近日，山西大学的陈君&张雷及其研究团队取得一项新进展。经过不懈努力，他们揭示倏逝波中介电球所受排斥梯度力的物理起源。相关研究成果已于2025年1月14日在国际知名学术期刊《物理评论A》上发表。

该研究团队采用多极展开理论，探讨了倏逝波中介电球所受的梯度力。研究发现，由于多极子与入射场的耦合，介电球可能会被推离高强度区域，无论是否激发回音壁模式（WGM）。然而，其背后的机制有所不同。对于s偏振入射的倏逝波，WGM的激发限制了特定阶次多极子对梯度力的贡献。由于电磁能量密度梯度贡献的差异，横磁模式激发时的梯度力明显强于横电模式。在没有WGM的情况下，多个多极子（有趣的是，仅磁多极子）对梯度力有贡献。

研究人员通过分析介电球内部的场分布和球内的光力密度，直观解释了介电球为何会受到方向相反的梯度力。此外，研究人员还通过分析反冲力和虚坡印廷动量密度的贡献，探讨了倏逝波的衰减率和偏振如何影响梯度力。这些深入研究加深了他们对近场光操控机制的理解，并为设计创新光学器件和系统提供了理论基础。

据悉，通常情况下，梯度力会将介电球吸引至高光强区域。然而，当瑞利粒子的有效极化率具有负实部，或介电球的回音壁模式（WGMs）被激发时，这种力可能会排斥介电球。对于一个比瑞利粒子大但不足以激发WGMs的介电球，是否会经历这种“异常”梯度力仍不确定。我们的研究采用多极展开理论，探讨了倏逝波中介电球所受的梯度力。

附：英文原文

Title: Physical origin of the repelling gradient force on a dielectric sphere in an evanescent wave

Author: Yaxin Li^1,*, Xinning Yu^2,3,*, Tiantao Qu¹, Lei Zhang^4,5,†, and Jun Chen^1,5,‡

Issue&Volume: 2025-01-14

Abstract: Generally, a gradient force draws a dielectric sphere towards the region of high light intensity. However, this force may repel the sphere when a Rayleigh particle's effective polarizability has a negative real part, or when a dielectric sphere's whispering gallery modes (WGMs) are excited. For a dielectric sphere larger than a Rayleigh particle but not large enough to excite WGMs, the possibility of experiencing this “anomalous” gradient force remains uncertain. Our study employs multipole expansion theory to investigate gradient forces on a dielectric sphere in an evanescent wave. We find that the sphere may be pushed away from the high-intensity region regardless of WGM excitation, due to multipole-incident field coupling. However, the underlying mechanisms differ. For an s-polarized incident evanescent wave, WGM excitation limits the contribution to the gradient force from a specific order of multipole. The gradient force is notably stronger for transverse magnetic mode excitation compared to transverse electric mode, due to differences in the electromagnetic energy density gradient's contribution. Without WGMs, multiple multipoles—interestingly, only magnetic ones—contribute to this gradient force. We provide an intuitive explanation for the dielectric sphere experiencing gradient forces in opposite directions by analyzing internal field distributions and the optical force density within the sphere. Additionally, we explore how evanescent wave decay rates and polarizations influence gradient forces by analyzing recoil forces and imaginary Poynting momentum density contributions. These in-depth studies deepen our understanding of near-field optical manipulation mechanisms and provide a theoretical foundation for designing innovative optical devices and systems.

DOI: 10.1103/PhysRevA.111.013516

Source: https://journals.aps.org/pra/abstract/10.1103/PhysRevA.111.013516