Scientists bend light the ‘wrong’ way

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Materials that bend light in unnatural ways are often touted as the path to futuristic technologies such as cloaking devices and super-powered lenses. But such materials are hard to make, but scientists have now discovered a simpler way using electrons.

At Harvard University's School of Engineering and Applied Sciences, a team of researchers led by Hosang Yoon and Donhee Ham showed that using ordinary semiconductors and confining electrons to a two-dimensional plane they could make a material with a so-called negative refractive index that bends radio waves the “wrong” way, and does so a hundred times better than other methods.

A refractive index is a measure of how much a material bends light. An index of 1 means no bending at all. Diamonds have that nice prism effect because they have an index of about 2.42, whereas air bends light hardly at all. Light – and that includes radio waves – bends because as it travels through anything other than a vacuum it slows down. Most materials always have a positive refractive index. That means that if light is approaching a denser, higher-index material from a lower-index one it gets bent to the right if the denser stuff is on that same side.

This all changes if the material has a negative index – as metamaterials do. In that case, the bend would be to the left. An object surrounded by a metamaterial would scatter the light away from it, making it invisible.

The Harvard team’s radio-wave metamaterial itself won't make you invisible, but it could be used to make a kind of "superlens" for radio waves, boosting signals. Or it could divert radar away from a target.

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The researchers set up one micrometer-wide strips of aluminum-gallium-arsenide (a common semiconductor) parallel to each other. They then cooled the device to a few degrees above absolute zero and ran a current through it, while simultaneously applying an electric field to one end.

The electric field accelerates the electrons in one of the strips. Those accelerating electrons couple to those in the strip next to it, and so on. That creates an effect like people in a stadium doing the wave – the electrons don't move but they do couple with others.

This differs from other methods of coupling electrons, which use magnetic fields. In this case, it's an electric field, and the coupling is due to the acceleration of the electrons themselves, a phenomenon called kinetic inductance.

Yoon and Ham then fired a beam of microwaves at frequencies of 1-50 gigahertz at the accelerating electrons. They found that the beam was refracted the "wrong" way, with an index of refraction at up to -700. For comparison, diamond, one of the most refractive materials known for visible light, has an index of 2.42. Most metamaterials developed so far have indexes of between -1 and -5.

Ham told InnovationNewsDaily that the wave of electrons is a key piece of the effect. The electric field applied to the strips creates an effective wave of a specific frequency, so the electrons will refract radio waves in a certain range. But that range can be adjusted by simply raising or lowering the frequency of the field.

This system wouldn't work for visible light, as the semiconductors used aren't transparent. So the technology won’t lead to the creation of invisibility cloaks. But that doesn't mean it won't be possible later on.

Ham said future experiments will look at proving that the apparatus works with higher frequencies, in the terahertz and far infrared range.

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