Anomalous amplified and bound-state-like optical transmissions via unidirectional interaction in parity-time symmetric metamaterials

We develop a coupled-mode theory on the optical transmission in parity-time ( PT) symmetric coaxial metamaterials. Modeled by coupled lossy Lorentzian oscillators, the theory provides a good fit to numerical full-wave simulation. In the scenario of unidirectional coupling, two polarization-sensitive anomalies are obtained: an amplified transmission and an ultra-narrow one analogous to bound states in continuum. We argue that these phenomena are associated with either a unidirectional-field-transfer process or an indirect unidirectional-field-trapping resonance. The broadening effect is shown to determine the magnitude and polarization of the transmission. Our theory and analysis provide a deep understanding on the importance of PT symmetry and dark helical modes and would contribute to applications such as light storage, field amplification, and even lasing.