Metamaterials Display Negative Phase and Group Velocity

A team made up of researchers in Europe and the US has confirmed that negative-refractive-index media also can exhibit exotic pulse propagation phenomena. In the May 12 issue of Science, the investigators from Universität Karlsruhe and Forschunszentrum Karlsruhe in der Helmholtz-Gemeinschaft, both in Germany, from Iowa State University in Ames, and from the University of Crete in Iraklion, Greece, present the findings of experiments with metamaterials that simultaneously display a negative phase velocity and a negative group velocity at 1500 nm.

For their work, the scientists used electron-beam lithography and evaporation techniques to fabricate 60 metamaterial samples constructed from two 25-nm-thick gold films sandwiching a 35-nm-thick MgF₂ spacer layer and atop glass substrates coated with ITO. By inserting a sample into one arm of a Michelson interferometer into which they injected 170-fs pulses of 1500-nm radiation from an optical parametric oscillator, they deduced from the effects on the interference fringes that the metamaterials have a negative phase and group velocity such that both the carrier wave and peak of a given pulse exit the samples before they have entered.