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

摘要

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 wavelengthλ.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 ～λ/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 superoscillatoryfield changes more than tenfold faster than a freepropagating plane wave.Areas with high local wavevectors are pinned to phase vortices and zones of energy backflow (～λ/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.