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Tuning the effective plasma frequency of nanorod metamaterials from visible to telecom wavelengths

Hyperbolic plasmonic metamaterials are important for designing sensing, nonlinear, and emission functionalities, which are, to a large extent, determined by the epsilon-near-zero behaviour observed close to an effective plasma frequency of the metamaterial. Here, we describe a method for tuning the effective plasma frequency of a gold nanorod-based metamaterial throughout the visible and near-infrared spectral ranges. These metamaterials, fabricated by two-step anodization in selenic acid and chemical post-processing, consist of nanorods with diameters of around 10 nm and interrod distances of around 100 nm and have a low effective plasma frequency down to a wavelength range below 1200 nm. Such metamaterials open up new possibilities for a variety of applications in the fields of bio- and chemical sensing, nonlinearity enhancement, and fluorescence control in the infrared.