arXiv subjects

Shiqi Jia

Publications and source records attributed to Shiqi Jia.

7 recordsLinked to original sources

arXiv2024

Optical polarization singularities in metallic cavities excited by electric dipole sources

Optical polarization singularities (PSs) in real space carry rich topological properties and can enable highly precise manipulations of light fields. Conventional studies focus on the PSs in the open space of optical systems. The properties of PSs inside optical cavities remain largely unexplored. By using full-wave finite-element simulations, we investigate the optical PSs inside metallic cavities excited by electric dipole sources. We determine the topological indices, morphology, and spatial evolutions of the singularities inside both spherical and torus cavities. We uncover the relationship between spatial symmetries and the PSs, as well as the mechanism underlying the emergence of polarization Mobius strips in the spherical cavity. In addition, we investigate the topological transitions of the PSs connecting two geometries (i.e., sphere and torus) with distinct topologies. The results provide insight into the singular and topological properties of light fields in optical cavities and can find applications in optical sensing and optical manipulation of small particles.

physics.optics↗

arXiv2024

Topological Dark Spots of Electric Near Field in Metal Structures

Electric dark spots are point singularities at which the electric field amplitude vanishes. These singularities usually emerge in real space accidentally and are unstable due to the vectorial property of the electric field. In this paper, we show that topologically protected electric dark spots can emerge in metal scatterers under external excitation. The material property of metal imposes a boundary condition that reduces the vectorial electric field on the metal surface to a scalar field. The phase singularity of this scalar field has zero amplitude and carries a well-defined topological charge corresponding to an electric dark spot. The topological electric dark spots give rise to the superoscillation phenomenon with a divergent local wavenumber. We uncover the global charge conservation property of the dark spots on the scatterers' surfaces and demonstrate their stability under different perturbations. We also demonstrate the manipulation of the dark spots' topological charge and spatial location. The results open a new avenue for nanophotonic near-field manipulations and may find applications in optical metrology, optical sensing, and super-resolution imaging.

physics.optics↗

arXiv2024

Near-field Spin Chern Number Quantized by Real-space Topology of Optical Structures

The Chern number has been widely used to describe the topological properties of periodic structures in the momentum space. Here, we introduce a real-space spin Chern number for the optical near fields of finite-sized structures. This new spin Chern number is intrinsically quantized and equal to the structure's Euler characteristic. The relationship is robust against continuous deformation of the structure's geometry and is irrelevant to the specific material constituents or external excitation. Our work enriches topological physics by extending the concept of Chern number to the real space, opening exciting possibilities for exploring the real-space topological properties of light.

physics.optics↗

arXiv2023

Broadband and large-area optical chirality generated by an achiral metasurface under achiral excitation

Optical chirality plays an essential role in chiral light-matter interactions with broad applications in sensing and spectroscopy. Conventional methods of generating optical chirality usually employ chiral structures or chiral excitations. Here, we propose to use an achiral metasurface consisting of gold disk array excited by a linearly polarized light to generate optical chirality. Using full-wave numerical simulations, we show that the metasurface can give rise to large-area optical chirality of the same sign for the wavelength ranging from 1.2 um to 1.5 um. The magnitude of the chirality is comparable to that of circularly polarized plane waves. The emergence of the optical chirality can be attributed to the asymmetric polarization singularity lines (C lines) in the near fields of the metasurface. We further explore the application of the proposed metasurface in chiral discriminations by simulating the absorption of chiral helix particles immersed in the near fields, and demonstrate that the left-handed and right-handed helix particles give rise to different absorptions. The phenomenon can be understood using an analytical theory based on the dipole approximation, which predicts differential absorption quantitatively agrees with the numerical simulation results. Our study uncovers the subtle relationship between near-field optical chirality, polarization singularities, and symmetry. The results can find applications in optical sensing, chiral quantum optics, and optical manipulations of small particles.

physics.optics↗

arXiv2022

Optical force and torque on small particles induced by polarization singularities

Optical forces in the near fields have important applications in on-chip optical manipulations of small particles and molecules. Here, we report a study of optical force and torque on small particles induced by the optical polarization singularities of a gold cylinder. We show that the scattering of the cylinder generates both electric and magnetic C lines (i.e., lines of polarization singularities) in the near fields, and the C lines can induce complex force and torque on a dielectric/magnetic particle. The force and torque manifest dramatic spatial variations with interesting symmetry properties, providing rich degrees of freedom for near-field optical manipulations. The study, for the first time to our knowledge, uncovers the effect of optical polarization singularities on light-induced force and torque on small particles. The results contribute to the understanding of chiral light-matter interactions and can find applications in on-chip optical manipulations and optical sensing.

physics.optics↗

arXiv2022

Topological near fields generated by topological structures

The central idea of metamaterials and metaoptics is that, besides their base materials, the geometry of structures offers a broad extra dimension to explore for exotic functionalities. Here, we discover that the topology of structures fundamentally dictates the topological properties of optical near fields and offers a new dimension to exploit for optical functionalities that are irrelevant to specific material constituents or structural geometries. We find that the nontrivial topology of metal structures ensures the birth of polarization singularities (PSs) in the near field with rich morphologies and intriguing spatial evolutions including merging, bifurcation, and topological transition. By mapping the PSs to non-Hermitian exceptional points and employing homotopy theory, we extract the core invariant that governs the topological classification of the PSs and the conservation law that regulates their spatial evolutions. The results have effectively bridged three vibrant fields of singular optics, topological photonics, and non-Hermitian physics, with potential applications in chiral sensing, chiral quantum optics, and beyond photonics in other wave systems.

physics.optics↗

arXiv2022

Chiral discrimination by polarization singularities of a metal sphere

The practical applications of chiral discrimination are usually limited by the weak chiral response of enantiomers and the high complexity of detection methods. Here, we propose to use the C lines (i.e., lines of polarization singularities) emerged in light scattering by a metal sphere to detect small chiral particles. Using full-wave numerical simulations and analytical multipole expansions, we determined the absorption dissymmetry of deep-subwavelength helices (i.e., chiral particles) at different positions on the C lines. We find that the absorption dissymmetry can be much larger than that induced by circularly polarized plane wave excitation, and it can be attributed to the asymmetric absorption of the induced electric and magnetic dipoles. We also show that the absorption dissymmetry strongly depends on the anisotropy of the helices. The results may find applications in optical manipulations, optical sensing, and chiral quantum optics.

physics.optics↗