Spectroscopic properties of near-stoichiometric In:Er:LiNbO3 crystals

The congruent and near-stoichiometric In(2 mol%):Er(1 mol%):LiNbO3 crystals have been grown by the Czochralski method from melts having compositions of 48.6 and 58 mol% Li2O, respectively. The OH− absorption spectra were characterized to investigate the structure defects of the crystals. The appearance of the 3508 cm−1 absorption peak manifests that the threshold concentration of In3+ is below or near 2 mol% in In:Er:LiNbO3 crystal with melt [Li]/[Nb] ratio of 1.38. The [Li]/[Nb] ratios in the crystals are estimated from the shift of ultraviolet absorption edge. In addition, the influence of [Li]/[Nb] ratio on the Judd–Ofelt intensity parameter Ωλ is analyzed using the Judd–Ofelt theory. The upconversion fluorescence spectra excited by an 800 nm femtosecond laser were measured at room temperature. The results show that three emission peaks centered at 535, 551 and 673 nm exist in both crystals but an additional 400 nm peak appears in the In:Er:LiNbO3 crystal with [Li]/[Nb] ratio of 1.38 in melt. The 400 nm emission peak is verified experimentally to be the second harmonic generation (SHG) of 800 nm light.