Li Yaxin, Yu Xinning, Qu Tiantao, Ng Jack, Lin Zhifang, Zhang Lei, Chen Jun
Opt Express. 2023 Dec 18;31(26):44004-44018. doi: 10.1364/OE.506758.
Evanescent waves, with their high energy density, intricate local momentum, and spatial distribution of spins, have been the subject of extensive recent study. These waves offer promising applications in near-field particle manipulation. Consequently, it becomes imperative to gain a deeper understanding of the impacts of scattering and gradient forces on particles in evanescent waves to enhance and refine the manipulation capabilities. In this study, we employ the multipole expansion theory to present analytical expressions for the scattering and gradient forces exerted on an isotropic sphere of any size and composition in multiple evanescent waves. The investigation of these forces reveals several unusual optomechanical phenomena. It is well known that the scattering force does not exist in counter-propagating homogeneous plane waves. Surprisingly, in multiple pairs of counter-propagating evanescent waves, the scattering force can arise due to the nonzero orbital momentum (OM) density and/or the curl part of the imaginary Poynting momentum (IPM) density. More importantly, it is found that the optical scattering force can be switched on and off by simply tuning the polarization. Furthermore, optical forces typically vary with spatial position in an interference field. However, in the interference field generated by evanescent waves, the gradient force becomes a spatial constant in the propagating plane as the particle's radius increases. This is attributed to the decisive role of the non-interference term of the electromagnetic energy density gradient. Our study establishes a comprehensive and rigorous theoretical foundation, propelling the advancement and optimization of optical manipulation techniques harnessed through multiple evanescent waves. Specifically, these insights hold promise in elevating trapping efficiency through precise control and manipulation of optical scattering and gradient forces, stimulating further explorations.
倏逝波具有高能量密度、复杂的局部动量和自旋的空间分布，是近期广泛研究的主题。这些波在近场粒子操纵方面具有广阔的应用前景。因此，深入了解倏逝波中散射力和梯度力对粒子的影响，以增强和完善操纵能力变得至关重要。在本研究中，我们采用多极展开理论，给出了在多个倏逝波中作用于任意尺寸和成分的各向同性球体上的散射力和梯度力的解析表达式。对这些力的研究揭示了几种不寻常的光机械现象。众所周知，在反向传播的均匀平面波中不存在散射力。令人惊讶的是，在多对反向传播的倏逝波中，由于非零轨道角动量（OM）密度和/或虚坡印廷动量（IPM）密度的旋度部分，散射力可能会出现。更重要的是，发现通过简单地调整偏振，光散射力可以打开和关闭。此外，光力通常在干涉场中随空间位置而变化。然而，在倏逝波产生的干涉场中，随着粒子半径的增加，梯度力在传播平面内成为空间常数。这归因于电磁能量密度梯度的非干涉项的决定性作用。我们的研究建立了一个全面而严谨的理论基础，推动了利用多个倏逝波的光学操纵技术的进步和优化。具体而言，这些见解有望通过精确控制和操纵光散射力和梯度力来提高捕获效率，激发进一步的探索。
