Absorption and emission characteristics of Er₃NbO₇ phosphor: A comparison with ErNbO₄ phosphor and Er:LiNbO₃ single crystal

Er₃NbO₇ phosphor was synthesized by sintering a mixture of Er₂O₃ and Nb₂O₅ powder in a molar ratio of 3:1 at 1600 °C over 55 h. Optical absorption and emission characteristics of Er³⁺ ions in the calcined Er₃NbO₇ powder were investigated and discussed compared with ErNbO₄ phosphor and a Z-cut congruent Er (2 mol%):LiNbO₃ single crystal. The absorption and emission studies show that, due to different crystal structures, the spectroscopic properties of these niobates have some differences in spectral shape, peak position, and relative intensity, especially at 1.5 μm. The most obvious spectral feature of the Er₃NbO₇ is that the spectral structure of band instead of peak is observed in its absorption or emission spectrum due to the existence of local structural disorder and multiple Er³⁺ sites. The Er₃NbO₄ shows stronger upconversion emission than the single crystal but weaker than the ErNbO₄. Experimental results show that energy transfer upconversion and/or excited state absorption play a dominant role in the upconversion emissions, and, at higher pump level (>200 mW), the thermal effect becomes significant and results in drop of the upconversion intensity. The 1.5 μm lifetimes of Er³⁺ ion in the Er₃NbO₇, ErNbO₄ phosphor, and in the Er:LiNbO₃ crystal are measured to be ∼5.3, 2.0, and 2.4 ms, respectively. In combination with the measured Raman spectra, the quantum efficiency, multiphonon nonradiative decay rate, and theoretical radiative lifetime of the 1.5 μm emission of the two powder materials are expected. The differences in upconversion intensity and measured 1.5 μm lifetime between the three materials are explained qualitatively.