Extremely anisotropic epsilon-near-zero media in waveguide metamaterials.

Extremely anisotropic media such as anisotropic zero-index media allow wave propagation in extraordinary ways that are absent in isotropic systems. Here, we propose an approach to realize a type of extremely anisotropic effective epsilon-near-zero media by exploiting anisotropic waveguide metamaterials. The metamaterial is composed of two kinds of dielectrics with metal wires in waveguides. Extreme anisotropy is realized by alternating layers of the two kinds of dielectrics, which lead to different cut-off frequencies for transverse electric modes in different directions. Anisotropic effective epsilon-near-zero media with low loss can be obtained around the cutoff frequencies. Based on the extreme anisotropy, the unique phenomena of directive emission, nearly perfect bending waveguides and arbitrary control of energy flux are demonstrated. Interestingly, subwavelength focusing of energy flux with a spotsize <0.2λ is observed (λ is the wavelength in free space). Our work provides a convenient approach for the realization of extremely anisotropic epsilon-near-zero media and unique applications in low-loss waveguide metamaterials.