Modulation of upconversion luminescence spectrum of single rare-earth-doped upconversion nanocrystal based on plasmonic nanocavity

Plasmonic nanocavities can effectively modulate the upconversion luminescence properties of lanthanide doped upconversion nanocrystals (UCNCs), which not only enhances the luminescence intensity, but also modifies the luminescence spectrum. However, currently reported studies of upconversion luminescence spectrum modulation by using nanocavities are mainly based on ensemble experiments. Compared with ensemble experiments, single-particle experiments facilitate the comparative studies for the same upconversion nanocrystal and therefore the influence of inhomogeneity in ensemble samples can be avoided. Here in this work, we couple a single particle of Yb³⁺/Tm³⁺ co-doped nanocrystal with a plasmonic nanocavity composed of a single gold nanorod by using the in-situ nano-manipulation technique based on an atomic force microscope. Experimentally, we compare the upconversion luminescence spectra, upconversion luminescence lifetimes and excitation-power dependent upconversion luminescence intensities of the same single nanocrystal before and after coupling with the single gold nanorod. The experimental measurements are consistent with the theoretical calculations from rate equations combined with electromagnetic simulations. The results indicate that the plasmaonic nanocavity modulated nanocrystal upconversion luminescence spectrum is the combined result of three effects: the excitation field enhancement effect, the Purcell effect and the change of radiation efficiency.