Light-Matter Interaction with Focused Optical Fields: Directional Scattering and Dynamics of Active Colloids

Abstract

Along with energy and linear momentum, angular momentum represents the most fundamental properties of light. These effect of these properties in various light-matter interactions become much more prominent upon focusing of the optical field to a small region. As a result, at the microscopic scale, the interaction of focused optical fields and the corresponding angular momentum of light with matter, broadly known as spin-orbit interactions (SOIs), can lead to various effects such as directional scattering, inter conversion of angular momentum as well as micro manipulation. On the other hand, illuminated optical fields intensity lead to optothermal heating of metallic objects, which can be utilized to trap as well as manipulate colloidal particles through thermophoresis, thermos-osmosis effects. In this thesis, broadly divided into two parts, we will discuss how these interactions can be modulated by the characteristics of the focal optical field properties. The first part discusses the numerical and experimental results on how angular momentum characteristics of the focal optical fields can be investigated by studying the forward scattering of a strongly focused optical field and the resultant SOIs. The second part involves discussion on the optical trapping and optothermal heating due to these optical fields, which leads to unconventional dynamics of thermally active colloids. Firstly, we address the question on how the polarization dependence of the Fourier plane scattering pattern can be utilized for simultaneous detection of spin and orbital angular momentum of light. In addition, we will discuss how the Fourier plane intensity distribution of the scattering of strongly focused linearly polarized light from a quasi-one-dimensional monocrystalline silver nanowire can be utilized to study the focal optical field characteristics. Through both numerical and experimental studies, we show detection of longitudinal spin density as well as the consequent optical spin-Hall effect. The following part discusses the optothermal heating due to focused and defocused optical field and its effect on the interaction with thermally active colloidal particles. We show that the thermophoretic behavior of the thermally active colloids can be exploited to form self-evolving dynamic assembly of the colloids for defocused laser beam illumination. Furthermore, it has been shown that polarization plays a key role in structural orientation of these assemblies. Driving these active colloids using optical orbital angular momentum also leads to collective dynamics, which unlike passive colloids, slows down and forms a colloidal matter while undergoing rotational motion. The thesis concludes by summarizing the key experimental results and outlines some possibilities for extrapolation of the results for future studies.