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Zero-index metamaterials exhibit exotic optical properties such as uniform spatial phase and infinite wavelength. These extreme properties can be utilized for integrated-optics applications. However, practical implementation of zero-index-based photonic devices requires compatibility with complementary metallic-oxide-semiconductor (CMOS) technologies. Zero-index metamaterials have been previously demonstrated in both out-of-plane and integrated configurations by taking advantage of a photonic Dirac-cone dispersion at the center of the Brillouin zone. Such metamaterials feature a square matrix of high aspect-ratio pillars and offer matched impedance through simultaneously zero effective permittivity and permeability. However, these configurations are inherently incompatible with integrated devices due to out-of-plane excitation, metallic inclusions, or high aspect-ratio structures.
This work demonstrates a CMOS-compatible zero-index metamaterial consisting of a square array of air-holes in a 220-nm-thick silicon-on-insulator wafer. To experimentally verify the refractive index, we measure the angle of refraction of light through a triangular prism consisting of the metamaterial. The index is extracted using Snell's Law to verify a refractive index of zero at a wavelength of 1625 nm. Through the air-hole in silicon configuration, the proportion of silicon is increased as compared to designs based on high aspect-ratio silicon pillars. This enables a platform with low-aspect-ratio features, improved confinement of transverse electric polarized light, as well as the original benefit of matched impedance.
Featuring a trivial monolithic fabrication and capacity for integration with the expansive library of existing silicon photonic devices, this metamaterial enables implementation of proposed zero-index devices and offers a powerful platform for exploring the future applications of zero-index materials.
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