A new method of
building materials using light, developed by researchers at the University of
Cambridge, could one day enable technologies that are often considered the
realm of science fiction, such as invisibility cloaks and cloaking devices.
Although
cloaked starships won't be a reality for quite some time, 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. Details are
published today (28 July) in the journal Nature Communications.
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. However, before
cloaking devices can become reality on a larger scale, researchers must
determine how to make the right materials at the nanoscale, and using light is
now shown to be an enormous help in such nano-construction.
The technique
developed by the Cambridge team 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. The method makes it
possible to produce materials in much higher quantities than can be made
through current techniques.
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," said Dr Ventsislav Valev of the
University's Cavendish Laboratory, one of the authors of the paper.
"Joining a few nanoparticles together is fine, but scaling that up is
challenging."
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.
"We have
controlled the dimensions in a way that hasn't been possible before," said
Dr Valev, who worked with researchers from the Department of Chemistry and the
Department of Materials Science & Metallurgy on the project. "This
level of control opens up a wide range of potential practical
applications."
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