  Semiconductor nanowires exhibit large polarization anisotropy for the
absorption and emission of light, making them ideal building blocks for novel
photonic metamaterials. Here, we demonstrate that a high density of aligned
nanowires exhibits giant optical birefringence, a collective phenomenon
observable uniquely for collections of wires. The nanowire material was grown
on gallium phosphide (GaP) (111) in the form of vertically standing GaP
nanowires. We obtain the largest optical birefringence to date, with a
difference between the in-plane and out-of-plane refractive indices of 0.80 and
a relative birefringence of 43%. These values exceed by a factor of 75 the
natural birefringence of quartz and a by more than a factor of two the highest
values reported so far in other artificial materials. By exploiting the
specific crystallographic growth directions of the nanowires on the substrate,
we further demonstrate full control over the orientation of the optical
birefringence effect in the metamaterial.
