Detecting the transverse spin density of light via electromagnetically induced transparency

For light that is transversely confined, its field vector spins in a plane not orthogonal to the propagation direction, leading to the presence of transverse spin, which plays a fundamental role in the field of chiral quantum optics. Here, we theoretically propose a scheme to detect the transverse spin density (TSD) of light by utilizing a multilevel atomic medium. The scheme is based on the electromagnetically induced transparency effect, which enables the TSD-dependent modulation of the susceptibility of the atomic medium by using a coupling field whose TSD is to be detected. The modulated susceptibility results in a spin-dependent absorption for a probe beam passing through the atomic medium. We show that there exists a corresponding relationship between the TSD distribution of the coupling field and the polarization distribution of the transmitted probe beam through a theoretical study of two typical cases, in which the coupling field is provided by a tightly focused field and a two-beam interference field, respectively. Based on this relationship, the key features of the TSD of the coupling field, such as the spatial distribution, the symmetry property, and the spin-momentum locking, can be inferred from the transmitted probe beam. Benefiting from the fast response of the atomic medium to the variation of the coupling field, the present scheme is capable of detecting the TSD in real time, offering new possibilities for developing transverse-spin-based techniques.