1月23日 香港科技大学张若洋博士学术报告：Impart orientations to topological line defects -- from non-Abelian and non-Hermitian band nodes to optical polarization singularities

报告题目：Impart orientations to topological line defects -- from non-Abelian and non-Hermitian band nodes to optical polarization singularities

报告人：张若洋

报告时间：1月23日（星期二）上午10:00

报告地点：唐仲英楼A213

报告人简介：现为香港科技大学研究助理教授 (陈子亭教授课题组)。于2016年博士毕业于南开大学陈省身数学研究所理论物理研究室。其研究领域涵盖拓扑光子学，非厄米物理以及电动力学的几何基础。目前已在Nature, Nature Physics, Nature Communications, Physical Review Letters等国际知名期刊发表SCI论文50余篇。

报告摘要：The theory of topological defects plays essential roles in many physical fields, benefiting our comprehension of the dynamics of ordered media, and the band structure degeneracies in crystalline materials. As a specific kind of topological defects in 3D space, the topological line defects exhibit charming features in their evolutions. When two line defects meet in space, they may either annihilate or repel each other, or even merge into a new defect. In this talk, I will show that these mysterious evolution phenomena of different kinds of line defects can be uniformly understood by assigning an orientation to the lines. First, I will introduce the recent breakthrough of the non-Abelian nodal line theory, where the orientations to the nodal lines imparted by the non-Abelian charges around them can successfully explain the formation of nodal chains and links in PT-symmetric crystals. Following this theory, we have experimentally realized the first 3D non-Abelian phononic crystal possessing earring nodal links and have achieved the first measurement of non-Abelian topological charges in well-designed transmission-line lattices. Second, by generalizing this idea to the lines of non-Hermitian band degeneracies, so-called exceptional points (EPs), we discover that the eigenvalue braiding around an exceptional line (EL) can endow it with a positive direction and propose several mechanisms for realizing symmetry-protected exceptional chains based on the source-free principle of ELs. Finally, I will reveal that the circular polarization singularities (C-points) in 3D vector optical fields can be directly mapped to the non-Hermitian EPs. Hence, the exceptional chain theory can also predict the robust intersection morphologies of the lines of C-points in certain symmetric light fields.