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 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.
Highlights
In recent years, plasmonics—coupled electromagnetic states of light and free electrons in metals—has become the dominant research direction in photonics
Considerable efforts have been devoted to searching for novel plasmonic materials with functionalities and losses improved beyond those offered by conventional plasmonic media, where the material characteristics can be better than those offered by conventional plasmonic metals, such as gold and silver
Our work goes further and explores one of the most practically important and fundamentally challenging questions of optics—how to focus light into a small spot— and exposes a previously unnoticed analogy between focusing by plasmonic nanostructures and superoscillatory focusing in free space
Summary
Plasmonics—coupled electromagnetic states of light and free electrons in metals—has become the dominant research direction in photonics. The main purpose of this paper is to bring to the attention of the growing nanophotonics research community that the attractive features of evanescent plasmonic fields—such as high localization and extremely rapid variations of fields, giant wavevectors, phase singularities, nanoscale vortices, and energy backflows—can be constructed in the immediate vicinity of a metallic plasmonic nanostructure and in free space, and in the absence of Joule losses.
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