Figure 1. Simplified energy-level scheme of Er³⁺ doped UCNCs and plasmonic properties of gold nanostructures.

(a) Simplified energy-level scheme of Er³⁺ doped UCNCs. Dashed, dotted and solid arrows indicate photon excitation, multi-phonon relaxation and emission processes, respectively. (b) Normalized extinction spectra of a gold nanoparticle with 100-nm diameter (blue curve) and a gold nanorod with 22-nm diameter and 100-nm length (red dashed curve). The insets in (b) show the excitation configuration for the gold nanoparticle and nanorod. Position A is on the longitudinal axis and position B is above the nanorod at a horizontal distance a quarter of the nanorod length to the nanorod end (c–e) Normalized E-field distribution in the GNR at wavelengths 980 (c), 650 (d), and 540 nm (e), corresponding to the excitation and emission wavelengths in (a).