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Optical metamaterials have been shown to offer a number of useful properties, including enhancement and control of spontaneous emission, chemical and biosensing and nonlinear optical control and manipulation. To date, however, the vast majority of research has been conducted into the properties of visible (or longer) wavelength systems. The materials typically chosen for this work, such as the coinage metals (Ag, Au, Cu) for visible wavelength applications become unsuitable for use in the ultraviolet due to inter- and intraband driven absorption. In order to develop metamaterials in the ultraviolet different materials need to be considered.
Ultraviolet metamaterials are proposed to have additional benefits and functionalities in addition to those present in previously considered systems. For instance, the autofluorescence of biological materials typically lies in the UV range (DNA fluoresces at around 260 nm) and this can be coupled to the optical response of UV metamaterials to allow label-free fluorescence (allowing the studying of biological materials and cells in a near-native state, without the use of potentially bio-perturbing dyes) as well as detection at lower concentrations. UV-range substrates are also of interest for SERS applications due to the fact that the enhancement scales as ν^4, dramatically increasing the efficiency of the process.
Here we demonstrate the development and characterisation of a large area, self assembled metamaterial for use in the ultraviolet wavelength range. Anodised aluminium oxide (AAO) provides a template for the growth of nanorods of deep-UV suitable metals, Aluminium and Gallium. These metamaterials consist of nanorods with geometric parameters smaller than the free-space wavelength of UV light (diameter around 25 nm, inter-rod separation around 60 nm) grown vertically from a UV-suitable substrate. The precise geometric parameters are controlled by the anodisation conditions, allowing tunability of the spectral response of the metamaterial. Aluminium is well documented to be the best choice of material for UV plasmonic and metamaterial use, due to its large, negative real permittivity and low imaginary permittivity in the UV range, and gallium presents interesting behaviour due to its relatively low melting point (30°C), with the liquid and solid state showing significant differences in their optical properties. Both systems have been optically characterised across the UV and visible wavelength ranges and compared with numerical modelling in order to analyse and describe their behaviour.
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