  Metamaterials offer unprecedented flexibility for manipulating the optical
properties of matter, including the ability to access negative index,
ultra-high index and chiral optical properties. Recently, metamaterials with
near-zero refractive index have drawn much attention. Light inside such
materials experiences no spatial phase change and extremely large phase
velocity, properties that can be applied for realizing directional emission,
tunneling waveguides, large area single mode devices, and electromagnetic
cloaks. However, at optical frequencies previously demonstrated zero- or
negative-refractive index metamaterials require the use of metallic inclusions,
leading to large ohmic loss, a serious impediment to device applications. 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 possesses a nearly isotropic
low-index response leading to angular selectivity of transmission and directive
emission from quantum dots placed within the material.
