Nonreciprocal transmission of vortex beam in double Laguerre-Gaussian rotational cavity system

Abstract

By constructing an optorotational system composed of two linearly coupled Laguerre-Gaussian rotational cavities, we realize the nonreciprocal transmission of the vortex beam with the orbital angular momentum. Two vortex beam cavity modes driven by strong fields are coupled with a rotational mirror via the torsion, and two cavity modes interact with each other via the optical fiber. A weak probe field is incident from one side of the system for examining the optical response along one propagating direction. With the Hamiltonian of the system and the Heisenberg-Langevin equation, we can obtain the transmission of the output light field from the input-output theory. The result shows that the optical nonreciprocity of the vortex beam arises from the quantum interference between the optorotational interaction and the linear coupling interaction between two vortex beam modes, and the phase difference can be used to adjust the optical nonreciprocity. The phase difference can determine not only the occurrence of the nonreciprocity but also the direction of nonreciprocity. Moreover, the ratio of the topological charges carried by the two vortex beam fields has an influence on the transmission. Under an appropriate topological charge ratio, the unidirectional transmission of the vortex beam can be realized in such a system. It is found that whether the topological charge ratio is positive or negative, i.e. whether the vortex beam is left-hand beam or right-hand beam, does not affect the transmission; the influence of the topological charge on the transmission amplitude actually comes from the topological charge number carried by the vortex beam, due to the fact that the coupling strength between the rotating mirror mode and the cavity mode depends on the topological charge number. In addition, we also obtain the condition that the system damping rates should meet for realizing the perfect nonreciprocal propagation of the vortex beam. Finally, we can achieve the nonreciprocal group velocity of the slow light. The direction of the nonreciprocal slow light can be controlled via phase modulation. Our work provides a possible application in manipulating the vortex beam propagation. Furthermore, we extend the nonreciprocity of ordinary beams in the optomechanical system to the nonreciprocity of the vortex beam in the optorotational system. The results are expected to be applied to fabricating the ideal optical isolators for the vortex beam carrying the orbital angular momentum in optical communication.