| 1. |
- Dufåker, Daniel, 1971-
(författare)
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Spectroscopy studies of few particle effects in pyramidal quantum dots
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis work two very similar processes have been studied both involving excitations of particles during recombination of exciton complexes in quantum dots, reducing the energy of the emitted photon. Different exciton complexes are defined according to the number of electrons and holes in the quantum dot upon recombination. The neutral exciton complexes with one electron and one hole (X–) and two electrons and two (X+) holes respectively are referred to as the exciton andthe biexciton. Accordingly the charged exciton complexes consisting of two electrons and one hole (X–) and one electron and two holes (X+), respectively, are referred to as negatively- and positively charged excitons, respectively. Whenever another particle is excited during the recombination of one electron-hole pair within these complexes, the result is a weak satellite peak, spectrally redshifted with respect to the main emission peaks related to the exciton complex.In the first part of this thesis work, described in the first two papers, the exciton-LO-phonon interaction is studied with a weak redshifted satellite peak as the signature, referred to as a phonon replica. The intensity ratio between the replicas and the corresponding main emission were determined from the obtained microphotoluminescencespectra. It was found that this ratio was significantly weaker for the positively charged exciton X+ compared to the neutral exciton, X, and the negatively charged exciton, X–. This experimentally obtained result was further supported by computations. Interestingly, the computations revealed that despite that X+ displays the weakest phonon replica among the investigated complexes, it possesses the strongest Fröhlich coupling to phonons in the lattice before recombination. In addition, the spectral broadening of the phonon replicas compared to the main emission is discussed. Also, the origin of the exciton-LO-phonon coupling is concluded to be from the QD itself, based on a comparison between quantum dots with different barriers.In the last paper an additional weak redshifted satellite peak in the recombination spectra is studied. The intensity of this weak satellite peak is correlated to the peak intensity of the positively charged exciton, X+, main emission peak. In addition to this photoluminescence excitation experiments, magnetic field measurement and calculations further support our interpretation that the satellite peak is related to the shake-up of the ground-state hole in the QD that is not involved in the optical recombination. This hole is thus excited by Coulomb interaction to an excited state yielding a photon energy reduced with the difference between the ground-state and the excited state of the spectator hole.
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| 2. |
- Jemsson, Tomas
(författare)
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Time Correlated Single Photon Spectroscopy on Pyramidal Quantum Dots
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Generation of non-classical light is both of fundamental interest and a common condition for quantum information applications (QIA). One feasible type of single photon emitter for QIA is based on semiconductor quantum dots (QDs), due to their atomic-like energy structure and their possibility to be integrated with other semiconductor devices on the same chip. Sitecontrolled QDs with highly linear polarized emission are a prerequisite for certain QIA and a close to room temperature operation is demanded for widespread applications.III-nitride QD can have the deep connement potentials needed for high temperature operation, and the demonstration of single photon emission at room temperature was recently reported for a GaN QD [Nano Lett. 14, 982 (2014)]. Asymmetric III-nitride QD emits light with a high degree of linear polarization. To make site-controlled nitride-based QDs a promising approach is to deposit a thin layer of InGaN on top of hexagonal GaN micropyramids. QDs formed on the apex of the pyramids grown with this approach have been shown to exhibit single and sharp InGaN related emission lines with a high degree of linear polarization [Nano Lett. 11, 2415 (2011)]. A simple elongation of the pyramid base gives control of the polarization direction [Light: Sci. Appl. 3, e139 (2014)].The work presented in this thesis deals with time correlation measurements, to measure, for the rst time, the single photon properties of these pyramidal QDs.A time correlated single photon spectroscopy (TCSPS) setup was assembled, tested and used to perform measurements on these pyramidal QDs. The TCSPS apparatus measures the time dierences between subsequent photons emitted from the sample. In the spectrally ltered light of one emission line in the emission spectra, e.g. exciton emission, of a QD two or more photons cannot be emitted simultaneously, i.e. the photons are sent out one by one. A histogram of the ensemble of measured time dierences (~106 events) will then for the ideal case have no events for τ = 0, and very few for close to zero. This histogram, when normalized, is under certain conditions equal to the second order coherence function g(2)(τ ). In reality, however, there are photons coming from other sources close to the QD, i.e. background emission, that reach the detector and reduce the dip in the correlation histogram for small τ. There is also an statistical uncertainty in the measured time dierences and nally the nite bin width used in the histogram that deteriorate the measured correlation function. To understand the in uence on g(2)(τ) from background emission, instrument response function and the bin width, on the measurement on excitonic emission, simulations and calculations were made. The crucial variables were, for our samples and setup, the level of the background emission and the instrument response function.A post growth process was developed to cover the lower parts of the pyramid sides as well as the area between the pyramids with a metal lm, to reduce the background emission. This reduces the background emission and largely improves the relative QD signal. As a result, signicant improved single photon characteristics were demonstrated.A measurement of the second order coherence function for the excitonic autocorrelation at a temperature of 12 K, gave for zero time delay ( = 0) a value of g(2)(0) = 0.24 and the residual value of the second order coherence function (0.24) could be in full explained by the three variables, background emission, instrument response function and bin width. The g(2)(0) value for correlation measurements at higher temperatures of 50 K and 80 K is also fully explained by the three variables, showing that the emission from the QD itself is ideal up to 80 K.This result underlines the great potential of these site controlled pyramidal dots as sources of fast polarized single photon emission, and provides the rst rigorous evidence of InGaN quantum dot formation on hexagonal GaN pyramids. We also show the rst proof of biexcitonic emission in this pyramidal QDs.
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| 3. |
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| 4. |
- Karlsson, Anders, 1974-
(författare)
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Vilket religionskunskapsämne? : Ämneskonstruktioner i religionskunskap på gymnasiet med samtalsförhandlingar i centrum.
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis aims to describe one educational sequence in religious education. The study shows articulations and negotiations in the religious education classroom, as well as on digital discussion forums. Two questions are the core of the explorative case study. What is communicated between teachers and students in a Re-classroom in Sweden? How does the content of the communication contribute to constructing the Re-subject in two different discussion forums: digital and analogue? The case study follows an upper secondary school class, with 18 years old students, during one teaching sequence.The theoretical foundation is based in a social-cultural perspective. Data is produced by ethnographic methods and consist of classroom observations and interviews with teachers and students. For the interpretation of speech Even’s relief theory is used concerning what is said and Michel Bakhtin’s thoughts on speech genre concerning how it is expressed and negotiated in the teaching sequence.The analysis of the different forums show that in all of them, teachers and students tend to focus on what religious persons do, their appearance, and how they are limited by their faith. The religious persons are compared to a way of living according to a secular norm that the students find more relevant and up-to-date. When speaking of religious persons the students tend to express it in a derisive and disparaging way. Negotiations on content and framework factors interrupt the actual teaching in the classroom environment whilst digital forums are self-regulating and discussions quickly return to the topic after irrelevant digressions.In the discussion part of the thesis the various discussion forums are evaluated didactically. The impact of gender and framework factors on the student-teacher discussions is also raised. An additional question that is problematized is the tension between the curriculum of the subject and the teacher’s desire to get students interested in religion as a subject, described as ‘the teacher’s dilemma’. It is discussed whether this dilemma controls the teacher’s choice of subject.
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| 5. |
- Karlsson, Ingrid, 1948-
(författare)
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Att leda i kommunal äldreomsorg : Om arbetsledares handlingsutrymme, handlingsfrihet och skilda lojaliteter - exemplet Halland
- 2006
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The middle management position in the care of the elderly is the focus of this licentiate dissertation. The aim was to develop knowledge about the constraints within which the middle manager is able to act and which freedom of action he/she has. The empirical data consisted of 17 interviews and a vignette study completed by 47 middle managers in social care in the county of Halland. The managers' perceptions of what exerts a control on their work and of the type of control that is exercised was investigated into in the interviews. In the vignette study, using three specific cases, the focus was on the research issue, i.e. how they thought they could act, mainly in terms of their apprehensions of constraints (the formal framework) within which to act and the freedom of action. Further, aspects of loyalty were analysed. It comes down to a view of what can be done and how it can be carried out. The middle managers' freedom was violated by control mechanisms that had a disciplinary cognitive orientation as well as mechanisms that constituted a normative control, such as budget issues and legislation. They described, mainly in the interviews, their experiences of being in a vulnerable position and of being trapped between interested parties. On the other hand the vignette study revealed an emphasis on being a manager and conveyed an administrative approach. By adopting this approach the middle managers relinquish to some extent their moral responsibility. A moral indignation is apparent, however, when the consequences for the workplace and the staff become serious as it does in one of the vignettes. Then the indignation in many cases passed on into a deliberation about quitting. The administrative concentration of middle management can, in its extension, lead to the need for an old group to re-enter into the field of the elderly care, namely, the assistant manager. In other words, a new intermediate level emerges that takes care of this dilemma of being trapped between the interested parties from a bottom up perspective. It is possible here to make a comparison with what in modern terms is called shared leadership.
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| 6. |
- Karlsson, Kristoffer
(författare)
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Gap discharge transducers applied to ultrasonic flow measurement
- 2012
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, the potential of the gap discharge transducer is investigated as both a sound pulse emitter and a sound pulse receiver in a gas flow measurement setup. The main objective is to use the gap discharge transducer as an integral part in a gas flow measurement system developed for harsh environments. The idea for the method considered in this study to determine the flow was taken from the time-of-flight technique used in ultrasonic flow measurements. The gap discharge transducer should then be used as both an emitter and a receiver in a setup that somewhat mimics those in ultrasonics. Earlier studies with the gap discharge transducer has shown that it is both very durable and a potent sound pulse emitter. This thesis continues these studies by incorporating the transducer into a measurement system as an emitter and also investigates its capabilities to be used as a sound pulse receiver. As an emitter in a flow measurement system the transducer was placed in a pipe with a variable flow in a laboratory environment. The transducer was set to generate sparks to create the sound pulse and standard piezoelectric receivers were used to capture the signal.To determine the possibilities to use the transducer as a receiver the transducer was placed in a vacuum chamber to test the dependence between breakdown voltage and pressure. Since a sound pulse is a change in pressure the pulse might cause breakdown in the gap between the electrodes of the transducer if an initial voltage between the electrodes is set close enough to breakdown. The investigation shows that the gap discharge transducer is a potential sound pulse emitter in a flow measurement system and, with some small calibration and more precise alignment, is capable to determine the flow quite accurately. On the other hand, to use the transducer as a receiver is concluded to be very difficult. The breakdown voltage is too unstable to allow an initial voltage close to breakdown to be set without risking spontaneous breakdown due to random events.
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| 7. |
- Karlsson, Niklas
(författare)
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Parametric model for astrophysical proton-proton interactions
- 2006
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Observations of gamma-rays have been made from objects such as active galaxies and supernova remnants and also as diffuse emission from the Galactic ridge. The study of gamma-rays can provide information about production mechanisms and tell us about cosmic ray acceleration. In the high energy regime, the dominant mechanism for gamma-ray production is the decay of neutral pions produced in interactions of accelerated cosmic ray nuclei and interstellar matter. Having an accurate model for this is an integral part in the study of gamma-rays and the sources in which they are produced. We present here a parametric model for calculations of inclusive cross sections for stable secondary particles (gamma-rays, electrons, positrons, nue, anti nue, numu and anti numu) produced in proton-proton interactions. The model is based on the up-to-date proton-proton interaction model by Kamae et al., which includes the diffraction dissociation process, violation of the Feynman scaling law and the logarithmically rising inelastic proton-proton cross section. To improve fidelity to experimental data for lower energies, two baryon resonance excitation processes were added to the model: one representing the Δ(1232) and the other multiple resonances with masses around 1600 MeV/c^2. The parametric model predicts the spectra for all secondary particles to be harder by about 0.05 in power-law index than that of the incident proton and their inclusive cross sections to be larger than those predicted by previous models based on Feynman scaling. The transverse momentum distributions for gamma-rays have also been calculated from the described proton-proton interaction model, showing the potential of also having a parametric model for the transverse momentum distributions.
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| 8. |
- Karlsson, Per
(författare)
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Quasi Resonant DC Link Converters - Analysis and Design for a battery Charger Application
- 1999
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Environmental aspects have during the last years made electric vehicles an interesting competitor to the present internal combustion engine driven vehicles. For a broad introduction of pure electric vehicles, a battery charging infrastructure is deemed necessary. However, to build and maintain such an infrastructure is costly. Active power line conditioning capabilities could be included in the battery charger, making the infrastructure advantageous from the distribution network point of view. Another option is that the battery charger could be able to support the grid with peak power during periods of stability problems or emergency situations. This means that energy is borrowed from the batteries of vehicles connected to a charger. The price for energy supplied by the batteries is likely to be several times higher than the normal electric energy price, due to the wear costs of the batteries. Therefore, the battery charger losses also represents a high cost. It is often stated that resonant converters have a high efficiency compared to hard switched. Since carrier based pulse width modulation is employed, quasi resonant DC link converters are of interest. Four of the most promising quasi resonant DC link topologies reported in the literature are compared. A fair comparison is obtained by designing them to meet certain common design criteria, in this case the duration of the zero voltage interval and the maximum output voltage time derivative. The derivation of the design expressions are given, and also the simulation results, by means of efficiency. A 10 kW battery charger, equipped with one of the quasi resonant DC links investigated is implemented. A hard switched battery charger with the same rating is also tested to compare the measured efficiency with the simulated. Both the simulations and measurements shows that the efficiency decrease for quasi resonant battery chargers compared to the hard switched case. Furthermore, low frequency harmonics appear in the battery charger input and output currents. However, full control of the output voltage derivatives is obtained.
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| 9. |
- Kotte, Hari Babu, 1979-
(författare)
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High Speed (MHz) Switch Mode Power Supplies (SMPS) using Coreless PCB Transformer Technology
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The most essential unit required for all the electronic devices is the Power Supply Unit (PSU). The main objective of power supply designers is to reduce the size, cost and weight, and to increase the power density of the converter. There is also a requirement to have a lower loss in the circuit and hence in the improvement of energy efficiency of the converter circuit. Operating the converter circuits at higher switching frequencies reduces the size of the passive components such as transformers, inductors, and capacitors, which results in a compact size, weight, and increased power density of the converter. At present the switching frequency of the converter circuit is limited due to the increased switching losses in the existing semiconductor devices and in the magnetic area, because of increased hysteresis and eddy current loss in the core based transformer. Based on continuous efforts to improve the new semi conductor materials such as GaN/SiC and with recently developed high frequency multi-layered coreless PCB step down power transformers, it is now feasible to design ultra-low profile, high power density isolated DC/DC and AC/DC power converters. This thesis is focussed on the design, analysis and evaluation of the converters operating in the MHz frequency region with the latest semi conductor devices and multi-layered coreless PCB step-down power and signal transformers.An isolated flyback DC-DC converter operated in the MHz frequency with multi-layered coreless PCB step down 2:1 power transformer has been designed and evaluated. Soft switching techniques have been incorporated in order to reduce the switching loss of the circuit. The flyback converter has been successfully tested up to a power level of 10W, in the switching frequency range of 2.7-4 MHz. The energy efficiency of the quasi resonant flyback converter was found to be in the range of 72-84% under zero voltage switching conditions (ZVS). The output voltage of the converter was regulated by implementing the constant off-time frequency modulation technique.Because of the theoretical limitations of the Si material MOSFETs, new materials such as GaN and SiC are being introduced into the market and these are showing promising results in the converter circuits as described in this thesis. Comparative parameters of the semi conductor materials such as thevienergy band gap, field strengths and figure of merit have been discussed. In this case, the comparison of an existing Si MOSFET with that of a GaN MOSFET has been evaluated using a multi-layered coreless PCB step-down power transformer for the given input/output specifications of the flyback converter circuit. It has been determined that the energy efficiency of the 45 to 15V regulated converter using GaN was improved by 8-10% compared to the converter using the Si MOSFET due to the gate drive power consumption, lower conduction losses and improved rise/fall times of the switch.For some of the AC/DC and DC/DC applications such as laptop adapters, set-top-box, and telecom applications, high voltage power MOSFETs used in converter circuits possess higher gate charges as compared to that of the low voltage rating MOSFETs. In addition, by operating them at higher switching frequencies, the gate drive power consumption, which is a function of frequency, increases. The switching speeds are also reduced due to the increased capacitance. In order to minimize this gate drive power consumption and to increase the frequency of the converter, a cascode flyback converter was built up using a multi-layered coreless PCB transformer and this was then evaluated. Both simulation and experimental results have shown that with the assistance of the cascode flyback converter the switching speeds of the converter were increased including the significant improvement in the energy efficiency compared to that of the single switch flyback converter.In order to further maximize the utilization of the transformer, to reduce the voltage stress on MOSFETs and to obtain the maximum power density from the power converter, double ended topologies were chosen. For this purpose, a gate drive circuitry utilising the multi-layered coreless PCB gate drive transformer was designed and evaluated in both a Half-bridge and a Series resonant converter. It was found that the gate drive power consumption using this transformer was less than 0.8W for the frequency range of 1.5-3.5MHz. In addition, by using this gate drive circuitry, the maximum energy efficiency of the series resonant converter was found to be 86.5% with an output power of 36.5W.
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| 10. |
- Nosratabadi, Ali Reza, 1964-
(författare)
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Novel Methods for Sampling, Characterization and Analysis of Airborne Street Particles in a Health Perspective
- 2022
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Over the last decades, there has been much attention on air quality, especially in urban environments. A significant factor effecting the air quality in the urban environment is airborne particulate matter (PM). Long-term exposure to PM causes increased risk for heart disease, decreased lung function, exacerbation of asthma, and lung cancer. Therefore, many countries have implemented exposure limits to the concentration of ambient PM in the urban environments. The toxicity of PM is dependent on several factors such as chemical composition, shape, adsorbed materials, and particle sizes (usually divided in ultrafine, fine and coarse particles). However, the relationship between different PM properties and developing health hazards are not clear. Therefore, further studies to investigate different properties of PM may contribute to understanding the influence of PM on human health. In the present work, different novel methods to investigate sampled airborne PM and to investigate potential health effects have been used to increase the knowledge regarding street and wear particles. In study I, a sampling plan involving collecting one filter from Tapered Element Oscillating Microbalance (TEOM) monitoring stations used for Environmental monitoring each month for 20 months were developed. Particles on the filters were extracted into a suspension. TEOM particles were then studied with respect to constituents and variables that reflect their toxicity. The constituent and toxicity was found to be spatial and seasonal dependent. As a follow-up, in study II, TEOM filters from three geographical different cities were collected once a month for a year. The variation in particle mass measured with TEOM monitor, cadmium and lead contents, as well as endotoxin levels between locations and time points over the year was studied. The correlation between studied variables and biological effect was investigated. The results show that the concentration of metals and endotoxin in TEOM particles have no relationship to particle mass, while endotoxin levels coincided with pro-inflammatory response. These studies show that results from analyzing different variables on obtained particles from TEOM filters in combination with information about the ambient particle concentration, could be helpful in the evaluation of differences in the risk of breathing air at various locations. The dominant road traffic particle sources are wear particles from the road and tyre interface, and from vehicle brake pads. The particle concentrations are highest in cities with high traffic amounts and a high frequency of braking. There are a few cell studies that have investigated the toxicological and biological effect of these wear particles, but there is a lack of knowledge regarding their effect on tissue level. Furthermore, the knowledge about importance of rock materials in pavement is deficient. To mitigate these knowledge gaps, the effect of different wear particles from pavement and brake pad were tested using a model with isolated perfused rat lungs in study III. The wear particles from the pavement showed a significant decrease of tidal volume compared to unexposed controls. The largest effect were found with quartzite stone material. Wear particles from brakes instead showed a larger effect on released proinflammatory cytokines. The study shows that the toxic effect on lungs exposed to airborne particles can be investigated using repetitive measurements of tidal volume. Furthermore, the study shows that the choice of rock material in road pavements has the potential to affect the toxicity of road wear particles. This should be considered in environments where the concentrations and exposures are high. The brake particles showed a different effect than stone particles, indicating the need to differentiate wear particles from different sources in relation to health effects. In summary, the present work have investigated different aspects of airborne particles collected from streets as well as generated wear particles. These indicate different important aspects of the particles that may be of importance to better understand their health effects.
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