Influence of vapor transport equilibration on spectroscopic properties of Er:LiNbO ₃ crystal heavily codoped with MgO

Z -cut MgO heavily codoped (6 mol %) Er (0.4 mol %): LiNb O3 crystals were thermally treated at 1110 and 1120 °C over 100, 120, and 220 h using vapor transport equilibration (VTE) technique. Visible and infrared-absorption and x-ray powder diffraction have been used to characterize their spectral, spectroscopic, and crystalline phase characteristics in comparison with the as-grown crystal. The experimental results show that VTE treatment results in the reduction of background absorption and 4-5-nm blueshift of the optical-absorption edge, qualitatively showing that these crystals have been brought closer to stoichiometric composition. The VTE procedure also results in remarkable Er3+ absorption changes. These spectroscopic changes imply that the VTE treatment may result in the formation of ErNb O4 precipitates in all VTE crystals. In addition, the VTE treatment also results in substantial spectral changes with respect to O H- absorption. These O H- spectral changes are explained from the viewpoint of VTE-induced site-occupancy alterations of intrinsic and extrinsic defects. Based upon the measured absorption spectra, the influences of VTE treatment on the Er3+ spectroscopic parameters were studied by using the Judd-Ofelt (JO) theory. The theoretical results show that VTE treatment has a little influence on the branch ratio, and a substantial influence on JO parameters and electron transition probability because of the phase transformation with respect to Er3+. The VTE treatment results in the increase of the JO parameters by more than 50% and theoretical radiative lifetime by 50%-120%.