Exceptional Points in Plasmonic Waveguides Do Not Require Gain or Loss

Exceptional points (EPs) in photonics are associated with non-Hermitian Hamiltonians with energy gain or loss, a notable example is EPs in parity-time symmetric Hamiltonians. We show that, counterintuitively, actual energy gain or loss is not required to generate this type of non-Hermitian degeneracy, since the eigenvalue of the optical Hamiltonian is the mode's propagation constant and not the energy. This is demonstrated by a simple three-layer insulator-metal-insulator (I-M-I) plasmonic waveguide, the eigenmodes of which are known to experience degeneracy at a certain point in the parametric space suggested to be used for rainbow trapping. We identify this point to be, in fact, an EP with the coalescence of the eigenmodes, despite the system having neither parity nor time symmetry. Furthermore, we demonstrate the manifestation of a third-order EP, which is generated by merging two separate EPs in the parametric space of the I-M-I waveguide. The presented results reveal unconventional properties of the Hamiltonian of a simple plasmonic waveguide and provide an insight into the nature of EPs in non-Hermitian plasmonic systems in general, suggesting the possibility of accessing even higher-order EP regimes in simple photonic structures without the need for optical gain or loss.