New technique to build 'invisible' materials with light

Researchers have developed a new method of building materials using light that could bring sci-fi concepts such as cloaking devices one step closer to reality.
The technique which researchers have developed for constructing materials with building blocks a few billionths of a metre across can be used to control the way that light flies through them, and works on large chunks all at once.
The key to any sort of 'invisibility' effect lies in the way light interacts with a material. When light hits a surface, it is either absorbed or reflected, which is what enables us to see objects.

However, by engineering materials at the nanoscale, it is possible to produce 'metamaterials': materials which can control the way in which light interacts with them.
Light reflected by a metamaterial is refracted in the 'wrong' way, potentially rendering objects invisible, or making them appear as something else.
Metamaterials have a wide range of potential applications, including sensing and improving military stealth technology.

The technique developed by researchers at Cambridge University involves using unfocused laser light as billions of needles, stitching gold nanoparticles together into long strings, directly in water for the first time.
These strings can then be stacked into layers one on top of the other, similar to Lego bricks.

In order to make the strings, the researchers first used barrel-shaped molecules called cucurbiturils (CBs).
The CBs act like miniature spacers, enabling a very high degree of control over the spacing between the nanoparticles, locking them in place.
In order to connect them electrically, the researchers needed to build a bridge between the nanoparticles. Conventional welding techniques would not be effective, as they cause the particles to melt.
"It's about finding a way to control that bridge between the nanoparticles. Joining a few nanoparticles together is fine, but scaling that up is challenging," said Dr Ventsislav Valev of the University's Cavendish Laboratory, one of the authors of the paper in the journal Nature Communications.

The key to controlling the bridges lies in the cucurbiturils: the precise spacing between the nanoparticles allows much more control over the process.
When the laser is focused on the strings of particles in their CB scaffolds, it produces plasmons: ripples of electrons at the surfaces of conducting metals.
These skipping electrons concentrate the light energy on atoms at the surface and join them to form bridges between the nanoparticles.
Using ultrafast lasers results in billions of these bridges forming in rapid succession, threading the nanoparticles into long strings, which can be monitored in real time.