Friday, September 19, 2008

Opening the door to Hogwarts
[This is how I originally wrote my latest story for Nature’s online news. It is about another piece of creative thinking from this group at Shanghai Jiao Tong University. I was particularly struck by the milk-bottle effect that John Pendry told me about – I’d never thought about it before, but it’s actually quite a striking thing. (The same applies to water in a glass, but it’s more effective with milk.) John says that it is basically because, as one can show quite easily, no light ray can pass through the glass wall that does not also pass through some milk.

Incidentally, I have to suspect that John Pendry must be a candidate for some future Nobel for his work in this area, though probably not yet, as the committee would want to see metamaterials prove their worth. The same applies to Eli Yablonovitch and Sajeev John for their work on photonic crystals. Some really stimulating physics has come out of both of these ideas.

The photo, by the way, was Oliver Morton’s idea.]

Scientists show how to make a hidden portal

In a demonstration that the inventiveness of physicists is equal to anything fantasy writers can dream up, scientists in China have unveiled a blueprint for the hidden portal in King’s Cross railway station through which Harry Potter and his chums catch the train to Hogwarts.

Platform Nine and Three Quarters already exists at King’s Cross in London, but visitors attempting the Harry Potter manoeuvre of running at the wall and trusting to faith will be in for a rude shock.

Xudong Luo and colleagues at Shanghai Jiao Tong University have figured out what’s missing. In two preprints, they describe a method for concealing an entrance so that what looks like a blank wall actually contains invisible openings [1,2].

Physicist John Pendry of Imperial College in London, whose theoretical work laid the foundations of the trick, agrees that there is a whiff of wizardry about it all. “It’s just magic”, he says.

This is the latest stunt of metamaterials, which have already delivered invisibility cloaks [3] and other weird manipulations of light. Metamaterials are structures pieced together from ‘artificial atoms’, tiny electrical devices that allow the structure to interact with light in way that are impossible for ordinary substances.

Some metamaterials have a negative refractive index, meaning that they bend light the ‘wrong’ way. This means that an object within the metamaterial can appear to float above it. A metamaterial invisibility shield, meanwhile, bends light smoothly around an object at its centre, like water flowing around a rock in a river. The Shanghai group recently showed how the object can be revealed again with an anti-invisibility cloak [4].

Now they have worked out in theory how to hide a doorway. The trick is to create an object that, because of its unusual interactions with light, looks bigger than it really is. A pillar made of such stuff, placed in the middle of an opening in a wall, could appear to fill the gap completely, whereas in fact there are open spaces to each side.

Pendry and his coworker S. Anantha Ramakrishna demonstrated the basic principle in 2003, when they showed that a cylinder of metamaterial could act as a magnifying lens for an object inside it [5].

“When you look at a milk bottle, you don’t see the glass”, Pendry explains. Because of the way in which the milk scatters light, “the milk seems to go right to the edge of the bottle.” He and Ramakrishna showed that with a negative-refractive index metamaterial, an object in the bottle could be magnified on the surface.

And now Luo and colleagues have shown that an even more remarkable effect is possible: the milk can appear to be outside the bottle. “It’s like a three-dimensional projector”, says Pendry. “I call it a super-milk bottle.”

The Chinese team opt for the rather more prosaic term “superscatterer”. They show that such an object could be made from a metal core surrounded by a metamaterial with a negative refractive index [1].

The researchers have calculated how light interacts with a rectangular superscatterer placed in the middle of a wide opening in a wall, and find that, for the right choice of sizes and metamaterial properties, the light bounces back just as it does if there was no opening [2].

If someone passes through the concealed opening, they find, it becomes momentarily visible before disappearing again once they are on the other side.

So “platform nine and three-quarters is realizable”, the Shanghai team says. “This is terrific fun”, says Pendry. He feels that the effect is even more remarkable than the invisibility cloak, because it seems so counter-intuitive that an object can project itself into empty space.

But the calculations so far only show concealment for microwave radiation, not visible light. Pendry says that the problem in using visible-light metamaterials – which were reported last month [6,7] – is that currently they tend to absorb some light rather than scattering it all into the magnified image, making it hard to project the image a significant distance beyond the object’s surface. So openings hidden from the naked eye aren’t likely “until we get on top of these materials”, he says.

1. Yang, T. et al. (2008).
2. Luo, X. et al. (2008).
3. Schurig, D. et al., Science 314, 977-980 (2006).
4. Chen, H., Luo, X., Ma, H. & Chan, C. T. (2008).
5. Pendry, J. B. & Ramakrishna, S. A. J. Phys.: Condens. Matter 15, 6345-6364 (2003).
6. Valentine, J. et al., Nature doi:10.1038/nature07247 (2008).
7. J. Yao et al., Science 321, 930 (2008).


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