In this study, we illustrate the effective medium theories in the designs of three-dimensional composite metama- terials of both negative permittivity and negative permeability. The proposed metamaterial consists of random coated spheres with sizes smaller compared to the wavelength embedded in a dielectric host. Simple design rules and formulas following the effective medium models are numerically and analytically presented. We demonstrate that the revised Maxwell-Garnett effective medium theory enables us to design three-dimensional composite metamaterials through the assembly of coated spheres which are random and much smaller than the wavelength of the light. The proposed ap- proach allows for the precise control of the permittivity and the permeability and guides a facile, flexible, and versatile way for the fabrication of composite metamaterials.
An actively enhanced resonant transmission in a plasmonic array of subwavelength holes is demonstrated by use of terahertz time-domain spectroscopy. By connecting this two-dimensional element into an electrical circuit, tunable resonance enhancement is observed in arrays made from good and relatively poor metals. The tunable feature is attributed to the nonlinear electric response of the periodic hole array film, which is confirmed by its voltage-current behavior. This finding could lead to a unique route to active plasmonic devices, such as tunable filters, spatial modulators, and integrated terahertz optoelectronic components.
