Singular systematic phases, transparencies, and invisibilities produced by parity-time-symmetric Thue–Morse optical waveguide networks

To study the effect of structural symmetry on both the quantity and quality of singular optical properties, a one-dimensional parity-time-symmetric (PT-symmetric) Thue–Morse (TM) optical-waveguide networks family is built and investigated. It is shown that this type of optical waveguide networks can produce 15 types of singular optical properties that have not yet been reported. These include 3 types of new systematic phases, 6 types of new unidirectional transparencies and 6 types of new bidirectional transparencies. Compared with a PT-symmetric periodic photonic crystal (structure 1), PT-symmetric periodic optical waveguide networks (structure 2) and PT-symmetric TM optical multilayers (structure 3), this type of optical system has lower symmetry. However, the number of singular optical properties for this system is higher. This strongly suggests that for an optical system, which contains a PT-symmetric substructure, with decreasing symmetry of the PT-symmetric optical system, the quantity and quality of systematic phase, transparency, and invisibility for the system tends to increase. This work deepens not only the understanding of the TM sequence and singular optical properties of PT-symmetric optical systems but also that of the influence of structural symmetry on the singular optical properties of PT-symmetric systems is clarified. In addition, the structure also allows the occurrence of previously-unreported bidirectional invisibility, which has potential applications for the manufacturing of invisibility-generating devices.