Agenda

Résumé : Topology leads to many robustly quantized physical phenomena ranging from particle physics to condensed matter physics. In solids, many topological phenomena appear in bands of nearly free electrons moving in a periodic potential. Recently, periodic media composing metamaterials enable photons carrying modulated momenta. These systems have highly tunable band structures and their dynamics are generically described by non-Hermitian matrices when dissipative effects are considered. The level crossings between such photonic bands are known as exceptional points. In this presentation, I will give a brief introduction on these nodal points, demonstrating they are described by non–Abelian topological charges. This leads to non-commuting operations of moving them in the reciprocal space and violation of the famous fermion-doubling Nielsen-Ninomiya theorem. I will show how these are verified experimentally. Then I will introduce parity-time symmetric systems, where the system can transit between Hermitian and non-Hermitian regimes by spontaneous symmetry breaking. This system has the great potential of exploring topological bands as the gauge structures are tunable through spontaneous symmetry breaking. This leads to some new ideas of preparing topological states via dissipation.