Realization of an all-dielectric zero-index optical metamaterial

Metamaterials offer unprecedented flexibility for manipulating the optical properties of matter, including the ability to access negative index(1-4), ultrahigh index(5) and chiral optical properties(6-8). Recently, metamaterials with near-zero refractive index have attracted much attention(9-13). Light inside such materials experiences no spatial phase change and extremely large phase velocity, properties that can be applied for realizing directional emission(14-16), tunnelling waveguides(17), large-area single-mode devices(18) and electromagnetic cloaks(19). However, at optical frequencies, the previously demonstrated zero- or negative-refractive-index metamaterials have required the use of metallic inclusions, leading to large ohmic loss, a serious impediment to device applications(20,21). Here, we experimentally demonstrate an impedance-matched zero-index metamaterial at optical frequencies based on purely dielectric constituents. Formed from stacked silicon-rod unit cells, the metamaterial has a nearly isotropic low-index response for transverse-magnetic polarized light, leading to angular selectivity of transmission and directive emission from quantum dots placed within the material.