Dynamically encircling an exceptional point in anti-parity-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. By steering microwaves in an anti-parity-time symmetric system, Chinese scientists have explored the topological physics and their wave-manipulation applications in optics and photonics. Recent developments in non-Hermitian systems, i.e., open systems that can exchange energy with surroundings, have given rise to new optical devices such as isolators, sensors, and absorbers. Such systems exhibit the so-called exceptional points, where two or more resonances have equal frequencies and dissipations. By sending microwaves along a waveguide with specially designed boundaries, which is equivalent to looping the waveguide resonance around an exceptional point in a parameter space, Xu-Lin Zhang from Jilin University and Che Ting Chan from the Hong Kong University of Science and Technology have demonstrated a new method for manipulating electromagnetic waves in waveguides. Their work could pave the way for on-chip optical devices with new functionalities.