Impurity sites of LiNbO3 : Er3+ codoped with MgO or ZnO

The optical properties of ~ r ~ ' doped LiNbO LiNb03(MgO) and 3' LiNbO (ZnO) have been studied. Polarized absorption and laser excited 3 luminescence spectra at liquid-He temperature show different structure dependent on the co-dopants (MgO, ZnO). The relationship between the optical characteristics and the impurity sites location is discussed. LiNb03 crystals doped with rare earth ions have been extensively investigated in the last years due to their potential use in electro-optics and laser technology [I-31. In particular, ~r~~ and ~ d ~ ' doped LiNb03 have been already demonstrated in active waveguides devices [4-61. Although, pure LiNb03 crystals are damaged under high intensity illumination, the problem is greatly reduced codoping the material with MgO at a concentration of about 5% in the melt [71. This additional doping not only affects the photorefractive characteristics of LiNbO but also the site location and the optical 3' properties of the active ions, as it has been already observed in several rare earth doped LiNb03(MgO) crystals [3]. Recently, it has been reported that photorefractive damage in LiNbO 3 can be prevented by codoping with ZnO more efficiently than with MgO [81. However, the effect on the optical properties of the rare earth dopants have not been studied yet In this work, the effect of codoping LiNbO with MgO or ZnO, on the 3 optical properties of ~ r ~ + ions is discussed through a comparative study of low temperature polarized absorption and luminescence experiments. (I) Permanent address: CIOp-La Plata, Argentina (2) Permanent address: Inst. of Physics and Tech. of Materials, Bucharest-Magurele, PO. Box MG-7, Romania Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:19944106 JOURNAL DE PHYSIQUE IV Er doped LiNb03, LiNb03:Mg0 and LiNbO :ZnO crystals were grown by the 3 Czrochalski method from grade I Johnson-Mathey powder. The erbium to niobium concentration was around 1% (molar fraction in the melt) in all cases. Magnesium and zinc oxide were incorporated in a ratio of 6% molar fraction in the melt. Polarized absorption spectra were obtained by using a Cary 14 spectrophotometer and a calcite polarizer. Luminescence spectra were obtained under dye laser excitation and a multichannel analyzer detection. Low temperature measurements were carried out with the crystal sample mounted at the cold station of a close-cycle helium cryostat. 3.-RESULTS AND DISCUSSION The polarized absorption spectra of ~r~~ ions in LiNb03 can be explained considering a C symmetry for ~ r ~ + ions. Nevertheless several 3 multiplets exhibit a two-components structure which was initially interpreted as due to multisite occupancy [I]. Although the term "ion-site" has been extensively used in the literature to denote the different optical bands observed in rare earth doped LiNb03, recent Rutherford Back Scattering (RBS) studies suggest that they may arise from slight distortions of a single lattice site, so that we shall adopt the term "optical centre" to label the different bands appearing in the optical spectra [91. Polarized absorption measurements at low temperature show that MgO or ZnO codoping affects the impurity distribution, and favors the appearance of a single optical centre. This effect is observed in the different transitions of EX-^+ ions in the visible and infrared range (350 nm 1700 4 4 nm) and is illustrated in Figure 1 for the 115/2+ S3/2 transit ion of LiNb03(ZnO) : ~ r ~ + . This transition (see inset in Figure 11, exhibits two opposite polarization components 1 + p (n-polarization) and 1 + q 3+ (o-polarization). It is apparent in Figure 1 that Er doped crystals show a doublet structure (Er-1, Er-21, while the absorption bands for ZnO co-doped crystals are substantially narrower showing the appearance of a single band (Er-1). A similar result is obtained for MgO co-doped crystals