Topological multiband photonic chain with one pair of loss and gain defects.

Multiband photonic chains own richer topological properties compared with a lattice of dimers. We investigate one pair of loss and gain defects located on the edges of the topological multiband photonic chain. A photonic chain is composed of unit cells with four elements. In the presence of one pair of loss and gain defects, the chain shows different topological behavior on certain restrictions on intra- and intercell couplings. From the properties of the eigenvalues spectrum, we find that multiple edge states only exist under certain intra- and intercell couplings. We hope our configuration can be useful for topologically protected mode-division multiplexing in photonic devices.