The trouble with optical invisibility cloaks


You could be forgiven for thinking that invisibility cloaks are a few R&D dollars away from hitting the high streets. Not so.

While it’s true that a number of high profile cloaks have been built, the best of these work only in the radio and microwave regions of the spectrum and then only in at a single frequency and in two dimensions . So unless you are a flatlander viewing the world through microwave eyes, these cloaks are not much use.

The one claim for an optical invisibility cloak works only inside a strange, exotic material made from gold nanorods and even then over a distance of a only few nanometres.

What an invisibility cloak has to do is steer light around an internal cavity in way that makes it appear to have passed straight through. That’s possible, in theory, if you can design a material in which its permeability and permittivity (the way it interacts with an electromagnetic wave) can be tailored throughout its structure.

That can be done relatively easily at microwave frequencies. The materials in question are extraordinary honeycombs of repeating patterns of split ring resonators and wires. The pattern has to be about the same size as the wavelength of the microwaves– a few centimetres or so.

So why not just make everything smaller to match the wavelength of visible light? The first reason is that we’re talking about a material with a feature size measured in nanometres and that is just beyond what’s possible today. The second is that optical frequencies tend to match the resonant frequency of electrons within these materials. What that means in plain English is that the materials absorb light rather than transmit it (which is why the one demonstration so far has worked only over a distance of a few nanometres before the light was absorbed).

So what to do? One idea is to make the cloaks out of lasing materials which constantly replace the light as it is absorbed. But a better one is to create a material that doesn’t absorb light in the first place. There’s no way to get rid of the electronic resonance that is responsible for absorbing light so the trick is to design a structure in which the resonance can be made to cancel itself out or help to propel the light through the material.

So physicists are desperately examining the properties of various nanostructures to see whether they might have the properties that fit the bill.

Today, Andrea Alu and Nadar Enghet at the University of Pennsylvania in Philadelphia, publish an analysis of just such a metamaterial made of nanoparticles arranged in a ring, as shown above. They say that this material gives “cleaner magnetic dipole response”.

Unfortunately, they are less clear over whether they’ve hit the jackpot with regards optical invisibility. In fact they can’t be sure whether this will have the required properties at optical frequencies or not.

The trouble is that it’s not possible to know the bulk properties of a material made from a particular nanostructure without some heavyweight calculating. And choosing which structure to investigate is little more than guesswork at the moment. As Alu and Enghet show with this work.

Back to the drawingboard, I’d say. Looks as if they’ll need to kiss a few more frogs before they find their prince.

Ref: Dynamical Theory of Artificial Optical Magnetism Produced by Rings of Plasmonic Nanoparticles

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