It is the phase velocity and group velocity of light that determines how it propagates in a material. Phase velocity dictates how the peaks and valleys of the wave move in the material, while the group velocity describes the transport of energy. Einstein’s laws dictate that the transport of the energy of light can not be faster than the speed of light, and so the group velocity is limited.
There are no physical limitations to the phase velocity though: when the phase velocity is at zero there is no movement of the peaks and valleys of the wave but when it is infinite the wavelength diverges to very large values. No materials with these properties exist in nature.
Researchers, from the FOM Institute AMOLF and the University of Pennsylvania, constructed a metamaterial made of a unit cell structure much smaller then the wavelength of light, by stacking silver and silicon nitride nanolayers. This new material allows light to ‘feel’ the optical properties of both layers. As the permittivity of silver is negative and that of silicon nitride is positive, the combined material has a permittivity which is effectively equal to zero. The light therefore experiences zero resistance, and propagates with an infinite phase velocity; the wavelength of the light is therefore nearly infinite.
The material was constructed using focused ion beam milling, a technique that allows the user control over the material’s structure at the nanoscale level. A specially built interferometer showed that light did propagate through the metamaterial with no significant change of phase, essentially corresponding to an almost infinite wavelength.
It is hoped the new material will have applications in new optical components or circuits, as well as the design of more efficient leds.