Mini-Seminar I: Topological transition in graphene-like photonic crystals

Speaker: Fei Gao, Zhejiang University

Time: 10:00 – 10:20 am, Jan. 19, 2020

Abstract: Graphene-like photonic crystals are arrayed electromagnetic honeycomb lattice. By introducing photonic spin-orbital coupling and inversion-symmetry breaking respectively, two analogues have been realized respectively as photonic quantum spin Hall (QSH) and quantum valley Hall (QVH) systems. These two photonic analogues can respectively support topological helical or chiral waveguiding modes on their edges. Spin-orbital coupling and inversion-symmetry breaking can compete each other while they are introduced into a single graphene-like photonic crystals. This competition enables us to observe a topological transition between photonic QSH and QVH, which is hardly to realize in condensed-matter systems. In addition, we also realize spin-locked valley splitting in the bulk of designed graphene-like photonic crystals.

Mini-Seminar II: Topological phases of quantized light

Speaker: Da-Wei Wang, Zhejiang University

Time: 10:20 – 10:40 am, Jan. 19, 2020

Abstract: Topological photonics is an emerging research area that focuses on the topological states of classical light. Here we reveal the topological phases that are intrinsic to the particle nature of light, i.e., solely related to the quantized Fock states and the inhomogeneous coupling between them. The Hamiltonian of two cavities coupled with a two-level atom is an intrinsic one-dimensional Su-Schriefer-Heeger model of Fock states. By adding another cavity, the Fock-state lattice is extended to two dimensions with a honeycomb structure, where the strain due to the inhomogeneity of the coupling strengths induces a Lifshitz topological phase transition between a semimetal and three band insulators within the lattice. In the semimetallic phase, the strain is equivalent to a pseudomagnetic field, which results in the quantization of the Landau levels and the valley Hall effect. We further construct a Haldane model where the topological phases can be characterized by the topological markers. This study demonstrates a fundamental distinction between the topological phases of bosons and fermions and provides a novel platform for studying topological physics in dimensions higher than three.