Enhancement of synthetic magnetic field induced nonreciprocity via bound states in the continuum in dissipatively coupled systems

The nonreciprocal propagation of light typically requires use of materials like ferrites or magneto-optical media with a strong magnetic bias or methods based on material nonlinearities which require use of strong electromagnetic fields. A simpler possibility to produce nonreciprocity is to use spatiotemporal modulations to produce magnetic fields in synthetic dimensions. In this paper we show that dissipatively coupled systems can lead to considerable enhancement of nonreciprocity in synthetic fields. The enhancement comes about from the existence of a nearly nondecaying mode---a bound state in continuum (BIC)---in dissipatively coupled systems. The dissipative coupling occurs in a wide class of systems coupled via transmission lines, waveguides, or nano fibers. The systems could be optical resonators or microscopic qubits. Remarkably we find that for specific choice of the modulation amplitudes, the transmission say in forward direction is completely extinguished whereas in the backward direction it becomes maximum. The synthetic fields produce transmission resonances which show significant line narrowing which owe their origin to the existence of BIC's in dissipative systems.