Waveguide offers new circuitry possibility

Within a nanometre-scale device, visible light travels infinitely fast - by one measure - a team of physicists and engineers reports.

The gizmo won't lead to instantaneous communication - the famous speed limit of Albert Einstein's theory of relativity remains in force - but it could have a variety of uses, including serving as an element in a type of optical circuitry.

"The demonstration of such a thing is definitely very interesting and possibly useful," said Wenshan Cai, an electrical engineer at the Georgia Institute of Technology in Atlanta, who was not involved in the work.

In empty space, light always travels at 300,000,000 metres per second.

In a material such as glass, it travels slower.The ratio of light's speed in the vacuum to its speed in a material defines the material's "index of refraction," which is typically greater than one.

However, scientists have begun to manipulate the interactions of light and matter to tune the index of refraction in weird ways, such as making it negative, which leads to an unusual bending 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 off a particularly odd feat.

They've developed a tiny device in which the index of refraction for visible light is zero - so that light waves of a particular wavelength move infinitely fast.

The device consists of a rectangular bar of insulating silicon dioxide 85 nanometres thick and 2000 nanometres long surrounded by conducing silver, which light generally doesn't penetrate.

The result is a light-conveying chamber called a waveguide.

Researchers fashioned different devices in which the width of the silicon dioxide ranged from 120 to 400 nanometres, as they report in a paper in press at Physical Review Letters.

Light behaves differently in such a waveguide, because the electromagnetic fields must obey certain "boundary conditions" on the sides of 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. Above a "cutoff" wavelength, light doesn't flow at all.

Right at the cutoff wavelength, things get interesting.

Instead of producing a banded pattern, the whole waveguide lights up.

That means that instead of acting as waves with equally spaced peaks, or "phase fronts," the wave behaves as if its peaks are moving infinitely fast and are everywhere at once.

So the light oscillates in synchrony along the length of the waveguide.

Engheta and company had previously created an index of refraction of zero for longer-wavelength radiation called microwaves.

Repeating the feat for visible light was harder, as the new widget is too small to contain a light source.

Instead, the researchers shot in a beam of electrons to generate light of all wavelengths within the waveguide and measured the light leaking out of it.

The amount of light shining out at a particular wavelength depends on whether the electron beam enters at a point where there should be a dark or a bright spot for that wavelength.

So by scanning the beam along the waveguide and monitoring the output, researchers traced the light pattern at each wavelength.

"It is nanofabrication and characterisation at its best," said Che Ting Chan, a physicist at the Hong Kong University of Science and Technology.

So how does an everywhere-at-once light wave not violate relativity?

Light has two speeds, Engheta explains.

The "phase velocity" describes how fast waves of a given wavelength move, and the "group velocity" describes how fast the light conveys energy or information.

Only the group velocity must stay below the speed of light in a vacuum, Engheta said, and inside the waveguide, it does.

The device could have various uses, Engheta said. Because the light leaking out of the waveguide is all in synch, the waveguide might be bent to form an antenna that emits light wave with sculpted phase fronts, he said.

It might also make a conduit for a hoped-for type of nanoscale optical circuitry, he said.

An array of such waveguides might even make a bulk material with zero index of refraction.

But fabricating that array would be very challenging, Cai said: "In theory it's easy; experimentally it's very hard."