Three-Dimensionally Coupled THz Octagrams as Isotropic Metamaterials

Split-ring resonator (SRR) based metamaterials have been studied for the development of highly sensitive, small-sized, low-power chemical and biomolecular sensors. However, the anisotropic behavior arising from their two-dimensional (2D) structure presents substantial challenges leading to ambiguity in their transmission spectra. In this paper, we present the design of a three-dimensional (3D) isotropic octagram split-ring resonator (OSRR) demonstrating a three-dimensionally coupled resonance behavior that overcomes the anisotropic response of conventional 2D SRRs, leading to a strong, distortion-free, and polarization-invariant transmission response. The OSRR undergoes 3D coupling through the splits at the corners of the 3D structure (cube), which remains invariant under any polarization along the coordinate axes. The strong coupling between resonant segments provides the OSRR with 25 times higher sensitivity than the corresponding 2D structure, allowing the resonant frequency to be reliably monitored for small changes in concentration of a targeted substance. The isotropic frequency response of the 3D OSRR, without ambiguity in the amplitude caused by the polarization dependence, also allows monitoring the amplitude for minute changes in concentration that are too small to cause any shift in resonant frequency. Thus, the detection range for the presented 3D OSRR stretches from large to minute variations of targeted substance.