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 B a 2 L a F 7 : Y b 3 + ( 90 m o l % ) , T m 3 + ( 5 m o l % ) upconversion nanocrystals from amorphous borosilicate glass to support emission at ∼ 263 n m under 980 nm ns laser excitation. The excitation threshold can be further reduced from ∼ 130 to ∼ 26.5 m J / c m 2 by using a cylindrical microcavity. We also found that the growth of defect-free B a 2 L a F 7 nanocrystals with a high concentration of codoping Y b 3 + and T m 3 + ions inside high optical damage threshold borosilicate glass is the key to achieving room-temperature UVC upconversion lasing under high-intensity excitation.