Ali Rfaqat, Dutra Rafael S, Pinheiro Felipe A, Rosa Felipe S S, Maia Neto Paulo A
Instituto de Física, Universidade Federal do Rio de Janeiro, Caixa Postal 68528, Rio de Janeiro, RJ, 21941-972, Brasil.
Applied Physics Department, Photonics Research Center, Gleb Wataghin Physics Institute, University of Campinas - UNICAMP, P.O. Box 6165, Campinas, SP, 13083-970, Brazil.
Sci Rep. 2020 Oct 5;10(1):16481. doi: 10.1038/s41598-020-73530-1.
We report for the first time the theory of optical tweezers of spherical dielectric particles embedded in a chiral medium. We develop a partial-wave (Mie) expansion to calculate the optical force acting on a dielectric microsphere illuminated by a circularly-polarized, highly focused laser beam. When choosing a polarization with the same handedness of the medium, the axial trap stability is improved, thus allowing for tweezing of high-refractive-index particles. When the particle is displaced off-axis by an external force, its equilibrium position is rotated around the optical axis by the mechanical effect of an optical torque. Both the optical torque and the angle of rotation are greatly enhanced in the presence of a chiral host medium when considering radii a few times larger than the wavelength. In this range, the angle of rotation depends strongly on the microsphere radius and the chirality parameter of the host medium, opening the way for a quantitative characterization of both parameters. Measurable angles are predicted even in the case of naturally occurring chiral solutes, allowing for a novel all-optical method to locally probe the chiral response at the nanoscale.
我们首次报道了嵌入手性介质中的球形介电粒子的光镊理论。我们开发了一种分波（米氏）展开式，以计算作用在由圆偏振、高度聚焦激光束照射的介电微球上的光力。当选择与介质具有相同手性的偏振时，轴向阱稳定性得到改善，从而能够捕获高折射率粒子。当粒子因外力偏离轴时，其平衡位置会因光扭矩的机械效应围绕光轴旋转。当考虑半径比波长几倍大时，在手性主体介质存在的情况下，光扭矩和旋转角度都会大大增强。在此范围内，旋转角度强烈依赖于微球半径和主体介质的手性参数，为这两个参数的定量表征开辟了道路。即使在天然存在的手性溶质的情况下，也预测了可测量的角度，从而产生了一种在纳米尺度上局部探测手性响应的新型全光学方法。
