Quench, glow, or stay silent: distance-controlled up-conversion emission near metallic nanowires

We demonstrate controlled transitions between competing radiative and nonradiative decay channels in the up-conversion luminescence of NaYF₄:Er³⁺/Yb³⁺ nanocrystals placed in proximity to metallic nanowires. The nanocrystal-nanowire separation is used as a key control parameter governing the optical response. An essential aspect of our approach is the reduction of inhomogeneous residual polymer layers from the nanowires, thereby reducing uncertainties associated with distance control and improving reproducibility in emitter-metal hybrid nanostructures. Replacing them with a well-defined polymer spacer yields controlled access to three qualitatively distinct interaction regimes: luminescence quenching, plasmonic enhancement, and effective decoupling. Transitions between these regimes are shown to reflect changes in the dominant energy relaxation pathways: from nonradiative losses in direct contact with the metal, through modification of the local photonic density of states and coupling to plasmon-mediated modes, to behavior characteristic of quasi-isolated emitters. The plasmonic origin of the emission enhancement in the intermediate regime is evidenced by an increase in luminescence intensity accompanied by shortened decay times, as revealed by fluorescence lifetime imaging microscopy of up-conversion nanocrystals. All experiments were performed on single nanostructures, thereby excluding artefacts arising from aggregation effects and strengthening the interpretation of the observed phenomena. The presented approach provides controlled access to plasmon-modified decay channels and offers a basis for the rational design of functional nanophotonic and sensing structures with tailored optical properties.