Orbital chirality of light in few-mode step-index optical fibers

A physical system is considered chiral when it can be distinguished from its mirror image, regardless of the applied translation and rotation. An electromagnetic wave can be chiral through its two essential forms of angular momenta: spin and orbital. Although the interaction between the spin angular momentum of light and chiral matter is now well known, the way the light possessing an orbital angular momentum interacts with matter is a topic of growing interest. In particular, specific signatures of light–matter interaction that depend on the sign of the angular momentum remain elusive. We demonstrate here that orbital chirality takes place as soon as light propagates within an inhomogeneous isotropic medium exhibiting cylindrical symmetry through quadrupolar effects. The theoretical predictions are supported by experimental evidence in multimode optical fibers. In particular, we show that the spin–orbit coupling taking place in these media leads to intriguing effects such as orbital angular momentum-based circular birefringence, orbital birefringence, or a q-plate-like effect.