Ultraviolet C lasing at 263 nm from Ba₂LaF₇:Yb³⁺,Tm³⁺ upconversion nanocrystal microcavities

It is a daunting challenge to realize ultraviolet C (UVC) lasing (i.e., has a wavelength range from 200 to 275 nm) from upconversion nanocrystals due to their low upconversion efficiency. Here, we fabricate Ba₂LaF₇:Yb³⁺(90 mol%), Tm³⁺(5 mol%) upconversion nanocrystals from amorphous borosilicate glass to support emission at ∼263 nm under 980 nm ns laser excitation. The excitation threshold can be further reduced from ∼130 to ∼26.5 mJ/cm² by using a cylindrical microcavity. We also found that the growth of defect-free Ba₂LaF₇ nanocrystals with a high concentration of codoping Yb³⁺ and Tm³⁺ ions inside high optical damage threshold borosilicate glass is the key to achieving room-temperature UVC upconversion lasing under high-intensity excitation.