光束正入射至界面时的自旋-轨道相互作用及其增强

光束正入射至均匀突变界面时的自旋-轨道相互作用表现为拓扑荷数为±2的、自旋可控的涡旋相位.然而,该涡旋相位的物理来源以及界面的性质在自旋-轨道相互作用过程中起到何种作用,这些问题还有待解决.首先建立一个简洁的菲涅耳琼斯矩阵来描述这种自旋-轨道相互作用,并揭示其中的涡旋相位其实是一种贝里(Berry)几何相位,它来源于光束本身的拓扑结构,而界面的性质影响自旋-轨道相互作用的转换效率.一般情况下,转换效率极低,限制了其应用.因此,基于上述理论,提出采用光轴平行于界面法线方向的单轴薄层材料,来极大地增强这种自旋-轨道相互作用.

Spin-orbit interaction of a light beam under normal incidence at a sharp interface and its enhancement

The spin-orbit interaction(SOI)of light refers to the mutual conversion and coupling between the spin angular momentum and orbital angular momentum.It is a fundamental effect in optics,and has been widely found in many basic optical processes,such as reflection,refraction,scattering,focusing,and imaging.So it plays an important role in the fields of optics,nanophotonics,and plasmonics,and has great potential applications in precision measurement and detection,information storage and processing,particle manipulation,and various functional photonic devices.Recently,it has been found that a circularly polarized light beam normally passing through an isotropic sharp interface can undergo an SOI process,that is,part of the incident beam experiences a spin-flip and acquires a spin-dependent vortex phase with a topological charge of.However,the physical origin of this phase and the role of the interface played in the SOI process are still unclear at present.In this work,a Fresnel Jones matrix is first established to describe the relationship between the incident beam and the transmitted beam,based on which we unveil that the vortex phase is in fact a spin-redirection Berry geometric 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 azimuthal Pancharatnam-Berry phase elements.The difference lies in the fact that the Pancharatnam-Berry phase originates from the external anisotropy of the composite material.Generally,the efficiency of this SOI is extremely low,which limits its applications.The existing method of enhancing this SOI employs an isotropic epsilon-near-zero slab,whose maximum efficiency can reach only about 20%.Since the anisotropic medium(such as birefringent uniaxial crystals)has more degrees of freedom,we further point out that the weak SOI can be greatly enhanced by an optically thin uniaxial slab whose optical axis is parallel to the normal direction of the interface.And under certain conditions,the conversion efficiency can reach 100%.Our study not only establishes a simple and convenient full-wave theory for this SOI,but also reveals the relevant underlying physics,and further provides a possible scheme to significantly enhance the SOI.