Transparent Terahertz Metamaterial with a Refractive Index of Zero

It is commonly believed that most of naturally occurring transparent materials have small positive refractive indices around 1 in the terahertz waveband. However, metamaterials have made a paradigm shift and achieve the design of man-made materials with both on-demand electromagnetic properties and unprecedented refractive indices. Here, inspired by the metamaterial concept, we report a transparent metamaterial with a refractive index of zero. Measurements demonstrate an effective refractive index of 0.037 + j0.072, near perfect transmission power of 94.5%, and an extremely low reflection of 0.8% at 0.50 THz. Electromagnetic properties at a refractive index of zero such as an infinite phase velocity and wavelength put many attractive applications exceeding the limitations of conventional materials within reach.