Simpson Stephen H, Hanna Simon
H. H. Wills Physics Laboratory, University of Bristol, Bristol, UK.
J Opt Soc Am A Opt Image Sci Vis. 2009 Mar;26(3):625-38. doi: 10.1364/josaa.26.000625.
It is well known that Laguerre-Gaussian beams carry angular momentum and that this angular momentum has a mechanical effect when such beams are incident on particles whose refractive indices differ from those of the background medium. Under conditions of tight focusing, intensity gradients arise that are sufficiently large to trap micrometer-sized particles, permitting these mechanical effects to be observed directly. In particular, when the particles are spherical and absorbing, they rotate steadily at a rate that is directly proportional to the theoretical angular momentum flux of the incident beam. We note that this behavior is peculiar to absorbing spheres. For arbitrary, axially placed particles the induced torque for rotation angle zeta is shown to be Gammaz=Asin(2zeta+delta)+B, where A, B, and delta are constants that are determined by the mechanisms coupling optical and mechanical angular momentum. The resulting behavior need not be directly related to the total angular momentum in the beam but can, nonetheless, be understood in terms of an appropriate torque density. This observation is illustrated by calculations of the torque induced in optically and geometrically anisotropic particles using a T-matrix approach.
众所周知，拉盖尔 - 高斯光束携带角动量，并且当这种光束入射到折射率与背景介质不同的粒子上时，这种角动量会产生机械效应。在紧聚焦条件下，会出现足够大的强度梯度，从而能够捕获微米级粒子，使得这些机械效应能够被直接观测到。特别地，当粒子是球形且具有吸收性时，它们会以与入射光束的理论角动量通量成正比的速率稳定旋转。我们注意到这种行为是吸收性球体所特有的。对于任意轴向放置的粒子，旋转角度为ζ时的诱导转矩被证明为Γζ = Asin(2ζ + δ) + B，其中A、B和δ是由光学和机械角动量耦合机制所确定的常数。由此产生的行为不一定与光束中的总角动量直接相关，但尽管如此，仍可以根据适当的转矩密度来理解。使用T矩阵方法对光学和几何各向异性粒子中诱导的转矩进行计算，说明了这一观察结果。
