  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.
