Spectroscopic properties of Er3+ ions in LiNbO3 crystals codoped with HfO2

The results of the spectroscopic analysis of transition strengths for Er3+ ions in a series of Hf:Er:LiNbO3 crystals with variable Hf content and fixed Er content are reported. Unpolarized UV-VIS-NIR absorption spectra, upconversion fluorescence spectra excited at 800 nm, and microsecond time-resolved spectra excited at 400 nm and 800 nm by 800 nm femtosecond laser were measured at room temperature. The HfO2 incorporation has influence on Er3+ radiative lifetimes, and fluorescence branching ratios. For Hf(4 mol %):Er(1 mol %):LiNbO3, Omega(2) = 2.63 x 10(-20) cm(2), Omega(4) = 2.86 x 10(-20) cm(2), and Omega(6) = 0.72 x 10(-20) cm(2). Omega(2) < Omega(4) is contrary to the Er3+ general trend of Omega(2) > Omega(4) > Omega(6) when the Hf content is below its threshold concentration. In addition, the sum of Omega increases with the Hf content when the HfO2 content below 6 mol % is unfamiliar. The upconversion mechanism is discussed in this work.