Fig. 2

The intensities and nonlinear properties of Er³⁺-doped and Tm³⁺-doped UCNPs. a The upconversion emission spectra of Er³⁺-doped UCNPs and Tm³⁺-doped UCNPs. b Left: Confocal images of Er³⁺-doped UCNPs and Tm³⁺-doped UCNPs under different excitation power. Right: Total power-dependent upconversion emission intensity obtained from single Er³⁺-doped and Tm³⁺-doped UCNPs. Scale bar: 1 μm. c Power-dependent upconversion emission intensity obtained from single Er³⁺-doped UCNPs at 650 nm and 550 nm and single Tm³⁺-doped UCNPs at 800 nm and 455 nm. d Super-resolution scaling ∆x of UCNPs as a function of the excitation power (intensity). Inset, cross section profile lines of 4% Tm³⁺-doped UCNPs at 1.8 × 10⁶ W/cm² and 2% Er³⁺-doped UCNPs at 4 × 10⁶ W/cm². Error bars indicate standard deviations from line profiles of several measurements. Scale bar: 500 nm