Tapered fiber coupling of nanoparticle-coated silica microspheres for multi-wavelength upconversion whispering gallery mode lasers

Upconversion nanoparticles (UCNPs) have attracted considerable interest due to their large anti-Stokes shift, offering promising applications in lasing. Here, multi-wavelength upconversion whispering gallery mode (WGM) lasing is demonstrated in silica microspheres coated with NaYF 4 @NaYbF 4 :1 % Tm 3+ @ NaYF 4 nanoparticles and coupled with tapered fibers. Under continuous-wave 980 nm pumping, low-threshold lasing is achieved across Tm 3+ transitions from near-infrared to visible, with an ultra-low threshold of 0.61 μW for the H 4 3→H3 6 transition. Additionally, upconversion laser output can also be achieved in Er 3+ - and Ho 3+ -activated microspheres. These results establish tapered fiber coupling as a versatile approach for enhancing upconversion microlasers.