Design, Fabrication and Characterization of Ordered and Disordered Photonic Materials

Academic or specialist Other
03.12.2021 15:00 - 16:00

In this thesis we describe the interaction of light with ordered and disordered photonic structures. Photonic materials influence light in peculiar ways. Often the center of interest are materials with a photonic band gap. This latter describes a range of frequencies where light is prohibited to enter the medium. For periodic dielec-tric structures, or photonic crystals, the emergence of the photon-ic band gap is well understood as the interference of electro-magnetic radiation reflected at the Bragg planes. This reasoning is not applicable for amorphous photonic materials due to the lack of long-range order, a fully satisfying explanation is still miss-ing. Furthermore, the influence of defects, which inevitably occur during production, on the scattering strength of the material is not well understood yet.
In the first part of the thesis, we design a writing protocol to fabri-cate two-dimensional photonic structures and we measure their transmittance. The structures are produced using direct laser writ-ing (DLW). DLW uses a two-photon absorption process to poly-merize a resist at the focal point of the laser. The comparison of the calculations and measurements show that we are able to create high quality 2D photonic structures with the predicted photonic features.
The second part is about reaching bandgaps with smaller wave-lengths by heat-shrinking polymer photonic hyperuniform struc-tures. Due to physical limitations, the minimum feature size reachable with the DLW set-up is limited. For some applications, smaller structures are required. We reduce the structure size by isotropically shrinking hyperuniform three-dimensional structures. By inverting them into a higher index material using atomic layer deposition and chemical vapor deposition, a band gap appears at wavelengths lower than 2$\mu$m.
The last part presents a study where we introduce defects in a woodpile photonic crystal to monitor its effects on the scattering properties. By adjusting the laser power in some segments of the woodpile structure we can generate thicker (positive defects) and thinner (negative defects) rod segments. We measure the reflectance and transmittance to calculate the scattering mean free path. The results show that for a low defect density, the de-fects act like a dilute gas of scatterers that can be described us-ing the Rayleigh-Gans-Debye approximation.

When? 03.12.2021 15:00 - 16:00
Where? PER 08 1.50
Chemin du Musée 3
1700 Fribourg
speaker Stefan Aeby, présentation publique de thèse de doctorat
Group Scheffold

Contact Département de Physique
Prof. Scheffold