Theses 2024
Assessing the Accuracy and Efficiency of Rigorous Coupled Wave Algorithms (RCWA) for Photonic Waveguide Applications
Abstract: The thesis endeavors to investigate the application of an existing implementation of rigorous coupled wave algorithm (RCWA) for the simulation of photonic waveguides and waveguide-based photonic components. To investigate and demonstrate novel applications of the RCWA, typical setups consisting of photonic waveguides and directional and grating couplers are implemented. Comprehensive simulation studies and comparison to literature data and the theory are used to identify critical simulation parameters, to devise appropriate simulation strategies, and to develop procedures and recipes for the application of the RCWA simulation practical photonic problems. This study provides insights into the capabilities and limitations of the RCWA module for designing waveguide structures.
Experimental Characterisation of Spatially and Temporally Focused Femtosecond Laser Pulses
Abstract: Simultaneous spatial and temporal focusing (SSTF) offers a wide variety of potential applications where light with a high energy density is required and needs to be controlled in all three dimensions. […] (It) enables its users to manipulate the duration, shape and propagation of light pulses in unique ways. […] When spatial and temporal focusing are combined to focus a pulsed laser, this […] can create a volume […] which has a shorter and a flatter intensity distribution along the optical axis. This technique is highly interesting for delicate applications like eye-surgery and high-precision micro-machining. The realisation of SSTF for ultrashort pulses of 35 fs at a repetition rate of 1 kHz was demonstrated and experimentally compared to a traditionally focused pulsed beam with a z-scanning two-photon excitation fluorescence measurement technique.
Development of an Optical Setup and an Image Analysis for the Measurement of Contact Angles and Contact Angle Hysteresis of Fluids with Low Surface Tension
Abstract: Dropwise condensation (DWC) is a condensation mode where condensate forms discrete droplets rather than a continuous film, as observed in filmwise condensation (FWC). DWC offers a significant advantage in that droplets of different sizes drain off the cooled surface continuously. This dynamic drain off process effectively prevents the formation of an insulating condensate layer and results in substantially higher heat transfer coefficients compared to FWC. Consequently, promoting DWC can significantly enhance the energy efficiency of equipment and processes. The successful promotion of DWC for a specific fluid depends on its interaction with the condensation surface, which is determined by the wetting behavior. This behavior is quantified by the contact angle at the three-phase boundary of a liquid droplet resting on a solid surface. In an idealized static scenario assuming a perfectly smooth and isotropic solid surface, Young’s law describes how the contact angle is influenced by the surface tension of the fluid and the surface free energy of the solid. Achieving DWC with discrete droplets necessitates a sufficiently high contact angle. When dealing with low-surface-tension fluids, this requires compensation by solid surfaces with low surface free energy. The contact angle influences the condensation heat transfer coefficient by affecting the contact areas at the liquid-vapor and liquid-solid interfaces. Additionally, it impacts the conduction resistance within droplets of a specific volume, thus playing a critical role in determining the efficiency of the condensation heat transfer process. To measure contact angles, the sessile drop technique, also known as the static drop method, is widely used. This technique involves observing a liquid droplet on a solid surface in equilibrium. Theoretical models serve as tools to analyze and predict the behavior of liquid droplets on various solid surfaces under different conditions, including, for example, the effects of surface roughness. In the context of DWC, also the dynamics of surface wettability, described by contact angle hysteresis, plays a crucial role. Contact angle hysteresis is defined as the difference between the advancing contact angle of the liquid advancing onto a previously unwetted solid area and the receding contact angle describing the situation when the droplet is removed from a previously wetted area. As a low hysteresis value indicates efficient droplet drain off and prevents the spreading of the droplets into a continuous film, minimizing the contact angle hysteresis is essential for optimizing the DWC process, especially of low-surface-tension fluids exhibiting small contact angles. To minimize hysteresis, the foremost task is to minimize surface roughness. This shows that the accurate measurement of the different types of contact angles is important for characterizing the wettability of solid surfaces designed to promote DWC and for connecting the corresponding droplet shapes with heat transfer. Therefore, this thesis aims to design and implement an optical system capable of capturing high-quality images and videos. This system enables the precise measurement of contact angles, particularly for low-surface-tension fluids, under carefully controlled conditions, preferably close to those occurring during condensation. To analyze the acquired images, specialized software using appropriate algorithms to identify droplet contours and accurately determine contact angles should be developed. This analysis will be conducted for static, growing, and shrinking droplets, focusing on the angles formed between the tangents to the liquid-gas interface at the three-phase line and the solid surface on both sides.