When it comes to invisibility cloaks, one of the trickiest problems is how to make the things.
The materials of choice are known as metamaterials which are created by assembling a repeating pattern of structures that interact with the light they are designed to cloak. This kind of detailed assembly means that making metamaterials is an expensive and time-consuming process. What’s more, the resulting invisibility cloaks are never perfect
So physicists have begun to wonder if they can do just as well with other materials that are easier and simpler to make.
Their approach is entirely different to the theoretical light-bending mathematics physicists have used until now. This current approach works by attempting to steer electromagnetic fields around an object in a way that hides it. The necessary material must be able to repeat this kind of light distortion in real life.
The new approach is to create a computer model of the cloak in the form of a conventional material with fixed light bending properties.
The model simulates how this conventional material distorts light as it passes by. The computer then changes the shape and topology of the material to reduce this distortion.
By repeating this process many times, it is possible to find a topology that minimises the distortion of light so that it remains more or less unchanged as it passes by. The result is an invisibility cloak; not a perfect one but one that can hold its own against many of those made of metamaterials.
At least, that’s the theory. So-called topological optimisation has been little more than a twinkling in a few applied physicists’ eyes. Until now.
Today, Lu Lan at Zhejiang University in China and a few pals have actually created the first invisibility cloak designed using topology optimisation. They carved it out of Teflon and it took them all of 15 minutes using a computer-controlled engraving machine. “The fabrication process of a sample is substantially simplified,” they say.
The resulting “Teflon eyelid” invisibility cloak hides a cylindrical disc of metal the size of poker chip from microwaves. But crucially, its performance closely matches the prediction of the computer simulation.
That significant because it brings invisibility cloaks into the realms of mass production. There is no reason why Teflon eyelids couldn’t be printed or moulded en masse.
What’s more, Lu and co so there is no reason why the same approach can work in optical wavelengths. “Such a cloaking setup won’t be a big problem to replicate in the THz or even optical spectrum,” they say.
Of course, there are challenges ahead. Lu and co want to develop the technique to create cloaks that work over a range of frequencies and at a range of angles. If they can make them cheaply and easily for a cost measured in pennies, there’s no reason why invisibility cloaks won’t soon be everyday objects.
Ref: arxiv.org/abs/1308.6049: Experimentally Demonstrated An Unidirectional Electromagnetic Cloak Designed By Topology Optimization