Selective transport of one-way topological edge modes by encircling an exceptional point in gyromagnetic photonic crystals

Dynamically encircling an exceptional point (EP) in non-Hermitian systems can exploit the intriguing energy topology around the EP, giving rise to non-Hermitian unique applications such as chiral mode switching. In this work, we introduce the concept of EP encirclement to time-reversal-symmetry broken topological systems towards the manipulation of topologically protected one-way edge modes. We design a three-layered non-Hermitian gyromagnetic photonic crystal, which, under specially distributed external magnetic fields, possesses two topological one-way edge modes. The proposed system is found to be equivalent to a two-level non-Hermitian system with an EP at the frequency of interest. We introduce a two-dimensional parameter space to dynamically encircle the EP, where the encircling direction can be switched by parameter designs, while the edge mode direction can be tuned by changing the direction of the magnetic fields. A selective transport of the topological edge modes is realized, that is, the output mode solely depends on the encircling direction, while the output direction depends on the magnetic field. The mode transport behavior is topologically protected such that the performance is robust to local defects and backward scatterings are avoided. Our findings provide a means for manipulating electromagnetic waves in non-Hermitian and topological photonic systems.