Blue-Pumped Deep Ultraviolet Lasing from Lanthanide-Doped Lu₆O₅F₈ Upconversion Nanocrystals

Deep ultraviolet (UV) lasers are essential for enormous use ranging from medical and life science to environmental and industrial applications. Herein, a class of blue-pumped microcavity lasers operating in the deep UV region is demonstrated by using Lu₆O₅F₈:Pr/Gd@Lu₆O₅F₈ upconversion nanocrystals as the gain medium. A hot-injection synthesis is developed for tuning dimensions of the Lu₆O₅F₈ nanocrystals. By synthesizing large-sized nanocrystals and using inert shell coating, deep ultraviolet emission at 315 nm by blue excitation at 447 nm is realized for the first time in colloidal solutions. Mechanistic investigations suggest that the upconversion stems from sequential absorption of two blue photons by Pr³⁺ ions followed by energy transfer to Gd³⁺ emitters. By virtue of their small particle size and high dispersibility, the upconversion nanocrystals are readily incorporated into silica resin microcavities with a bottle-like geometry. Lasing emissions of tunable optical characteristics are achieved from the microcavities through the formation of whispering gallery modes. These findings pave the way for constructing compact lasers operating in the deep UV regime by pumping with sophisticated long-wavelength lasers.