Figure 1.

(a,b) are visible PL spectra of UCNPs on silver nanograting (blue, refer to left y-axis) and on flat silver film (red, refer to right y-axis) measured under 980 nm laser excitation with 1 kW/cm² and 102 kW/cm² power density, respectively. The photoluminescence is under arbitrary unit with the same scale. (c) NIR PL spectra of UCNPs on silver nanograting (blue, 5 s integration time), on silver film (red, 10 s integration time), and the 980 nm laser excitation source (yellow, 10 s integration time). The PL spectra are modified by the transmittance of a 997 nm long pass edge filter, which attenuates the scattered laser light. (d) Energy levels of Yb³⁺ and Er³⁺ ions relevant to the energy transfer upconversion processes. The black solid arrow indicates initial absorption of the incident photons. Dashed and dotted arrows indicate subsequent energy transfer processes and non-radiative relaxations, respectively. The green and red arrows indicate the final upconverted luminescence.