The Study Of Spin-orbit Interactions Of A Light Beam Under The Normal Incidence At Optical Interfaces

Light is an electromagnetic wave whose physical properties include energy and momentum.Momentum can be divided into linear momentum and angular momentum.Angular momentum includes spin angular momentum determined by polarization,intrinsic orbital angular momentum determined by spatial distribution of light field,and extrinsic orbital angular momentum related to the propagation trajectory of light beam.Coupling and transformation between spin and orbital angular momenta is referred to as spin-orbit interaction(SOI),which results in phenomena such as vortex generation and photonic spin Hall effect.It plays an important role in the fields of optics,nanophotonics,and plasmonics,and has a great application potential in precision measurement and detection,information storage and processing,particle manipulation,and various functional photonic devices.When light beam normally passes through an isotropic sharp interface,part of the incident beam experiences a spin-flip and acquires a spin-dependent vortex phase with a topological charge of ±2.That is,the conversion from spin angular momentum to intrinsic orbital angular momentum occurs.Although this phenomenon has been studied in literature,the physical origin of the vortex phase and the role of interface properties played in SOI process are still unclear.However,the efficiency of this SOI is extremely low,which limits its application.In this work,a Fresnel Jones matrix is first established to describe the relationship between the incident and transmitted beams,based on which we unveil that the vortex phase is in fact a spin-redirection Berry phase,originating from the topological structure of the beam itself.The properties of the interface affect the conversion efficiency of the SOI.This kind of SOI is very similar to that in the PB phase elements.Both of them are due to the spin-reversal of a portion of incident beam caused by SOI.The difference lies in the fact that the vortex phase generated by Pancharatnam-Berry(PB)elements originates from the external anisotropy of the composite materials.We further point out that the SOI can be greatly enhanced by an optically thin uniaxial slab whose optical axis is parallel to the normal direction of the interface.Based on the above theory,we found that the vortex phase generated by paraxial beam propagating in uniaxial crystal is also a spin-redirection Berry phase,and the anisotropy of uniaxial crystal affects the efficiency of vortex generation.We found that uniaxial crystals can greatly enhance the conversion efficiency of vortex,even reach 100%.Our findings not only establishes a simple and convenient theoretical framework for the above-mentioned SOI system,but also reveals the underlying physics,and further provides a possible scheme to significantly enhance it,paving the way for the future applications.