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Backward bending light key to stealth
Scientists have for the first time demonstrated a reversal of the optical Doppler effect, an advance that could lead to the development of 'invisibility cloak' technology.

The Doppler effect describes the change in frequency of waves whenever there is a relative movement between an observer and a wave's source.

Most people would know of it in relation to sound - for example, the change in pitch of a police siren as it nears and then passes. As it gets closer the sound frequency increases, and as it moves away the frequency decreases.

Light behaves in a similar way. When an object and an observer move closer together, light frequency increases from red wavelengths to blue.

When the light source moves further away, light frequency decreases from blue to red.

In a paper published today in the Nature Photonics journal, researchers from Swinburne University in Melbourne and their collaborators from the University of Shanghai for Science and Technology, have demonstrated the reversal of this effect, which does not occur naturally.

That is, when an object and a light wave detector moved closer together, they were able to decrease the light frequency from blue wavelengths to red ones, and vice versa.

"This is the first time that the inverse Doppler effect has been demonstrated in the optical region," says Professor Min Gu, Director of Swinburne's Centre for Micro-Photonics.
Concept realised after 45 years

Prior research in the United Kingdom showed the phenomenon was possible with microwaves. Although scientists had suspected since the 1960s that the reverse Doppler effect was possible with light waves, the technology didn't exist to prove the concept.

The researchers achieved this by creating an artificial nanostructured crystal - known as a 'photonic crystal' - out of silicon.

"In our super prism the dispersion of light was twice the magnitude of a standard Newton prism," Gu says.

"This large angle makes the prism's refractive index - a property that determines how fast light travels through it - change to negative."

All materials that occur in nature have a positive refractive index. This means whenever they move in respect to an observer, they will exhibit the standard Doppler effect.

"By creating this artificial material, with a negative refractive index, we were able to reverse this natural phenomenon," Gu says.

Swinburne University Senior Research Fellow Baohua Jia, a co-author on the paper, explains the phenomenon.

"For example if you have stick and you put that into water, what you expect to see is that the stick is bending towards you. If you have a negative index material the bend will be opposite," she says. "It's counterintuitive."

By projecting a laser beam onto the photonic crystal 'super prism' and changing the distance between it and the detector, the researchers were able to create an inverse Doppler effect phenomenon.
Invisibility cloaks

The researchers say that being able to reverse the Doppler effect is a promising sign for the future development of technology such as invisibility cloaks, which effectively bend light around an object. According to Professor Gu this technology, which US researchers have already demonstrated on a micro-scale, may be closer to becoming a reality than most people think.

The technology may see earlier deployment in optical telecommunications and medical imaging.

But Jia says at this stage it's just a demonstration of the fundamental physics.

"This paper is not really trying to emphasise the potential application, but rather to demonstrate that we can actually achieve it by using this artificial negative index material," she says.

"Although you know from principle it should be possible, when you really observe it it's very exciting."

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