Because of it's highly ionized state, the state of matter called plasma has a negative index of refraction. In 1968, the Russian physicist Victor Veselago found that in order for a material to have a negative index of refraction, it must behave as a perfect plasma. The index of refraction in plasma is affected by the plasma density and magnetic field. The field causes faraday rotation, which is a change in orientation of the right and left hand polarized vectors of an electromagnetic wave.

Sheldon Schultz, David Smith, and Richard Shelby at the University of California at San Diego constructed a device consisting of copper rings connected by wires that displayed a negative index of refraction in 2000.