Optical transitions and frequency upconversions of Ho3+ and Ho3+/Yb3+ ions in Al(NO3)3–SiO2 sol–gel glasses

Infrared-to-visible upconversion processes and Judd Ofelt intensity parameters were analyzed for Ho3+ singly doped and Ho3+/Yb3+ co-doped Al(NO3)3–SiO2 glasses with a fixed Ho3+ and Yb3+ concentrations prepared by sol–gel method. Blue and intense green upconversion emissions centered at 467 and 538 nm, corresponding to the F53→I58 and F54(S52)→I58 transitions, respectively, were observed under 800 nm excitation. The analysis of the dynamics of upconversion emissions suggest excited state absorption, energy transfer and back transfer as the possible causes for the observed transitions. Significant enhancement of upconversion intensities in Ho3+/Yb3+ co-doped glass compared to the Ho3+ singly doped one confirms efficient energy transfer between Yb3+ and Ho3+ ions. Intense upconversion emissions shown by the glasses in the present study indicate their potential in upconversion device applications.