Resonant Control and Enhancement of Upconversion Luminescence of NaYF₄:Yb,Er Nanoparticles on Metal Gratings

Abstract Large plasmonic enhancements of upconversion luminescence (UCL) of lanthanide‐doped materials are achieved usually by plasmon resonances at excitation wavelengths. Here, moderately high‐Q plasmon resonance modes at emission wavelengths are used to control and enhance the UCLs of NaYF₄:Yb,Er nanoparticles on metal gratings. It is experimentally shown that, as Bloch‐type plasmon resonance modes locate at/near the green‐ (≈540 nm) or red‐emission (≈654 nm) wavelengths, the UCL is strongly enhanced (e.g., up to ≈117 times for the green and ≈272 times for the red), as well as large modifications of the green‐to‐red intensity ratios. The enhancement factors are comparable to or even much larger than those for plasmon resonances at excitation wavelengths reported in literatures. Importantly, it is disclosed that localized plasmon resonance modes in grooves of the metallic gratings, usually invisible in far‐field characterization, can be excited by the emission dipoles (Er³⁺) in vicinity (i.e., in the near‐field) to play an important role on UCL emissions. It is also inferred that the plasmon resonance modes at the emission wavelengths improve not only the Purcell factor for UCL emissions but also the energy transfer (Yb³⁺→Er³⁺) rates in excitation.