Reversing Doppler Effect holds promise for invisibility cloak

If scientists carry on at this rate, we’ll soon have more types of invisibility cloak to choose from than Imelda Marcos had shoes.

This season’s model comes courtesy of Swinburne University and the University of Shanghai for Science and Technology, which have for the first time ever demonstrated a reversal of the optical Doppler Effect.

This describes how, when an object and an observer move closer together, light frequency increases from red wavelengths to blue. When they move further apart, the frequency decreases from blue to red.

But the Swinburne team has demonstrated the reversal of this effect – which doesn’t occur naturally – whereby moving an object and a light wave detector closer together decreases the light frequency from blue wavelengths to red and vice versa.

“This is the first time in the world that the inverse Doppler Effect has been demonstrated in the optical region,” says Professor Min Gu, Director of Swinburne’s Centre for Micro-Photonics.

The researchers were able to achieve this by creating an artificial nanostructured crystal – known as a ‘photonic crystal’ – from silicon, and projecting a laser beam onto the unique photonic crystal ‘super prism’. By changing the distance between it and the detector, they were able to create an inverse Doppler Effect phenomenon.

“In our super prism, the dispersion of light was twice the magnitude of a standard Newton Prism. This large angle makes the prism’s refractive index – a property that determines how fast light travels through it – change to negative,” says Gu.

All naturally-occuring materials have a refractive index greater than one – in other words, whenever they move in respect to an observer, they exhibit the standard Doppler Effect.

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

It’s a promising sign for the future development of invisibility cloaks, he says, which may be closer to becoming a reality than most people think.