Figure 2: Fast-slow “on-off” optical modulation of blue/green UC fluorescence from Tm³⁺ (Er³⁺) single-doped GCs upon two-step excitation of two-wavelengths.

(a,b) Continuous “on-off” cycling of the blue UC fluorescence from Tm³⁺ single-doped GCs with 800 nm (33.37 KW/cm²) and 1064 nm (3.65 MW/cm²) laser. The fluorescence signal follows the modulation of 1064 nm (a) or 800 nm laser (b), and negligible blue UC fluorescence signal is detected with only single 800 nm laser switched on (red lines in (b)). (c) Time-dependent fluorescence of Tm³⁺ single-doped GCs following repeated pulse sequence of 1064 and 800 nm laser light (upper panel in (c)). (d,e) Continuous “on-off” cycling of the green UC fluorescence from Er³⁺ single-doped GCs with 1530 nm (22.88 KW/cm²) and 850 nm (3.57 MW/cm²) laser. The fluorescence signal follows the modulation of 850 nm (d) or 1530 nm laser (e), and negligible green UC fluorescence signal is detected with only single 1530 nm laser switched on (red lines in (e)). (f) Time-dependent fluorescence of Er³⁺ single-doped GCs following repeated pulse sequence of 850 and 1530 nm laser (upper panel in (f)). The fluorescence decay time is fitted with the single exponential function: , where I and I₀ are the fluorescence intensity at time t and 0, A is constant, t is the time, and τ is the fluorescence decay time for the exponent^39,40.