High-aspect-ratio mushroom-like silica nanopillars immersed in air: epsilon-near-zero metamaterials mediated by a phonon-polaritonic anisotropy.

Epsilon-near-zero metamaterials offer opportunities for intriguing electromagnetic-wave phenomena. Here we experimentally demonstrate that silica perpendicular nanopillars immersed in air exhibit a uniaxial epsilon-near-zero response mediated by phonon polaritons in the mid-infrared range. Unique mushroom-shaped heads on nanopillars play a crucial role to realize SiO metamaterials over a large area in our self-assembled fabrication process with block copolymers, polystyrene--poly(dimethylsiloxane) (PS--PDMS). SiO nanopillars having heights of 80 nm, 200 nm, and 300 nm (aspect ratios up to ∼13) are obtained after calcination at 450 °C and the electromagnetic responses are evaluated using a mid-infrared ellipsometric apparatus. For nanopillars with 200 nm height, the permittivity of the perpendicular component approaches to near zero (0.2) while the parallel component shows a value of 1.8. The measured uniaxial epsilon-near-zero responses are excellently reproduced by the effective medium theory. Our results, therefore, indicate that SiO nanopillars/air uniaxial epsilon-near-zero metamaterials in the mid-infrared range can be amenable to large scale fabrication.