Investigating exceptional points in dark-bright mode-coupled plasmonic systems.

Exceptional points (EPs) of non-Hermitian systems are gaining more and more attention due to their important applications in unidirectional transmitters, sensors, etc. However, theoretical studies on EPs of reflection, transmission, and absorption spectra are less available. In this paper, in the dark-bright mode-coupled plasmonic systems, the variations of EPs of reflection, transmission, and absorption spectra are numerically investigated using temporal coupled-mode theory, and an assumption is given using the representation transformation theory. The intermediate representation (IR) is firstly proposed and related to the reflection spectrum, while the normal representation (NR) is associated with the absorption spectrum. In the region far from EPs, the IR (or NR) describes the reflection (or absorption) spectrum well. Near EPs, modified formulas similar to the representation transformation theory are given. In order to verify the correctness of the assumption, two metasurfaces are designed. And the simulation results are in good agreement with the assumption and it is found in the near-infrared and visible-light band that the absorption loss of the dark mode is linearly related to the EPs of reflection, transmission, and absorption spectra, while the radiation loss of the bright mode is only linearly related to the EPs of the absorption spectrum. These laws can help to manipulate the splitting of spectral lines for reflection, transmission, and absorption by adjusting the radiation loss and absorption loss of bright mode, the absorption loss of dark mode, and the coupling coefficients between two resonant modes. This research provides a guiding scheme for the design of micro and nano photonics devices.