Dynamically encircling an exceptional point in antiparity-time symmetric systems:asymmetric mode switching for symmetry-broken modes

Dynamically encircling an exceptional point(EP)in parity-time(PT)symmetric waveguide systems exhibits interesting chiral dynamics that can be applied to asymmetric mode switching for symmetric and anti-symmetric modes.The counterpart symmetry-broken modes(i.e.,each eigenmode is localized in one waveguide only),which are more useful for applications such as on-chip optical signal processing,exhibit only non-chiral dynamics and therefore cannot be used for asymmetric mode switching.Here,we solve this problem by resorting to anti-parity-time(anti-PT)symmetric systems and utilizing their unique topological structure,which is very different from that of PT-symmetric systems.We find that the dynamical encircling of an EP in anti-PT-symmetric systems with the starting point in the PT-broken phase results in chiral dynamics.As a result,symmetry-broken modes can be used for asymmetric mode switching,which is a phenomenon and application unique to anti-PT-symmetric systems.We perform experiments to demonstrate the new wave-manipulation scheme,which may pave the way towards designing on-chip optical systems with novel functionalities.