Theses 2017

Abstract: A laser-induced fabrication of novel carbon flake intercalated with Au-Ag nanoparticles was presented at St Petersburg State University under the supervision of Dr. Alina Manshina recently. The resulting carbon flakes feature a crystalline structure intercalated with metal nanoparticles which have never been discussed and presented before. In this report, the optical properties of these nanostructures, namely attenuation, birefringence and diattenuation are discussed. These optical properties were calculated from the microscopic Müller matrix based polarization measurements. The measurements were done primarily for a carbon flake of dimension 5.1μm x 2μm with a height of 340nm. As a comparison of the optical properties, measurements were performed for two other nanostructures with heights 176nm and 140nm. This master thesis acts as a stepping stone to further investigate and understand these complex carbon flakes and also enabling future applications. 

Abstract: Recently, the advance in terahertz (Thz) technology have offered unique opportunities in the application of THz frequency range in spectroscopy, sensing and imaging. Large progress has been achieved in this area and some birefringent materials have been applied for THz applications, for instance, the liquid crystals are used as THz wave plates, and the crystals of lithium niobate (LiNbO₃) have been employed in the THz wave polarizer. The optical components conduction at the THz frequency range are still not widely demonstrated due to some technical issues on THz. Recently, some doped optical crystals have been found with higher laser damage threshold, thermal values and remarkable features like pure YVO₄ crystal and the currently used LiNbO₃crystal, for instance, or Neodymium (Nd)-doped YVO₄. Besides, several parameters, such as temperature-dependent electro-optical constants, thermos-optical coefficient and Sellmeier formula parameters, which are vital to utilize the features in the THz polarizers and wave plates, are rarely reported in the published articles. For instance, the uniaxial optical crystal yttrium vanadate (YVO₄), a well-developed laser material, has strong anisotropy in thermal and optical properties with birefringence in the THz range. Yet, the thermal effect and optical crystal yttrium vanadate (YVO₄), a well-developed laser material, has strong anisotropy in thermal and optical properties with birefringence in the THz range. Yet, the thermal effect and optical constants are still not elaborately investigated. In addition, the comparison between ordinary and extraordinary refractive indices of YVO₄ to thermos-optical coefficients and optical constants would be of interest and useful for applications as polarizers in the THz frequency range. Furthermore, numerous modern materials with interesting properties in the THz range (e.g. superconductors, charge density waves, ferrot and antiferromagnets) are often grown on birefringent substrates (e.g. sapphire). Therefore, unveiling birefringent optical properties of different substrate materials is also important, when analyzing material’s electronic and lattice properties, such as the conductivity and phonon vibrations, using THz spectroscopy. To summarize, in order to characterize physical properties in the THz range, investigation of birefringent materials is required, and accurate analysis methods need to be applied within the THz range.

The purpose of this thesis is to characterize THz components and properties of different substrates and optical crystals. To achieve the aim, we carried out the following experiments. Firstly, we built the (polarization-sensitive) time domain terahertz spectroscopy (THz-TDS) system in our lab. Different cuts (a, r-cut) of sapphire single crystal substrate were investigated to collect birefringent properties in the THz range. Secondly, c-cut magnesium fluoride (MgF2) single crystal as a substrate and c-cut LiNbO₃ single crystal as THz generator as well as 3% Nd-doped YVO4 were examined to collect the parameters to compare the properties of different (birefringent) optical crystals. On the other hand, MgF₂ and LiNbO₃ did not show any birefringence. Finally, both, graphene and metallic antiferromagnet Mn₂Au were investigated in the THz range. 

Motivation: Simultaneous Laser Beam Melting is a new Additive Manufacturing technology to build parts that consist of different materials. By the means of SLBM polymer powders with different properties are processed in order to achieve multi-material parts with different mechanical, optical, and chemical properties. The SLBM process is still on the basic research stage.
Until now, a lot of work have been done to optimize SLBM process and to study variety of the applicable polymers and their compatibility in simultaneous melting. Besides all that, the problem that limits the progress in the SLBM process is the fixed rectangular form of the built-up specimens. This is mainly because a DOE (diffractive optical element) was used to achieve a fixed form simultaneous laser energy deposition. This step of the process is essencial to compensate the preheating temperature difference between two polymers. Moreover due to the DOE usage the energy density over the region of interest was limited since the laser energy was spread over the entire surface through the DOE mask. This prevented the ability to compensate the high preheating temperature difference and thus limited the possible polymer combinations.
The aim of the project is to modify the CO2 radiation delivery system that will allow a preheating of the flexible shapes. Plus, it will allow the preheating of the polymer combinations with a big melting temperature difference. The aim of this modification is to obtain a homogeneous temperature distribution on the surface of the powder bed through quasi-simultaneous laser energy deposition with single defocused laser beam prior for melting. Therefore, a high speed galvanometric laser scanner is integrated and its operation is investigated. Moreover, the LabVIEW software program is developed for the synchronized control of the scanner and the CO2 laser during the process. Afterwards, the quasi-simultaneous energy deposition parameters are analyzed and optimized. Furthermore, the aim of this project is to use the quasi-simultaneous energy deposition method to try to compensate a preheating temperature difference in 60 K between polyamide based TPE and PA12 polymers. Therefore, required experiments are done. The analysis of the three-dimensional temperature field of the powder bed during the preheating stage of the process is delivered, using thermal camera system and thermocouples positioned inside the powder bed. The final objective of this study is to develop a process protocol to preheat the PA12 polymer powder in combination with polyamide based TPE using a quasi-simultaneous energy deposition.

Abstract: Flight hardware on satellites underlies enormous quality demands. Therefore every single part is checked several times in order to assure that only non-damaged components are launched to the orbit. The core part of the power supply of a satellite is the solar cell that generates the electricity for the payload of a satellite. During production, space solar cells are checked for mechanical cracks using electroluminescence (EL). However, the main disadvantage of EL is that an electrical connection to the solar cell is necessary. Therefore the principle of EL should be substituted by photoluminescence (PL), where no electrical connection is necessary but the current is induced contactless to the solar cell by light. In the following thesis the principle of PL and the successful production integration of a PL method checking triple junction (3J) space solar cells for mechanical defects is shown. 

Abstract: One of the main challenges of EUV lithography is low numerical aperture of the exposure systems. This problem arises from geometrical design limitations. Anamorphic imaging is one unconventional way to solve the problem of the low NA. The consequences of using high NA imaging systems on the most relevant lithography metrics and process characteristics are investigated. Simulation results for isomorphic imaging systems are shown. The impact of both imaging systems on the lithography metrics is inspected.

The large NA achieved by anamorphic imaging increases the importance of investigating 3D mask effects in EUV lithography. It is not known, which of these 3D mask effects can be addressed to the absorber or the multilayer respectively. A hybrid mask simulation approach addresses this question. Simulations results for a hybrid of real and ideal mask elements are shown in an attempt to try to understand their individual effects of the mask elements and which mask element cases these effects. 

Abstract: In the near future, microspectrometers will play an important role in various field of application. By integrating plasmonic filter technology it is possible to reduce costs and time of production as well as to improve the versatile usability of microspectrometers.

Specific filter properties cause a sensor response to suboptimally represent the incident spectrum. The main issue is to reconstruct the incident spectrum with suitable algorithms.

In the scope of this thesis multiple approaches for solving the linear, inverse problem are examined and implemented using the programming language Phython. Besides simple algorithms of linear algebra and a regularization, the approach was to map the real sensor properties to optimum ones. Simulations with various filter curves were performed to analyse and rate the use algorithms.

Finally, the spectra of real LEDs are reconstructed from measured values of the sensor validating the realized algorithms. 

Abstract: In this master thesis it is demonstrated that thermal and mutual diffusion coefficients of binary mixtures relevant for biofuels can be accessed simultaneously by dynamic light scattering. Thereby, measurements are performed in the compressed liquid phase and at a constant distance to the bubble point line. The dynamic light scattering setup used for investigations, as well as a hydrodynamic circuit for the sample handling, have been developed. With the here presented arrangement it is possible to perform investigations up to temperatures of 524 K with a stability of +/-0.4 mK and pressures of 5 MPa with a stability of +/-0.003 MPa over the complete range. Equimolar mixtures of ethanol, isopentane, isooctane, n-decane and toluene were observed and represent the first data on diffusion coefficients in literature for most mixtures. Moreover, a concentration study (10-mol.% to 90-mol.%) on the diffusion coefficients of isooctane/toluene mixtures was realised. The experimentally determined mass diffusion coefficients exhibit an average expanded (k=2) uncertainty of 2%, which has not been demonstrated before in literature. 

Abstract: In the last few decades, research on new laser systems with high output power has strongly progressed. The required pump power for such high power lasing also induces a significant amount of heat load within the gain medium. This thermal influence leads to different effects, the most important are thermal lensing, beam degradation and depolarization. The purpose of this thesis is to develop a ray tracing model for the simulation of thermal lensing, beam degradation and depolarization in single pass solid state amplifiers. The therefore necessary theory is given in the first part of the thesis. The simulation model, which traces rays through the thermally influenced media, is described in detail, just as the evaluation of the data for determining the thermal focal length, beam degradation and depolarization. With this model, the influences of pump and seed beam size, quantum defect and heat distribution, crystal properties and amplifier dimensions are investigated. Those give a general idea how to prevent and compensate for the thermal effects in amplifiers. The thermally induced focal power was shown to rise linear with the pump power and decreases linear with the quantum defect. Also the dioptric power decreases exponentially with increasing pump beam radius. The most dominant influences on the beam quality degradation are the ratio between seed and pump radius, the pump beam shape and the pump power as well. Ratios of seed to pump close to unity lead to stronger beam degradation, while too small ratios may significantly decreases the efficiency of the amplifier. Since ray tracing is already an approximation of the actual ray trajectory, its limits are investigated carefully. Other effects, such as gain guiding or gain saturation, are not included in this simulation, which further limit the simulation results. However, if those limits are not exceeded, the simulation gives good results, as is shown by comparison with experimental data.

Abstract: Talbout-Lau X-ray grating interferometry applied within a polychromatic setup suffers from additional artifacts compared to conventional attenuation imaging. Among those are beam hardening and dispersion effects due to the complex coupling of different physical effects involved in the image formation process. In computed tomography these effects lead to image degradation such as cupping and streak artifacts, hampering diagnostic use.

This thesis seeks to reduce these artifacts in an iterative reconstruction framework. To this purpose, we define a model of the polychromatic forward projection that includes prior knowledge about the physical setup. Using this model we derive a maximum likelihood algorithm for simultaneous reconstruction of the attenuation, phase and scatter images.

In our experiments on a synthetic ground-truth phantom, we compare filtered back projection reconstruction with the proposed approach. The proposed method considerably reduces strong beam hardening artifacts in the phase images, and almost completely removes these artefacts in the absorption and scatter images. Reconstruction with real data has not been successful because the proposed model does not reproduce the measured reality. Further research is required to resolve this discrepancy.

Furthermore, an optimized reconstruction algorithm for grating based tomography is proposed. Last, an in-depth analysis of an iterative reconstruction framework for Talbot-Lau imaging data is provided.