Index-tunable terahertz metamaterials based on double-layered closed-ring resonator arrays

We demonstrate index-tunable metamaterials working in the terahertz (THz) frequency range based on double-layered closed-ring resonator (CRR) arrays. The double-layered CRR arrays have a narrow-band transmission peak in a relatively wide stop band, and that peak shows spectral shift by slightly shifting relative position of the arrays or changing dielectric constant of the dielectric media inserted between CRR arrays. We show by numerical simulations that the effective refractive index can be widely tuned from un-naturally high positive to near-zero and negative values. Our approach may be utilized to develop THz active devices.