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Metamaterials have a number of interesting and potentially useful applications in a variety of fields, such as chemical and biological sensing, enhancement of spontaneous emission, nonlinear optics and as substrates for use in surface enhanced Raman spectroscopy (SERS). However, to date the low-wavelength cutoff for the majority of work at the higher frequency end of the spectrum has been determined by use of the coinage metals, which intrinsically prohibit their implementation below a vacuum wavelength of approximately 500nm for gold and 350nm for silver.
Producing nanostructured plasmonic media that exhibit metamaterial functionalities in the ultraviolet will have a number of benefits. Not only will working in a new range of the electromagnetic spectrum allow for higher energy photons to be controlled, but a number of other benefits arise from the behaviour of different materials in the ultraviolet. For instance, many biological molecules, including DNA, exhibit fluorescence in the UV range, allowing for label-free detection and analysis of biological material; the intrinsic electronic absorption can be used to increase this label-free bio-sensitivity as well as enable the possibility of SE(R)RS, a process further enhanced by the frequency dependence on the efficiency of this scattering process.
Here, we demonstrate the fabrication and characterisation of metamaterials operating in the deep-near UV. By using alternatives to the coinage metals, including aluminium and gallium, we have measured optical responses in the system down to approximately 200 nm. Sample preparation utilises a self-assembly method, allowing for the production of macroscopic-sized assemblies (> 1 cm2) of nanometric elements (radius ~ 25 nm, separation ~ 100 nm). Careful control of the fabrication conditions allows fine control of the structural parameters, which in turn allows tunability of the optical properties over a wide range of wavelengths (> 200 nm). The structures produced include gallium nanorods, oriented with their long axis perpendicular to the substrate and having a sub-wavelength interrod distance, fabricated via oxygen- and water-free electrodeposition into nanoporous anodised alumina (AAO), and aluminium nanohole arrays fabricated using AAO as a mask for ion milling, with elements at the same nanometric size scale. 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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