Theoretical and computational studies of the scattering of light from randomly rough dielectric surfaces

We are surrounded by light being scattered from surfaces all around us, both natural and man-made. Improving our understanding of exactly how light (and more generally, electromagnetic waves) interacts with and scatters from or through surfaces, such as a solar cell, a telescope mirror, paint or a glass window, is of value and importance to both industry and society as a whole. It gives us a better understanding of the world around us and how we perceive it, and it can also enable us to develop new technologies and improve upon existing ones. This thesis is a collection of work where we have tried to better understand a few of these interactions through the use of theory, experimental results and computer simulations. We have investigated the scattering of polarized light from two-dimensional randomly rough dielectric interfaces, in order to look for scattering patterns of interest in the angular intensity distributions of the diffusely scattered light. The basis for our investigations has been the reduced Rayleigh equations and their numerical solutions. Our overall contribution is towards an increased understanding of diffuse scattering from randomly rough surfaces, especially for three-dimensional systems where we allow for cross-polarized scattering. This can be useful in a wide range of optical systems, since the non-invasive method of surface characterization through the analysis of scattering data is interesting for both industry and research. When light is scattered diffusely in either reflection or transmission from or through a weakly rough interface, two phenomena of interest can be observed in the scattering intensity distributions. These are the Yoneda phenomenon, relatable to the idea of total internal reflection from a planar interface, and the Brewster scattering phenomenon, relatable to the polarizing angle observed for a planar interface. These scattering phenomena have only partially been investigated in the past, and their study has been the core of this thesis. We investigate these phenomena thoroughly through perturbative and non-perturbative numerical and theoretical work, also with the aid of new experimental results. We show, describe, explain and predict the behavior of both phenomena based on a lowest non-zero order perturbative approach, and as such we conclude that they are so-called single-scattering phenomena. We also investigate the physical mechanisms that underpin these phenomena, and attempt to describe them in terms of simple notions such as scalar waves, oscillating and rotating dipoles and geometrical arguments. If you let sunlight reflect from the layer of water vapor hovering some micrometers above the reflective surface of your morning cup of tea, you might observe some colored rings of light when you look into the reflection. These rings are a variety of Selenyi rings, an interesting interference effect that emerges when light is scattered diffusely by thin dielectric films. We investigate this effect thoroughly in this thesis, and describe the Selenyi phenomenon theoretically and numerically. Lastly, when medium interfaces are randomly rough, it is of value if we can infer the statistical properties of the roughness along with the properties of the scattering media based purely on the non-invasive scattering of light. Through the use of numerical phase perturbation theory based on the reduced Rayleigh equations, we investigate the reconstruction of such properties through a minimization method based on the reflected intensity distributions

Atomistic Implications of Stacking Fault Energy on Dislocation - Void Interactions

Irradiation hardening due to voids can be a significant result of radiation damage in metals, but treatment of this by elasticity theory of dislocations is difficult when the mechanisms controlling the obstacle strength are atomic in nature. Copper has long been used to approximate austenitic stainless steels in computer simulations because of their shared face-centered cubic structure and similar Stacking Fault Energy (SFE). Their stacking fault properties are however not identical; the SFE in stainless steel is significantly lower than that in Cu. Low values of SFE lead to wide dissociation of dislocations in their glide planes into Shockley partial dislocations, severely affecting the hardening process of the metal.Molecular Dynamics simulations have been conducted in order to highlight the implications of stacking fault energy on the interaction between dissociated dislocations and nanoscale voids. A recently developed set of interatomic potentials with a range of stacking fault energies based on FCC copper was used in order to investigate the Critical Resolved Shear Stress (CRSS) and other interaction details for a range of void sizes, temperatures, impact parameters and void separation distances for both edge and screw dislocations.Changes in SFE were found to affect the pinning interactions for dissociated edge dislocations in a relatively weak but systematic manner. The CRSS needed for any given dislocation to overcome the array of voids was in all cases shown to decrease with decreasing SFE and vice versa. This was also concluded for dissociated screw dislocations, but with an extra layer of complexity: The advent of complex cross-slip mechanisms sometimes lead to highly unpredictable void pinning dynamics, mainly through multiple cross-slip and its consequences; the creation of temporary immobile dislocation structures. The value of SFE is shown to be very influential on the distribution, probability and form of complex cross-slip mechanisms, which may double or triple the pinning strength of voids

Numerical studies of the transmission of light through a two-dimensional randomly rough interface

The transmission of polarized light through a two-dimensional randomly rough interface between two dielectric media has been much less studied, by any approach, than the reflection of light from such an interface. We have derived a reduced Rayleigh equation for the transmission amplitudes when p- or s-polarized light is incident on this type of interface, and have obtained rigorous, purely numerical, nonperturbative solutions of it. The solutions are used to calculate the transmissivity and transmittance of the interface, the mean differential transmission coefficient, and the full angular distribution of the intensity of the transmitted light. These results are obtained for both the case where the medium of incidence is the optically less dense medium and in the case where it is the optically more dense medium. Optical analogues of Yoneda peaks observed in the scattering of x-rays from metallic and non-metallic surfaces are present in the results obtained in the former case. For p-polarized incident light we observe Brewster scattering angles, angles at which the diffuse transmitted intensity is zero in a single-scattering approximation, which depend on the angle of incidence in contrast to the Brewster angle for flat-surface reflection

Experimental and numerical studies of the scattering of light from a two-dimensional randomly rough interface in the presence of total internal reflection: Optical Yoneda peaks

The scattering of polarized light from a dielectric film sandwiched between two different semi-infinite dielectric media is studied experimentally and theoretically. The illuminated interface is planar, while the back interface is a two-dimensional randomly rough interface. We consider here only the case in which the medium of incidence is optically more dense than the substrate, in which case effects due to the presence of a critical angle for total internal reflection occur. A reduced Rayleigh equation for the scattering amplitudes is solved by a rigorous, purely numerical, nonperturbative approach. The solutions are used to calculate the reflectivity of the structure and the mean differential reflection coefficient. Optical analogues of Yoneda peaks are present in the results obtained. The computational results are compared with experimental data for the in-plane mean differential reflection coefficient, and good agreement between theory and experiment is found