"Plasmonics" in free space: observation of giant wavevectors, vortices, and energy backflow in superoscillatory optical fields

• Published in: Light, science & applications Vol. 8; no. 1; pp. 2 - 9
• Main Authors: Yuan, Guanghui, Rogers, Edward T F, Zheludev, Nikolay I
• Format: Journal Article
• Language: English
• Published: England Springer Nature B.V 03.01.2019; Nature Publishing Group UK
• Subjects: Disease hot spots; Electromagnetic fields; Nanoparticles; Vortices
• ISSN: 2047-7538; 2095-5545; 2047-7538
• DOI: 10.1038/s41377-018-0112-z

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.