Ion beam etching of anodic aluminium oxide barrier layer for Au nanorod-based hyperbolic metamaterials

Porous anodic aluminium oxide (AAO) is widely used as a template for electrodeposition of ordered arrays of one-dimensional nanostructures. Prior to template filling, the barrier layer, which blocks the pores at their bases, should be removed. However, an increase in the AAO porosity during wet-chemical etching of the barrier layer is unavoidable. This makes it difficult to control the geometric parameters of the AAO films and limits their further application as templates. In this study, we demonstrate that ion beam etching of the AAO barrier layer is an effective and convenient technique for preparing AAO templates with the lowest possible porosity, which is equal to 8% in the case of the films obtained in 0.3 M oxalic acid at 40 V. The kinetics of the etching process is studied in detail. Based on the proposed ion beam based approach, Au nanorod-containing hyperbolic metamaterials (HMMs) with a certain spectral position of epsilon-near-zero point are fabricated. Transmission spectra measurement and simulation give self-consistent values of the gold volume fraction in the range from 7.3% to 8.4% confirming the high reproducibility of ion beam etching of the AAO barrier layer. In contrast, we show that chemical etching of the same AAO templates leads to a higher metal volume fraction in HMMs of more than 11% and a weaker dependence of the epsilon-near-zero position on the nanorod length. Reaching low volume fractions of metal makes Au nanorod-based HMMs promising candidates for use in nanophotonic devices with tuneable properties that are sensitive to the geometric parameters of structural elements.