| 1. |
- Alegret, Joan, 1977
(författare)
-
Numerical Simulations of Plasmonic Nanostructures
- 2008
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis focuses on the study of metallic nanostructures that support plasmons. Special emphasis is devoted to two specific numerical methods that allow us to predict plasmon characteristics: the discrete dipole approximation (DDA) and the Green's tensor (GT) method.DDA is an approximate method that produces fast and accurate results, but it can only be applied to systems in which the nanostructure is situated in a homogeneous background. In this thesis, DDA has been applied to predict the field enhancement and field decay around nano-rings, showing that the structure is well suited for biosensing; to obtain the spectral characteristics of silver trimers, showing that the actual plasmon modes are closely related to symmetry-adapted coordinates derived from group-theory; and to calculate the optical forces between two spherical particles illuminated by a plane wave, showing that the illumination wavelength determines the separation between the particles.The GT method, on the other hand, is an exact method, in the sense that the system can be solved to arbitrary precision depending on the size of the discretization elements. Its major drawback is the long time it takes to perform the calculations. To tis end, this thesis introduces a novel algorithm, called the top-down extended meshing algorithm (TEMA), that speeds up GT calculations by reducing the number of elements in the discretization process. This decreases the total time needed to perform the calculations, while keeping the precision of the result essentially unaltered. The GT method with TEMA meshes has successfully been used to study single holes of different sizes and shapes (circular and ellipsoidal) in the near- and far-field regime, as well as hole pairs as a function of their separation distance. The results compare very well with experiments, demonstration that the GT method is well suited for predicting the behavior of nano-holes.
|
|
| 2. |
|
|
| 3. |
- Eriksson, Martin, 1989
(författare)
-
There's Plenty of Room in Higher Dimensions - Nonlinear Dynamics of Nanoelectromechanical Systems
- 2017
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Nanoelectromechanical systems (NEMS) couple the dynamics of electrons to vibrating nanostructures such as suspended beams or membranes. These resonators can be used in for instance nanoelectronics and sensor applications. NEMS are also of fundamental interest since electrons exhibit strong quantum effects when confined in nanoobjects. Furthermore, NEMS such as graphene resonators are strongly nonlinear, which opens the door for complex dynamical response. The operation of nanoresonators often rely on actuation of mechanical vibrations driven by an electric ac-field. The first part of this thesis theoretically investigates high-frequency nonresonant actuation relying on electromechanical back action (Papers I-II). The nonresonant phenomenon can be utilized to study nonlinear dissipation and to selectively actuate different vibrational modes, also asymmetric ones, even though the driving field is homogeneous (Paper III). Another nonresonant actuation mechanism converts heat into mechanical energy and relies on electron-electron interaction in a movable quantum dot (Paper IV).Furthermore, parametric actuation of a nanoresonator can be used to generate a supercurrent through a superconducting weak link even though the superconducting phase difference across the link is zero (Paper V). The excitation leads to a spontaneous symmetry breaking, which allows for a new possibility to switch between the two current directions.Actuation of mechanical vibrations is also used to study nonlinear dynamics and mode coupling in nanoresonators. The strength of nonlinearities and vibrational frequencies can be tuned by electrostatic means (Paper VI). This tunability and the low dissipation in nanoresonators make it possible to selectively address individual or combinations of modes. Coupled modes allow for much richer nonlinear dynamics, such as internal resonances (Paper VII), due to the increased dimensionality of the relevant phase space. Furthermore, exotic dynamical regions may be hidden and not observed in standard experiments. However, bifurcation theory can help to construct maps which reveal the hidden regions. A lot more is therefore to be expected from coupled mode dynamics, since there’s plenty of room in higher dimensions.
|
|
| 4. |
- Schmidt, Falko, 1992
(författare)
-
Active Matter in a Critical State: From passive building blocks to active molecules, engines, and active droplets
- 2020
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The motion of microscopic objects is strongly affected by their surrounding environment. In quiescent liquids, motion is reduced to random fluctuations known as Brownian motion. Nevertheless, microorganisms have been able to develop mechanisms to generate active motion. This has inspired researchers to understand and artificially replicate active motion. Now, the field of active matter has developed into a multi-disciplinary field, with researchers developing artificial microswimmers, producing miniaturized versions of heat engines and showing that individual colloids self-assemble into larger microstructures. This thesis taps into the development of artificial microscopic and nanoscopic systems and demonstrates that passive building blocks such as colloids are transformed into active molecules, engines and active droplets that display a rich set of motions. This is achieved by combining optical manipulation with a phase-separating environment consisting of a critical binary mixture. I first show how simple absorbing particles are transformed into fast rotating microengines using optical tweezers, and how this principle can be scaled down to nanoscopic particles. Transitioning then from single particles to self-assembled modular swimmers, such colloidal molecules exhibit diverse behaviour such as propulsion, orbital rotation and spinning, and whose formation process I can control with periodic illumination. To characterize the molecules dynamics better, I introduce a machine-learning algorithm to determine the anomalous exponent of trajectories and to identify changes in a trajectory’s behaviour. Towards understanding the behaviour of larger microstructures, I then investigate the interaction of colloidal molecules with their phase-separating environment and observe a two-fold coupling between the induced liquid droplets and their immersed colloids. With the help of simulations I gain a better physical picture and can further analyse the molecules’ and droplets’ emergence and growth dynamics. At last, I show that fluctuation-induced forces can solve current limitations in microfabrication due to stiction, enabling a further development of the field towards smaller and more stable nanostructures required for nowadays adaptive functional materials. The insights gained from this research mark the path towards a new generation of design principles, e.g., for the construction of flexible micromotors, tunable micromembranes and drug delivery in health care applications.
|
|
| 5. |
- Tornsö, Marcus, 1993
(författare)
-
Plasma Oscillations in Holographic Quantum Matter
- 2021
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis we explore strongly correlated matter in the framework of holographic duality. Specifically, we examine the quasinormal modes of such systems, and we extend the current framework to efficiently and naturally cover plasmons and other collective modes that may be found within strongly correlated matter. The interest in strongly correlated matter is motivated by the presence of a “strange metal” phase both in high temperature superconductors and in near charge neutral graphene, both being materials of immense scientific interest. The strange metal phase is a phase characterized by the absence of quasi-particles. This implies that conventional methods, such as perturbation theory in quantum field theory and Monte Carlo methods fall short of being able to describe the dynamics. Perhaps surprisingly, string theory provides a novel method, capable of precisely describing such systems - the holographic duality. With the holographic duality, strongly coupled matter is mapped onto a weakly coupled gravity theory in one additional dimension, allowing for a conventional treatment of the dual system. In this thesis, we extend the existing framework to also describe polarizing media. This is explicitly done in the form of new boundary conditions on the holographic dual, which deviate from previous holographic studies, and we contrast the quasinormal modes previously studied with the emergent collective modes we find for some studied models. We find new results, as well as confirm the predictions of less general models in their respective regions of validity and pave the way for more complex future models.
|
|
| 6. |
- Persson, Daniel, 1978
(författare)
-
Arithmetic and Hyperbolic Structures in String Theory
- 2009
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis consists of an introductory text followed by two separate parts which may be read independently of each other. In Part I we analyze certain hyperbolic structures arising when studying gravity in the vicinity of spacelike singularities (the BKL-limit). In this limit, spatial points decouple and the dynamics exhibits ultralocal behaviour which may be mapped to an auxiliary problem given in terms of a (possibly chaotic) hyperbolic billiard. In all supergravities arising as low-energy limits of string theory or M-theory, the billiard dynamics takes place within the fundamental Weyl chambers of certain hyperbolic Kac-Moody algebras, suggesting that these algebras generate hidden infinite-dimensional symmetries of gravity. We investigate the modification of the billiard dynamics when the original gravitational theory is formulated on a compact spatial manifold of arbitrary topology, revealing fascinating mathematical structures known as galleries. We further use the conjectured hyperbolic symmetry E10 to generate and classify certain cosmological (S-brane) solutions in eleven-dimensional supergravity. Finally, we show in detail that eleven-dimensional supergravity and massive type IIA supergravity are dynamically unified within the framework of a geodesic sigma model for a particle moving on the infinite-dimensional coset space E10/K(E10). Part II of the thesis is devoted to a study of how (U-)dualities in string theory provide powerful constraints on perturbative and non-perturbative quantum corrections. These dualities are typically given by certain arithmetic groups G(Z) which are conjectured to be preserved in the effective action. The exact couplings are given by moduli-dependent functions which are manifestly invariant under G(Z), known as automorphic forms. We discuss in detail various methods of constructing automorphic forms, with particular emphasis on a special class of functions known as (non-holomorphic) Eisenstein series. We provide detailed examples for the physically relevant cases of SL(2,Z) and SL(3,Z), for which we construct their respective Eisenstein series and compute their (non-abelian) Fourier expansions. We also discuss the possibility that certain generalized Eisenstein series, which are covariant under the maximal compact subgroup K(G), could play a role in determining the exact effective action for toroidally compactified higher derivative corrections. Finally, we propose that in the case of rigid Calabi-Yau compactifications in type IIA string theory, the exact universal hypermultiplet moduli space exhibits a quantum duality group given by the Picard modular group SU(2,1;Z[i]). To verify this proposal we construct an SU(2,1;Z[i])-invariant Eisenstein series, and we present preliminary results for its Fourier expansion which reveals the expected contributions from D2-brane and NS5-brane instantons.
|
|
| 7. |
- Scott, Pat, 1982-
(författare)
-
Searches for Particle Dark Matter : Dark stars, dark galaxies, dark halos and global supersymmetric fits
- 2010
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The identity of dark matter is one of the key outstanding problems in both particle and astrophysics. In this thesis, I describe a number of complementary searches for particle dark matter. I discuss how the impact of dark matter on stars can constrain its interaction with nuclei, focussing on main sequence stars close to the Galactic Centre, and on the first stars as seen through the upcoming James Webb Space Telescope. The mass and annihilation cross-section of dark matter particles can be probed with searches for gamma rays produced in astronomical targets. Dwarf galaxies and ultracompact, primordially-produced dark matter minihalos turn out to be especially promising in this respect. I illustrate how the results of these searches can be combined with constraints from accelerators and cosmology to produce a single global fit to all available data. Global fits in supersymmetry turn out to be quite technically demanding, even with the simplest predictive models and the addition of complementary data from a bevy of astronomical and terrestrial experiments; I show how genetic algorithms can help in overcoming these challenges.
|
|
| 8. |
- Larsson, Irina, 1975
(författare)
-
Helium measuring system for on-line fuel and control rod integrity surveillance in BWRs
- 2012
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Accurate supervision of fuel and control rod integrity in a nuclear reactor core is very important for maintaining a safe and reliable operation of the reactor. Fuel rod failures can cause increased doses to personnel in the power plant, increased backend costs, limitations of the reactor operation and unplanned shut downs, which can lead to large economic losses. Control rod failures are normally not as severe as fuel rod failures, but can lower the reactor efficiency, cause operational restrictions and increase a need for control rod inspections during outage. Additional inspections can cause a prolonged outage, leading to economic losses.A project with the main objective to improve the detection of fuel and control rod failures in boiling water reactors (BWRs) by implementing continuous helium measurements in the off-gas system was initiated in 2007. The work was performed in collaboration between Chalmers University of Technology in Sweden, Forsmarks Kraftgrupp AB in Sweden and Kernkraftwerk Leibstadt (KKL) in Switzerland. During the project, helium detector systems have been installed in the reactor’s off-gas systems at both of the nuclear power plants (NPPs). The helium measuring system is a part of a combined on-line system which also includes nuclide specific measurements of gamma emitting noble gases. This combined system provides continuous information of both the fuel and control rod integrity. The work described in this thesis can be divided into two parts. One part includes evaluation of helium measurements for detection of control rod failures at KKL, which was the main purpose for the installation of the system at KKL in 2008. The helium measuring system at KKL has shown to be able to identify and follow control rod failures on-line. Since 2012 the helium measurement system is part of the process control and is included in the KKL core supervision system, which monitors the thermal operational parameter of the reactor core and all the relevant chemical parameters of the reactor water. The second part of the work was to investigate the ability of the helium measuring system to detect the small amount of helium that is expected to leak out in case of a fuel failure. This was also the main objective for the installation of the system at Forsmark 3 (F3) in 2010. Some helium releases from fuel failures at Forsmark 3 have been detected with the current system, however, further measurements might be needed to optimize the technique. Several factors, including operational parameters, which are influencing the helium measurements, have been identified and implemented in the developed equation for calculation of the helium background level in the off-gases. When being programmed in the core supervision system, such an automatic calculation of the helium background level, will give an immediate estimation of the released amount of helium if the helium concentration increase relative to the background level was detected.
|
|
| 9. |
- Dall` Olio, Daria, 1981
(författare)
-
Magnetic fields around massive protostars as traced by masers and dust emission
- 2020
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- It is not fully clear how the magnetic field acts during the first stages of star formation. A possible way to clarify its role is to observe the polarized light coming from masers and thermal dust emission. By measuring linear polarization angles and Zeeman splitting of different maser species it is possible to study the magnetic field morphology and strength in different parts of the protostar. Polarized emission of thermal dust has also been used extensively to probe the magnetic field at the onset of star formation. In this thesis we study the magnetic field properties of two well-known sources: the massive protostar IRAS18089-1732, showing a hot core chem- istry and a disc-outflow system, and the high-mass star forming complex G9.62+0.19, presenting several cores at different evolutionary stages. We also investigate the polarization properties of selected methanol masers, con- sidering newly-calculated methanol g-factors and hyperfine components. We compare our results with previous maser observations and we evaluate the contribution of preferred hyperfine pumping and non-Zeeman effects. We make use of MERLIN and ALMA observations and we analyse the polarized emission by 6.7 GHz methanol masers and thermal dust. Simulations were run using the radiative transfer code CHAMP for different magnetic field values, hyperfine components and pumping efficiencies. We observe that the large scale field probed by dust continuum emission is consistent with the small scale magnetic field probed by masers. Moreover, in the G9.62+0.19 complex we resolved several cores showing polarized emission. We propose an evolutionary sequence of magnetic field in this complex, where the less evolved stellar embryo exhibits a magnetic field stronger than the more evolved one. From our simulations, we find that preferred hyperfine pumping can explain some high levels of linear and circular polarization. We also notice that non-Zeeman effects need to be considered in magnetic field studies. In conclusion, our work indicates that there is a link between the magnetic field at different scales. More masers observations will help in evaluating the relevance of non-Zeeman effects and obtain good estimates of magnetic fields close to the protostar. Future multi-wavelength and multi-scale observations, aimed at detecting polarized light from masers, thermal dust and thermal molecular lines, will help to constrain magnetic field properties around massive protostars.
|
|
| 10. |
- Jones, Steven, 1990
(författare)
-
Mass Transport via Thermoplasmonics
- 2020
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- When a metallic nanoparticle is illuminated with light under resonant conditions, the free electron gas oscillates in such a way that substantial amplification of the local electric field amplitude is achieved – this is known as a plasmonic resonance. This resonance enhances both the optical scattering as well as absorption. In many applications, the enhanced scattering can facilitate efficient coupling between the near-field and the far-field, which enables optical interrogation of nanoscale volumes. Simultaneously, however, the enhanced absorption results in localized heating and substantial temperature gradients. The resulting temperature profile can drive other thermal processes, some beneficial others detrimental. Thermoplasmonics is the study of these plasmonically enhanced thermal processes. Elevated temperatures increase the Brownian motion of small particles. Moreover, if large temperature gradients are present, then a process known as thermophoresis is likely to occur. Thermophoresis tends to cause a local depletion of Brownian particles around a hot region. From the context of “conventional” plasmonic applications (like molecular sensing), these thermally driven mass transport mechanisms are adverse side effects since they reduce the interaction rate between the plasmonic system and the analyte. An investigation of thermal effects in plasmonic optical tweezers showed that the increased Brownian motion essentially negated the optical tweezing effect, resulting in an overall insensitivity between the resonance condition of the antenna and the particle confinement when evaluated in terms of the local temperature increase. Additionally, a significant thermophoretic depletion of analytes occurred, extending tens of microns from the plasmonic structure. This depletion acts in opposition to the plasmonically enhanced optical forces, which are restricted to a region of only a few hundred nanometres. However, thermoplasmonic effects can also be used for advantageous means. Once example is by driving thermocapillary flows directed towards the plasmonic system, thereby facilitating the efficient accumulation of analytes. One method of employing this effect is to superheat a plasmonic particle to a high enough temperature such that a bubble is nucleated. Once a bubble is formed, thermocapillary effects at the bubble interface drive fluid motion with a flow profile similar to that of a Stokeslet. This fluid flow can be utilized for analyte accumulation near the plasmonic structure. In addition to the thermocapillary induced flow, it was found that even more intense flow speeds were achieved immediately upon nucleation due to the mechanical action of the bubble. This transient peak in flow speed was approximately an order of magnitude faster than the subsequent persistent (thermocapillary) flow. By designing the plasmonic nanoparticle so that the Laplace pressure restricted the ultimate bubble size, these bubbles could be kept small enough to permit high modulation rates and maximize the relative effect of the peak transient flow.
|
|
