Fig. 1: Continuous-wave upconversion lasing in liquid-quenched upconversion microspheres (LQUM).

a Schematic of the fabrication of an LQUM via the liquid quenching of silica-coated β-NaYF₄:Yb³⁺, Er³⁺ (20%, 2%) UCNPs on a TEM grid using a highly focused CW 980-nm laser beam. b SEM images of the LQUM at low and high magnifications. Because the direction of light circulation must be aligned perfectly with the circumference of the microsphere, an LQUM dangling on a TEM grid wall was selected for the formation of WGM resonators. c Schematic of upconversion lasing in an LQUM using a free-space beam of a 980-nm continuous-wave pump laser. The LQUM accommodates upconversion lasing through the light circulation along the circumference of the microsphere upon the upconversion luminescence facilitated by energy transfer upconversion of an activator (Er³⁺) with a sensitizer (Yb³⁺). d Upconversion luminescence of UCNPs (below) and an LQUM with upconversion lasing (above). The doping concentrations of Yb³⁺ and Er³⁺ were 20% and 2%, respectively. The color inset shows an optical microscopy image of an illuminated LQUM. e The proposed upconversion lasing pathway supported by energy transfer upconversion in the LQUM. Short-dashed, dot-dashed, colored solid, long-dashed, and dotted lines indicate absorption, energy transfer, spontaneous emission, stimulated emission, and multiphonon relaxation, respectively.