Monolithically integrated EP-based optical isolator.

Exceptional points (EPs) display peculiar degeneracies, where complex eigenvalues and associated eigenvectors coalesce simultaneously, resulting in a defective Hamiltonian. Meanwhile, the negative imaginary part of the energy eigenvalues related to a finite spectral linewidth at the resonant energy, which could provide a solution to tackle the isolation bandwidth limitation of MRR-based optical isolators without sacrificing the insertion loss. Here, a second-order EP system constructed by SiN-based cascaded racetrack resonators is proposed, while the metal strip operating as an integrated electromagnet provides magnetic fields required for non-reciprocal phase shifting (NRPS). Owing to the existence of the NRPS perturbation, the system is pushed away from EP and consequently triggers complex frequency splitting, resulting in the isolation bandwidth proportional to the square-root perturbation instead. The results show that the isolation bandwidth of the EP isolator is increased by 163% and 22% compared to single-racetrack and cascaded-racetrack isolators with 2.85 dB insertion loss and 34.3 dB isolation ratio, respectively. The presented EP-based optical isolator shows tremendous potential for high-density monolithic integration and packaging.