Enhanced multiphoton ultraviolet and blue upconversion emissions in Y 2O3 : Er3+ nanocrystals by codoping with Li+ ions

Ultraviolet (UV) and blue upconversion (UC) radiations with large and anomalous enhancement were observed in Y 2O3:Er3+ nanocrystals under laser excitation of 970 nm. Three-photon UV and blue UC radiations were enhanced by about 60 times via codoping with Li+ ions, in remarked contrast to the 45 times enhancement for two-photon green UC radiation. The integrated intensity of enhanced UV and blue UC radiations in Y 2O3:Er3+, Li+ nanocrystals can reach about half that in Y 2O3:Er3+ ceramic bulk. Such anomalous enhancement behavior for UV and blue UC radiations originates from the fact that the tailored lifetime of the S3/24 state was further involved for their generation, contrasted to the green UC emission involving only the tailored lifetime of the I11/24 state. Theoretical calculations based on steady state equations evidenced these conclusions.