2.1. Metal Metasurface Fabrication

Asymmetric metal nanoantenna metasurface arrays were fabricated by electron beam lithography and optically characterised by micro-Fourier transform infrared spectroscopy. A SiO₂ surface, thickness 192 nm, was thermally grown by standard PECVD on a commercially available double-side-polished silicon wafer, of thickness 525 μm (MicroChemicals, Ulm, Germany). The wafer was diced into chips of dimensions 10 × 10 or 20 × 20 mm². The nanostructured arrays were then defined with a Raith eLine Plus electron beam lithography system (Raith, Dortmund, Germany) using polymethyl methacrylate (950PMMA A4 PMMA, Micro Resist Technology, Berlin, Germany) spin-coated at 4000 r.p.m. for 1 min, baked at 180 °C. Initial geometrical design parameters were determined with input from FDTD studies. The metal nanostructures were deposited in an Edwards Auto 306 thermal evaporator (Edwards, Burgess Hill, UK) (vacuum pressure 10⁻⁶ mBar), to cover large surface areas (2 × 2 to 10 × 10 mm²) of the SiO₂ substrates. Following metal lift-off and solvent based cleaning (acetone, isopropanol), the surface geometries were analysed by scanning electron microscopy (SEM), using the same eLine system.

Dimensions of the metal surface arrays were determined with an approximate measurement uncertainty of 0.025 μm, originating from geometric non-uniformity of fabricated metal nanoantenna and SEM measurement precision. Both the array-to-array and wafer-to-wafer repeatability was found to be within this same uncertainty range. Deposited metal thicknesses were verified with a Dektak 150 Surface Profiler (Veeco, Santa Barbara, CA, USA) within 2 nm. Thicknesses of the deposited Au nanoantenna structures were 50 ± 5 nm.