Absorption and emission characteristics of Er 3NbO 7 phosphor: A comparison with ErNbO 4 phosphor and Er:LiNbO 3 single crystal

Abstract

Er 3NbO 7 phosphor was synthesized by sintering a mixture of Er 2O 3 and Nb 2O 5 powder in a molar ratio of 3:1 at 1600 °C over 55 h. Optical absorption and emission characteristics of Er 3+ ions in the calcined Er 3NbO 7 powder were investigated and discussed compared with ErNbO 4 phosphor and a Z-cut congruent Er (2 mol%):LiNbO 3 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 Er 3NbO 7 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 Er 3+ sites. The Er 3NbO 4 shows stronger upconversion emission than the single crystal but weaker than the ErNbO 4. 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 Er 3+ ion in the Er 3NbO 7, ErNbO 4 phosphor, and in the Er:LiNbO 3 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.