Nonreciprocity in optical fiber radiation modes induced by spin–momentum locking

Nonreciprocity in optical fibers are opening new avenues for fields such as quantum computing and quantum photonics. In this study we explore the chiral properties of the radiation modes of optical fibers and show that whispering gallery mode resonances, as part of radiation modes, carry specific transverse spin angular momenta. The transverse spin angular momentum is different for forward and backward propagating radiation modes, hence indicating spin–momentum locking. As a result of spin–momentum locking, a nonreciprocity in the emission coupling of an atomic transition with a specific spin into the forward and backward propagating modes is observed. Modeling an atomic transition by a classical dipole that rotates clockwise or anticlockwise, we optimize the position of the dipole within the optical fiber to achieve maximum nonreciprocity between the coupling of the dipole emission into the forward or backward propagating mode. We find near-perfect nonreciprocity in both radiation and guided modes and further outline the fiber diameter and dipole position to achieve this state. This study not only shows the rich physics of fiber radiation modes within the context of light–matter interaction but also complements previous studies of nonreciprocity in subwavelength waveguides.