Fig. 1. SEM images of the microdisk. (a) Top view; (b) enlarged SEM image of the microdisk indicated by a rectangle in (a), showing a smooth sidewall; (c) oblique view.

Fig. 2. (a) Schematic of experimental setup. Inset, the optical microscope image of the upconversion fluorescence distributed in the microdisk, exhibiting a hexagon pattern. OSA, optical spectrum analyzer; PD, photodetector; PC, polarization controller; CCD, charge-coupled device; OSC, oscilloscope. (b) Lorentz fitting (red curve) of the pump mode around 974.79 nm, revealing a loaded Q factor of 4.79 × 10⁵; (c) Lorentz fitting (red curve) of the lasing mode around 1531.40 nm, exhibiting a loaded Q factor of 1.28 × 10⁶.

Fig. 3. (a) Spectrum of the pump light at 974.79 nm wavelength; (b) evolution of the output power of the microlaser under different pump power levels; (c) spectrum of lasing signal at 1531.40 nm wavelength, showing a side mode suppression ratio of 32.4 dB; (d) output power dependence on the pump power, exhibiting an ultralow threshold of 1.04 µW and a high conversion efficiency of 4.06 × 10^-3.