Double Negative Index of Refraction Observed in a Single Layer of Closed Ring Resonators*

We report the results of a spectroscopic study of a single layer of nanoscale metallic single closed ring resonators on a free- standing thin membrane at near-normal and grazing angles of incidence$^{[1]}$. When the magnetic component of the light is perpendicular to the ring plane, we observe a so-called ``double'' negative index of refraction at near-infrared frequencies attributed to a strong magnetic dipolar resonance and a broad electric resonance in this metamaterial. We experimentally identify the different resonance modes and the spectral region of negative refractive index on a series of samples with different feature and lattice sizes, using multi- oscillator fits and comparing to electromagnetic simulations. \newline [1] Z. Hao, M. C. Martin, B. Harteneck, S. Cabrini, E. H. Anderson, Appl. Phys. Lett., in press (2008).