Universal Behavior of the Scattering Matrix Near Thresholds in Photonics.

Scattering thresholds and their associated spectral square root branch points are ubiquitous in photonics. In this Letter, we show that the scattering matrix has a simple universal behavior near scattering thresholds. We use unitarity, reciprocity, and time-reversal symmetry to construct a two-parameter model for a two-port scattering matrix near a threshold.

Topological complex-energy braiding of non-Hermitian bands.

Effects connected with the mathematical theory of knots emerge in many areas of science, from physics to biology. Recent theoretical work discovered that the braid group characterizes the topology of non-Hermitian periodic systems, where the complex band energies can braid in momentum space. However, such braids of complex-energy bands have not been realized or controlled experimentally.