Symmetric and asymmetric photonic spin-orbit interaction in metasurfaces

Photonic spin and orbital angular momenta, which are determined by the polarization and spatial degrees of freedom of photons, are strongly coupled with each other in subwavelength structured metasurfaces. The photonic spin-orbit interaction (PSOI) results in the splitting of the degenerated system states. In this review, we focus on the principles of symmetric PSOI associated with the conjugated geometric phase modulation as well as the asymmetric PSOI resulting from the additional localized phase manipulation. Recent advances and important applications of symmetric and asymmetric PSOI in metasurfaces are also discussed. We finally highlight with our perspective on the remaining challenges and future trends in this field.