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

TL;DR

This paper demonstrates that superoscillatory optical fields can achieve nanoscale focusing beyond the diffraction limit, revealing features similar to plasmonic hotspots and enabling advanced nanoscale imaging.

Contribution

It introduces a new integrated metamaterial interferometry technique for mapping superoscillatory fields with deep subwavelength resolution, linking superoscillation to superfocusing.

Findings

Mapping of superoscillatory hotspots with ~{}100 resolution

Observation of nanoscale phase singularities and energy backflow zones

Identification of high local wavevectors pinned to phase vortices

Abstract

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 {\lambda}. Nevertheless, precisely tailored interference of multiple waves can form a hotspot in free space of arbitrarily small size known as superoscillation. Here, we report a new type of integrated metamaterial interferometry that allows for the first time mapping of fields with deep subwavelength resolution ~ {\lambda}/100. It reveals that 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 (~ {\lambda}/50 in size) and zones where the phase 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 zones of energy backflow (~ {\lambda}/20 in size) that contribute to tightening of the main focal spot size beyond the Abbe-Rayleigh limit. Our observations reveal the analogy between plasmonic nano-focusing of evanescent waves and superoscillatory nano-focusing of free-space waves, and prove the fundamental link between superoscillations and superfocusing offering new opportunities for nanoscale metrology and imaging.