Giant Enhancement of Upconversion Fluorescence of NaYF 4 :Yb 3+ ,Tm 3+ Nanocrystals with Resonant Waveguide Grating Substrate

By embedding NaYF4:Yb3+,Tm3+ nanocrystals into the top cladding layer of a resonant waveguide grating structure, we demonstrate that the upconversion fluorescence of Tm3+ ions can be greatly enhanced, by a factor of up to 104. The resonant waveguide grating structure consists of an SU8 bottom layer with sinusoidal grating morphology coated with a thin TiO2 waveguide layer and then covered with a poly(methyl methacrylate) cladding layer doped with NaYF4:Yb3+,Tm3+ nanocrystals. The giant enhancement of the upconversion fluorescence is achieved first by coupling the excitation light with a guided mode of the resonant waveguide grating structure and then the fluorescent light with a second guided mode. Our numerical simulation results obtained by rigorous coupled-wave analysis indicate that the electric field of the incident light is strongly enhanced near the interface of the TiO2 layer and the poly(methyl methacrylate) layer at guided mode resonance, and this is the major effect of the observed enhancement ...