Negative index materials using simple short wire pairs

Negative refraction is currently achieved by a combination of artificial ``electric atoms'' (metallic wires with negative electrical permittivity $\ensuremath{\epsilon}$) and artificial ``magnetic atoms'' (split-ring resonators with negative magnetic permeability $\ensuremath{\mu}$). Both $\ensuremath{\epsilon}$ and $\ensuremath{\mu}$ must be negative at the same frequency, which is not easy to achieve at higher than THz frequencies. We introduce improved and simplified structures made of periodic arrays of pairs of short metal wires and continuous wires that offer a potentially simpler approach to building negative index materials. Using simulations and microwave experiments, we have investigated the negative index $n$ properties of short wire-pair structures. We have measured experimentally both the transmittance and the reflectance properties and found unambiguously that $n<0$. The same is true for $\ensuremath{\epsilon}$ and $\ensuremath{\mu}$. Our results show that short wire-pair arrays can be used very effectively in producing materials with negative refractive indices.