Nonreciprocal Transmission in Nonlinear PT-Symmetric Metamaterials Using Epsilon-Near-Zero Media Doped with Defects

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, results 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.