Spin-Selective Topological Effects without Encircling Exceptional Points

Exceptional points (EPs), namely non-Hermitian spectral singularities, enable unconventional light-matter interactions, leading to intriguing phenomena, such as chiral mode transfer, state flip, and chiral phase accumulation. Yet considerable previous EP effects in topological photonics based on a standard strategy, i.e., evolution encircling EPs, require fine tuning of multiple parameters. To go beyond this approach for designing unconventional topological devices with more convenient tunabilities, it is essential to study how to control open trajectories by tuning fewer or even a single parameter without encircling EPs. This route remains largely unexplored. Here, we demonstrate 2π optical phase control with open evolution trajectories by tuning a single geometrical parameter of the structure in a terahertz metasurface featuring a pair of EPs in a complex-frequency space, while only π phase control can be achieved with open trajectories in the metasurface with a single EP. We find that such full 2π-phase control without encircling EPs is realized due to the open trajectories passing between the two EPs with identical chirality but opposite topological charges. Furthermore, we demonstrate topological spin-selective beam deflection and realize dynamical control of EP positions in the complex-frequency space by tuning the angle of split-ring resonators. Our Letter drives the field of EP topological photonics into a broader regime with open trajectory approaches, opening up perspectives for achieving unconventional photonic devices that are more controllable with exciting applications, such as novel wavefront control and polarization multiplexing.