Exceptional points in parity-time symmetric plasmonic Huygens' metasurfaces

Parity-time (PT) symmetric optical structures exhibit several unique and interesting characteristics, with the most popular being exceptional points. While the emerging concept of PT-symmetry has been extensively investigated in bulky photonic designs, its exotic functionalities in nanophotonic non-Hermitian plasmonic systems still remain relatively unexplored. Towards this goal, in this work we analyze the unusual properties of a plasmonic Huygens' metasurface composed of an array of active metal-dielectric core-shell nanoparticles. By calculating the reflection and transmission coefficients of the metasurface under various levels of gain, we demonstrate the existence of reflectionless transmission when an exceptional point is formed. The proposed new active metasurface design has subwavelength thickness and can be used to realize ultracompact perfect transmission optical filters.