Abstract
Plasmonic colour technology has been used by humans since the 4th century, in the form of coloured metal nanoparticles. Recently, advancements in fabrication technology have allowed the precise determination of the size and arrangement of these particles, leading to the new technology of plasmonic colour printing. With the advantages of high resolution and capability of generating many types of colours using one type of structure, academic and commercial interest in this field has grown rapidly, and materials like aluminium and copper have been studied besides the traditional silver and gold. In this thesis, we discuss new types of nanostructure for generating colours– all-metallic nanoposts and nanopits – and apply metal-insulator-metal (MIM) gap plasmonic structures to multi-level security prints by manipulating their infrared and darkfield reflectances. The nanostructures were fabricated and characterised to understand the colours produced, and numerical simulations were performed to understand the plasmon resonances responsible for the features in the reflectance. A theoretical model is also developed to explain the darkfield spectra for the MIM structures. The all metallic nanostructures were fabricated using silver, and shown to give a wide range of colours. The effects of varying the structural width and array pitch were studied, and it was observed that the nanopost structure supports a local plasmon resonance that cycles between electric and magnetic modes. Interestingly, the simulations showed that for certain structural widths, the energy absorption is nearly-perfect, which would give rise to saturated colours. The silver structures were also protected from degradation via contact with air by encapsulation in glass. The MIM structures were made using aluminium and aluminium oxide, so that an array of aluminium disks stands on a thin layer of native aluminium oxide that is formed on bulk aluminium. The dielectric intermediary layer creates a gap between the metals, so that a gap plasmon is formed. The gap plasmon has a useful feature of being tunable across the ultraviolet, visible and infrared, which was exploited to create a two-level security print – with one visible brightfield colour image, and one infrared image. In addition, we showed that the MIM disk structures display pitch-dependent blue and cyan colours under darkfield illumination, and used rectangular arrays of disks to create a four-level security print, with two linearly polarised darkfield colour images besides the visible brightfield and infrared images. The darkfield colour gamut was extended by changing the disk arrangement, so that clusters of four disks were formed while maintaining the same total areal density, which gave rise to a plasmon mode and a reflectance minimum at a wavelength around 500 nm.