Polarization Singularities in Light Scattering by Metal Particles

金屬微粒光散射中的偏振奇點

Student thesis: Doctoral Thesis

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Award date15 Aug 2022

Abstract

Using full-wave numerical simulations and analytical multipole expansions, we investigated the properties of real-space polarization singularities that emerge in light scattering by subwavelength particles. We considered metal particles with various geometry and topology under the excitation of an incident light. We determined the topological indices and the trajectories of the electric-field and magnetic-field polarization singularities in both near-field and far-field regions. In the far-field region, a total of four polarization singularities of the scattering electric field are identified, and the sum of their polarization topological indices is two, independent of the particles’ geometric shapes. In the near-field region, the polarization singularities of the scattering electric field strongly depend on the particles’ shapes and the polarization of incident light, and their index sum is not governed by the Poincaré-Hopf theorem anymore due to the non-transverse nature of the fields. While for magnetic-field polarization singularities on the particles’ surfaces, the sum of their indices is bounded by the Euler characteristic of the particles’ geometry. In addition, when particles and incident light possess rotational symmetries, higher-order polarization singularities can emerge in the near field. We found that the far-field properties of the singularities can be well explained by the interference of the excited multipoles in the particles, but their near-field properties are strongly affected by the evanescent fields that are not captured by the multipole expansions. The three-dimensional evolutions (including merging, bifurcation, and topological transition) of the polarization singularities can be well understood with the topological invariant (i.e., the phase index). We further explored the application of the polarization singularities in optical manipulations and found that the near-field polarization singularities can induce complex configurations of optical force and torque fields, providing rich degrees of freedom for near-field manipulations of small particles. The results in this thesis uncover the important relationship between particles’ geometric properties and the polarization singularities of their scattering field, as well as the physics of chiral light-matter interaction enabled by the polarization singularities. The results can generate novel applications in optical sensing, chiral quantum optics, and on-chip optical manipulations of small particles/molecules.