Robust low threshold full-color upconversion lasing in rare-earth activated nanocrystal-in-glass microcavity

Visible light microlasers are essential building blocks for integrated photonics. However, achieving low-threshold (μW), continuous-wave (CW) visible light lasing at room temperature (RT) has been a challenge because of the formidable requirement of population inversion at short wavelengths. Rare-earth (RE)-activated microcavities, featuring high-quality factor (Q) and small mode volume of whispering gallery modes, offer a great opportunity for achieving infrared-to-visible upconversion (UC) lasing. Here, we report that batch-produced nano-glass composite (GC) microspheres incorporating RE-doped fluoride nanocrystals show efficient UC emissions. These multi-phase composite microspheres exhibit a high Q value (≥10⁵), comparable to that of conventional multi-component glass microspheres. The UC lasing with pure red, green, and blue (RGB) emissions are demonstrated based on a highly efficient tapered fiber-microsphere system. More importantly, the GC microspheres manifest reduced (by 45%) lasing threshold and enhanced (more than four times) slope efficiency. These characteristics, together with excellent long-term stability, suggest a promising solution to achieving highly robust, stand-alone, low-threshold, and versatile UC microlasers.