Machfuudzoh Izzah, Hinamoto Tatsuki, García de Abajo F Javier, Sugimoto Hiroshi, Fujii Minoru, Sannomiya Takumi
Department of Materials Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503 Japan.
Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University, Kobe 657-8501, Japan.
ACS Photonics. 2023 Mar 15;10(5):1434-1445. doi: 10.1021/acsphotonics.3c00041. eCollection 2023 May 17.
A spherical dielectric particle can sustain the so-called whispering-gallery modes (WGMs), which can be regarded as circulating electromagnetic waves, resulting in the spatial confinement of light inside the particle. Despite the wide adoption of optical WGMs as a major light confinement mechanism in salient practical applications, direct imaging of the mode fields is still lacking and only partially addressed by simple photography and simulation work. The present study comprehensively covers this research gap by demonstrating the nanoscale optical-field visualization of self-interference of light extracted from excited modes through experimentally obtained photon maps that directly portray the field distributions of the excited eigenmodes. To selectively choose the specific modes at a given light emission detection angle and resonance wavelength, we use cathodoluminescence-based scanning transmission electron microscopy supplemented with angle-, polarization-, and wavelength-resolved capabilities. Equipped with semi-analytical simulation tools, the internal field distributions of the whispering-gallery modes reveal that radiation emitted by a spherical resonator at a given resonance frequency is composed of the interference between multiple modes, with one or more of them being comparatively dominant, leading to a resulting distribution featuring complex patterns that explicitly depend on the detection angle and polarization. Direct visualization of the internal fields inside resonators enables a comprehensive understanding of WGMs that can shed light on the design of nanophotonic applications.
球形介电粒子能够维持所谓的回音壁模式（WGMs），这种模式可被视为循环电磁波，从而导致光在粒子内部的空间限制。尽管光学WGMs作为一种主要的光限制机制在众多实际应用中被广泛采用，但模式场的直接成像仍然缺乏，仅通过简单的摄影和模拟工作得到了部分解决。本研究通过展示从激发模式中提取的光的自干涉的纳米级光场可视化，全面填补了这一研究空白，该可视化通过实验获得的光子图直接描绘激发本征模式的场分布。为了在给定的光发射检测角度和共振波长下选择性地选择特定模式，我们使用基于阴极发光的扫描透射电子显微镜，并辅以角度、偏振和波长分辨能力。配备半解析模拟工具后，回音壁模式的内部场分布表明，球形谐振器在给定共振频率下发射的辐射由多种模式之间的干涉组成，其中一种或多种模式相对占主导地位，导致最终的分布呈现出明确依赖于检测角度和偏振的复杂图案。对谐振器内部场的直接可视化能够全面理解WGMs，这有助于纳米光子应用的设计。
