Figure 3.

(a) Single-photon fluorescence spectrum of the nanoparticles in solution (5 mg/mL), excited at 480nm. The peak can be attributed to the ⁵S₂,⁵F₄ ➝ ⁵I₈ transitions of holmium(III⁺) atoms; (b) Two-photon fluorescence image of the nanoparticles embedded in a PMMA thin film. When excited with 850 nm light, the particles emit in the 520–560 nm region, as was also proven in (a). An intensity profile (of the red line) is shown as inset in (b), to show the presence of aggregates with elevated signal compared to the rest of the film.