By Jeff Hecht: A nano-sized bar of glass encased in silver allows
visible light to pass through at near infinite speed. The technique may
spur advances in optical computing.
Metamaterials
are synthetic materials with properties not found in nature. Metal and
glass have been combined in previous metamaterials to bend light
backwards or to make invisibility cloaks. These materials achieve their bizarre effects by manipulating the refractive index, a measure of how much a substance alters light's course and speed.
In a vacuum the refractive index is 1,
and the speed of light cannot break Einstein's universal limit of
300,000 kilometres per second. Normal materials have positive indexes,
and they transmit at the speed of light in a vacuum divided by their
refractive index. Ordinary glass, for instance, has an index of about
1.5, so light moves through it at about 200,000 kilometres per second.
The new material contains a nano-scale
structure that guides light waves through the metal-coated glass. It is
the first with a refractive index below 0.1, which means that light
passes through it at almost infinite speed, says Albert Polman at the
FOM Institute AMOLF in Amsterdam, the Netherlands. But the speed of light
has not, technically, been broken. The wave is moving quickly, but its
"group velocity" – the speed at which information is travelling – is
near zero.