A team of international physicists have made a nano-sized device which can allow the phase velocities of certain wave of visible light travel infinitely fast. No, this doesn’t translate into instant communication, nor does it mean that Einstein’s Theory of Relativity has been broken. It’s safe and sound. Read on, however, about the potential uses this sort of experiment may render as well as the process that produced these findings.

In vacuum, light travels at 300,000 kilometers/second. If it hits a refracting medium like water or glass, light is bent and travels slower. The ratio between the velocity of light in vacuum and that in the refracting medium is known as the refraction index. If light travels through vacuum, this index is equal to 1, else it’s greater than one.

For some years now, various scientists have claimed they’ve developed negative refraction index mediums, as in smaller than 1. A decade or so ago, John Pendry of London’s Imperial College published a paper proposing a “perfect lens” with a negative refractive index. Light wavelengths normally limit lens resolution, but Pendry’s perfect lens suffered no such limitations. Besides,  in 2006, Pendry collaborated with David Smith at Duke University to develop a theory to hide an arbitrary object from electromagnetic fields. Realizations of this concept have succeeded at radar and at visible wavelengths.

Pendry’s paper was heavily criticized at the time of publishing, in the same year, physicists David Smith and Sheldon Schultz of the University of California at San Diego measured the transmission angle of microwaves they sent through an unusual grid of thin copper wires and split copper rings mounted on a circuit board. These measurements, they claimed, showed negative refraction for the first time.

A zero refraction index of light

Now, Albert Polman, a physicist at the FOM Institute for Atomic and Molecular Physics in Amsterdam; Nader Engheta, an electrical engineer at the University of Pennsylvania; and colleagues have pulled out something similar and entirely different at the time – a medium with a refraction index of 0! This means light of particular wavelengths can travel infinitely fast.

Within the carefully sculpted waveguide, (left) light waves typically overlap to make a banded pattern (middle). However, depending on the width of the waveguide, waves of a certain wavelength travel infinitely fast, making the whole waveguide light up. (c) AMOLF and University of Pennsylvania

The device in question is comprised of a rectangular bar made out of insulating silicon dioxide, 85 nanometers thick and 2000 nanometers long, surrounded by conducing silver that blocks light. The resulting set-up  is called a waveguide since it conveys light. The researchers performed multiple experiments in which the width of the silicon dioxide ranged from 120 to 400 nanometers.

Because of its extremely compact size, light behaves in an odd manner inside the device. Short-wavelength light bounces back and forth between the ends of the guide, and the peaks and troughs of the counter-propagating light waves overlap to create a pattern of bright and dark bands much like the pressure patterns with a ringing organ pipe. It seems that light instead of traveling like it regularly does, it appears to be everywhere at once – in perfect synchronicity.

How does this not violate the laws of physics? The authors explain that light travels in two speeds – that is, the “phase velocity”, which describes how fast waves of a given wavelength move, and the “group velocity”, which describes how fast the light conveys energy or information. Only the group velocity must stay below the speed of light in a vacuum, Engheta says, and inside the waveguide, it does.

Applications for such a device could range from an antenna that emits light wave with sculpted phase front or used in nanoscale optical circuits, since the light leaking out of the waveguide is all in synch.

Results were published in the journal Physical Review Letters.

source: Science Mag

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