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

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 λ. 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 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 (~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.Plasmonic-like electromagnetic field structures can exist in free spaceA novel metamaterial-based technique can generate tailored optical fields for nanoscale optical applications. In plasmonics, highly-structured electromagnetic states can be generated by the coupling of light and free electrons in metals. This enables researchers to localize light in applications such as photovoltaics and is used to miniaturize optical devices. Until recently, plasmonic fields have been generated near the surfaces of metallic nanostructures, but now Guanghui Yuan and Nikolay Zheludev at Nanyang Technological University, Singapore, and Southampton University, UK have demonstrated that the similar field structure can be created in free space, through the interference of multiple waves forming a “superoscillatory” focused hotspot. The team developed a new form of interferometry to map the features of optical fields at resolutions orders of magnitude shorter than the light wavelength. They showed that the electromagnetic field near the superoscillatory hotspot had similar characteristics to those generated at plasmonic nanostructures.