RUI: Optical Excitation of Nonradiating Nanosphere for Lossless Device

金属纳米粒子将光限制在非常小的体积(甚至几十纳米)的能力已经发展成新型的小型化光学和电子设备。然而，与贵金属和加热相关的高水平损耗一直是限制光学器件效率的挑战。在这方面，有一种“新面孔”，即电动“极点”模式(即希腊语中的“无极”)，它可以通过最大限度地减少辐射损失来克服这些问题。该提案计划探索与高指数介电纳米球(硅纳米颗粒)相关的变极模式，这种纳米球可以作为无辐射源，并通过最小化辐射损失来有效限制能量。从我们的研究中获得的知识可能会转化为原型，可以开发成新型的光学和光子器件，如纳米激光、宽带光电探测器、传感器等。我们的研究还将通过提供本科生参与尖端实验研究、将研究整合到课程中以及与外部合作者和科学家建立网络机会来加强本科教育，这将有助于激励我们的学生选择STEM职业道路，包括来自代表性不足社区的学生。介电粒子的共振光激发为未来的光学和纳米光子器件提供了独特的机会，因为它们降低了损耗，并对电场和磁场的近场产生了很大的共振增强。在这方面，在高折射率介质材料中发现了作为非辐射源的电动力学“极点模式”，为实现新型纳米光子器件提供了一个独特的游乐场。在特定条件下，高折射率介电纳米结构的内模(磁模和环模)的叠加可以产生无辐射损失的非辐射态，称为“极点”。尽管对非辐射物体的研究长期以来一直是基础物理的一部分，但与光学频率中随时间变化的振荡电荷-电流分布相对应的动态极点直到2015年才被实验证明。由于球面几何结构不适合在平面波照射下激发变极模式，因此在演示结构中激发变极模式依赖于高度专业化的结构(硅纳米盘)的设计。然而，尽管针对特定的变极条件构建了纳米盘，但该纳米盘不能产生理想的变极模式。这里，我们建议在各向同性纳米球中激发变极模式，而不是专门设计的结构。由于平面波光不适合在纳米球中激发极点模式，我们将利用柱面矢量束的偏振对称性来激发各向同性纳米球中的“理想”极点模式。更具体地说，我们将利用径向极化柱面矢量束的径向电场分布和焦平面上的无磁场来激发理想的极点模式。由于激发的性质将负责产生变极模式，我们的方法将提供一种简单、直接的替代路径来激发变极模式，该模式已被预测会引起增强的非线性效应、纳米激光、理想的磁散射以及极高的Q因子和近场增强。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

The ability of metal nanoparticles to confine light to a very small volume (down to a few tens of nanometer) has been developed into novel miniaturized optical and electronic devices. However, the high level of losses associated with the noble metals and heating have always been a challenge limiting the efficiency of optical devices. In this regard, there is a "new kid on the block", namely electrodynamic "anapole" mode (i.e. "without poles" in Greek), that can overcome these issues by minimizing the radiative loss. This proposal plans to explore anapole mode associated with high-index dielectric nanosphere (Silicon nanoparticles) that can act as a radiationless source and confine energy efficiently by minimizing the radiative loss. The knowledge gained from our research can potentially be translated into prototypes that can be developed into novel optical and photonic devices, such as nano-lasers, broadband photo-detectors, sensors, etc. Our research will also enhance the undergraduate education by providing undergraduate participation in cutting-edge experimental research, integration of research into the curriculum, and networking opportunities with external collaborators and scientists, which will help motivate our students to choose a STEM career path, including students from under-represented community. Resonant optical excitation of dielectric particles offers unique opportunities for future optical and nanophotonic devices because of their reduced dissipative losses and large resonant enhancement of both electric and magnetic near-fields. In this regard, the discovery of the electrodynamic "anapole mode" as a non-radiating source in high index dielectric materials provides a unique playground to realize new nanophotonic devices. Under specific conditions, the superposition of internal modes (magnetic and toroidal) of high-index dielectric nanostructures can generate non-radiating states, called "anapoles", that are free from radiative loss. Even though the study of non-radiating objects has been part of fundamental physics for a long time, the dynamic anapole corresponding to the time-varying oscillating charge-current distributions in the optical frequencies was only experimentally demonstrated in 2015. Since spherical geometry is not suitable for excitation of the anapole mode under plane wave illumination, excitation of anapole mode in the demonstrated structure relied on the design of a highly specialized structure (Si nanodisk). However, in spite of constructing the nanodisk for the specific anapole condition, the nanodisk was unable to produce an "ideal" anapole mode.Here, instead of specifically designed structures, we propose to excite the anapole mode in isotropic nanosphere. Since plane wave illumination is not suitable for anapole mode excitation in a nanosphere, we will exploit the polarization symmetry of cylindrical vector beam to excite "ideal" anapole mode in isotropic nanosphere. More specifically, we will use the radial electric field distribution and absence of magnetic field in the focal plane of the radially polarized cylindrical vector beam to excite the ideal anapole mode. Since the nature of the excitation would be responsible for generating the anapole mode, our approach would provide a simple, straightforward alternate path to excite anapole mode that has been predicted to give rise to enhanced nonlinear effects, nanolasers, ideal magnetic scattering, as well as extremely high Q-factor and near-field enhancements.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.