A novel strategy for improving upconversion luminescence of NaYF₄: Yb, Er nanocrystals by coupling with hybrids of silver plasmon nanostructures and poly(methyl methacrylate) photonic crystals

The coupling of upconversion nanophosphors (UCNPs) with the surface plasmonic resonance (SPR) of noble metals is a promising way to improve luminescent efficiency of UCNPs; however, it is still a challenge to achieve stable, reproducible and effective upconversion luminescence (UCL) enhancement through such coupling. In this work, we present a novel strategy to improve UCL of NaYF₄: Yb³⁺, Er³⁺ UCNPs, by combining the near-field coupling of SPR of silver and the far-field coupling of poly(methyl methacrylate) (PMMA) opal photonic crystals (OPCs) with the UCNPs. In order to control the effective interaction distance between the UCNPs and the SPR, a porous silver film consisting of randomly distributed silver nanoparticles (NPs) (> 100 nm) was prepared which demonstrated strong SPR over a broad wavelength range, and its coupling to the UCNPs was found to be much stronger than that of a dense film. In the far-field coupling of OPCs, the photonic stop band (PSB) of the PMMA OPCs was tuned to 980 nm, matching exactly the excitation light. By modulating the particle size of the UCNPs, and the direction and excitation power of the incident light, a maximum enhancement of 60-fold was observed, which is an important advance for metal-induced UCL enhancement systems.