Large enhancements of NaYF4:Yb/Er/Gd nanorod upconversion emissions via coupling with localized surface plasmon of Au film.

Four-layered plasmonic structures of glass/Au/TiO2/NaYF4:Yb, Er, Gd nanorods were fabricated and tremendous improvement in upconversion luminescence (UCL) was observed under infrared 980 nm excitation. The TiO2 film was used as an oxide spacer. The emission intensity of the upconversion (UC) nanorods was strongly modulated by the thickness of the TiO2 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 TiO2 film was used; 150-fold was observed for the other green emission (521 nm) at the same thickness of TiO2; and 105-fold was observed for the red emission (654 nm) when a 7.5 nm-thick TiO2 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 distances from the Au layer. The light interaction and coupling between metal Au and UC nanorods is numerically studied. The UCL mechanisms of multilayer plasmonic structures are discussed. Experimental results are explained and correspond well with those of theoretical calculations.