The goal of the project is to assess whether the Off-Resonance Saturation (ORS) method is able to differentiate and quantify different mineralized iron forms, in particular magnetite and ferritin....Show moreThe goal of the project is to assess whether the Off-Resonance Saturation (ORS) method is able to differentiate and quantify different mineralized iron forms, in particular magnetite and ferritin. Samples containing agarose and iron nanoparticles will be prepared and studied with a pre-clinical 7T MRI scanner at the LUMC. First, the samples will be characterized with commonly-used MRI sequences to obtain relaxation time maps and spectra. The main part of the project is to apply the ORS method to acquire positive contrast. Different nanoparticles will be used and the parameters of the ORS sequence will be optimized. Finally, a simulation is made to verify the validity of the ORS theory.Show less
In this study we measured eye deformations with Magnetic Resonance Imaging for different patient positions to improve radiotherapy planning for patients with uveal melanoma. By comparing the shape...Show moreIn this study we measured eye deformations with Magnetic Resonance Imaging for different patient positions to improve radiotherapy planning for patients with uveal melanoma. By comparing the shape of the eye for the superman position and supine position in the MRI for different volunteers we found an overall deformation smaller than 0.5 mm in all directions.Show less
The availability of information about complex networks is severely restrained by issues, such as confidentiality and privacy. This poses a problem when analysing properties of networks that are of...Show moreThe availability of information about complex networks is severely restrained by issues, such as confidentiality and privacy. This poses a problem when analysing properties of networks that are of relevance to the general public. One example is the study of the resilience of banking networks to financial distress. We discuss a random graph model that reconstructs such unavailable networks, based on information that is either node specific or specific to groups of nodes. The procedure is determined by enforcing renormalizability, i.e. consistency when modelling renormalizations of networks. Then we propose a weighted semi-renormalizable extension of this model. Both models are tested on an empirical trade network, by analysing how well they captures properties that are commonly used to characterize networks. Their performances are shown to closely resemble that of the (weighted) fitness-induced Configuration Model.Show less
In this thesis we will try to find explicit examples and characterisations of the quantum moduli algebras on ribbon graphs with one vertex. First, we will study the classical case of group gauge...Show moreIn this thesis we will try to find explicit examples and characterisations of the quantum moduli algebras on ribbon graphs with one vertex. First, we will study the classical case of group gauge theory, in which we identify the moduli algebra with the function algebra on the moduli space of flat connections. Secondly, the group gauge theory case will be extended to the group algebra case, for which we show that the quantum moduli algebra is isomorphic to the moduli algebra in the group gauge theory case. Thirdly, we will give a general construction of how to obtain quantum moduli algebras of semisimple finite-dimensional Hopf algebras, and we will identify this construction with the construction of the quantum moduli algebra in the group algebra case. Fourthly, we will be examining the situation in which our Hopf algebra is the Drinfel'd double of a group algebra. After giving some examples, we will show that the quantum moduli algebra in the case of the Drinfel'd double is isomorphic to the quantum moduli algebra in the group algebra case.Show less
This thesis deals with the use of conducting AFM to image the topography and conducting properties of graphene on SiO2. Specifically,the current image will be used to distinguish graphene from SiO2...Show moreThis thesis deals with the use of conducting AFM to image the topography and conducting properties of graphene on SiO2. Specifically,the current image will be used to distinguish graphene from SiO2 and the height image to identify the edge of the wafer. These together can show how far graphene reaches this edge. For testing the usability of the Conducting AFM module measurements were also made on gold and on graphite. Lastly, specific settings were tested and discussed for optimal current imaging results.Show less
To make direct detection of planets like Proxima Centauri b possible, a high contrast coronagraph with a low inner-working angle is needed. With that in mind Por et al. designed the SCAR...Show moreTo make direct detection of planets like Proxima Centauri b possible, a high contrast coronagraph with a low inner-working angle is needed. With that in mind Por et al. designed the SCAR coronagraph. In SCAR the starlight is nulled using a SCAR phase plate and a single mode multicore fiber. In this thesis we describe the process of building a spectrograph for this instrument. After a small trade-off study, we have decided to build a prism spectrograph. Due to the proximity of the fibers, resolution increases in spite of bandwidth. The ultimate design has a resolution of 110 and a bandpass from 525 to 700 nm. The spectrograph was set-up and tested using a laser and a broadband source. The design induces a lot of crosstalk, therefore we used a special extraction method for the spectra described by Bolton and Schlegel. This technique creates a 2D model of the data and extracts accordingly. Finally we have been been able to retrieve spectra using this spectrograph and extraction method. This is a proof of concept for both the SCAR and the spectrograph behind it.Show less
Context. Much data from the integral field spectroscope (IFS) of SPHERE, the high contrast imager of the VLT, of circumstellar disks is yet unpublished, since this data is always collected in...Show moreContext. Much data from the integral field spectroscope (IFS) of SPHERE, the high contrast imager of the VLT, of circumstellar disks is yet unpublished, since this data is always collected in parallel with IRDIS, which is much easier to reduce and analyze. Aims. We search for a good and reliable way to reduce raw IFS data and study the effects of different post-processing methods on the morphology of protoplanetary disks. Methods. We used the common pipeline of ESO to reduce spatially resolved spectral IFS data (YJ band) of RXJ1615.3-3255 and applied classical ADI, classical SDI and classical RDI on the data. Results. We detected a ring, an arc and an inner disk component in both the ADI and SDI image, the ring is detected in the RDI image as well. We conclude that we can trust the SDI data the best around the minor axis of the ring since ADI and RDI have to deal with self-subtraction and over subtraction in that region, the other parts of the ring can be trusted the best in the ADI and RDI data. The disk signal appears to be red, but further research is needed to conclude whether this effect is astrophysical or not.Show less
This research paper presents the investigations on the impact of Earth’s magnetic field on the efficiency of 3-inch photomultiplier tubes used in the KM3NeT neutrino telescope. The research...Show moreThis research paper presents the investigations on the impact of Earth’s magnetic field on the efficiency of 3-inch photomultiplier tubes used in the KM3NeT neutrino telescope. The research includes measurements of three Hamamatsu PMTs. The measurements show that the efficiency of the PMTs are directional-dependent. However, this significance is most probably not caused by Earth’s magnetic field only. Further investigation is needed to determine where these external effects come from.Show less
When Van derWaals materials are reduced to two-dimensional atomic crystals, their physical properties start to change. For most materials these properties and phenomena are still unresearched. Van...Show moreWhen Van derWaals materials are reduced to two-dimensional atomic crystals, their physical properties start to change. For most materials these properties and phenomena are still unresearched. Van der Waals materials can be created to be atomically thin, to layers of a single atom thick. One class of these materials are transition metal dichalcogenide materials (TMDs). There exist different crystalline structures of TMDs, which are called polytypes. Here, TaS2 flakes are fabricated in order to study charge density waves (CDWs). A protocol to fabricate large thin flakes is developed along with a simple and quick method to determine the layer thickness with the use of an optical microscope. The calculated thicknesses are then verified with Atomic Force Microscopy (AFM) measurements. Low Energy Electron Microscopy (LEEM) measurements are performed on thick flakes in order to research CDWs. Although confirmation of CWDs is absent, different domains of reflective electron intensity and various structures are observed. These features are compared with AFM measurements. The flake existing of different polytypes could be the cause for the observed contrasts. The reflection of electrons is observed until an energy of 150 eV.Show less
Prior research has demonstrated by theory and simulation that the creation of holes in two-dimensional superconductors by periodic nanopaterning can increase the critical temperature. To test this...Show morePrior research has demonstrated by theory and simulation that the creation of holes in two-dimensional superconductors by periodic nanopaterning can increase the critical temperature. To test this theory it must be implemented experimentally and the resulting critical temperature must be measured. Preceding the modification of these superconductors, the cryogenic resistance-temperature measurement must be tested and optimized. This thesis discusses the steps, and the problems in them, of the measurement process and provides possible explanations and solutions.Show less
In order to improve the the polarimetric performances of VLT’s SPHERE/ZIMPOL polarimeter instrument, polarization dependent beam shifts due to reflection of a coated aluminum mirror need to be...Show moreIn order to improve the the polarimetric performances of VLT’s SPHERE/ZIMPOL polarimeter instrument, polarization dependent beam shifts due to reflection of a coated aluminum mirror need to be taken into account. Therefore measurements are performed to characterize these beam shifts using a cylindrically shaped beam. In plane-of-reflection beam shifts, as described by the Goos-Hänchen effect, and out of plane-of-reflection beam shifts, described by the Imbert-Federov effect, have been measured. A measurement setup has been built which measures relative beam shifts of orthogonal polarizations states for different angles of incidence. Using the measurements of the Goos-Hänchen effect a beam shift is detected in the plane-of-reflection, where no shift in out of the plane-of-reflection is detected. An effective refractive index of n = 1.12 and k = 2.57 has been found. Measurements of the Imbert-Federov effect show a significant shift out of the plane-of-reflection, where no shift is detected in the plane-of-reflection, yet a complex refractive index could not be determined.Show less
New methods of DNA sequencing using tunnelling currents are being investigated, in order to speed up the process of reading out longer strands. It has been demonstrated that a tunnelling junction...Show moreNew methods of DNA sequencing using tunnelling currents are being investigated, in order to speed up the process of reading out longer strands. It has been demonstrated that a tunnelling junction can be created by bringing two twisted graphene sheets together. This junction may provide a setup for DNA sequencing using tunnelling currents. Various improvements in steps in the preparation of the graphene electrodes are investigated to optimise their quality for further use in sequencing applications. Moreover, the junction must be immersed in a liquid for use in DNA sequencing. Several issues arise from this, including the formation of leakage currents throughout the liquid. Various methods of reducing the leakage currents by insulating the electrodes are investigated.Show less
Currently still under construction, the KM3NeT neutrino telescope in the Mediterranean Sea will be used to detect atmospheric and cosmic neutrinos. KM3NeT will consist of large cubic arrays of...Show moreCurrently still under construction, the KM3NeT neutrino telescope in the Mediterranean Sea will be used to detect atmospheric and cosmic neutrinos. KM3NeT will consist of large cubic arrays of large amounts of optical sensors. The detector will exploit Cherenkov radiation emitted by energetic secondary particles. To have an understanding of the performance of the detector, knowing about the performance of the different digital optical modules (DOMs) is vital. Using various methods (mainly potassium decay, 40K -> 40Ca), relative sensitivities of the different DOMs have been measured. However, only rough estimates for absolute efficiencies are known. A novel method will be developed to cross check the results previously found. This research will exploit atmospheric muons and probe the sensitivity of the KM3NeT neutrino telescope. Events in which multiple DOMs in a row detect a muon are used. Both threefold and fourfold combinations are used. Ratios are introduced to describe the fraction of times all DOMs are hit over the outer DOMs. This scales with efficiency. Simulations are used to describe extinction and propagation of Cherenkov photons and muons in water.Show less
We will discuss the results from Vancea and construct some concrete examples of the general solutions that are proposed in that paper. To this end we will develop the necessary theory of manifolds...Show moreWe will discuss the results from Vancea and construct some concrete examples of the general solutions that are proposed in that paper. To this end we will develop the necessary theory of manifolds and the basic framework of general relativity in terms of the 3+1 formalism. After which we examine the method proposed by Vancea and construct concrete cases of such solutions.Show less
This thesis studies the origin of large signals detected in the KM3NeT optical modules. The origin of the signals is directly studied with the single PMT and indirectly with the DOM. The results of...Show moreThis thesis studies the origin of large signals detected in the KM3NeT optical modules. The origin of the signals is directly studied with the single PMT and indirectly with the DOM. The results of both the single PMT and the DOM point toward a high energetic particle origin. Further measurements are required to conclude this with certainty.Show less
We explore the implementation of a vector-Apodizing Phase Plate for crowded field observation on the European Extremely Large Telescope using the MICADO instrument. In this joint effort between...Show moreWe explore the implementation of a vector-Apodizing Phase Plate for crowded field observation on the European Extremely Large Telescope using the MICADO instrument. In this joint effort between Leiden Observatory and the Kapteyn Institute, we design an APP specifically suited for crowded fields in a two stepped approach: we first assume a circular symmetric aperture which allows us to perform cone optimization. This yields a global optimal APP design. Secondly, we use an implementation of Gerchberg-Saxton to optimize our design for the non-circular symmetric aperture of the ELT. We use METIS SCAO phase screens to generate a realistic PSF (with and without an APP) and convolve the resulting PSFs with two delta functions to create a toy model crowded field. We performed basic astrometric and photometric fitting by fitting two Airy disks on the resulting field, to map the quality of fit with and without an APP in place. We can conclude that implementing a vector-APP seems to improve the fitting quality, but better fitting techniques (such as PSF-modelling) need to be used to provide a definite conclusion.Show less
We simulated the time evolution of plasma torus knots in resistive, viscous MHD. These torus knots are stationary solutions to the ideal MHD equations, as proposed by Kedia et al. These magnetic...Show moreWe simulated the time evolution of plasma torus knots in resistive, viscous MHD. These torus knots are stationary solutions to the ideal MHD equations, as proposed by Kedia et al. These magnetic fields are parameterised by the winding numbers np and nt and exist of several families of nested magnetic fields around a core field line. In ideal MHD, the topological structure of these solitons is conserved and these fields form stationary solutions, but these properties are not carried over to resistive MHD. We will look at the structure of the magnetic field of such a plasma. We find that a new magnetic surface family arises whose topology depends on the poloidal winding number $n_p$ of the initial magnetic field. The time evolution of the corresponding magnetic energy and helicity is strongly influenced by these np. When np > 1, the new magnetic surfaces have a non-zero Euler characteristic and depend on a zero magnetic field line along the z-axis. The toroidal winding number nt is of lesser influence, and the corresponding zero line contracts and disappears. Both the old and the new structures are preserved over time and we observe the formation of magnetic islands between magnetic surfaces.Show less
We introduce a new technique called 3D-ARRES, with which the full free elec- tron eigenstates above the vacuum energy of van der Waals materials can be imaged by resolving the landing energy and in...Show moreWe introduce a new technique called 3D-ARRES, with which the full free elec- tron eigenstates above the vacuum energy of van der Waals materials can be imaged by resolving the landing energy and in-plane momentum of reflected electrons in LEEM. Compared to ARRES, 3D-ARRES measures the whole first Brillouin zone and not only along the high symmetry axes. This is done by inte- grating the LEED pattern on the camera at different incidence angles and land- ing energies and fitting the data using image analysis techniques. By comparing the three-dimensional (3D) band structures ARRES measurements on both bulk MoS 2 and h-BN, 3D-ARRES is shown to be effective.Show less
In this thesis paper the possibility of using Convolutional Neural Networks (CNN) for wavefront reconstruction is studied. This is tested with the Generalised Optical Differentiation Wavefront...Show moreIn this thesis paper the possibility of using Convolutional Neural Networks (CNN) for wavefront reconstruction is studied. This is tested with the Generalised Optical Differentiation Wavefront Sensor (g-ODWFS), a nonlinear pupil plane wavefront sensor that uses spatially varying polarisation rotators in the focal plane. Network architectures are developed for modal reconstruction in the Zernike and actuator basis. Both simulations and experimental work show that using a CNN for wavefront reconstruction improves the estimation of large phase aberrations as compared to the currently used linear method. This increases the convergence rate of the adaptive optics system, which is demonstrated in closed loop simulations.Show less
Rabi oscillations between the excited state and the ground state of an exciton in a quantum dot can be used to coherently control the state of a two-level system. Manipulating the state of a two...Show moreRabi oscillations between the excited state and the ground state of an exciton in a quantum dot can be used to coherently control the state of a two-level system. Manipulating the state of a two-level systems has applications in quantum computing and creating single photon sources. Rabi oscillations have been amply observed in atoms, free-space quantum dots, and ion traps. However, observing this phenomenon in cavity quantum electrodynamics has shown to be much more demanding. In this thesis we show that Rabi oscillations can also be observed in polarisation non-degenerate cavity-quantum dot systems.Show less
