Barth, Carlo: Analysis of photonic crystals for interaction with near-surface emitters. , 2018
Technische Universität Berlin
Open Accesn Version

The tailorable optical properties of photonic crystals enable a broad spectrum of applications beyond just the photonic bandgap. Leaky modes of photonic crystal slabs which couple to the external light field can facilitate light extraction from emitters located inside the slab or near the surface, known as the enhanced extraction effect. This effect can enhance light-emitting diodes or the output of light-emitting tags used throughout life science research and in vitro diagnostics. Moreover, the photonic crystal slabs can generate massively increased field energy densities. This allows for the enhancement of the emission rate of near-surface light emitters such as fluorescent dyes, molecules or quantum dots. This excitation enhancement effect makes the photonic crystal surface act as a wavelength-selective optical resonator and is widely used in the field of label-free biosensing. To design photonic crystals for applications that use excitation enhancement, the absolute increase in near-field energy must be taken into account together with the spatial distribution of the inducing fields. The huge parameter spaces and amounts of data that arise from these needs necessitate powerful numerical methods for the analysis and the systematic tailoring of such systems. In this thesis numerical techniques based on simulations using a versatile and error-controlled finite element Maxwell solver are studied in view of the design and analysis of photonic crystal for the interaction with near-surface emitters. Related experiments are presented and analyzed numerically in order to explain the measured effects, also considering a novel approach of machine learning-based classification of photonic crystal mode profiles. A numerical platform for the treatment of photonic crystals based on finite element simulations is created and optimized regarding accuracy and performance. Experimentally, the symmetry dependence of anticrossing phenomena in photonic crystal slabs, and fluorescence enhancement of lead sulfide quantum dots by excitation enhancement on a photonic crystal surface are presented. In the first experiment, the deactivation of band-anticrossing by restoring a symmetry of the system is demonstrated and traced back to the orthogonality properties of the corresponding modes. In the second experiment, the measured wavelength- and illumination direction-dependent fluorescence enhancement is compared to the numerically obtained field energy enhancement and 3D field distributions. For both experiments, a systematic analysis is conducted using a clustering technique that reduces the vast field distribution data to a minimal set of representative modes. The set of tools that has been developed and evaluated allows for the optimization of photonic crystal slabs for virtually any application based on excitation enhancement. In biosensing, such applications are relevant for high sensitivity cancer biomarker detection or for label-free high-resolution imaging of cells and individual nanoparticles, where the mentioned methods can be used to further increase the sensitivities. The techniques are potentially suited to systems designed for extraction enhancement, such as for light-emitting diodes, or a combination of excitation and extraction enhancement. Moreover, a proposal is made for a photonic crystal enhanced photochemical upconversion system relevant in various applications (e.g. solar energy harvesting or photodetectors).