  We present a nanoscale active plasmonic waveguide system consisting of an
array of periodic slits that can exhibit exceptional points and spectral
singularities leading to several novel functionalities. The proposed symmetric
active system operates near its cut-off wavelength and behaves as an effective
epsilon-near-zero (ENZ) medium. We demonstrate the formation of an exceptional
point (EP) that is accessed with very low gain coefficient values, a unique
feature of the proposed nanoscale symmetric plasmonic configuration.
Reflectionless ENZ transmission and perfect loss-compensation are realized at
the EP which coincides with the effective ENZ resonance wavelength of the
proposed array of active plasmonic waveguides. When we further increase the
gain coefficient of the dielectric material loaded in the slits, a spectral
singularity occurs at the ENZ resonance leading to super scattering (lasing)
response at both forward and backward directions. These interesting effects are
achieved by materials characterized by very small gain coefficients with
practical values and at subwavelength scales due to the strong and homogeneous
field enhancement inside the active slits at the ENZ resonance leading to
enhanced light-matter interaction. We theoretically analyze the obtained EP, as
well as the divergent spectral singularity, using a transmission-line model and
investigate the addition of a second incident wave and nonlinearities in the
response of the proposed active ENZ plasmonic system. Our findings provide a
novel route towards interesting nanophotonic applications, such as
reflectionless active ENZ media, unidirectional coherent perfect absorbers,
nanolasers, and strong optical bistability and all-optical switching
nanodevices.
