  Nonreciprocal transmission forms the basic operation mechanism of optical
diodes and isolators and requires the tantalizing task of breaking the Lorentz
reciprocity law. In this work, strong nonreciprocal transmission is
demonstrated by using a compact nonlinear parity-time (PT) symmetric system
based on epsilon-near-zero (ENZ) materials photonically doped with gain and
loss defects and separated by an ultrathin air gap. The nonlinear response of
this scalable configuration is triggered at relatively low optical intensities
due to the strong electric field confinement in the defects. The extreme
asymmetric field distribution achieved upon excitation from opposite incident
directions, combined with the enhanced nonlinear properties of the proposed
system, result in a pronounced self-induced nonreciprocal transmission. Cascade
configurations with optimized geometrical dimensions are used to achieve
self-induced nonreciprocal transmission with a maximum contrast, ideal for the
design of new all-optical diodes. The presented robust nonreciprocal response
occurs by operating at a frequency slightly shifted off the exceptional point
but without breaking the PT-symmetric phase, different compared to previous
works. The findings of this work can have a plethora of applications, such as
nonreciprocal ultrathin coatings for the protection of sources or other
sensitive equipment from external pulsed signals, circulators, and isolators.
