Narrow‐Gap‐Integrated Ring Arrays (GRA) as Ultrahigh Field‐Enhancement Optical Resonators

Abstract Gold nanoring nanoantenna are increasingly being utilized in biological and chemical sensing owing to their favorable spatially expanded hotspot distribution and distinctive field focusing capabilities. However, it is extremely challenging to further elevate the local electric field of nanorings for higher sensitivity without trading off the two aforementioned features. Here, a new nanoantenna design integrates conventional nanorings with nanogaps is introduced: in conjunction with metal sputtering and ion milling on a nanopillar template, narrow‐gap‐integrated ring arrays (GRA) are successfully nanofabricated featuring monolithic integration of nanoring arrays with sub‐5 nm nanogaps on‐chip. This new GRA nanophotonic structure simultaneously preserves the intrinsic benefits of the nanoring and amplifies the local electric field, achieving a field enhancement factor of up to 10⁴ magnitudes. Using both top‐down and bottom‐up thin‐film processes, the GRA shows high degree‐of‐freedom in nanoantenna geometry, leading to tunable resonances throughout visible and near‐infrared wavelengths. These results combined showcased the potential of using GRA's tunable resonance, high e‐field enhancement, and wide hotspot‐distribution features in the creation of a universal molecular sensing platform for enhanced fluorescence.