An Invisibility Cloak With An On-Off Switch
The first invisibility cloaks appeared about a decade ago. Since then, the theory behind these devices and the technology used to implement them has developed at a breathtaking pace.
We’ve looked at ways to make optical cloaks, at self assembling cloaks and even examined ‘illusion cloaks’ that make one thing look like another.
Today, Darran Milne and Natalia Korolkova at the University of St Andrews in Scotland outline another idea. These guys have worked out how to make an optical invisibility cloak that you can turn on and off.
What makes this possible is a process known as electromagnetically induced transparency–a phenomenon in which certain materials become transparent when zapped by light from two carefully tuned lasers.
This works for materials with atoms that can exist in three different electronic states–say a, b and, the highest, c. The idea here is that the first laser beam is absorbed by the material because it excites electrons from state a to state c. The second laser is also absorbed because it excites electrons from state b to state c.
If the frequencies of the lasers are close together, they can be tuned in a way that makes them interfere destructively. And when this happens, their ability to excite electrons cancels out.
When this happens, the laser photons suddenly pass through the material unimpeded, sometimes at dramatically reduced at speeds (which is how experiments that stop light are performed).
This effect has a number of applications. Physicists recently used it to create electromagnetically induced lenses in which a material’s refractive index is made to change in a way that focuses light.
What’s more, other physicists have shown how the same effect might be used to make a material’s refractive index negative.
This kind of manipulation of refractive index is exactly what’s needed for an invisibility cloak because it must steer light around an object, giving the impression that it isn’t there.
The problem, however, is that although this is all possible in theory with electromagnetically induced transparency, the extreme changes in refractive index haven’t been possible in practice.
Now Milne and Korolkova have worked out how to do it. Their trick is to use atoms that can exist in five electronic states rather than three. This allows additional control over the refractive index called magneto-electric cross-coupling.
The bottom line is that this allows an external magnetic field to modulate the change in refractive index. As a result, it is possible to steer light by placing the material in a field that varies in the required way.
Such a material ought to be straightforward to make by doping a crystal with the required atoms, say Milne and Korolkova.
Even still, the new material will have important limitations. For example, the refractive index manipulation is only enough for a certain special class of invisibility cloak called carpet cloaks. These only cloak an object on a surface but it can’t cloak an object in free space (at least not yet).
The big advantage of this technique is that make cloaks much simpler to make. Conventional cloaks must be engineered on the nanoscale in a way that bends light in the required way.
By contrast, an electromagnetically induced cloak would be made like any other doped material and simply placed in a magnetic field that is shaped in the required way. This, of course, can be switched on and off with the flick of a switch.
The “much simpler technique of electromagnetically induced transparency can be used to achieve a partial, carpet cloaking at optical frequencies in atomic vapours or solids,” say Milne and Korolkova, who as theorists, must now be waiting for somebody to have a crack at making one of these devices. A practical demonstration would be yet another interesting advance in cloaking technology.
We’ve yet to see commercially viable applications for cloaking but at this rate of development, we won’t have to wait much longer.
Ref: arxiv.org/abs/1206.3944: Electromagnetically Induced Invisibility Cloaking
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