Spectroscopic Imaging and Power Dependence of Near-Infrared to Visible Upconversion Luminescence from NaYF4:Yb3+,Er3+ Nanoparticles on Nanocavity Arrays

The spatial variations in upconversion luminescence from NaYF4:Er3+,Yb3+ nanoparticles embedded in PMMA on Au nanocavity arrays are investigated over a wide range of excitation powers, spanning the nonlinear and saturation power-dependence regimes. Spatially resolved upconversion spectra on these arrays show a minimum of ≈3× luminescence enhancement compared to the adjacent smooth Au surface under high-intensity excitation, with progressively higher enhancement ratios, up to 30×, at excitation intensities below 100 W/cm2. It is found that the average upconversion luminescence enhancement, obtained by spectroscopic imaging and far-field measurements, can be almost entirely accounted for by an effective multiplicative shift in the excitation intensity, P eff = F·P, which is robust over 5 orders of magnitude variation in excitation intensity. We reconcile this constant excitation enhancement factor, F = 4.46, with the wide range of observed luminescence enhancement factors, ranging from 3× to 30×, using an analytical model for a three level system, and by numerically solving a system of coupled rate equations for the Yb3+, Er3+ system. By analyzing the statistical distributions of luminescence intensities in the spectroscopic images on and off the nanocavity arrays, estimates of the luminescence enhancement factor independent of fluctuations in nanoparticle density are obtained. The results clearly relate observed enhancement factors to the kinetics of the energy-transfer upconversion process, suggesting the primary upconversion enhancement from these substrates is in the Yb3+ absorption channel, and demonstrate these self-assembled enhancing substrates as a low-cost and scalable route toward efficient near-infrared to visible upconversion at low excitation intensities.