Wideband zero-index metacrystal with high transmission at visible frequencies

Abstract

Materials with zero refractive index exhibit unprecedented optical properties including no spatial phase change and infinitely large phase velocity. Several zero-index material designs including artificial layered metallic/dielectric medium were proposed and demonstrated at microwave, terahertz, and IR wavelengths. However, realizing a zero-index material with low-losses, none-dispersion, and relatively wide bandwidth operation at visible frequencies is quite challenging due to optical losses in metals. Here, we propose and numerically demonstrate a three-dimensional zero-index metacrystal (ZIM) with low loss, low dispersion, and wide bandwidth at visible frequencies. The ZIM simply consists of periodic Ag nanocube arrays embedded inside a dielectric medium with same lattice constant in all directions. The calculated effective refractive index using a parameter retrieval method reveals a relatively wide band (∼40  nm) of near-zero index (<0.02) and achromatic behavior for designed metacrystal in the visible frequency. Using full-field electromagnetic (EM) simulations, we have theoretically demonstrated that the EM wave always propagates normal to the ZIM–air interface in spite of oblique incidence cases or any arbitrary wavefront of illumination. Our proposed zero-index metacrystal for visible frequencies could find use in many practical applications of wide-bandwidth and low-loss achromatic photonic devices for steering light propagation, arbitrary wavefront conversion, directional emission, and obstacle-free light guiding.