Sub-wavelength optical phenomena and their applications in nano-fabrication
This dissertation presents the numerical study of sub-wavelength optical phenomena and experimental demonstration of their applications in nanoscale patterning. The optical near-field enhancement associated with sub-wavelength scale nanostructures, such as nano-ridges and nano-tips, were utilized to produce nanoscale patterns. Numerical simulation using finite difference time domain (FDTD) method were employed as a modeling tool to predict and optimize a scheme to achieve parallel patterning by near-field enhanced direct nano-molding. In order to pattern the whole area of the substrate, the laser light was chosen to be transparent to the substrate and was shine from the back with an incident angle. The near-field enhancement facilitates the local ablation to form line or dot patterns on the thin film coated on the substrate. Fabrication process to manufacture the nanostructure used as the mold was also developed. Further investigation into the near-field enhancement phenomena leads to the study of underlying physics: surface plasmons (SPs) and SP-light coupling. Previous research efforts have shown that transmission through sub-wavelength apertures can be orders of magnitude higher than predicted by aperture theory due to SP-light coupling. In this dissertation study, the SPs excited on the nano apertures were combined with the SPs on the substrate to overcome the two fundamental restraints of light at sub-wavelength scale, transmission and diffraction. The discovery was exploited for nanolithography, coined Surface Plasmon Assisted Nanolithography (SPAN), and one-to-one nanoscale pattern transfer has been achieved using both laser light and UV lamp without losing convenience and simplicity of traditional photolithography technique. High contrast optical near-field interference generated by SP-light coupling was also studied and combined with the photolithography technique to produce threedimensional (3D) nanostructures. Different multi-layer 2D/3D periodic polymeric nanostructures have been directly fabricated using such a mechanism in a typical photolithography setup. The nanostructures fabricated can be easily controlled in terms of size, layout, and defects by designing the mask. This so-called Surface Plasmon Assisted 3D Nanolithography (3D-SPAN) was demonstrated in experiments to offer flexible and convenient 3D nanofabrication capabilities.