Towards the physical vacuum of cosmic inflation

There have been long debates about the initial condition of inflationary perturbations. In this work we explicitly show the decay of excited states during inflation via interactions. For this purpose, we note that the folded shape non-Gaussianity can be interpreted as the decay of the non-Bunch-Davies initial condition. The one loop diagrams with non-Bunch-Davies propagators are calculated to uncover the decay of such excited states. The observed smallness of non-Gaussianity keeps the window open for probing inflationary initial conditions and trans-Planckian physics

Laser Surface Texturing, Crystallization and Scribing of Thin Films in Solar Cell Applications

Thin films have been considered for use in terrestrial solar cell applications because of their significantly reduced cost compared with bulk crystalline silicon. However, their overall efficiency and stability are less than that of their bulk crystalline counterpart. The work presented in this thesis seeks to investigate these issues via a series of experimental and numerical analysis of the influences of laser processing on microstructure, optical and electrical properties of two absorber materials, a-Si:H (hydrogenated amorphous silicon) and CdTe (cadmium telluride). a-Si:H thin film solar cells suffer from disadvantages of low efficiencies and light induced degradation. A one-step laser processing is investigated for introducing light-trapping structure and crystallization on a-Si:H thin films, which can potentially simultaneously alleviate the two weaknesses of a-Si:H. The nanoscale conical and pillar-shaped spikes formed on the surface of a-Si:H films by irradiation of both femtosecond (fs) infrared and nanosecond (ns) excimer lasers enhanced light absorption, while the formation of a mixture of hydrogenated nanocrystalline silicon (nc-Si:H) and a-Si:H after crystallization suggests that the overall material stability can potentially improve. It is shown that growth is a more dominant spike formation mechanism in excimer laser processing, rather than ablation which is dominant during fs laser texturing. Experimental and analytical approaches are also developed revealing the effect of hydrogen on texturing behavior and crystallization during excimer laser irradiation, and a step-by-step crystallization process is proposed to prevent the hydrogen from diffusing out in order to reduce the defect density. In addition, a comparison of absorptance spectra for various surface morphologies and crystallinity is developed and the absorptance across the solar spectrum shows that the combination of surface texturing and crystallization induced by laser processing is very promising for a-Si:H thin film solar cell applications. CdTe thin-film solar cells are the basis of a significant technology with major commercial impact on terrestrial photovoltaic production, since CdTe leads to substantial cost reduction. Laser scribing is a key process used to increase thin-film solar panel efficiency through the formation of serial interconnections to reduce photocurrent and resistance losses. Currently, scribing is performed using glass-side laser processes which have led to increased scribe quality. Defects formed during scribing such as micro cracks, film delamination, thermal effect and tapered sidewall geometries, however, still keep solar panels from reaching their theoretical efficiencies. In this study, a ns Nd:YAG laser operating at the fundamental (1064nm) or frequency-doubled (532nm) wavelengths is employed for pattern 1 (P1) and 2 (P2) scribing on CdTe thin-film solar cells. The experimental investigation shows that film removal mechanisms for different materials are due to laser-induced ablation, thermal stress and micro-explosion processes. The formation mechanisms and mitigation techniques of the defects during micro-explosion process are studied. A fully-coupled thermal and mechanical finite element model is developed to analyze the laser-induced spatio-temporal temperature and thermal stress distribution responsible for SnO2:F film removal, and a plasma expansion model is also investigated to simulate the film removal of CdTe/Cds multilayer due to the micro-explosion process. The characterization of removal qualities will enable the process optimization and design required to enhance solar module efficiency

Numerical simulation of subcontinent lithosphere dynamics: craton stability, evolution and formation

Through geodynamical modelling, two hypotheses about the craton stability and evolution were revisited and an important process of cratonization is investigated. Unlike most previous, related numerical studies, non-Newtonian rheology with composition dependence was used in these studies, and the rheological parameters are thus directly comparable with laboratory experiment of mantle. The first hypothesis, that the cratonic lithosphere is “isopycnic”, is found to be not strictly necessary for craton stability and longevity. The high viscosity of the cratonic litho- sphere due to compositional effects on the mantle rheology is found to be essential to maintain a thickness difference between cratonic and non-cratonic lithosphere for over billions of years and it allows a modest negative buoyancy of the cratonic root, depending on the strengthening factor due to the compositional effects. The second hypothesis to be tested is that mantle plume im- pingements cause rapid, significant removal of subcontinental lithosphere. The results presented in this thesis show that the erosion caused by a plume impact on a continent that is strong enough to have survived billions of years of Earth’s history is rather limited. A special weaken- ing mechanism of such highly viscous and buoyant roots is required to reactivate this cratonic lithosphere and thus cause significant thinning within 10s of Myrs. The fluid/melt-rock interac- tion during mantle metasomatism is probably the most likely mechanism to modify and weaken depleted cratonic lithosphere. Therefore, metasomatic weakening is essential for the significant thinning of subcontinental lithosphere observed, e.g.at North China Craton and Namibia, south- ern African, no matter whether caused by a plume impact or another tectonic event. Using the reasonable compositional effects on the buoyancy and rheology of mantle rocks from the above studies, numerical experiments are performed to study the formation of thick cratonic lithosphere from a layered, depleted mantle material. In this scenario, substantial tec- tonic shortening and thickening of previously depleted material seems to be an essential ingre- dient to initiate the cratonization process. Afterwards, gravitational self-thickening will cause further thickening. Compositional buoyancy resists Rayleigh-Taylor instability collapse and stabilizes the thick cratonic root, while the secular cooling also has a stabilizing effect on the cratonic root by reducing the thermal buoyancy contrast between lithosphere and asthenosphere and increasing mantle viscosity. The presented numerical results are consistent with the vertical movement of cratonic peridotite as suggested on petrological grounds