Optical nonreciprocity and unidirectional transmission by loss engineering*

As the basic units in optical systems, nonreciprocal devices play important roles in optical communication and optical information processing. They prohibit a light field from returning along its original path after passing through an optical system in one direction, implying the breaking of the Lorentz reciprocity theorem. Conventional principles for realizing nonreciprocity rely on magneto-optical properties (e.g. Faraday rotation). However, the magnetic fields required make it difficult for integration on a small scale. To overcome this problem, it is crucial to realize optical nonreciprocity also unidirectional energy transmission by developing magnetic-free approaches. In this work, we will propose and analyze a generic method to nonreciprocity generation by loss engineering. When multiple dissipative coupling channels exist, the phase lag induced by the loss, results in different interference outcomes to implement nonreciprocity and unidirectional energy transmission. This study paves the way for nonreciprocal device design in lossy systems without stringent conditions and provides opportunities for studying chiral and topological properties in systems with lossy coupling.