Ultrabroadband Unnaturally High Effective Refractive Index Metamaterials in the Terahertz Region

We demonstrate that an ultrabroadband, extremely high index of refraction can be realized from the thin window-type terahertz metamaterials composed of strongly multilayer coupled unit cells. By drastically increasing the effective permittivity by means of intense capacitive coupling and reducing the diamagnetic effect using a thin metamaterial structure, a peak refractive index of 70 at the resonant frequency, along with a low-frequency quasi-static value of over 18 were obtained for a single-layer thin ring terahertz metamaterial. A relative bandwidth of 0.5 THz at the index more than 20 can be observed for the single-layer thin window-type metamaterial with the refractive index peak achieving to 45. Based on the coupling effect of multilayer window-type metamatarials, the bandwidth of 1.25 THz with the index of more than 20 for the five-layer structure can be reached, and the refractive index peak reach to 50 while maintaining low losses. Importantly, the ultrabroadband performance with the bandwidth of 2.25 THz at the index larger than 20 was surprisingly indicated for multilayer coupling structure by a small incident angle. Simultaneously, the refractive index peak of more than 70 can be obtained.