Upconversion Properties of LiNbO3 Single Crystals Co-doped with Ho and Yb

The IR-to-visible upconversion fluorescent crystals, Yb:Ho:LiNbO3, with a constant Ho3+ concentration (0.1 mol%) and different doping concentrations of Yb3+ (0.5, 1.5, 2.0, 2.5, 3.0 mol%) are synthesized by Czochralski method in air atmosphere. X-ray diffraction shows that the obtained crystal is a single phase of LiNbO3 and the rare-earth ions occupied the Li+ or Nb5+ sites instead of the interstitial sites. Under 980 nm excitation, green and red emission bands due to the Ho3+ (S-5(2), F-5(4))/I-5(8) and Ho3+ F-5(5)/I-5(8) energy transitions are observed in these samples, respectively. Power dependence studies on these samples with different Yb3+ dopant concentrations indicate that the red and green emissions are based on a two-photon process. The intensities of the red and green upconversion fluorescence increase with Yb3+ ions of 0 similar to 2.0 mol% because of an increased Yb3+ sensitization, but decrease at higher concentrations owing to the back-energy transfer between the Yb3+ and Ho3+ ions.