Coherence vortices in Mie scattered partially coherent light
Creators: name: Marasinghe Sumanasirige, Madara Lakshika Marasinghe
Publisher: name: Monash University
Year: 2017
Subjects: Uncategorized

description: Although the fields of coherence optics and Mie scattering have been separately subjected to extensive investigations for some time, no work has been carried out combining these two hitherto distinct concepts. In particular, despite the fact that many scientists have performed numerous studies on optical singularities in various systems, no work is reported on correlation singularities in Mie scattered systems. It is interesting to investigate the coherence variation of Mie scattered realistic fields such as nonparaxial beams. This dissertation presents numerical investigations carried out in order to bridge this knowledge gap and present a model for the coherence variation and correlation singularities present in Mie scattered partially coherent electromagnetic fields. The project presented in this dissertation was carried out in three major phases. In the first phase, a simple Mie scattering system consisting of a single spherical dielectric particle was considered. This system was illuminated with partially coherent electromagnetic plane waves and features of both the internal and scattered fields were studied. In order to carry out this study, a mathematical model was developed to represent the Mie scattering of partially coherent plane waves. In this phase, coherence vortices were identified in the correlation function in both the internal and scattered fields. It was shown that as the scattering system parameters such as particle size and pointing-stability of the incident field are changed, the resulting coherence patterns also change generating different coherence vortex-antivortex structures. Another interesting discovery was the identification of spontaneous annihilating coherence vortices as the scattered field propagated in free space. In real life systems, scattering particles usually do not exist in isolation, but rather often appear as sets or groups. Hence an investigation on scattering of electromagnetic fields by several particles will be of much interest. Motivated by this fact and the discovery of coherence vortices in the first phase of the research, several multi-particle scattering systems were analyzed in the second phase. However, in this phase special care was taken to avoid multiple scattering by placing particles sufficiently far apart from each other, in order to reduce the complexity of the scattering scenario. These systems were also illuminated with partially coherent electromagnetic plane waves. The superposition principle along with partial coherence theory concepts developed for Mie scattering in the first phase of the research, were employed to analyze these scattering phenomena. The key result obtained in this phase was the identification of networks/lattices of coherence vortices in the Mie scattered field. Furthermore, it was found that by suitably arranging scattering particles in space, differently structured coherence vortex networks can be generated. Finally, the propagation characteristics of coherence vortices in the three-dimensional space were analyzed using a nodal line plot. This nodal line plot revealed that coherence vortices exhibit interesting characteristics such as hairpin structures and oscillatory behaviors as the scattered field propagates in free space. Although interesting results were obtained in the previous two phases, the incident fields employed in those two phases were ideal plane waves which are far from reality. Hence in the last phase of the research, a more realistic electromagnetic field was utilized to illuminate the scattering system. Thus, illumination of a single dielectric particle with a partially coherent nonparaxial electromagnetic focused beam was considered in this investigation. In this situation also, coherence vortices were identified in the scattered field. Furthermore, it was noted that as the incident field properties were varied, the resulting coherence variation patterns also changed. Here also a nodal line plot depicted the propagation characteristics of coherence vortices in the scattered field. Another interesting analysis was carried out in the final phase of the research where results of different coherence and polarization metrics defined by various research groups were numerically calculated and compared. The physical difference between these competing coherence metrics was elucidated. These metrics were seen to produce different albeit related spatially distributed measures of coherence, with only the complex measures being able to support coherence vortices.