Giant birefringence is demonstrated in ensembles of randomly grown, vertically aligned semiconductor nanowires. Bottom-up fabrication of epitaxial semiconductor nanowires grown using the vapor-liquid-solid mechanism yields a highly anisotropic photonic metamaterial. The birefringence in the material is shown to depend on the nanowire length. Short nanowires with a volume filling fraction around 50% exhibit the strongest birefringence, in agreement with effective medium theories. A large difference is found between the in-plane and out-of-plane refractive indices of Δn=0.8, exceeding by a factor of 75 the natural birefringence of quartz and by more than a factor of 2 that of inverted artificial materials.

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