Yuan Guanghui, Rogers Edward T F, Zheludev Nikolay I
1Centre for Disruptive Photonic Technologies, The Photonic Institute, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore, 637371 Singapore.
2Optoelectronics Research Centre and Centre for Photonic Metamaterials, University of Southampton, Highfield, Southampton, SO17 1BJ UK.
Light Sci Appl. 2019 Jan 3;8:2. doi: 10.1038/s41377-018-0112-z. eCollection 2019.
Evanescent light can be localized at the nanoscale by resonant absorption in a plasmonic nanoparticle or taper or by transmission through a nanohole. However, a conventional lens cannot focus free-space light beyond half of the wavelength . Nevertheless, precisely tailored interference of multiple waves can form a hotspot in free space of an arbitrarily small size, which is known as superoscillation. Here, we report a new type of integrated metasurface interferometry that allows for the first time mapping of fields with a deep subwavelength resolution /100. The findings reveal that an electromagnetic field near the superoscillatory hotspot has many features similar to those found near resonant plasmonic nanoparticles or nanoholes: the hotspots are surrounded by nanoscale phase singularities and zones where the phase of the superoscillatory field changes more than tenfold faster than a free-propagating plane wave. Areas with high local wavevectors are pinned to phase vortices and zones of energy backflow (/20 in size) that contribute to tightening of the main focal spot size beyond the Abbe-Rayleigh limit. Our observations reveal some analogy between plasmonic nanofocusing of evanescent waves and superoscillatory nanofocusing of free-space waves and prove the fundamental link between superoscillations and superfocusing, offering new opportunities for nanoscale metrology and imaging.
倏逝光可以通过等离子体纳米颗粒或锥形结构中的共振吸收，或者通过纳米孔的透射，在纳米尺度上实现局域化。然而，传统透镜无法将自由空间光聚焦到超过波长一半的程度。尽管如此，多个波的精确调控干涉可以在任意小尺寸的自由空间中形成一个热点，这就是所谓的超振荡。在此，我们报道了一种新型的集成超表面干涉测量方法，首次实现了具有深度亚波长分辨率（约为λ/100）的场映射。研究结果表明，超振荡热点附近的电磁场具有许多与共振等离子体纳米颗粒或纳米孔附近发现的特征相似之处：热点被纳米级相位奇点和区域所包围，在这些区域中，超振荡场的相位变化比自由传播的平面波快十倍以上。具有高局部波矢的区域被固定在相位涡旋和能量回流区域（尺寸约为λ/20）上，这些区域有助于将主焦点尺寸压缩到阿贝 - 瑞利极限以下。我们的观察揭示了倏逝波光的等离子体纳米聚焦与自由空间波的超振荡纳米聚焦之间的一些相似性，并证明了超振荡与超聚焦之间的基本联系，为纳米级计量和成像提供了新的机会。
