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Nonreciprocal transmission is the fundamental process behind unidirectionalwave propagation phenomena. In our work, a compact and practical parity-time(PT) symmetric metamaterial is designed based on two Silicon Carbide (SiC)media separated by an air gap and photonically doped with gain and lossdefects. We demonstrate that an exceptional point (EP) is formed in thisPT-symmetric system when SiC operates as a practical epsilon-near-zero (ENZ)material and by taking into account its moderate optical loss. Furthermore andeven more importantly, strong self-induced nonreciprocal transmission isexcited due to the nonlinear Kerr effect at a frequency slightly shifted offthe EP but without breaking the PT-symmetric phase. The transmittance from onedirection is exactly unity while the transmittance from the other direction isdecreased to very low values, achieving very high optical isolation. Theproposed active nonlinear metamaterial overcomes the fundamental physicalbounds on nonreciprocity compared with a passive nonlinear nonreciprocalresonator. The strong self-induced nonreciprocal transmission arises from theextreme asymmetric field distribution achieved upon excitation from oppositeincident directions. The significant enhancement of the electric field in thedefects effectively decreases the required optical power to trigger thepresented nonlinear response. This work can have a plethora of applications,such as nonreciprocal ultrathin coatings for the protection of sources or othersensitive equipment from external pulsed signals, circulators, and isolators.

Strong self-induced nonreciprocal transmission by using nonlinear PT-symmetric epsilon-near-zero metamaterials