Affiliation:
1. Department of Physics, Stockholm University, AlbaNova University Center , 106 91 Stockholm, Sweden
Abstract
Symmetry and non-Hermiticity play pivotal roles in photonic lattices. While symmetries, such as parity-time (PT) symmetry, have attracted ample attention, more intricate crystalline symmetries have been neglected in comparison. Here, we investigate the impact of the 17 wallpaper space groups of two-dimensional crystals on non-Hermitian band structures. We show that the non-trivial space group representations enforce degeneracies at high symmetry points and dictate their dispersion away from these points. In combination with either T or PT, the symmorphic p4 mm symmetry and the non-symmorphic p2mg, p2gg, and p4gm symmetries protect exceptional chains intersecting at the pertinent high symmetry points.
Funder
Vetenskapsrådet
Knut och Alice Wallenbergs Stiftelse
Göran Gustafssons Stiftelse för Naturvetenskaplig och Medicinsk Forskning
Subject
Physics and Astronomy (miscellaneous)