Practical applications for metamaterials beyond the invisibility cloak

In the late 1960s, Russian physicist Victor Veselago pondered whether two key electromagnetic properties could ever be negative. In conventional materials, permeability and permittivity are always positive. But he proposed that if both permeability and permittivity were negative, so too would the refractive index of that medium.

This demanded a new look at some of the equations that determine electromagnetic behaviour and opened up intriguing possibilities for materials that might have negative refractive indices.

One result was the prediction that a light ray entering a transparent material with negative refractive index would bend the 'wrong' way relative to the surface normal. The reason behind this change lies in the group and phase velocities of a wave. Refractive index is a ratio of phase velocities. So, the phase velocity of a light wave has to turn negative when the wave encounters a medium with a negative refractive index, while its group velocity can remain positive.