Engineering Absorption for Foil Applications

Light matter interactions such as extinction, reflection, and transmission can be described by classical optics. Dur-ing the past decades, research focused on a novel resonant extinction type called plasmonics that allows for en-hancement of both types of extinction, absorption and scattering, depending on the size of the involved nanostruc-tures. Plasmonics is a resonant interaction of light with conducting matter, especially metals with a free electron density in excess of 1022 cm-3. The incident light excites oscillating electrons movement and this leads to absorption cross sections far exceeding the geometrical ones for sub-micron sized metal particles.In this thesis, I research resonant absorption phenomena in three different and rather classical systems that can be principally manufactured on industrial scale using modern foil production. The chosen systems are carefully studied by simulations to reveal important parameters for tuning absorption for the application at hand. I reveal that the underlying absorption mechanisms of these three systems are different and, depending on the application, one may select the most suitable of these three systems to exploit narrow or broadband absorption.The first system is based on interacting and randomly arranged metal nanoparticles placed atop a mirror surface forming a lossy Fabry-Perot cavity. Its sensitivity to parameters such as mirror distance, particle shape, polarization, spacing, metal type, and dielectric environment are investigated. This random system maximizes absorption across the entire visible spectrum and offers the flexibility to alter various system parameters including metal type and filling fraction to shift the absorption band spectrally at will. The focus of the second system is a simplification of the first one to facilitate mass production and reproducibility of the system, while still achieving near perfect absorption across the visible spectrum. This initially black appearing system is then modified locally by laser processing to realize high reflection or full transmission depending on laser pulse energy. The influence of the system parameters on the laser process window are investigated in detail and the findings are applied to realize a colorful 3D art print consisting of black, reflective, and transparent microscopic elements. The third and last investigated system en-hances absorption of three dye lacquers atop corrugated surfaces. It is found that the interaction of a metal crossed grating with these dye lacquers can enhance absorption locally up to a factor of 20. Compared to applying the same lacquers on flat surfaces, this strong enhancement turns otherwise barely visible dye lacquers colorful and vivid.The results demonstrate that very different systems can be used for absorption optimization. Each system uses a unique mechanism for local field enhancement to achieve strong absorption, inter-particle coupling, interference, or interaction of grazing propagating as well as evanescent grating orders. The systems studied in this thesis can be mass produced using cost effective and scalable roll-to-roll foil production. This makes them especially attractive for applications in document authenticity protection and visual sensing, but also any other application that de-mands tuning of absorption to a selected spectral band.