Optical torque on small chiral particles in generic optical fields
Optical torque on small chiral particles in generic optical fields
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一般光学领域中小手性粒子的光学扭矩
DOI：
10.1364/oe.25.032867
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发表时间：
2017
期刊：
影响因子：
3.8
通讯作者：
Zhifang Lin
中科院分区：
文献类型：
--
作者：
Huajin Chen;Wanli Lu;Xinning Yu;Chunhua Xue;Shiyang Liu;Zhifang Lin
在偶极近似下，我们导出了一般单色光场中手性粒子的光学扭矩的解析表达式。除了粒子上激发的偶极子与入射场相互作用产生的消光项外，我们的表达式还包括一些长时间缺失的项，这些项被理解为粒子上激发的偶极子辐射引起的反冲。在许多情况下，反冲项对OT有很大的贡献。受此公式的启发，我们进一步证明了任意尺寸的无损各向同性手性球形粒子在任意单色光场照射下的净OT为零。最后，利用我们的解析表达式，我们在零阶矢量贝塞尔光束中的一个手性小球上追踪了OT的起源。结果表明，垂直于光束传播方向的方位角动量来自入射场的自旋角动量向粒子的传递，而沿照明方向的纵向角动量来源于粒子的手性，它在光束上产生纵向角动量，从而使粒子受到反冲产生的纵向角动量。因此，纵向OT倾向于将螺旋度相反的手性粒子朝着相反的方向旋转，而横向OT几乎不依赖于粒子手性的利手性。我们的结果可能有助于理解OT作为光学镊子中粒子操纵的额外句柄。
We derive an analytical expression of the optical torque (OT) on chiral particles in generic monochromatic optical fields within the dipole approximation. Besides the extinction terms owing to the interaction between the dipoles excited on the particle and the incident field, our expression includes also some long missing terms that are understood as recoil due to the radiation of the dipoles excited on the particle. The recoil terms are shown to have a significant contribution to the OT in many situations. Inspired by our expression, we further proved that the net OT vanishes for a lossless isotropic chiral spherical particle of any size illuminated by arbitrary monochromatic optical fields. Finally, we trace the origin of OT on a small chiral sphere immersed in a zeroth-order vector Bessel beam, taking advantage of our analytical expression. It is found that the azimuthal OT perpendicular to the beam’s propagation direction comes from the transfer of the spin angular momentum of the incident field to the particle, while the longitudinal OT along the illumination direction originates from the particle chirality, which generates longitudinal angular momentum on the optical beam and thus makes the particle subject to a longitudinal OT by recoil. Thus the longitudinal OT tends to rotate the chiral particle with opposite helicities in opposite directions, while the transverse OT shows little dependence on the handedness of particle chirality. Our results may help in understanding OT as an extra handle for particle manipulations in optical tweezers.