Nanomanufacturing of functional nanostructured surfaces for efficient light transport

Abstract

Nanostructured surfaces have given rise to many unique optical properties, such as broadband anti-reflectivity, structural coloring effects, and enhanced light extraction from high refractive index materials due to their potential to modulate optical behavior on their surfaces. This thesis focuses on design, analysis, and fabrication of functional nanostructured surfaces for efficient light transport, seeking optimized optical performance, high mechanical robustness, and manufacturability, with the aim of increasing the practicality of the photonic nanostructures. First, for the case when light propagates from a low-index material to a high-index material, I designed and fabricated an array of inverted nanocones that realizes anti-reflectivity with robust mechanical strength. The surface exhibits broadband, omnidirectional anti-reflectivity due to the axially varying effective refractive index of the inverted nanocone arrays. The surface also maintains its optical performance after being externally loaded, thanks to low stress concentration and small deflection of the inverted nanocone structure. In addition, for multi-optical interfacial surfaces, double-gradient- index nanostructures are proposed and demonstrated in order to achieve ultimate anti-reflectivity. The top surface, textured with inverted nanocones, maintains high mechanical robustness. Second, for the case where light has to be extracted from high-index materials, a conical photonic crystal is proposed and demonstrated. The tapered conical geometry suppresses Fresnel reflections at the optical interfaces due to adiabatic impedance matching. Periodicity of the arrays of cones diffracts light into higher-order modes with different propagating angles, enabling certain photons to overcome total internal reflection (TIR). After optimizing the structural geometries to balance Fresnel reflection and TIR, light yield efficiency is characterized experimentally on scintillator surfaces. In order to enhance the adaptability to industrial manufacturing, the fabrication methods are based on replicating the photonic nanostructures into a UV-curable polymer, with the help of laser interference lithography as a method of fabricating a master mold. Advanced techniques such as vacuum assisted-filling and a selective delaminating method are also developed to produce nanostructures more effectively. The novel nanostructured surfaces designed in the thesis, and the ability to imprint these topographies through several generations, are promising for large-scale commercial applications where efficient light transport is important.