Plasmonic enhancement of lanthanides luminescence using metallic nanoparticles

This work summarizes how plasmonic phenomena can enhance the fluorescence of lanthanides (Ln(3+)) coupled to metallic nanoparticles (MNP). Lanthanide-ions emission lines, based on 4f-4f transitions, are weak due to Laporte selection rules for optical transitions. This effect results in low absorption cross sections for excitation and long lifetimes for emission processes. We propose to use metallic nanoparticles in order to determine how plasmonic nanoparticles can enhance absorption and emission of two emblematic lanthanides ions used for bio-labeling and energy conversion, i.e., Eu3+ and Er3+. This work quantifies the average enhancement factor (AEF) is expected for different geometries of nanoparticle structures and compares it to previous studies. Then, we theoretically and numerically investigate metal-enhanced fluorescence of plasmonic core-shell nanoparticles doped with lanthanides ions. The shape and size of the particles are engineered to maximize the average enhancement factor of the overall doped shell. We show with theoretical considerations and numerical studies that the highest enhancement (11 in the visible and 7 in the near-infrared) is achieved by tuning either the dipolar or the quadrupolar particle resonance to the rare-earth ion's excitation wavelength. Additionally, the calculated AEFs are compared to experimental data reported in the literature, obtained under similar conditions (plasmon-mediated enhancement) or when a metal-Ln(3+) energy-transfer mechanism is involved.