the team used spacer molecules called cucurbiturils. These hydrocarbons form a scaffold that keeps the particles just the right distance apart so that ripples of electrons can flow along the surface of the chain. The plasmons concentrate laser energy on the particles and reinforce billions of nanoparticle connections simultaneously.
This research is the first step toward practical applications for a great number of interesting nanoparticles that we didn’t have any way to assemble before. You won’t be invisible tomorrow, but maybe sooner than you think.