We study the technique of photothermal microscopy by which we can detect single nano-objects by their absorption at room temperature. We optimize the sensitivity of this technique and demonstrate... Show moreWe study the technique of photothermal microscopy by which we can detect single nano-objects by their absorption at room temperature. We optimize the sensitivity of this technique and demonstrate the first optical detection of a single molecule by its absorption at room temperature. Moreover, we combine photothermal, luminescence and scattering of individual nano-objects (organic dye nanoparticles and gold nanoparticles) at single-particle level to gain insight into their radiative and nonradiative properties. Single organic nanoparticles exhibit a complex excitation power-dependent luminescence quantum yield due to singlet-singlet or singlet-triplet annihilation, and their luminescence quantum yield can be as high as 10^(__2). In contrast to organic dye nanoparticles, gold nanoparticles yield very stable optical signals. Gold nanoparticles are also easily detectable by their photoluminescence. We find that the luminescence quantum yield of single gold nanoparticles is nearly independent of their volumes and can be as high as ~10^(-5) for nanorods with a plasmon resonance of ~650 nm. We further investigate the sensitivity of a single gold nanorod to an approaching dielectric surface. We show that the nanorod exhibits significant red-shift in its plasmon resonance wavelength for distances less than 400 nm pointing the way towards the possible application of nanorods as distance sensors. Show less
In this thesis, we describe the latest advances in SQUID-detected Magnetic Resonance Force Microscopy (MRFM). We have developed a new MRFM setup, which we describe in great detail, in particular... Show moreIn this thesis, we describe the latest advances in SQUID-detected Magnetic Resonance Force Microscopy (MRFM). We have developed a new MRFM setup, which we describe in great detail, in particular our efforts to remove vibrational noise from our dry dilution refrigerator, whilst maintaining the lowest possible operating temperatures, and our solution to reduce the crosstalk between the B1 field and SQUID (Superconducting QUantum Interference Device) detection. We have used the setup to further investigate the MRFM signals of copper nuclei, with a specific focus on the usage of higher modes of the cantilever as source for the B1 field, resulting in an MRFM frequency shift signal from the Boltzmann polarization of spins in a voxel as small as (40 nm)3. Furthermore, we have investigated the spin system in diamond, where we found evidence of the suppression of spin-diffusion in the layer of surface spins due to our high magnetic field gradients. Show less
Mechanical interactions between cells and their environment play an important role in many biological processes. These interactions are often anisotropic in nature, but most mathematical models in... Show moreMechanical interactions between cells and their environment play an important role in many biological processes. These interactions are often anisotropic in nature, but most mathematical models in the field of cell mechanics describe cells as isotropic entities. In this thesis we theoretically study the role of anisotropic forces in cell mechanics, and compare our predictions to experimental data. Show less
We have studied the impact of particle shape anisotropy, multivalent interactions and flexibility on systems of micron-sized colloidal particles. In short, we have characterized the diffusive... Show moreWe have studied the impact of particle shape anisotropy, multivalent interactions and flexibility on systems of micron-sized colloidal particles. In short, we have characterized the diffusive properties of anisotropic dumbbell particles near surfaces. Furthermore, by using experiments and simulations, we have uncovered marked flexibility-induces effects in the Brownian motion of reconfigurable colloidal structures. Our work demonstrates the rich dynamics and possibilities for applications of shape-changing colloidal systems. We hope our findings further the study of the diffusivity of flexible objects found in complex mixtures relevant in, for example, the cosmetic, pharmaceutical and food industries, as well as in biological and drug-delivery systems. For example, our results may have implications for understanding both the diffusive behavior and the most likely conformations of macromolecular systems such as polymers, single-stranded DNA and other chain-like molecules. Show less
This thesis addresses a variety of systems in which the diffusion is anomalous, mainly motivated by recent experimental developments. The main topics discussed are: * We look at the consequences... Show moreThis thesis addresses a variety of systems in which the diffusion is anomalous, mainly motivated by recent experimental developments. The main topics discussed are: * We look at the consequences that subdiffusion on fractals has for shot noise. * The effects correlations have on superdiffusion in one dimension are examined. * We develop and demonstrate the usefulness of a method to simulate the anomalous diffusion of Dirac fermions in a computer. * A spin precession experiment in topological insulators is proposed and analyzed. * We present the mechanism for the conversion of an ordinary insulator into a topological insulator by disorder which was reported in the literature, on the basis of computer simulations, but had remained unexplained. Show less
In my thesis I study Majorana fermions from three different perspectives. The first one corresponds to non-interacting Majorana fermions that, however, pose non-trivial topological properties. The... Show moreIn my thesis I study Majorana fermions from three different perspectives. The first one corresponds to non-interacting Majorana fermions that, however, pose non-trivial topological properties. The second one corresponds to strongly interacting Majoranas and the third to relic neutrinos. No one knows for sure if they have Dirac or Majorana nature. Show less
As one of the most important developments in string theory, the AdS/CFT correspondence(or in general gauge/gravity correspondence) encodes a way of using string theory to perform non-perturbative... Show moreAs one of the most important developments in string theory, the AdS/CFT correspondence(or in general gauge/gravity correspondence) encodes a way of using string theory to perform non-perturbative calculation in gauge theory which is still a complicated problem. It was firstly formulated as the correspondence between weakly coupled gravity theory with AdS (Anti de Sitter) space-time background and strongly coupled gauge theory with conformal symmetry in one lower dimension. One application of AdS/CFT is to compute some of observables of sQGP such as photon and dilepton production rates, mean-free path time of the plasma constituents, and anisotropic drag force effect to the elliptic flow. Show less
Graphene nanoribbons (GNRs) are used as a current carrying substrate in investigation of current-induced forces in a low-temperature STM (chapter 2). We demonstrate induced migration of Co adatoms... Show moreGraphene nanoribbons (GNRs) are used as a current carrying substrate in investigation of current-induced forces in a low-temperature STM (chapter 2). We demonstrate induced migration of Co adatoms on GNRs and on Au(111) using voltage pulses from the STM tip and we argue that motion is due to thermal excitations rather than the wind force. In chapter 3 we show that voltage signal is induced in a graphene strip when a droplet of ionic liquid is moved across its surface. Here we show that even deionized water can induce voltage over charged graphene surface due to the polarizability of water molecules. In chapter 4 we present a method for fabrication of graphene nanoelectrodes which we further test electrically in a modified STM. For the first time we demonstrate that the gap between two graphene nanoelectrodes can be tuned with subnanometric precision Show less
Proteins and enzymes play a key role in all biological systems. Understanding the mechanism of biological functions and reactions in which proteins and enzymes are involved requires a detailed... Show moreProteins and enzymes play a key role in all biological systems. Understanding the mechanism of biological functions and reactions in which proteins and enzymes are involved requires a detailed characterization of protein structure and dynamics. Structure refers to geometrical structure, as a result of the local arrangement of amino-acid side chains, and electronic structure, in particular at the active site of proteins and enzymes. Dynamics refers to structural changes that proteins undergo to perform their function. The work reported in this thesis concerns both methodological developments and the application of electron paramagnetic resonance (EPR) to study protein structure and dynamics. To this end, both continuous wave (cw) and pulsed microwave excitation have been applied. In the research described in this thesis transition-metal ions, such as Cu(II) and Fe(III), and nitroxide spin labels have been used as paramagnetic probes. Show less
The focus of the dissertation "Aspects of cosmic acceleration" is the study of possible mechanisms responsible for the late-time accelerated expansion of the universe. It has 5 main chapters. In... Show moreThe focus of the dissertation "Aspects of cosmic acceleration" is the study of possible mechanisms responsible for the late-time accelerated expansion of the universe. It has 5 main chapters. In the first chapter I have given an overview of modern cosmology. Particularly, an introduction to cosmological perturbation theory, and a general overview of the cosmological standard model, as well as an overview to beyond-standard-model scenarios is presented. Chapter two discusses cosmological models based on the so-called alpha-attractor framework. Chapter three is dedicated to the study of constraints imposed by gravitational wave observations on the so-called doubly-coupled massive bimetric gravity theory. Chapter four studies the cosmological stability of the so-called massive mimetic gravity theory. Finally, Chapter five studies the effect of the Symmetron gravity on the so-called splashback radius of dark matter halos. Show less
This PhD-thesis presents a study on micron-sized particles, so-called colloids. By controlling the chemical and physical properties of these particles, such as the interparticle interaction... Show moreThis PhD-thesis presents a study on micron-sized particles, so-called colloids. By controlling the chemical and physical properties of these particles, such as the interparticle interaction and the particles’ shape, colloids can act as building blocks that self-assembly into larger structures. This could lead to the development of materials with novel properties such as ‘smart’ materials with the ability to adapt their structure to the environment. In this thesis spherical colloids are used as a starting point to make complex colloidal building blocks and larger microstructures. Anisotropic particles were formed by introducing surface roughness, dents, protrusions and chemical functionalization on the particle surface. Complex structures were obtained by assembling and reconfiguring clusters of spheres. Here, a balance of several phenomena including, the interfacial and potential energy, entropy and geometric constraints, determined the final geometry of the assembled structure. The work also shows how anisotropic elongated particles distorted the hexagonal order in crystals of spheres, either locally or over long distances. These distortions are known to influence the optical, mechanical and electronic properties of colloidal crystals. The complex particles and assemblies made in this study are therefore an important step towards the development of materials with novel and adaptable properties. Show less
Quantum dots (QDs) are nm-size semiconductor structures that hold promise for applications in quantum information. One important requirement, however, is to achieve near-unity interaction between... Show moreQuantum dots (QDs) are nm-size semiconductor structures that hold promise for applications in quantum information. One important requirement, however, is to achieve near-unity interaction between photons and (singly charged) QDs. For this purpose, we make use of oxide-apertured micropillars that confine light in a small volume and thereby enhance the interaction. A QD transition coupled to the cavity mode can turn a transmittive cavity into a reflective one, and this property can be used to create entanglement between the spatial state of a photon and the spin state of a charge in the QD. This thesis consists of two parts: 1) in the first part we demonstrate techniques to monitor and fine tune the oxide aperture size and shape. By controlling the oxide shape we show we can fabricate polarization degenerate microcavities. 2) In the second part, perform cryogenic experiments with such a QD-cavity system. We investigate neutral and singly charged QDs as function of polarization and find this offers a way to assess the QD coherence. Next, we demonstrate a novel effect where charges around the QDs have a strong feedback with the QD properties. Finally, we present a homodyne detection technique of the QD coherence and phase shift. Show less
This thesis presents a viable route towards the implementation of quantum computing utilizing quantum dots embedded in optical microcavities. Following the introduction of the big picture and long... Show moreThis thesis presents a viable route towards the implementation of quantum computing utilizing quantum dots embedded in optical microcavities. Following the introduction of the big picture and long-term visionary goal, general concepts fundamental to this field of research are described: quantum dots and microcavities, forming the physical system explored; and cavity quantum electrodynamics, the theoretical language used to describe their interaction. The physical structure and the optical mode composition in oxide-apertured micropillar cavities is analyzed. Permanent tuning methods achieving polarization degenerate cavities resonant with a quantum dot transition are illustrated. Active positioning of single quantum dots is developed providing an accuracy suitable to measure the interaction between a quantum dot and a cavity in the strong coupling limit. The possibility to waveguide-couple photonic crystal cavities on the same sample is explored. A theoretical description of the quantum-dot confined electron dynamics is presented. Presented are ideas how a hybrid quantum system could serve for implementation of a controlled NOT gate, and therewith be the building block for a quantum computer, exploiting the weak coupling regime. A Bell-state analyzer is the second scheme that is discussed. Results from reflection spectroscopy measurements on single quantum dots in a micropillar cavity are presented. Show less
This thesis starts with an introduction to the quantum electrodynamical description of the interaction between light and matter. The role of optical cavities is discussed and the basic properties... Show moreThis thesis starts with an introduction to the quantum electrodynamical description of the interaction between light and matter. The role of optical cavities is discussed and the basic properties of rare-earth ions are reviewed. In Chapter 2 a bare ring resonator that is coupled to a waveguide is studied. Transmission spectra are measured, from which optical properties of the ring resonator and the waveguide are characterized. Chapter 3 addresses two technical issues that are essential for the research presented in this thesis: the implantation of rare-earth ions into the ring resonator and the permanent fiber connections to the waveguides. Chapter 4 is devoted to the research on the enhancement of the spontaneous emission rate in an ytterbium-doped ring resonator in the temperature range of 5.5-295 K as a result of the Purcell effect. Chapter 5 presents the results of measurements performed on an ytterbium-doped ring resonator in a dilution refrigerator in the range of 12 mK-4.7 K. In Chapter 6 collective effects of an ensemble of emitters in a cavity are theoretically studied with different initial states and pure dephasing rates by using the quantum Monte Carlo method. Chapter 7 concludes the thesis and presents an outlook for future work. Show less
Life’s building block is a cell. Different cell types are differentiated by specific functional properties. A white blood cell, for instance, can get rid of bacteria and many muscle cells contract... Show moreLife’s building block is a cell. Different cell types are differentiated by specific functional properties. A white blood cell, for instance, can get rid of bacteria and many muscle cells contract together for proper muscle function. Deformation and force exertion play important roles in these processes. Bacteria have to be physically engulfed by the white blood cell, and the muscle cell has to contract in the right way. In this research we measured how much force cells exert and simultaneously visualized specific proteins. A newly developed technique enabled the visualization of the nanometer-structure of cellular adhesions. We also examined the relationship between cellular shape and orientation of an intracellular network of protein (actin). We discovered that the signal of yet another protein (p130Cas) alters the mechanical behavior of the cell when the stiffness outside the cell changes. Finally, we also examined the structure of other proteins (tubulin and H2B) during cell division. In all these processes we measured how much force a cell exerts on its environment. The results provide important insights in the mechanical component of cellular function and their role in life Show less
In the research presented in this thesis, the effects of humidity on a series of Ru-complex molecular layers is investigated with the Conductive Atomic Force Microscopy technique. One specific... Show moreIn the research presented in this thesis, the effects of humidity on a series of Ru-complex molecular layers is investigated with the Conductive Atomic Force Microscopy technique. One specific molecule out of this series called '2-Ru-N' shows remarkable humidity sensitive diode behavior: in low humidity it behaves as a conventional resistive tunneling junction, wheras in high humidity, it behaves as a diode. The rectification ratios found for 2-Ru-N are among the highest reported for molecular junctions in the literature. By comparing the behavior of 2-Ru-N with its other counterparts, a model is presented that is based on two localized molecular orbitals that are misaligned during the addition of water. This is due to the water pulling on the counter-ions that are found in the molecular layer and hence, effectively gate one of the two localized molecular orbitals, resulting in a misalignment and thus, a diode-like behavior. Show less
In this work the reconstruction of a tau neutrino signal in the KM3NeT detector is discussed. Tau neutrinos leave a two shower signature in the detector, which is a unique signature among all... Show moreIn this work the reconstruction of a tau neutrino signal in the KM3NeT detector is discussed. Tau neutrinos leave a two shower signature in the detector, which is a unique signature among all neutrino interactions. By identifying and reconstructing these tau signatures the KM3NeT detector will be able to distinguish all three neutrino flavors. In addition, cosmic tau neutrinos suffer significantly less from atmospheric backgrounds, makign them ideal messengers for cosmic events. The presented reconstruction is the first tau reconstruction for the KM3NeT detector and resulted in a total expected observation rate of half a tau event per year for one unit of the KM3NeT detector. Show less
In eukaryotic cells, genomic DNA is organized in chromatin fibers composed of nucleosomes as structural units. A nucleosome contains 1.7 turns of DNA wrapped around a histone octamer and is... Show moreIn eukaryotic cells, genomic DNA is organized in chromatin fibers composed of nucleosomes as structural units. A nucleosome contains 1.7 turns of DNA wrapped around a histone octamer and is connected to the adjacent nucleosomes with linker DNA. The folding of chromatin fibers effectively increases the compaction of genomic DNA, but it remains accessible for enzymatic reactions. This apparent paradox motivates a detailed study of the dynamics of chromatin. A structural model at the molecular level will shed light on how cells regulate the compaction and dynamics of genomic DNA. This thesis presents the results of an experimental study on the dynamics of chromatin higher-order folding. Using magnetic tweezers, we observed force-dependent structural changes within chromatin fibers at the single nucleosome level. Show less
The central topic of the Thesis concerns light scattering experiments with entangled photons. Specifically, we study the effect of scattering processes on polarization-entanglement of twin-photons.... Show moreThe central topic of the Thesis concerns light scattering experiments with entangled photons. Specifically, we study the effect of scattering processes on polarization-entanglement of twin-photons. The main idea is that scattering generally couples polarization and spatial degrees of freedom of photons. The details of this coupling depend on the characteristics of the scattering medium. Such coupling, in turn, can reduce the entanglement of twin-photons, if the photon pairs are detected in a momentum-insensitive way. We have investigated a broad range of optical scattering media ranging from milk to polymer fibers. By manipulating the parameters of these samples we were able to generate a broad range of quantum states, proving for the first time that scattering processes are a substantial tool for mixed-state engineering. This is of great importance for quantum information since any real-world application thereof has to deal with mixed (as opposed to pure) states. Show less
This thesis explores the basic mechanism of the CO oxidation reaction on a palladium catalyst. Two new setups are presented to investigate catalytic reactions in general. Using a variety of... Show moreThis thesis explores the basic mechanism of the CO oxidation reaction on a palladium catalyst. Two new setups are presented to investigate catalytic reactions in general. Using a variety of techniques, reaction oscillations are analysed and modeled to examine the basic structure of the palladium catalyst in operating conditions. Show less
