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(Color online) NIR PL spectra of NaYF4:1%Ho3+ upon excitation of (a) 287 nm, (b) 535 nm, and (c) 880 nm. The insets of Figs. 1(a) and 1(b) show the corresponding visible PL spectra.
(Color online) PLE spectra monitored at various PL wavelengths of NaYF4:1%Ho3+ phosphor.
(Color online) Schematic energy-level diagram of Ho3+ showing the concept of sequential three-step NIR-QS under excitation of 287, 535, and 808 nm, respectively. (1) the first step, 3D3 → 3K8,5F2 at 850 nm; (2) the second step, 5S2,5F4 → 5I6 at 1015 nm; and (3) the third step, 5I6 → 5I8 at 1180 nm. Solid arrow represents optical transition, and short-dotted arrow is NR process.
(Color online) Visible (a) and NIR (b) time resolved emission spectra excited at 287 nm with a μF900 microsecond xenon lamp; # refers to that recorded in a fine time interval of 0.2 μs.
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