Imaging Single Particle Enhanced Upconversion Luminescence from NaYF4:Yb³⁺,Tm³⁺ Nanoparticles on Plasmonic Substrates

Author: Kim Yip Chiok, Department of Nanoscience & Nanoengineering

Mentor: Dr. Steve Smith, Department of Nanoscience & Nanoengineering

Upconversion is an anti-Stokes process that converts lower energy photons into higher energy photons through the energy transfer upconversion (ETU) mechanism. Lanthanide-doped β-NaY₄:Yb³⁺,Tm³⁺ upconverting nanoparticles are one of the most efficient upconversion nanoparticles (UCNPs) available. Due to their high photostability, large anti-Stokes shifts, zero-autofluorescence interference, and nontoxicity, UCNPs have many useful applications, such as deep-tissue imaging, optical displays, security printing, and spectral conversion for solar cells. However, their relatively low energy conversion efficiency has limited their use in practical applications, due in part to surface quenching of the excitation energy via nonradiative recombination. Studies have shown that luminescence enhancement can be achieved through coupling of UCNPs with metallic nanostructures as the surface plasmons resonance (SPRs) at a metal-dielectric interface can drastically alter the electromagnetic environment of UCNPs and enhance their emission profiles without changing the particles composition, size, or morphology. In this work, we examined the emission intensity of single UCNPs on gold films and gold microcavity arrays. The 644 nm emission of UCNPs on the gold microcavity arrays was enhanced by 8X in comparison with the gold film. To assess the mechanism of the luminescence enhancement, we measure the change in radiative lifetime of the fluorophore, and relate this to the modification of the local photonic density of states due to the plasmon resonance.