Nanostructured polymer- and metal surfaces: Antireflective and colour functionality

This Ph.D. thesis explores the optical properties of nanostructured dielectric and metallic surfaces. Focusing on scalable fabrication methods for antireflective nanostructures, this experimental study has resulted in the proof of concept of inexpensive, large area antireflective nanostructures, as well as structural enhancement of light absorption in thin metal films, deposited on nanostructured substrates.

Large areas of nanostructures were realized using the black silicon (BSi) method: a mask less reactive ion etching process, resulting in tapered nanostructures with tunable dimensions, on wafer scale. The BSi structures were replicated into the UV curable organicinorganic hybrid polymer Ormocomp, in order to further assess the optical properties of the structures.

BSi structures with lateral dimensions of around 1 μm would selectively scatter specific bands of wavelengths, resulting in a structural colour filter for specularly transmitted light. By reducing the height and lateral size, the structures enter a regime where scattering of visible light becomes insignificant.

In this regime, the BSi structures were shown to be antireflective. An empirical relation between the characteristic length scale of the nanostructured surface, and the wavelength at which scattering becomes significant, was shown. The result is thus a design criterion for the use of random nanostructures for non-scattering antireflective surfaces.

Antireflective BSi nanostructures were fabricated using injection moulding in polypropylene. A Ni shim was electroplated from a BSi master, and inserted in an injection moulding tool. The reflectance of the injection moulded parts was reduced from 4.5 % to 2.5% in the visible spectrum. The reflectance was calculated from the gradient in the refractive index from AFM data, using effective medium theory, and the measured and theoretical reflectance showed good agreement.

Binary antireflective nanostructures were fabricated in Ormocomp by replication from a Si master which was patterned using fast e-beam lithography. The reflectance was decreased from 4% down to 1%. Finally, the optical properties of thin metal films deposited on nanostructured surfaces was studied. When a metal film was deposited on BSi structures in Ormocomp, the reflectance of the metal film was lowered significantly, and the absorption was increased.

In contrast to their reflective planar counterparts, these thin metal films appear completely black. Two effects causing the increased absorption were suggested: a gradient effect, causing the metal film to be antireflective, and that the structured surface allows for coupling to surface plasmons in the metal film.