The common thread in metamaterials is negative refraction, the researchers noted, but in contrast, all materials found in nature have a positive refractive index, a measure of how much electromagnetic waves are bent when moving from one medium to another. "Natural materials do not respond to the magnetic field of light, but the metamaterial we created here does," said Jason Valentine, A U.C. Berkeley graduate student and co-lead author of the Nature paper. "It is the first bulk material that can be described as having optical magnetism, so both the electrical and magnetic fields in a light wave move backward in the material."

To better explain how refraction works, researchers gave the example of looking at the submerged part of a pole inserted into water, which will appear as if it is bent up towards the water's surface. If water exhibited negative refraction, the submerged portion of the pole would instead appear to jut out from the water's surface. Or, to give another example, a fish swimming underwater would instead appear to be moving in the air above the water's surface, they said.