Optimization of an Optical Thermal Absorber Plasmonic Metamaterial: Understanding the Two Particle Interactions

In the recent years, the emergence of electromagnetic metamaterials (MMs) have ushered in an exciting new field with many promising applications in direct energy conversion. One of the more promising applications of MMs is the ability to fully harness the solar energy through the design of a perfect electromagnetic absorber. Electromagnetic MMs posses the ability to manipulate a material\u27s response resulting in augmented properties such as negative index of refraction, artificial permittivity, and permeability. One of the proven methods for the construction of electromagnetic metamaterials is to use a metal-dielectric composite whereby the electromagnetic response is governed by oscillating surface plasmons and the geometry of the dielectric environment. However, the majority of these materials exhibit a peak absorptivity at a single frequency, therefore, a deep understanding of their response becomes necessary to target approaches to broadening the absorptivity.;In this research the reflectance, transmittance, absorbance, and heat generation in a square nano-antenna composed of a metal-dielectric-metal construction was investigated. The wavelength of interest was in the visible regime from 390-700nm (430-790 THz). The dual focus of this research was 1) the development of metamateial with high absorbance characteristics and 2) the development of a broadband response. The focus was on tailoring the particle geometry and dielectric environment. The nano-antenna was constructed from a silver nano-particle situated on top of a controlled thickness aluminium oxide dielectric material followed by a thick silver ground plane. For this study a transverse electric (TE) plane wave was propagated in the negative z direction and the resulting scattered parameters were monitored. In order to evaluate the electromagnetic response (absorbance) at the sub-wavelength scales, a finite-difference time-domain (FDTD) method was implemented. In addition, the interaction from the neighboring particles en route to a perfect broadband absorber was studied. Our simulation results indicate a complex response between neighboring particles that could be categorized into four independent effective interactions. Furthermore, by controlling the distance between particles, the absorptivity achieved an increase by seven fold

Solitons on lattices and curved space-time

This thesis is concerned with solitons (solutions of certain nonlinear partial differential equations) in certain cases when the underlying space is either a lattice or curved. Chapter 2 of the thesis is concerned with the outcome of collisions between a kink (a 1-dimensional soliton) and an antikink for certain topological discrete (TD) systems. The systems considered are the TD sine-Gordon and the TD ø(^4) For the TD sine-Gordon system it is found that the kink can support an internal shape mode which plays an important role during the collisions. In particular, this mode can be excited during collisions and this leads to spectacular resonance effects. The outcome of any particular collision has sensitive dependence on the initial conditions and could be either a trapped kink-antikink state, a "reflection" or a "transmission”. Such resonance effects are already known to exist for the conventional discrete ø(^4) system, and the TD ø(^4) system is no different, though the results for the two are not entirely similar. Chapter 3 considers the question of the existence of explicit travelling kink solutions for lattice systems. In particular, an expression for such a solution for the integrable lattice sine-Gordon system is derived. In Chapter 4, by reducing the Yang-Mills equations on the (2 + 2)-dimensional ultrahyperbolic space-time, an integrable Yang-Mills-Higgs system on (2 + 1) dimensional de Sitter space-time is derived. It represents the curved space-time version of the Bogomolny equations for monopoles on R(^3) . Using twister methods, various explicit solutions with gauge groups U(l) and SU(2) are constructed. A multi-solution SU(2) solution is also presented

A Numerical Algorithm For Simulating Two Species Plasma

An algorithm for modeling two species plasmas, which evolves the number density, flow velocity, and temperature equations coupled to Maxwell\u27s electric and magnetic field equations, is discussed. Charge separation effects and the displacement current are retained. Mathematical derivations of normal modes in cold and hot plasmas, as represented by dispersion relations resulting from a linear analysis of the two fluid equations, are presented. In addition, numerical theory in relation to the ideas of geometry, temporal and spatial discretization, linearization of the fluid equations, and an expansion using finite elements is given. Numerical results generated by this algorithm compare favorably to analytical results and other published work. Specifically, we present numerical results, which agree with electrostatics, plasma oscillations at zero pressure, finite temperature acoustic waves, electromagnetic waves, whistler waves, and magnetohydrodynamics (MHD) waves, as well as a Fourier analysis showing fidelity to multiple dispersion relations in a single simulation. Final consideration is given to two species plasma stability calculations with a focus on the force balance of the initial conditions for a resistive MHD tearing mode benchmark and a static minimum energy plasma state

Observation of exciton redshift-blueshift crossover in monolayer WS2

We report a rare atom-like interaction between excitons in monolayer WS2, measured using ultrafast absorption spectroscopy. At increasing excitation density, the exciton resonance energy exhibits a pronounced redshift followed by an anomalous blueshift. Using both material-realistic computation and phenomenological modeling, we attribute this observation to plasma effects and an attraction-repulsion crossover of the exciton-exciton interaction that mimics the Lennard-Jones potential between atoms. Our experiment demonstrates a strong analogy between excitons and atoms with respect to inter-particle interaction, which holds promise to pursue the predicted liquid and crystalline phases of excitons in two-dimensional materials

Nonlinear optics

Nonlinear light-matter interactions have been drawing attention of physicists since the 1960's. Quantum mechanics played a significant role in their description and helped to derive important formulas showing the dependence on the intensity of the electromagnetic field. High intensity light is able to generate second and third harmonics which translates to generation of electromagnetic field with multiples of the original frequency. In comparison with the linear behaviour of light, the nonlinear interactions are smaller in scale. This makes perturbation methods well suited for obtaining solutions to equations in nonlinear optics. In particular, the method of multiple scales is deployed in paper 3, where it is used to solve nonlinear dispersive wave equations. The key difference in our multiple scale solution is the linearity of the amplitude equation and a complex valued frequency of the mode. Despite the potential ill-posedness of the amplitude equation, the multiple scale solution remained a valid approximation of the solution to the original model. The results showed great potential of this method and its promising wider applications. Other methods use pseudo-spectral methods which require an orthogonal set of eigenfunctions (modes) used to create a substitute for the usual Fourier transform. This mode transform is only useful if it succeeds to represent target functions well. Papers 1 and 2 deal with investigating such modes called resonant and leaky modes and their ability to construct a mode transform. The modes in the first paper are the eigenvalues for a quantum mechanical system where an external radiation field is used to excite an electron trapped in an electrical potential. The findings show that the resonant mode expansion converges inside the potential independently of its depth. Equivalently, leaky modes are obtained in paper 2 which are in close relation to resonant modes. Here, the modes emerge from a system where a channel is introduced with transparent boundaries for simulation of one-directional optical beam propagation. Artificial index material is introduced outside the channel which gives rise to leaky modes associated with such artificial structure. The study is showing that leaky modes are well suited for function representation and thus solving the nonlinear version of this problem. In addition, the transparent boundary method turns out to be useful for spectral propagators such as the unidirectional pulse propagation equation in contrast to a perfectly matched layer