School of Physics - Theses
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ItemTowards Scalable Fabrication of Plasmonic Devices via Nanoimprint Lithography (NIL)( 2021)Advances in theoretical and computational techniques for advancing concepts in nanophotonics along with major development in nanofabrication technologies has enable the realization of nanoscale optical device with unprecedented ability to control light such as plasmonic metasurface. Fabrication of plasmonic devices, however, has long been relying on conventional lithography technique such as electron beam lithography (EBL), focused ion beam (FIB) milling and photolithography (PL). While these lithography techniques have enable construction of sub-wavelengths nanostructures, their reliance on the interaction between charged particles or light beam with polymer resist during pattern writing process require intricate control systems and usually require high operating and maintenance cost. In fact, for FIB and EBL, the writing time can be very slow depending on the pattern area. These approaches, therefore, are unsuitable for industrial scale production thus hindering commercial uptake of metasurfaces. Therefore, there is a need for an alternative low-cost, high-throughput nano-manufacturing approach to overcome the bottleneck in conventional lithography methods. With this motivation in mind, this thesis focused on utilizing the nanoimprint lithography (NIL) technique for fabrication of the designated plasmonic device. NIL uses pattern transfer approach whereby a predefined pattern on a mould is used to replicate the pattern onto another surface. NIL offers scalability and high-throughput large area manufacturing of nanostructures. In this thesis, the versatility and capability of NIL for producing plasmonic device, particularly plasmonic colours, with a potential of scalability is investigated. This includes the investigation of the dynamic flow of the resist during the nanoimprinting process through various process parameters. With this information, it will be shown that NIL is capable of producing multi height, grayscale-like structures through a careful control of the flow of the resist, without relying on control of charged particle or light beams. This novel technique was then utilised as a method to print the plasmonic colouration device, with ability of controlling the hue and saturation of the resulting colours via tuning of the vertical gap size of the structure. Additionally, an investigation of multispectral, polarisation-selective iridescence plasmonic colour will also be shown in this thesis. This was achieved via the elongation of the spatial dimension of plasmonic structure which results in multiple resonances in both visible and infrared region of the spectrum. Such multispectral system will be very useful especially for optical security device. Finally, a novel plasmonic colour designed to preserve the vividness of the resulting colour under unpolarised ambient lighting condition will be demonstrated in this thesis. This involves production of polarization-independent plasmonic structure featuring symmetric cross structures. It will be shown that this device produced excellent colours with preserved hue and vividness, regardless of polarization state of light