Four-channel parity-time symmetry

In this letter, we propose a general scheme for four-channel parity-time symmetry. We theoretically demonstrate how to achieve novel features via exploiting the parity-time symmetry for four-channel coupled-mode equations and study the spontaneous symmetry breaking manifold, separating the parity-time symmetric and parity-time broken regimes, for various parameter configurations. We also propose a possible candidate (an optoelectronic oscillator) which can demonstrate the theoretically derived features, which include broadband tunability, eigenfrequency selection and flipping, and single/dual frequency operation regimes.