Optical parity-time induced perfect resonance transmission in zero index metamaterials.

Non-Hermitian photonic systems with balanced gain and loss have become significantly more popular due to their potential applications in communications and lasing. In this study, we introduce the concept of optical parity-time (PT) symmetry to zero-index metamaterials (ZIMs) to investigate the transport of electromagnetic (EM) waves through a PT-ZIM junction in a waveguide system. The PT-ZIM junction is formed by doping two dielectric defects of the same geometry in the ZIM, with one being the gain and the other being the loss. It is found that the balanced gain and loss can induce a perfect transmission resonance in a perfect reflection background, and the resonant linewidth is controllable and determined by the gain/loss. The smaller the gain/loss, the narrower the linewidth and the larger the quality (Q) factor of the resonance. This finding originates from the fact that the introduced PT symmetry breaks the spatial symmetry of the structure, leading to the excitation of quasi-bound states in the continuum (quasi-BIC). Additionally, we also show that the lateral displacements of the two cylinders play a crucial role in the electromagnetic transport properties in ZIMs with PT symmetry, which breaks the common sense that the transport effect in ZIMs is location-independent. Our results provide a new approach to manipulate the interaction of EM waves with defects in ZIMs using gain and loss to achieve anomalous transmission, and a pathway to investigate non-Hermitian photonics in ZIMs with potential applications in sensing, lasing, and nonlinear optics.