Exceptional point induced unidirectional radiation from non-Hermitian plasmonic structures

Non-Hermitian (NH) photonics has attracted considerable attention from researchers owing to exotic properties that originate from the parity-time ($\mathcal{PT}$) phase transition and exceptional points (EPs). To date, the $\mathcal{PT}$ phase transition, EPs, and circling around EPs have been investigated in many photonic systems. However, few studies focused on the singular nature of the EP eigenstates of the Hamiltonian matrices. Moreover, the switching of an EP eigenstate based on the sign of the coupling constant and its manifestation in physical phenomena have not yet been investigated. In this paper, we propose and numerically demonstrate a unidirectional radiation phenomena manifested by the formation of Huygens dipoles using the singular EP eigenstates in coupled plasmonic systems. Two types of EPs corresponding to positive and negative signs of the coupling constants are realized using dipole-dipole coupling. We show that the Huygens dipole is formed at the EP condition and its radiation direction can be controlled by choosing the sign of the coupling. The unique photonic functionality, which originates directly from the singular eigenstate at the EP uncovers a new aspect of NH photonic physics and devices.