<p>For a long time spectrophotometry has been a powerful method of determining optical material properties. Since the technique measures the parameters of interest, reflectance and transmittance, it is in general easy to use and interpret. Certain materials, such as miniature structures or scattering materials, must be given a more careful analysis to avoid incorrect interpretation. General solutions to measurement challenges for thick scattering samples and anisotropically scattering samples are presented.</p><p>Thorough knowledge about the components of a spectrophotometer gives a solid base which is necessary when trying to design or modify an instrument for the characterisation of miniature components. Focusing optics and pinhole apertures are two methods investigated for studying samples on a millimeter scale. Focusing optics retain a high intensity but might cause internal reflection. Pinhole apertures are easy to build into a sample holder, but they will reduce light intensity which can give problems with signal to noise ratio. Using a microscope as a focusing lens system permits the measurement of samples of a size down to the order of ten micrometers. However, absolute measurements are difficult due to the strong focusing properties of the microscope.</p><p>Translucent sheets are of interest for daylighting applications, a growing field in today's energy-conscious society. If sunlight is to be used for indoor illumination it is preferable to make it diffuse. By using Transparent Refractive Index Matched Micro (TRIMM) particles in a transparent polymer sheet, it is possible to obtain high transmittance in combination with tailorability of the scattering profile. Such sheets have been characterised experimentally, as well as by Monte Carlo raytracing simulations. The good agreement between simulation and experiment shows that this type of simulation can be used in the materials design process. A more theoretical study of patterns in multiple Mie scattering has been carried out using the Monte Carlo program developed for characterisation of the TRIMM particle sheets.</p>

For a long time spectrophotometry has been a powerful method of determining optical material properties. Since the technique measures the parameters of interest, reflectance and transmittance, it is in general easy to use and interpret. Certain materials, such as miniature structures or scattering materials, must be given a more careful analysis to avoid incorrect interpretation. General solutions to measurement challenges for thick scattering samples and anisotropically scattering samples are presented. Thorough knowledge about the components of a spectrophotometer gives a solid base which is necessary when trying to design or modify an instrument for the characterisation of miniature components. Focusing optics and pinhole apertures are two methods investigated for studying samples on a millimeter scale. Focusing optics retain a high intensity but might cause internal reflection. Pinhole apertures are easy to build into a sample holder, but they will reduce light intensity which can give problems with signal to noise ratio. Using a microscope as a focusing lens system permits the measurement of samples of a size down to the order of ten micrometers. However, absolute measurements are difficult due to the strong focusing properties of the microscope. Translucent sheets are of interest for daylighting applications, a growing field in today's energy-conscious society. If sunlight is to be used for indoor illumination it is preferable to make it diffuse. By using Transparent Refractive Index Matched Micro (TRIMM) particles in a transparent polymer sheet, it is possible to obtain high transmittance in combination with tailorability of the scattering profile. Such sheets have been characterised experimentally, as well as by Monte Carlo raytracing simulations. The good agreement between simulation and experiment shows that this type of simulation can be used in the materials design process. A more theoretical study of patterns in multiple Mie scattering has been carried out using the Monte Carlo program developed for characterisation of the TRIMM particle sheets.

The field of Information and Communication Technologies is witnessing a development speed unprecedented in history. Moore’s law proves that the processor speed and memory size are roughly doubling each 18 months, which is expected to continue in the next decade. If photonics is going to play a substantial role in the ICT market, it will have to follow the same dynamics. There are mainly two groups of components that need to be integrated. The active components, including light sources, electro-optic modulators, and detectors, are mostly fabricated in III-V semiconductors. The passive components, such as waveguides, resonators, couplers and splitters, need no power supply and can be realized in silicon-related semiconductors. The prospects of silicon photonics are particularly promising, the fabrication is mostly compatible with standard CMOS technology and the on-chip optical interconnects are expected to increase the speed of microprocessors to the next generation. This thesis starts with designs of various silicon-based devices using finite-difference time-domain simulations. Parallel computation is a powerful tool in the modeling of large-scale photonic circuits. High Q cavities and resonant channel drop filters are designed in photonic crystal platform. Different methods to couple light from a single mode fiber to silicon waveguides are studied by coupled-mode theory and verified using parallel simulations. The performance of waveguide grating coupler for vertical radiation is also studied. The fabrication of silicon-based photonic devices involves material deposition, E-beam or optical lithography for pattern defining, and plasma/wet-chemistry etching for pattern transfer. For nanometer-scaled structures, E-beam lithography is the most critical process. Depending on the structures of the devices, both positive resist (ZEP520A) and negative resist (maN2405) are used. The proximity and stitch issues are addressed by careful dose correction and patches exposure. Some examples are given including photonic crystal surface mode filter, micro-ring resonators and gold grating couplers. In particular, high Q (2.6×105), deep notch (40 dB) and resonance-splitting phenomenon are demonstrated for silicon ring resonators. It is challenging to couple light into photonic integrated circuits directly from a single-mode fiber. The butt-coupled light-injecting method usually causes large insertion loss due to small overlap of the mode profiles and large index mismatch. Practically it is not easy to cleave silicon sample with smooth facet where the waveguide exposes. By adding gold gratings to the waveguides, light can be injected and collected vertically from single-mode fiber. The coupling efficiency is much higher. There is no need to cleave the sample. The access waveguides are much shortened and the stitch problem in E-beam lithography is avoided. In summary, this thesis introduces parallel simulations for the design of modern large-scale photonic devices, addresses various issues with Si-based fabrication, and analyses the data from the characterization. Several novel devices using silicon nanowire waveguides and 2D photonic crystal structures have been demonstrated for the first time. / QC 20100923

This project is concerned with both the study of different Multiple Quantum Wells (MQWs) structures using the In0.53Ga0.47As/In0.52Al0.48As material system lattice matched to InP and a systematic investigation of the properties of InAs QD systems within GaAs with the aim of achieving enhancements of solar cell performance. The key challenge is the growth of QDs solar cell structures which exhibit sufficient absorption (enhanced infrared absorption) to increase short circuit current density (Jsc) but which can still maintains a high open circuit voltage (Voc). The research consists of epitaxial growth using state-of–the-art MBE, optical absorption, photoluminescence and high resolution x-ray diffraction measurements as well as device fabrication and characterization of novel solar cell structures. Optimization was performed on these novel cells to further improve their efficiency by inserting stacks of QD into different regions of the device. The effect of localized doping of such structures was used in an attempt to maintain and enhance the open-circuit voltage which in turn increases the device efficiency. The fabricated devices were characterized using measurements of the dark/light current-voltage (I-V) characteristics and spectral response (50-480 K). Solar cell external quantum efficiencies under standard air mass (AM) 1.5 spectrum were determined and the suitability of these new cells under solar concentration were assessed. Full physical simulations are performed using SILVACO semiconductors modelling software to generate models of multi-junction solar cells that were crucial in informing iterations to growth and fabrication and help to reconcile theory with experiment. One of the key findings, of this thesis, is the fact that Intermediate band photovoltaic devices using material based on InAs/GaAs vertically stacked quantum dot arrays, can be used in applications according to specific configuration criteria such as high temperature operation conditions. The intermediate band cell, including an inter-dot doped configuration, has been found to be a potential candidate as the inter dot doping profile reduces the efficiency degradation below the GaAs values including an enhancement in the open circuit voltage. It has been proved that these devices not only have a good performance at high temperatures but also by changing the vertical stacking QD layer periodicity can enhance the short circuit current density while keeping a large open circuit voltage. It was confirmed in practical device operation that thermal energy is required to enable the intermediate band in InAs/GaAs QD materials. The impact of this works can help in the future improvements of the intermediate band solar cells based on InAs on GaAs QD. The best overall efficiency of 11.6 % obtained in this work is an excellent value for so simple devices configuration. The Si3N4, tested for the first time on InAs/GaAs QD materials, reduces the reflectance on the device surface to a value of 2% and the operational wavelength can be tuned by controlling the layer thickness. A 100 nm Si3N4 antireflective coating proved to be an excellent coating from 700 to 1000 nm. In terms of short circuit current density a 37% enhancement was achieved.