Side-coupled resonators with parity-time symmetry for broadband unidirectional invisibility

We analyze the scattering properties of a PT-structure made of a waveguide and a finite chain of side-coupled resonators. Typical 1D PT-structures exhibit unidirectional invisibility (also called anisotropic transmission resonances), meaning unit-transmission and zero-reflection for incidence from one direction. The side-coupled nature of our structure provides different features than the traditional tight-binding chain, which we explore rigorously. For example, we can achieve a broadband unidirectional invisibility with only two resonators, and we observe rich dispersions for ATRs with four resonators.