Reconfigurable unidirectional propagation of electromagnetic waves in photonic crystal waveguides

We demonstrate reconfigurable unidirectional propagation of electromagnetic waves in waveguide channels sandwiched by two dielectric photonic crystal (PC) slabs or magnetic PC (MPC) slabs, where the extrinsic and intrinsic responses emerge in two kinds of systems. Concretely, the unidirectionality of the MPC system originates from the time-reversal symmetry breaking nature of magnetic material, while in the dielectric PC system, the unidirectionality is achieved by active control of two separated line sources. The results indicate that in the MPC based system, not only the amplitude but also the directionality of the guiding electromagnetic waves is flexibly regulated by controlling the separation and phase difference of two active line sources. However, the directionality in the MPC based system is determined by the magnetization due to the intrinsic unidirectionality of the magnetic system, but for the dielectric PC based system, directionality is controlled by active sources. The introduction of a second line source in the MPC based system can result in two remarkably different consequences. On one hand, the directionality can be switched from forward to backward propagation by reversing magnetization. On the other hand, the same operation results in the transition from unidirectional propagation to localization of the electromagnetic waves by dynamically controlling two active sources. The functionality and flexibility in the present systems might find potential applications in microwave photonics and integrated photonics.