Nano-magnets in metamaterials pave the way to invisibility cloaks
January 6, 2010
A Harry Potter-style invisibility cloak is one more step closer to reality thanks to the work of a research team at the FOM Institute for Atomic and Molecular Physics (AMOLF) in the Netherlands, which has successfully harnessed the magnetic field of light to develop meta-materials that can deflect light in every possible direction.
Metamaterials are a broad class of materials which have been specifically engineered to exhibit peculiar properties, particularly with regard to how light behaves when traveling in them: metamaterials with negative refractive indexes could even reflect light so to make entire objects invisible, and scientists have been making progress towards an invisibility cloak that, a few years back, belonged more to the pages of a fantasy novel than to those of a scientific paper. There's certainly still a long road ahead, but continous advancements have consistently made this dream less and less laughable in the recent past.
As with all electromagnetic waves, light has two oscillating components, an electrical and a magnetic one, meaning that — theoretically — both electricity and magnetism can be used to control how it propagates within an object. However, atoms in standard materials interact only weakly with magnetic fields oscillating over 500THz. Because visible light ranges approximately from 400THz to 800THz, this means we simply can't hope to exploit magnetism here to help us in our quest for invisibility.
But, the AMOLF team found out, the picture changes dramatically when metamaterials are involved. Set to find out more about the behavior of magnetic fields at this threshold frequencies, they engineered very small U-shaped metamaterials called "nano-rings" and studied how they interacted with light.
The electromagnetic field of light, they found, drives electrical charges back and forth the nano-rings, generating an alternating current that transforms each of them into a small electromagnet whose polarity alternates 500 billion times every second. Unlike classical materials, metamaterials show a strong interaction with the magnetic component of light as well as the electrical one.
This particular piece of research is notable not only because it shows that metamaterials look once more like the right way to go, but also because it provides scientists with a mechanism — the nano-rings — to actually manipulate light in them. It also appears that the nano-magnets can influence and can transfer power to each other, which is sure to be a handy variable in follow-up research.
Now, in fact, the team will need to figure out how to perfect this technique to direct light appropriately around an object containing these nano-rings. If this is achieved, the world could soon see — or, rather, not see — its first invisibility cloak.
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