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In this paper, we propose a zero-refractive index metamaterial (ZIM) with a simple structure, which is composed of dielectric spheres. The effective medium theory was used to invert the equivalent electromagnetic parameters of metamaterials. Metamaterials have nearly zero refractive index in the three bands of 17. GHz, respectively. Through monitoring the electric field and phase distribution at three zero frequencies, the zero refractive index characteristic is verified. Based on the proposed ZIM, a three-dimensional polarization-independent invisibility cloak within three wavebands was designed and numerically demonstrated.

Zero-index metamaterials are widely used in electromagnetic devices due to their favorable optical properties such as infinite large-phase velocity and quasi-infinite wavelength. However, insufficient bandwidth and high loss seriously limit the wide application of zero refractive index metamaterials (ZIMs), especially in the terahertz band. Here, we propose a kind of supercell microstructure metamaterial to realize a broadband near-zero refractive index. This new superlattice structure contains four subunit structures of different sizes in a single unit. Dual broadband characteristics of the near-zero refractive index can be demonstrated in our design. The bandwidth of the near-zero index can reach about 30 GHz with low loss. The resonance principle of the supercell microstructure and the generation mechanism of the near-zero refractive index are discussed in detail. The influence of the thickness of the medium, of the thickness of the metal sheet, and of the period of the central area and the metal line width on the near-zero refractive index is analyzed. Dual broadband zero-refractive metamaterials bring wider applications to various optoelectronic devices for arbitrary wavefront conversion, directional radiation, and obstacle-free light guiding.

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We propose a rectangular column two-dimensional square lattice photonic crystal to realize zero refractive index. Through analysis of the energy band structure of the photonic crystal structure, the lattice constant and side length of the rectangular columns can be optimized, and the Dirac cone dispersion appears at the center of the Brillouin zone. The Dirac cone is formed by the interaction of a monopolar eigenstate and a dipolar eigenstate to form a triple accidental degenerate state. The effective medium theory is used to invert the effective electromagnetic parameters of the photonic crystal with a double zero refractive index. The zero-phase change and the focusing characteristic of the concave lens of this kind of zero-refractive-index material are verified. Importantly, we have achieved transmission and reflection cloaking with this zero-index medium. Through the analysis of the amplitude and phase distribution characteristics of the electromagnetic field, it is proved that the designed cloaking devices have obvious cloaking effect.