Sub-10 nm BaYF₅:Yb³⁺,Er³⁺ core-shell nanoparticles with intense 1.53 μm fluorescence for polymer-based waveguide amplifiers

Sub-10 nm BaYF₅:Yb³⁺,Er³⁺ core-shell nanoparticles (NPs) were synthesized by a high boiling solvent process. Under a 980 nm laser excitation, up- and down-conversion fluorescence spectra from BaYF₅:Yb³⁺,Er³⁺ NPs were recorded and analyzed. After coating the active-shell (containing Yb³⁺ ions) around the BaYF₅:Yb³⁺,Er³⁺ NPs, the intensity of the 1.53 μm fluorescent band was enhanced by 11 times owing to surface passivation effect and efficient energy transfer between Yb³⁺ ions in active-shell and Er³⁺ ions. Furthermore, the NPs showed excellent redispersibility in common nonpolar solvents. By dispersing BaYF ₅:Yb³⁺,Er³⁺ active-core-active-shell NPs into an SU-8 2005 polymer, we constructed polymer-based optical waveguide amplifiers and measured their performance. For an input signal power of 0.1 mW and a pump power of 200 mW, a relative optical gain of ∼6.3 dB was obtained at 1535 nm in a 13 mm-long waveguide.