Imaging Upconversion from NaYF 4 :Er:Yb Nanoparticles on Au and Ag Nanostructured Substrates

Near-infrared-to-visible upconversion materials have many promising applications, including use in luminescent solar concentrators, in next-generation displays, and as biological labels. NaYF4 nano-particles doped with Yb and Er exhibit efficient upconversion and are easily deployed in these applications. It is known that a rough metal surface may increase the yield of fluorescence of a nearby fluorophore, by local field enhancement due to plasmonic resonances, and by modification of the radiative rate(s) of the fluorophore. Thus, properly chosen metallic nanostructures can potentially increase the upconversion efficiency of lanthanide-doped nanoparticles, yet the optimal design of these nanostructures is still an active area of research. In our experiments, we use a spectroscopic imaging system to study the upconversion efficiency of NaYF4: Er3+/ Yb3+ through spatially-resolved upconversion spectra, using a custom-built scanning confocal microscope system with infra-red excitation, and wide-field fluorescence imaging. We present spectrally-resolved upconversion images of NaYF4:Yb3+/Er3+ nanoparticles on plasmonic substrates, including silver nanowires and patterned substrates of gold and silver, which show localized regions (∼ 1μm) of relatively stronger intensity and modified upconversion spectra, and compare these to wide-field fluorescence images of samples with and without plasmonic substrates.