Empowering Polarization-Independent Fluorescence Enhancement with a C4v-Symmetry Plasmonic Nanocross Antenna Array.

Conventional bow-tie or dimer gap plasmons with C2v-symmetry are highly polarization dependent, which is unforvable in practice for unpolarized incident light sources and randomly dispersed fluorescence emitters. A nanocross configuration with C4v-symmetry could yield polarization-independent fluorescence enhancement, but the severe proximity effect during the nanofabrication process limits the gap size and the ultimate fluorescence enhancement. In this study, an advanced fabrication protocol is developed based on the concentric contour exposure strategy to overcome the notorious proximity effect, realizing a densely packed nanocross array with small gaps that is beyond the limit of conventional area exposure method. The proposed device consists of a top layer nanocross array and a bottom layer Au thin film, forming a metal-insulator-metal (MIM) cavity to maximize the fluorescence signal. The overall plasmonic resonance, primarily contributed by the electric dipolar and quadrupolar modes, is designed to match both the excitation and emission bands of the Cy5.5 dye with a broad hot-spot region not only within the antenna gap but also around the four antenna arms. An average fluorescence enhancement over 50 is demonstrated uniformly over the entire nanocross array, irrespective of the incident light polarization. Our work may pave the way for nanophotonic fluorescence sensing under relaxed lighting conditions.