We design and analyze a novel multiband left-handed metamaterial based on a fishnet-like structure at terahertz (THz) frequencies. The metamaterial exhibits simultaneous negative refractions around the frequencies of 0.48, 1.05, and 1.19 THz for the electromagnetic (EM) wave normal incidence, and around the frequencies of 0.20, 0.79, and 1.13 THz for parallel incidence. The simulated results verify the left-handed properties. A particularly important observation is the capability of the proposed metamaterial with a single geometrical structure to display multifrequency operations in a unit cell. The compact metamaterial is a major step toward the miniaturization of THz materials and devices suitable for multifrequencies.

We present a theoretical investigation of THz long-range surface plasmon polaritons propagating on thin layers of InSb. The metallic behavior of doped semiconductors at THz frequencies allows the excitation of surface plasmon polaritons with propagation and confinement lengths that can be actively controlled. This control is achieved by acting on the free carrier density, which can be realized by changing the temperature of InSb.

We numerically investigate the trade-offs between the dispersion properties, coupling efficiency, and geometrical constraints in dual-wire (twin-lead) terahertz (THz) waveguides. In particular, we show that their inherent linearly polarized quasi-transverse electromagnetic (TEM) modes exist for waveguide transverse dimensions comparable with the wavelength, enabling significant end-fire coupling (>10%) for numericalaperture limited Gaussian beams while supporting a relatively low-dispersion propagation of below 0.5 ps2/m, as desired for short-pulse time-domain spectroscopy applications. Starting from the dual-wire structure, we also demonstrate that low-dispersion tapers can be designed to improve coupling efficiency.

An equivalent circuit model for the analysis and design of terahertz (THz) metamaterial filters is presented. The proposed model, derived based on LMC equivalent circuits, takes into account the detailed geometrical parameters and the presence of a dielectric substrate with the existing analytic expressions for self-inductance, mutual inductance, and capacitance. The model is in good agreement with the experimental measurements and full-wave simulations. Exploiting the circuit model has made it possible to predict accurately the resonance frequency of the proposed structures and thus, quick and accurate process of designing THz device from artificial metamaterials is offered.