Description
Topological singular lines in three-dimensional parameter space – nodal lines and exceptional lines – are fundamental to wave physics and hold promise for advanced photonic control. However, their observation, especially in the optical regime, has been hindered by the challenge of constructing the required parameter space without complex structural engineering. Here, we demonstrate that the scattering matrix of a simple two-dimensional photonic crystal provides such a parameter space through frequency and in-plane momenta, enabling the first observation of chiral exceptional lines in the visible regime. Using high-precision momentum-space Mueller matrix spectroscopy, we map these lines and reveal their key topological features, including self-intersecting Riemann surfaces, phase vortices, and polarization half-vortices, with distinct responses to left- and righthanded circular polarizations. The exceptional lines exhibit characteristic square-root eigenvalue splitting and extend continuously across the frequency-momentum space, demonstrating their topological robustness. This achievement establishes a robust platform for investigating non-Hermitian topological physics at visible frequencies, opening pathways for chiral light-matter interactions, polarization-selective devices, and advanced sensing applications.
