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Functional structures as well as materials provided by nature have always been a great source of inspiration for new technologies. Adapting and improving the discovered concepts, however, demands a detailed understanding of their working principles, while employing natural materials for fabrication tasks requires suitable functionalization and modification.
In this thesis, the white scales of the beetle Cyphochilus are examined in order to reveal unknown aspects of their light transport properties. In addition, the monomer of the material they are made of is utilized for 3D microfabrication.
White beetle scales have been fascinating scientists for more than a decade because they display brilliant whiteness despite their small thickness and the low refractive index contrast. Their optical properties arise from highly efficient light scattering within the disordered intra-scale network structure.
To gain a better understanding of the scattering properties, several previous studies have investigated the light transport and its connection to the structural anisotropy with the aid of diffusion theory. While this framework allows to relate the light scattering to macroscopic transport properties, an accurate determination of the effective refractive index of the structure is required. Due to its simplicity, the Maxwell-Garnett mixing rule is frequently used for this task, although its constraint to particle and feature sizes much smaller than the wavelength is clearly violated for the scales.
To provide a correct calculation of the effective refractive index, here, finite-difference time-domain simulations are used to systematically examine the impact of size effects on the effective refractive index. Deploying this simulation approach, the Maxwell-Garnett mixing rule is shown to break down for large particles. In contrast, it is found that a quadratic polynomial function describes the effective refractive index in close approximation, while its coefficients can be obtained from an empirical linear function. As a result, a simple mixing rule is reported that unambiguously surpasses classical mixing rules when composite media containing large feature sizes are considered. This is important not only for the accurate description of white beetle scales, but also for other turbid media, such as biological tissues in opto-biomedical diagnostics.
Describing light transport by means of diffusion theory moreover neglects any coherent effects, such as interference. Hence, their impact on the generation of brilliant whiteness is currently unknown. To shed a light on their role, spatial- and time-resolved light scattering spectromicroscopy is applied to investigate the scales and a model structure of them based on disordered Bragg stacks. For both structures the occurrence of weakly localized photonic modes, i.e., closed scattering loops, is observed, which is further verified in accompanying simulations. As shown in this thesis, leakage from these random photonic modes contributes at least 20% to the overall reflected light. This reveals the importance of coherent effects for a complete description of the underlying light transport properties; an aspect that is entirely missing in the purely diffusive transport presumed so far. Identifying the importance of weak localization for the generation of brilliant whiteness paves the way to further enhance the design of efficient optical scattering media, an issue that recently drawn great attention.
Unlike their plant-based counterparts, rigid carbohydrates, such as chitin, are currently unavailable for 3D microfabrication via direct laser writing, despite their great significance in the animal kingdom for the construction of functional microstructures. To overcome this gap, the monomeric unit of chitin, N-acetyl-D-glucosamine, is here functionalized to serve as a photo-crosslinkable monomer in a non-hydrogel photoresist. Since all previous photoresists based on animal carbohydrates are in the form of hydrogel formulations, a new group of photoresists is established for direct laser writing.
Moreover, it is exhibited that the sensitization effect, previously used only in the context of UV curing, can be successfully transferred to direct laser writing to increase the maximum writing speed. This effect is based on the beneficial combination of two photoinitiators.
In this, one photoinitiator is an efficient crosslinking agent for the monomer used, but a rather poor two-photon absorber. The other photoinitiator (called sensitizer) possesses, conversely, a much higher two-photon absorption coefficient at the applied wavelength but is not well suited as a crosslinking agent. In combination, the energy absorbed by the sensitizer is passed to the photoinitiator, resulting in the formation of radicals needed to start the polymerization. As this greatly increases the rate at which the photoinitiator is radicalized, resists containing a photoinitiator and a sensitizer are shown to outperform resists containing only one of the components. Deploying the sensitization effect in direct laser writing therefore offers a simple way to individually tune the crosslinking ability and the two-photon absorption properties by combining existing compounds, compared to the costly chemical synthesis of novel, customized photoinitiators.
The scales of white beetles strongly scatter light within a thin disordered network of
chitin filaments. There is no comparable artificial material achieving such a high scat-
tering strength within a thin layer of low refractive index material. Several analyses
investigated the scattering but could not explain the underlying concept. Here a model
system is described, which has the same optical properties as the white beetles’ scales
in the visible wavelength range. With some modification, it also explains the behavior
of the structures in the near infrared range. The comparison of the original structure and
the model system is done by finite-difference time-domain calculations. The calcula-
tions show excellent agreement with the beetles’ scales with respect to the reflectance,
the time-of-flight, and the intensity distribution in the far-field.