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Evanescent light can be localized at the nanoscale by resonant absorption ina plasmonic nanoparticle or taper or by transmission through a nanohole.However, a conventional lens cannot focus free-space light beyond half of thewavelength {\lambda}. Nevertheless, precisely tailored interference of multiplewaves can form a hotspot in free space of arbitrarily small size known assuperoscillation. Here, we report a new type of integrated metamaterialinterferometry that allows for the first time mapping of fields with deepsubwavelength resolution ~ {\lambda}/100. It reveals that electromagnetic fieldnear the superoscillatory hotspot has many features similar to those found nearresonant plasmonic nanoparticles or nanoholes: the hotspots are surrounded bynanoscale phase singularities (~ {\lambda}/50 in size) and zones where thephase of the wave changes more than tenfold faster than in a standing wave.These areas with high local wavevectors are pinned to phase vortices and zonesof energy backflow (~ {\lambda}/20 in size) that contribute to tightening ofthe main focal spot size beyond the Abbe-Rayleigh limit. Our observationsreveal the analogy between plasmonic nano-focusing of evanescent waves andsuperoscillatory nano-focusing of free-space waves, and prove the fundamentallink between superoscillations and superfocusing offering new opportunities fornanoscale metrology and imaging.

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