Invisibility cloaks, thin lenses as powerful as a complete microscope and much better solar cells. These are just a few examples of the applications of optical metamaterials. Manufacturing is unfortunately difficult and expensive, but this new discovery may well change that.
Metamaterial requires difficult editing
The first metamaterial ever was designed not for light but for radio waves that have a much longer wavelength, namely centimeters. This material was made of C-shaped pieces of metal and wire and soldered together in the form of a honeycomb the size of a table. While this was a lot of work, it can be done by people. That's because the wavelength of radio waves is so great. Parts of a few millimeters are also sufficiently small to be able to serve as metamaterial. Light is a much bigger challenge. Metamaterials that can manipulate light must have structures much smaller than the wavelength of light, 400 to 700 nanometers. This is smaller than most bacteria. This is impossible for humans and nanotechnology has not yet reached the point where we can easily achieve this with devices, other than on a flat surface. Our best lithography techniques - used for the latest chips and processors - come in at around 20 nanometers.
To deform light waves requires structures that are a few atoms wide
In order to develop metamaterials for visible light, even slightly smaller structures have to be developed. We are talking about structures that are a few dozen atoms wide (an atom has, depending on the type, about 0.06 to 0.6 nanometers in diameter). This is much more difficult to achieve, especially if you do not want a surface, but a three-dimensional metamaterial. In itself, the components are easy to make using all kinds of well-known bulk processes from the chemical industry. The problem is putting them together. We don't have nanoscale robot hands. It is true that we can drag atoms, for example with the tip of a scanning tunneling electron microscope, but that is an extremely difficult and time-consuming job. No wonder Harry Potter is so careful with his invisibility cloak, fans of the famous book series by author JK Rowling will say.
DNA structure assembles itself
Up to now. Anton Kuzyk from Munich University of Technology and some colleagues have found a way to crack this problem. The technique is called DNA origami and means that gold particles are covered with short pieces of single DNA. At the same time, the matching pieces of DNA are built into a larger DNA structure. When the DNA puzzle pieces fall into place, the gold particles are entrained and an atomic structure is created that, with careful design, can have almost any shape.
Kuzyk and his colleagues have used this process to bind nine nanoparticles of gold only ten nanometers in diameter to pieces of DNA. With this they formed the steps of a spiral staircase on a nanoscale. More good news is that the process is self-organizing. In one solution, they can actually fabricate millions of these nano-spiral staircases. The process is also surprisingly accurate: about eighty percent of the stairs are the perfect shape.
The result is that a liquid is created with the optical properties of the spiral nanoparticles. Light particles, photons, consist of an electric field that generates a magnetic field perpendicular to the electric field, which in turn generates a new electric field, opposite to the first, and so on. Photons therefore appear to rotate on their axis. Light that rotates like the spiral staircase (polarized light) is absorbed. Namely, it is converted into plasmons, surface vibrations in the gold particles. Light that turns against the spiral staircase, springs the dance. This effect is circular dichroism and this is exactly what the researchers observed. They can manipulate the effect by spiraling the DNA in the opposite direction and also by applying a layer of silver to the gold nanoparticles. This changes the frequency of light that the spirals are sensitive to. This is the first time anyone has succeeded in fabricating an optical metamaterial on a large scale.
The liquids can even be converted into solids after using crystallization techniques (well known in the chemical industry). This is not easy, but the first steps have been taken. We will hear more about this.
Construction site on which houses assemble themselves
In this way materials with a negative refractive index could be made. If you put a straw in a liquid with a negative refractive index, it appears to be cut off in a strange way (right glass in the picture). With this you can make Potter-like invisibility cloaks or microscopes that fit in a wallet. The importance of the discovery does not end here. In fact, this technique provides a self-assembling nanostructure. Imagine a construction site where you give each part of a house a certain code, after which it assembles itself. That's what's happening here at the nanoscale. With this we would have radically solved the nano construction problem.