Invisibility cloaks: watch this space

A coating that can make an object appear "disappear" has been developed by scientists in the US. Science writer Mark Peplow has been taking a look at how it works. He revealed all to Chris Smith. 

Mark - It's Harry Potter time again, Chris! Researchers in California have created a tiny invisibility cloak which seems to be more effective at concealing objects than previous attempts to make these sorts of things. Because scientists have been working on invisibility cloaks since the mid-2000s. Many of them depend on meta materials. As light passes through them, it actually bends back on itself almost as if it was being reflected by mirror. So in theory, you cover an object with these materials and light from behind the object is bent around to the front so it looks as though the object isn't there anymore.

Chris - So, what was the problem with those particular formulations?

Mark - They tend to be relatively bulky compared to the objects that they're concealing. They were really difficult to make and they're very hard to scale up to work for everyday objects. Crucially, they change the phase of the light coming through. So, if you think of a light wave, the sort of peaks and troughs going up and down, in that exiting light wave, peaks have become troughs and vice versa. Now, it's pretty easy for instruments to see that phase change. So, if you really wanted to detect something that was being cloaked, it will be very easy to spot it because you can see this characteristic phase change. But these scientists at Berkeley say they have a solution to actually many of these problems. They made a small 3D object which is actually only the width of like a fine blonde hair. It's about 36 microns across and then they covered it in thin layers of gold and magnesium fluoride and topped it off with tiny pillars of gold that sit pround of the surface just about 30 nanometres high. Now, these act as antennae that reroute light around the object. Crucially, they also ensure that a phase of the light is unchanged. So the result, abra cadabra! The object disappears.

Chris - Does it really disappear? I mean, it sounds like it could be ideal if you have a head of hair with one hair on it and you've got your root showing or something. But no, could you scale this? Could you crucially make a big version of this to do something practical?

Mark - Well, that's one of the things that they're suggesting is different about their technique. They made them using electron beam lithography which is widely used in the semiconductor industry to make computer chips. They point out that you can do this sort of nanofabrication on meta long sheets now and their calculations, although they haven't done it yet suggests that you don't have to directly coat the object with this cloak. You could make a free-standing cloak that could be draped over it. For the moment, one of the main drawbacks is that this only works well with red lights. So again, that's a key thing that they are going to be working on in the future to try and make this work across the visible spectrum.

Chris - Why does it work at all and why does it get around that problem of not flipping the light waves back to front like the previous tries that people have had at making these things?

Mark - It's because it's bending the light in a slightly different way. The thing with the nano antennae is that the light is interacting with them and it's causing, if you like, ripples in the electrical charge around the surface of these nano antennae. Those ripples are what are then producing a beam of light which sort of shoots out. So, it's a slightly different process of channelling the light. If you like, it's translating the light from being light to being an electrical wave on the surface and then back out again as light.

Chris - Do you think they will be able to surmount the colour issue? I presume this comes down to how big those antennae are on the surface because a red light wave is much bigger than a smaller blue light wave. So, can they get around that?

Mark - That's exactly right, Chris. What they might have to do for example is tailor different sizes of nano antennae so that you're having some with one shape, some are a little bigger, some are a little smaller so that they can cope with different wavelengths of light to have this same effect overall. It's not impossible to do. It makes it more complex to make this. So, it remains to be seeing whether they can do that or not.