Exploring the Applications of Subwavelength Nanostructures Fabricated by Solid Immersion Lloyd's Mirror Interference Lithography

Abstract

Nanofabrication is a powerful tool in modern scientific and technological advancement and plays a key role in the production of consumer electronics, sensors and actuators, and nanophotonic applications. Developed originally from the microelectronics industry, the fabrication techniques have been driven to push the boundary of nano-scale manipulation. However, constrained by the resolution-defining procedure - photolithography, as the fabrication scale approaches the fundamental diffraction limit, further reduction of nanostructure geometries becomes difficult. Integrated with advanced nanophotonic ideas, the most recent developments in plasmonic lithography enabled low cost convenient patterning of structures with a variety of nanometre-scale geometries and materials. By utilising the evanescent plasmonic near-field containing high frequency information, sub-diffractional features have been achieved. However the evanescent field decays exponentially into almost any medium, strictly limiting its penetration depth, and consequently prohibiting high aspect ratio fabrication. It has been previously demonstrated that an all-dielectric solid immersion Lloyd's mirror interference lithography (SILMIL) technique could create 56 nm linewidth features with 2:1 (height to half-pitch) aspect ratio using 405 nm illumination. The high resolution arises from the ultra-high numerical aperture (UHNA) accessed by the total internal reflection (TIR) induced evanescent field at the immersion prism interface. The high aspect ratio comes from a dielectric resonance underlayer which enhances evanescent field intensity. This thesis looked into the further improvement of resolution with 193 nm exposure and found that it is obstructed by the lack of suitable low-absorption high refractive index materials at this wavelength. However carefully tuning numerical aperture (NA) and dosage could refine aspect ratio (height:halfpitch) to an unprecedented 4:1. Capabilities of patterning 2D arrays of nano-pillars and nano-holes were also demonstrated. Deep subwavelength structures has been widely applied in a variety of fields such as sensing, communication, medicine, and many others. This work focuses on nanophotonic applications of SILMIL fabricated metallic nano-gratings. Reflectance of the metamaterial exhibited distinct polarisation dependence. Investigation of the birefringence in reflection mode using rigorous coupled wave analysis (RCWA) and finite difference time-domain (FDTD) simulations revealed complex coupling of excited surface plasmon polaritons (SPPs) with multiple Fabry-Perot resonances in the metamaterial. Switching from dielectric resonator to metallic resonator, the SILMIL technique shows theoretically the capability of phase retardation control, providing a new scheme of reflective thin-film waveplates. Another utilisation of the metallic nano-gratings links SPPs to spontaneous emission enhancement in the field of quantum information. Rare-earth ion doped crystal, as a popular candidate for optical quantum memory devices, suffers from low absorption and emission cross-section which limits its efficiency. Surface plasmons, owing to their fast response and highly concentrated energy, can increase the fluorescent intensity and decay rate. The plasmonic gratings on thin-film rare-earth ion doped crystal (Pr:YSO) sample were prepared by lifting o a silver coatings from the SILMIL fabricated gratings with 500 nm pitch. Spontaneous emission was investigated with a confocal fluorescent photo-counting setup. Least mean square curve fitting was used for data analysis as statistical modelling with Bayes theorem failed to cope with the low signal-to-noise ratio (SNR). Experimental results showed improvements of fluorescent intensity but not of the decay rate. There have also been a few other applications of SILMIL technique explored in this research, such as vertical organic transistors, and pyrolysed photoresist films. The complexity and the resonance nature of SILMIL procedure severely limited its compatibility with these processes. Integrating SILMIL into a wider scope of device fabrication would therefore be impractical unless future development such as a robust pattern transfer method could be established.