Enhancement of Near-Infrared-to-Visible Upconversion Luminescence Using Engineered Plasmonic Gold Surfaces

An engineered plasmonic gold surface, specifically designed to couple with 980 nm radiation, is shown to enhance near-infrared-to-visible upconversion luminescence from a monolayer of beta-NaYF4: 17%Yb, 3%Er nanocrystals in poly(methyl methacrylate) on that gold surface. Confocal imaging of upconversion luminescence from the surface is used to characterize the nature of the enhancement. It is shown that the luminescence data were acquired below the so-called high power limit for excitation, but some saturation was evident, as the observed power dependence was less than quadratic. Over the range of excitation power densities used, the intrinsic enhancement factor for upconversion from the patterned surface was greater than a factor of 3 but decreased slowly with increasing excitation power. The red and green upconversion were enhanced by similar factors, which would support the intensification of the excitation field by the plasmonic surface as being the mechanism of enhancement. In the absence of other enhancement or quenching mechanisms, the data imply an approximate 2-fold magnification of the excitation field intensity relative to smooth gold.