Tip-Enhanced Upconversion Luminescence in Yb³⁺–Er³⁺ Codoped NaYF₄ Nanocrystals

Lanthanide doped upconversion nanoparticles reveal enormous potential for biomedical applications. However, they are limited by low upconversion efficiency. In this paper, we demonstrate tip-enhanced upconversion luminescence (UCL) from a single Yb 3+ –Er 3+ -codoped NaYF 4 nanoparticle with a maximum enhancement factor of 11 by the plasmonic effect, which is a solution for improving the conversion efficiency and will become a potential technique for the applications in sensitive imaging and detection via speeding up both the absorption and the emission processes of lanthanide doped upconversion nanoparticles. By investigating the optical properties of the tip-enhanced UCL at 550 and 660 nm separately in Yb 3+ –Er 3+ -codoped NaYF 4 nanoparticles, we observe that the gold-coated tip influences the upconversion process in Yb 3+ –Er 3+ -codoped NaYF 4 particles by improving the reception and the transmission of incident electromagnetic fields.