  Dynamically encircling an exceptional point (EP) in parity-time (PT)
symmetric systems shows an interesting chiral dynamics, leading to asymmetric
mode switching in which the output modes are different when the encircling
direction is reversed. Here we show that the dynamical encircling of an EP in
anti-PT-symmetric systems can also result in chiral dynamics if the
starting/end point lies in the PT-broken phase, in contrast to PT-symmetric
systems where chiral dynamics emerges if the starting/end point lies in the
PT-unbroken phase. For many applications, such as signal processing using
waveguides, the asymmetric mode switching of symmetry-broken modes in
anti-PT-symmetric systems is more useful since each eigenmode is localized in
one waveguide only. We develop an analytic theory for anti-PT-symmetric chiral
dynamics and perform experiments using three waveguides to demonstrate the
asymmetric mode switching. The new wave-manipulation phenomena observable in
anti-PT-symmetric systems may pave the way towards designing on-chip optical
systems with novel functionalities.
