Four-layered plasmonic structures of glass/Au/TiO 2 /NaYF 4 :Yb, Er, Gd nanorods were fabricated and tremendous improvement in upconversion luminescence (UCL) was observed under infrared 980 nm excitation. The TiO 2 film was used as an oxide spacer. The emission intensity of the upconversion (UC) nanorods was strongly modulated by the thickness of the TiO 2 layer. The extent of modulation depended on the separation distance between the Au layer and UC nanorods. A maximum UCL enhancement of 192-fold was observed for one green emission (540 nm) when a 10 nm-thick TiO 2 film was used; 150-fold was observed for the other green emission (521 nm) at the same thickness of TiO 2 ; and 105-fold was observed for the red emission (654 nm) when a 7.5 nm-thick TiO 2 film was used. Alteration of the radiative decay rate was demonstrated for the first time in measurements of the decay times of UC nanorods positioned at various d...
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