Singer Wolfgang, Nieminen Timo A, Gibson Ursula J, Heckenberg Norman R, Rubinsztein-Dunlop Halina
Centre for Biophotonics and Laser Science, Department of Physics, The University of Queensland, Brisbane QLD 4072, Australia.
Phys Rev E Stat Nonlin Soft Matter Phys. 2006 Feb;73(2 Pt 1):021911. doi: 10.1103/PhysRevE.73.021911. Epub 2006 Feb 21.
While the alignment and rotation of microparticles in optical traps have received increased attention recently, one of the earliest examples has been almost totally neglected--the alignment of particles relative to the beam axis, as opposed to about the beam axis. However, since the alignment torques determine how particles align in a trap, they are directly relevant to practical applications. Lysozyme crystals are an ideal model system to study factors determining the orientation of nonspherical birefringent particles in a trap. Both their size and their aspect ratio can be controlled by the growth parameters, and their regular shape makes computational modeling feasible. We show that both external (shape) and internal (birefringence) anisotropy contribute to the alignment torque. Three-dimensionally trapped elongated objects either align with their long axis parallel or perpendicular to the beam axis depending on their size. The shape-dependent torque can exceed the torque due to birefringence, and can align negative uniaxial particles with their optic axis parallel to the electric field, allowing an application of optical torque about the beam axis.
虽然近年来光学阱中微粒的排列和旋转受到了越来越多的关注，但最早的例子之一却几乎被完全忽视了——即粒子相对于光束轴的排列，而非围绕光束轴的排列。然而，由于排列扭矩决定了粒子在阱中的排列方式，它们与实际应用直接相关。溶菌酶晶体是研究决定非球形双折射粒子在阱中取向因素的理想模型系统。它们的尺寸和纵横比都可以通过生长参数来控制，并且其规则的形状使计算建模成为可能。我们表明，外部（形状）和内部（双折射）各向异性都对排列扭矩有贡献。三维捕获的细长物体根据其尺寸，其长轴要么与光束轴平行，要么与光束轴垂直排列。形状相关的扭矩可能超过双折射产生的扭矩，并且可以使负单轴粒子的光轴与电场平行，从而实现围绕光束轴的光学扭矩应用。
