We present a theoretical study of the optical angular momentum transfer from a circularly polarized plane wave to thin metal nanoparticles of different rotational symmetries. While absorption has been regarded as the predominant mechanism of torque generation on the nanoscale, we demonstrate numerically how the contribution from scattering can be enhanced by using multipolar plasmon resonance. The multipolar modes in non-circular particles can convert the angular momentum carried by the scattered field, thereby producing scattering-dominant optical torque, while a circularly symmetric particle cannot. Our results show that the optical torque induced by resonant scattering can contribute to 80% of the total optical torque in gold particles. This scattering-dominant torque generation is extremely mode-specific, and deserves to be distinguished from the absorption-dominant mechanism. Our findings might have applications in optical manipulation on the nanoscale as well as new designs in plasmonics and metamaterials.
Optical multipolar torque in structured electromagnetic fields: on `helicity gradient' torque, quadrupolar torque and the spin of field gradient
结构光通过光学力和扭矩与物质机械地相互作用。传统上，通过进入包围物体的体积中的总角动量（AM）的通量来计算光学扭矩。在[Phys. Rev. A 92，043843（2015）]中，提出了一种强大的方法来计算光学AM的自旋通量和轨道部分的光学扭矩，提供了有用的物理见解。然而，该方法预测了一种新类型的偶极扭矩依赖于光束的螺旋密度的梯度，与先前的扭矩计算不一致。在这项工作中，我们打算澄清这一矛盾，消除混乱。我们重新推导，从第一原理和详细的推导，传统的偶极总扭矩使用总AM通量，和相应的AM贡献的基础上的自旋和轨道扭矩分量，确保其总和与总扭矩一致。我们还测试了我们推导的解析表达式对数值积分，与精确的协议。我们发现，“螺旋度梯度”扭矩项确实存在于自旋和轨道组件分开，但我们提出了纠正前因子，使添加它们后，他们取消了，和“螺旋度梯度”项消失的总偶极扭矩，调和文献结果。我们还推导出四极矩的解析表达式，表明它与EM场梯度的自旋成正比，而不是文献中有时错误假设的局部EM场自旋。我们提供了反直觉的情况下，电磁场梯度的自旋行为非常不同的本地EM自旋的例子。天真地使用本地EM场自旋导致错误的预测的扭矩大粒子的四极和高阶多极子的强大贡献，特别是在一个结构化的入射场。
Optical forces, helicity, angular momentum and how they are all intertwined
Centro de Física de Materiales (CFM), CSIC-UPV/EHU（材料物理中心，西班牙科学研究中心-巴斯克国立大学）；Donostia International Physics Center（圣塞巴斯蒂安国际物理中心）；Basic Sciences Department, Mondragon Unibertsitatea（蒙德拉贡大学基础科学系）；IKERBASQUE, Basque Foundation for Science（伊克尔巴斯基，巴斯克科学基金会）
Singapore University of Technology and Design (SUTD)（新加坡科技设计大学）；Quantum Innovation Centre (Q. InC), Agency for Science Technology and Research (A*STAR)（科学技术研究局量子创新中心）；Department of Materials Science and Engineering, National University of Singapore（新加坡国立大学材料科学与工程系）；Department of Electrical and Computer Engineering, National University of Singapore（新加坡国立大学电气与计算机工程系）；Singapore-HUJ Alliance for Research and Enterprise (SHARE), The Smart Grippers for Soft Robotics (SGSR) Programme, Campus for Research Excellence and Technological Enterprise (CREATE)（卓越研究与科技企业园新加坡-希伯来大学科研联盟智能软体机器人夹爪项目）；Graduate School of Artificial Intelligence, Pohang University of Science and Technology (POSTECH)（浦项科技大学人工智能研究生院）；Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR)（科学技术研究局材料研究与工程研究所）；School of Electronic Science and Engineering, Southeast University（东南大学电子科学与工程学院）；Department of Mechanical Engineering, Pohang University of Science and Technology (POSTECH)（浦项科技大学机械工程系）；Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH)（浦项科技大学化学工程系）；Department of Electrical Engineering, Pohang University of Science and Technology (POSTECH)（浦项科技大学电气工程系）；POSCO−POSTECH-RIST Convergence Research Center for Flat Optics and Metaphotonics（浦项-浦项科技大学-RIST平面光学与超光子学交叉研究中心）；Research School of Physics, Australian National University（澳大利亚国立大学物理研究学院）
Revealing time characteristics of optical excitations in dielectric and plasmonic structures through cathodoluminescence interferometry
NWO-Institute AMOLF（荷兰皇家艺术与科学学院AMOLF研究所）；Kobe University（神户大学）；Universitat Rovira i Virgili（罗维拉-伊-维尔吉利大学）；ICREA-Institució Catalana de Recerca i Estudis Avançats（加泰罗尼亚研究与高等研究院）；ICFO-Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology（巴塞罗那科技研究所光子科学研究所）
