Fabian Michael Lütolf
Since most of the academic photonic and plasmonic nanostructures are based on slow and expensive electron beam lithography processes, there is a need for innovative alternatives suitable for industrial manufacturing. Large areas need to be patterned with a high throughput, which is for example achieved with roll-to-roll machines. Up-scalable designs of photonic and plasmonic nanostructures are therefore studied in this thesis. Typical industrial processes include embossing and evaporation, which are consequently used throughout the thesis. I propose oblique evaporation of high refractive index materials to render binary gratings highly efficient for first order transmission. Zinc sulfide coatings are employed to diffract close to 70% of unpolarized green light. Simulations further show that they can be encapsulated to protect them from environmental influences like humidity, wear or dust, while retaining their exceptional diffractive properties, which is very appealing for outdoor applications. I also show that thin metallic coatings can attain similar efficiencies for TE polarized light. The effect is asymmetric and shows a maximum at the Wood-Rayleigh anomaly, which results in orientation dependent coloration of the zero order as well as first order transmittances. A large part of the standard RGB gamut can be covered through proper adjustment of the grating parameters. Combination of zero order and first order effects allows creation of color appearances that switch when rotating or flipping the device. I finally present how floating images become apparent when a patterned light source like e.g. a mobile phone is used in conjunction with the metallized grating. Direct transfer of plasmonic technology from universities to industry is often not possible: in academia, metallic nanostructures often require a lateral resolution of a few nanometers, which is challenging to achieve in up-scalable processes. Thicknesses on the other hand can be controlled in this regime using evaporation techniques, which are hence powerful methods for high-throughput production. In this thesis, Fano-resonant, U-shaped nanowires are created with oblique metal deposition. In order to make them available for mass-production, aluminum is chosen as the plasmonic material. The surface integral equation method is used to investigate near-fields and charge distributions, which shed light onto the physics behind the present resonances. A surface plasmon polariton is found to couple to a localized plasmonic mode with a hexapolar charge distribution. It is finally shown that the Fano-resonance can be accurately tuned by adapting evaporation angle and metal thickness. These two parameters can easily be accessed and would allow for good control over the optical response even in an industrial environment. The applicability of the above insights is then demonstrated by creating a strain sensor. To that end, the process is transferred to a stretchable polymer and when elongating the structure perpendicular to the wires, the polymeric spacing between them is expanded. The sensitivity of the Fano-resonance to this change in inter-wire distance is investigated and a strong damping is observed. Through careful design, a clearly visible color switch from purple to green is achieved for elongations less than 20%. The sensor was deemed to be very durable, as no deterioration in the color or the spectral response was observed even after several strain cycles.
EPFL2015
