Fig. 2

Optical depletion mechanism of the 455-nm upconversion luminescence in NaYF₄:18% Yb³⁺, 10% Tm³⁺ nanoparticles. a Proposed optical emission depletion mechanism of the 455-nm upconversion band of Tm³⁺ of NaYF₄:18% Yb³⁺, 10% Tm³⁺ nanoparticles. b Schematics of net stimulated depletion (left) and net synergistic enhancement (right) induced by 810-nm irradiation in high and low Tm³⁺-doped UCNPs, respectively. c Decay lifetimes of the 455-nm emission of NaYF₄:18% Yb³⁺, 10% Tm³⁺ UCNPs at different power densities (Supplementary Table 1) of the depleting 810-nm laser. d The intensity change of different emission bands of NaYF₄:18% Yb³⁺, 10% Tm³⁺ UCNPs under 975-nm excitation with/without 810-nm irradiation. The spectra were individually normalized in each spectral range, including 360–400, 570–615, 680–715, 980–1140, and 1300–2100 nm, separated by cutoff marks, in order to clearly display the relative intensity changes of different emission bands with and without the 810-nm irradiation. e The Tm³⁺ doping concentration-dependent 1470-nm and 1800-nm downconversion emission spectra of NaYF₄:18% Yb³⁺, x% Tm³⁺ UCNPs (λ _exc = 975 nm). f Dependence of 455-nm depletion efficiency (DE, \(\left( {I_{455}^{975} - I_{455}^{975\&{\rm STED}}} \right)/I_{455}^{975}\))) of NaYF₄:18% Yb³⁺, 10% Tm³⁺ on the depletion laser wavelength (Δλ = 4 nm) ranging from 700 to 815 nm. Negative depletion efficiency represents emission enhancement. The intensity of the 975-nm excitation laser and the depletion laser were kept at 700 kW cm⁻² and 17.7 MW cm⁻², respectively. g The excitation spectrum for the 455-nm emission of NaYF₄:18% Yb³⁺, 10% Tm³⁺ solely excited by the depletion laser with wavelength ranging from 700 to 815 nm; the inset is an enlarged view. Error bars represent ± 1 s.d.