Slow light at the nanoscale based on active epsilon-near-zero plasmonic waveguides

Plasmonic waveguides exhibit an effective epsilon-near-zero (ENZ) response at their cut-off frequency. In this work, we demonstrate the formation of an exceptional point (EP) in a nanoscale open and lossy (non-Hermitian) nanophotonic system consisting of an array of periodic nanowaveguides loaded with a very low gain coefficient material. We theoretically analyze the obtained EP, as well as its topological properties, by using a transmission-line model adapted to the plasmonic properties of the proposed device. The dispersion of the active ENZ mode and the group velocity are thoroughly investigated. Reflectionless transmission (perfect loss compensation) and ultraslow group velocity values at the nanoscale are realized at the EP, which coincides with the ENZ cut-off frequency of the proposed plasmonic system. This special spectral degeneracy (EP) is a unique feature of the presented nanoscale symmetric plasmonic ENZ configuration, different from most of the previous works that were mainly focused on asymmetric bulky micron-scale active photonic configurations.