# Angle-Resolved Fluorescence of a Dye Coupled to a Plasmonic Nanohole Array
## 1 Introduction
GNAs are bi-dimensional metastructures, also called plasmonic metasurfaces, supporting plasmonic modes able to provide enhanced scattering properties characterized by a strong localization of the electromagnetic field on their surface. The angular distribution properties of a dye fluorescence signal with respect to the surface orientation have also been considered in some cases. Further, a complete study of the connection between the angle-dependent plasmonic behavior in the pump and in emission and the dye features was performed in [26], although it focused on a single plasmonic delocalized resonance. At the same time, the majority of the most recent papers on this topic are focused on specific bio-detection problems with an empirical approach [15; 27; 28; 29; 30]. In this view, the present work is aimed at verifying the suitability of a GNA for engineering a comprehensive PEF platform. The GNA exhibits an optical response determined by a mix of either localized, i.e., LSPRs, or propagating, i.e., surface plasmon polaritons (SPPs), modes [31; 32]. The GNA was developed by Plasmore SrI as a surface plasmon resonance (SPR) sensor [33] and can be customized to a homemade computational algorithm [34], ensuring the plasmonic response is tailored to specific needs. Moreover, the GNA has been considered for the development of a double-signal optical detection platform combining both SPR and PEF [35; 36; 37]. The synergic combination of SPR and PEF allows for exploiting the benefits while mitigating the drawbacks of each optical method, obtaining breakthrough characteristics combined into the same portable sensor:  
* Extended dynamic-range and reliable semi-quantitative measurements. PEF has a limited dynamic range (for analytes in a high concentration) due to self-quenching. This can be overcome since SPR is not affected by intermolecular interactions (being sensitive to the mass of the analyte molecules). Such a solution can provide the extension of the dynamic range and a real-time double-check of the sensor response in the concentration region where both SPR and PEF methods are applied. As reported in [38] when considering, e.g., Hg\({}^{2+}\), the dynamic range could span from 0.5 nM (the lower limit for the PEF method) up to 50 uM (the higher limit for the SPR method). * High sensitivity for multiple analytes. Both SPR and PEF detection methods can reach a high sensitivity for microbiological contaminants with a limit of detection (LOD) of up to tens of CFU/mL [39]. While low LOD (ppt-ppb) is expected, for example, in SPR bio-sensors in the case of heavy metals, SPR may also show low sensitivity for other low-molecular-weight compounds in low concentrations (ppb), such as pesticides. A double-signal optical detection platform will fill this gap in plasmonics-based methodology by exploiting the PEF detection mode, enabling low-LOD detection by collecting signals with respect to a non-fluorescent background [40]. In consideration of all the above, we identified two main goals deserving our interest:  
1. To analyze the spectral weight of fluorescence mediated by plasmonic resonance behavior and its angular dispersion;
2. To define the enhancement factor for each measurement configuration.