Metamaterials offer a path through the looking glass
Now you see it, now you don't. Top: An electric current from a source
ripples outwards and through a standard corridor. Below: One wall of the
corridor comprises a trapezoid chunk that behaves like a metamaterial
with an angled mirror behind (at point A). The electric current
interacts with the "metamaterial" and mirror, and seems to reflect off a
mirror angled across the corridor's entrance.
A house of mirrors can fool you into thinking a wall
is really an open corridor; using metamaterials, the opposite is
possible. The first practical demonstration of illusion optics shows
that an open passageway can take on the appearance of a mirror wall –
suggesting that in future it may be possible to emulate Alice and pass
through the looking glass.
The demonstration, conducted in a
two-dimensional electrical version of an optical system, relies on
replicating the negative refractive index of some metamaterials. This
property causes light entering or exiting the metamaterial to refract
"backwards" – as if reflected off a mirror passing at a right angle through the surface of the metamaterial.
If a chunk of such a metamaterial
formed one wall of a corridor, the magic would begin. Shining a light
down the corridor would illuminate the walls as normal, but instead of
bouncing off the metamaterial wall and continuing down the corridor, the
light would be refracted at the interface and enter the metamaterial.
Inside the metamaterial, the light
travels diagonally backwards before hitting an angled mirror directly
behind the metamaterial chunk. It is reflected back through the
metamaterial and is negatively refracted once more as it leaves the wall
and returns to the corridor. Consequently, on re-entering the corridor,
the light travels back towards the light source.
A viewer looking down the corridor
would be unaware of the unusual path taken by the light entering it.
They would simply see themselves apparently reflected in a mirror
blocking the corridor at an angle – although in reality the mirror
reflecting them is embedded behind the metamaterial that forms one wall
of the corridor.
Circuit training:
So much for the theory: now Huanyang Chen
of Soochow University in Suzhou, China, and colleagues have shown that
the illusion works in practice too. Metamaterials that control light
perfectly without absorbing it exist, but are difficult to make ,
so the physicists instead simulated their illusion using an electrical
system of inductors and capacitors. An alternating current passing
through this system can be made to behave in much the same way as light would if it was travelling through an optical system that included metamaterials with a negative index of refraction.
Instead of travelling freely through
the circuits, the current was blocked off at the entrance of the
"passage" as though reflected off an "angled wall" lying there, exactly
as the optical system is expected to behave.
Jason Valentine
of Vanderbilt University in Nashville, Tennessee, calls the experiment
an excellent proof of principle. "With future advances in technology it
may be possible to some day achieve such concepts at higher, maybe even
visible [light] frequencies," he says.
Chen's team is not new to the field of illusion optics: last year they suggested how metamaterials could theoretically help one object take on the appearance of another.
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