Analysis of defect modes in a one-dimensional periodic parity-time symmetric optical waveguide network

We investigate the behavior of defect modes arising in a one-dimensional periodic parity-time (PT) symmetric optical waveguide network with defects. Utilizing the transfer matrix method, the generalized eigenfunction method, and the waveguide network equation, we investigate the unique optical properties of defect modes with exceptional point (EP) feature, including unidirectional and bidirectional transparency, as well as coherent perfect absorption-lasing (CPAL). The defect segments are introduced at the center of the structure, which is inherently PT-symmetric, ensuring that the overall system retains its PT symmetry. Our study demonstrates that the number, position, and nature of exceptional points can be adjusted by tuning the physical parameters of the defect segments. In particular, we identify optimal conditions for achieving CPAL frequencies. These results are significant for the development of all-optical devices and the design of broadband and narrowband optical filters.