We present a theoretic analysis on (azimuthal) spin momentum- dependent orbital motion experienced by particles in a circularly-polarized annular focused field. Unlike vortex phase-relevant (azimuthal) orbital momentum flow whose direction is specified by the sign of topological charge, the direction of (azimuthal) spin momentum flow is determined by the product of the field's polarization ellipticity and radial derivative of field intensity. For an annular focused field with a definite polarization ellipticity, the intensity's radial derivative has opposite signs on two sides of the central ring (intensity maximum), causing the spin momentum flow to reverse its direction when crossing the central ring. When placed in such a spin momentum flow, a probe particle is expected to response to this flow configuration by changing the direction of orbital motion as it traversing from one side to the other. The reversal of the particle's orbital motion is a clear sign that spin momentum flow can affect particles' orbital motion alone even without orbital momentum flow. More interestingly, for dielectric particles the spin momentum-dependent orbital motion tends to be 'negative', i.e., in the opposite direction of the spin momentum flow. This arises mainly because of spin-orbit interaction during the scattering process. For the purpose of experimental observation, we suggest the introduction of an auxiliary radially-polarized illumination to adjust the particle's radial equilibrium position, for the radial gradient force of the circularly-polarized annular focused field tends to constrain the particle at the ring of intensity maximum.

中文翻译：

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自旋动量相关的轨道运动

我们对圆极化环形聚焦场中粒子所经历的（方位角）自旋动量相关的轨道运动进行了理论分析。与涡旋相位相关（方位角）轨道动量流的方向由拓扑电荷的符号指定不同，（方位角）自旋动量流的方向由场的极化椭圆度和场强的径向导数的乘积决定。对于具有确定偏振椭圆度的环形聚焦场，强度的径向导数在中心环的两侧（强度最大值）具有相反的符号，导致自旋动量流在穿过中心环时反转其方向。当置于这样的自旋动量流中时，预计探针粒子会通过改变轨道运动方向来响应这种流动配置，因为它从一侧穿过到另一侧。粒子轨道运动的逆转是一个明显的迹象，即自旋动量流可以单独影响粒子的轨道运动，即使没有轨道动量流。更有趣的是，对于介电粒子，与自旋动量相关的轨道运动趋于“负”，即与自旋动量流方向相反。这主要是因为散射过程中的自旋轨道相互作用。出于实验观察的目的，我们建议引入辅助径向偏振照明来调整粒子的径向平衡位置，