True negative-index metamaterial operates at 780 nm

Besting their own previous world record by almost a factor of two, researchers at the Universität Karlsruhe (Karlsruhe, Germany), Iowa State University (Ames, IA), and the Forschungzentrum Karlsruhe in der Helmholtz-Gemeinschaft (Karlsruhe, Germany) have shortened the operating wavelength of a true negative-index optical metamaterial from 1.4 μm to 780 nm-reaching the visible spectrum (a negative refractive index is harder to achieve than a negative magnetic permeability, which has already been achieved in the visible). But confirming the results was not a simple matter.

The metamaterial consisted of two 40 nm layers of silver sandwiching a single 17 nm layer of magnesium fluoride, all on a glass substrate; a rectangular lattice of nearly square holes with a period of 300 nm was formed through the layers using electron-beam lithography. Determining the refractive index required phase-sensitive time-of-flight measurements based on a Michelson interferometer and 125 fs pulses from a Ti:sapphire laser. The index was found to be -0.6. The material is somewhat lossy; stacking several layers may reduce the loss. Contact Gunnar Dolling at gunnar.dolling@physik.uni-karlsruhe.de.