Successive switching among four states in a gain-loss-assisted optical microcavity hosting exceptional points up to order four

The implementation of exceptional points (EPs), a special type of topological singularities, has emerged as a new paradigm for engineering the quantum-inspired or wave-based photonic systems. Even though there exists a range of investigations on EPs of order two and three (say, EP2s and EP3s, respectively), the hosting of fourth-order EPs (EP4s) in any real system and the exploration of associated topological features are lacking. Here we have designed a simple Fabry-Pérot type gain-loss-assisted open optical microcavity to host EPs up to order four. The scattering-matrix formalism has been used to analyze the microcavity numerically. With the appropriate modulation of the gain-loss profile in the same cavity geometry, we have encountered multiple different orders of EPs by investigating the simultaneous interactions among four coupled cavity states via level-repulsion phenomena. Besides affirming the second-order and third-order branch-point behaviors of the embedded EP2s and EP3s, the fourth-order branch-point functionality of an EP4 has been manifested by encircling three connecting EP2s simultaneously in the closed gain-loss parameter space. We have established a unique successive state-switching phenomenon among four coupled states by implementing such an EP4-encirclement scheme in the system's parameter space. The proposed scheme indeed offers potential applications in state-switching and control in quantum-inspired integrated photonic circuits, where the presence of an EP4 serves as a new light manipulation tool.