Back in 2003, the first metamaterial was designed to bend microwaves in ways that ordinary materials can never achieve. The material was made from c-shaped pieces of metal and wires assembled into a kind of honeycomb structure the size of a table top.
Size in is important. The active components in metamaterials and their repeating structure have to be smaller than the wavelength of light they are designed to influence. So the c-shaped pieces of metal–split ring resonators, as physicists call them–were a few millimetres across, big enough to allow the entire structure to be painstakingly assembled by hand.
But that raised an obvious question: how to build similar devices that work for smaller wavelengths.
For infrared light, the active components have to be assembled into a repeating structure on the scale of a few hundred nanometres. That’s easy enough with current lithographic techniques.
For visible light, however, the repeating structure must have a scale of just a few nanometres. That’s much harder to achieve. A number of groups have made such metamaterials using flat carpets of nanorods, as we’ve discussed on this blog. But more ambitious, three-dimensional devices are much more difficult to achieve.
That’s not because the components are hard to make–it’s straightforward to make optically active nanoparticles that are a few nanometres across. The difficult thing is assembling them into 3D structures, like the honeycomb for microwave metamaterials .
Now.Anton Kuzyk at the Technical University of Munich and a few pals have found a way to solve this problem using a technique known as DNA origami.
The idea here is to cover gold nanoparticles with short strands of single strand DNA. At the same time, the complement of this strand is built into a bigger DNA structure called a scaffold. When the nanoparticles are placed in solution with the DNA scaffold, the complementary DNA strands bond together, attaching the nanoparticles to the scaffold.
With careful design, DNA origami techniques can make all kinds of shapes.
Kuzyk and co have used this process to bind nine gold nanoparticles just 10nm across to strands of DNA, forming a helical shape. So the particles form the steps in a tiny spiral staircase.
And because this process is self organising, the assembly is massively parallel. In solution, they can construct millions of spiral staircases at the same time. And the process is surprisingly accurate with up to 80 percent of the helices forming perfectly.
The result is a fluid that takes on the optical properties of the helical nanoparticle structures. Any circularly polarised light travelling through the spiral will excite electronic waves called plasmons on the surface of the gold nanoparticles.
If the handedness of the light matches the handedness of the spiral, then it will be absorbed as it encounters one nanoparticle after another. However, light of the opposite handedness will pass straight through. So the fluid preferentially absorbs light with a particular handedness, a phenomenon called circular dichroism.
That’s exactly what Kuzyk and co observe. And they say they can tune the effect first by changing the handedness of the DNA scaffold and second by growing a layer of silver on the gold nanoparticles, which changes the frequency of light they are sensitive too.
That’s a significant advance. It’s the first time that anybody has achieved the large-scale assembly of an optically active metamaterial or metafluid, as they call it.
The team now hope to create much more ambitious devices. “Using the presented method, strategies can be conceived to create materials of negative refractive index, which in turn would allow for applications such as cloaking or the construction of perfect lenses,” they say.
And that opens up all kinds of opportunities. The DNA origami technique allows much more complex structures to be assembled. And these structures could even by turned into solids using crystallisation techniques. “The potential…is immense,” they say.
It won’t be easy, however. DNA origami is still an emerging technology and good quality crystallisation has never been easy.
But that won’t stop them trying. Kuzyk and buddies say the work paves the way for a brand new class of optically active metamaterials. Expect to see more examples soon.
Ref: arxiv.org/abs/1108.3752: DNA-based Self-Assembly of Chiral Plasmonic Nanostructures with Tailored Optical Response
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