Wideband zero-index metacrystal with high transmission at visible frequencies [Invited]

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.