Absorption characteristics of vapor transport equilibrated Er:LiNbO ₃ crystals

The visible and infrared transmission spectra of vapor transport equilibration (VTE) treated Er:LiNbO ₃ crystals, which have different doping levels (0.2%, 0.4%, and 2.0% Er per cation site), different cut orientation (X and Z cut) and different VTE duration (80, 120, 150, and 180 h), were recorded at room temperature in the wavelength range of 250-3700 nm. All of 2.0 mol% doped VTE crystals have precipitated whether X cut or Z cut, while the others have not. Their absorption characteristics were summarized and discussed in contrast to those of corresponding as-grown crystals. The OH ^- absorption feature of VTE treated Er:LiNbO ₃ is found to be different from that of pure VTE LiNbO ₃ crystal. The significant reduction of OH ^- absorption band implies that the hydrogen content in the VTE crystals has been reduced substantially whether the crystal precipitates or not. The electron transition absorption characteristics of the lower-doped, not precipitated crystals mainly include the higher transmittance, slight shift of peak or band position, slight absorption intensity change, the appearance of some additional peaks or bands, the narrowing of the peak width (full width at half maximum), and the definite blueshift of the optical absorption edge. The spectral changes are associated with the redistribution of Er ³⁺ spectroscopic sites induced by the VTE procedure. In comparison with those lower-doped VTE crystals, the highly doped VTE crystals display more significant absorption characteristics: the significant enhancement of 1480 nm pumping band and the obvious weakening of 1531 nm peak, the appearance of many additional peaks in the infrared region, and the interesting evolution of the transmittance with the wavelength. These substantial spectral changes are unambiguously conducted with the formation of a precipitate ErNbO ₄ induced by the VTE treatment in these crystals. The mechanism for the formation of the precipitate has been tentatively explained from the viewpoint of both microscopic and macroscopic domains. The higher transmittance, the narrowing of the peak width, the blueshift of the optical absorption edge, as well as the significant reduction of the OH band are the indicatives of the VTE treatment having brought these crystals closer to stoichiometric composition.