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The thermal diffusion of Er3+ into X- and Z-cut congruent LiNbO3 crystal in Li-enriched atmosphere [i.e., vapor transport equilibration (VTE)], created by Li3NbO4–LiNbO3 two-phase powder at the temperature around 1130 °C, was attempted. Single-crystal x-ray diffraction, micro-Raman, photoluminescence spectroscopy, and secondary ion mass spectrometry (SIMS) were used to study the crystalline phase with respect to Er3+ ion and the Er3+ diffusivity. The results show that the thickness of the Er film coated should not be thicker than 10 nm for an X-cut plate and 15 nm for a Z-cut plate. In this case, the diffusion is complete if the duration is long enough (>150 h) and the Er3+ ions in the diffused layer still retain the LiNbO3 phase. On the other hand, if the initial thickness of the Er metal film is thicker than 10 nm for the X-cut plate and 15 nm for the Z-cut plate, the diffusion will be incomplete no matter how long the duration is. This is because the residual Er3+ ions form irremovable ErNbO4 grains on the surface of the crystal. SIMS analysis on an X-cut VTE (1130 °C/192 h) and a Z-cut VTE (1129 °C/158 h) crystal coated, respectively, with 10 and 15 nm thick Er film reveals that the Er diffusion shows obvious anisotropy with the mean diffusion coefficients of 0.0155 and 0.0957 μm2/h, respectively. The surface concentrations are 1.5×1020 and 1.0×1020 at./cm3, respectively. The diffused Er3+ ions follow the stretched-exponential decay profile with a stretching factor of 1.85 and 3.5, respectively. The Li/Nb ratio in the Er-diffused layer is similar to 99.4% for the X-cut sample coated with 10 nm thick Er film and 99.3% for the Z-cut crystal coated with 15 nm thick Er film. The rms roughness of the diffused surface is better than 6 and 4 nm for the X-cut and Z-cut samples, respectively. [ABSTRACT FROM AUTHOR]

Multi-mode Ti:LiNbO 3 planar waveguide was fabricated by in-diffusion of 100-nm-thick Ti-metal film coated onto a near-stoichiometric LiNbO 3 substrate prepared by vapor transport equilibration. The crystalline phase and composition in the waveguide were characterized. The guided modes in the planar waveguide were characterized by prism coupling technique. The refractive index profile is constructed from the measured mode indices using the inverse Wentzel–Krames–Brillouin method, and correlated with the Ti 4+ - concentration profile, which was obtained from secondary-ion-mass-spectroscopy analysis. The results show that the waveguide still retains the LiNbO 3 phase, its composition is near-stoichiometric. The index change and Ti 4+ -concentration follow an exponential relationship with a power index 0.4 for an ordinary ray and 1.07 for an extraordinary ray. A comparison of various congruent and near-stoichiometric bulk materials or waveguides allows to conclude that the relationship is different from congruent to near-stoichiometric composition, from one fabrication method to another, and from bulk material to waveguide configuration. The difference is explained qualitatively. [ABSTRACT FROM AUTHOR]

Diffusion properties of Sc3+ in LiNbO3 crystal were studied, together with other two related issues: Sc3+-doping effect on LiNbO3 refractive index and Li2 O out-diffusion arising from Sc3+ in-diffusion. To reach the goal, some Sc3+-doped LiNbO3 crystal plates were prepared by in-diffusion of Sc2 O3 film coated onto Z-cut congruent substrates in air at different temperatures ranging from 1030°C to 1130°C. After diffusion, the refractive indices at the doped and undoped parts of surface were measured by prism coupling technique and the surface composition was evaluated from the measured index. The results show that the Sc3+ doping has little contribution to the substrate index and the Li2 O out-diffusion is not measurable. The Sc3+ profile was analyzed by secondary ion mass spectrometry. Some characteristic parameters such as temperature-dependent diffusivity, diffusion constant, activation energy, and surface concentration are obtained. The Sc3+/ Ti4+ co-diffusion characteristics were also studied. Comparison shows that in the single-diffusion case, the Sc3+ diffusivity is considerably smaller than the Ti4+ diffusivity. In the Sc3+/ Ti4+ co-diffusion case, the Ti4+ co-diffusion results in substantial increase in Sc3+ diffusivity while the Ti4+ diffusivity changing little. The diffusion characteristics are qualitatively explained. The Ti4+-assisted Sc3+ diffusion would be utilizable in shortening the fabrication period of an optical-damage-resistant Ti-diffused LiNbO3 waveguide doped with Sc3+, and some considerations for the fabrication are given. [ABSTRACT FROM AUTHOR]

Co-diffusion characteristics of Er3+ and Tm3+ ions in LiNbO3 crystal have been studied, together with two other properties: doping effect on refractive index of substrate, and Li out-diffusion. Er3+/ Tm3+-codoped LiNbO3 single-crystals were prepared by co-diffusion of stacked Er- and Tm-metal thin films, coated onto parts of the surfaces of X- and Z-cut congruent LiNbO3 crystal plates, at different temperatures and different durations in air. After diffusion, the surface refractive indices of the doped and undoped parts were measured by prism coupling technique, and the surface composition was evaluated from the measured indices. The results show that the dopings have negligible effect on the substrate index and the Li out-diffusion depends mainly on the diffusion temperature. The Er3+ and Tm3+ profiles were studied using secondary ion mass spectrometry, and a co-diffusion dynamical model is proposed and verified experimentally. From the measured profiles, the temperature-dependent Er3+/ Tm3+ diffusivity and solubility were obtained together with other relevant parameters, such as diffusion constant, activation energy, solubility constant and heat of solution. The Er3+ and Tm3+ co-diffusion exhibits similar temperature dependent diffusivity, and the dependence is similar to that of single-diffusion. In the co-diffusion case, the total solubility consists of two parts: Er3+ and Tm3+ parts, there is no anisotropy, and the value is equal to that of single-diffusion case, in which both ions have the similar solubility. The Er3+ and Tm3+ parts of solubility change relatively with the initial metal film thicknesses and hence have different temperature dependences. Nevertheless, their sum, i.e., the co-diffusion solubility, remains a constant at a given temperature. [ABSTRACT FROM AUTHOR]

Abstract: A thermodynamic study on Li-poor vapor transport equilibration (VTE) in MgO-doped LiNbO3 crystal was carried out. A thorough thermodynamic VTE model is established that considers all of possible factors including the depth relative to crystal surface, VTE temperature and time. To solve the model, the composition on crystal surface was studied as a function of VTE temperature and time. To achieve that, a number of Li-deficient MgO(5 mol% in melt):LiNbO3 crystals were produced by the VTE under different temperatures from 1010 to 1130 °C for different durations up to 395 h, the crystalline phase contained was determined by X-ray powder diffraction and their surface compositions were determined from the measured birefringence. The results show that the Li-deficient crystal keeps the LiNbO3 phase. The surface Li2O-content has an Arrhenius relationship to the VTE temperature and a square-root dependence on the VTE duration. Based upon the VTE temperature and time dependences of surface Li2O-content, the solution to the thermodynamic VTE is obtained and verified experimentally. The solution is crucial to design and produce a Li-deficient MgO:LiNbO3 crystal with desired Li2O-content profile. By using the solution, one can predict the Li2O-content depth profile after a certain VTE time for a given VTE temperature. [Copyright &y& Elsevier]

Abstract: Secondary ion mass spectrometry (SIMS) was used to study the diffusion properties of Mg2+ and Ti4+ ions in a peculiar optical-damage-resistant near-stoichiometric (NS) Ti:Mg:LiNbO3 waveguide structure that a number of strip waveguides are embedded in a planar waveguide. The structure was fabricated on an initially congruent, undoped Z-cut substrate with a technological process in sequence of planar waveguide, strip waveguide, MgO diffusion and post Li-rich vapor transport equilibration (VTE) treatment. The results show that the Mg2+-profiles in both strip and planar waveguide layers are desirably homogeneous with a concentration 1.7±0.3mol% above the optical damage concentration threshold. The Ti4+-profile in the strip waveguide follows a sum of two error functions along the lateral direction with a width 13±1.0μm and a sum of two Gaussian functions in the depth direction with a 1/e depth of ∼11μm all. The Ti4+ ions in the planar waveguide obey a Gaussian profile. From the measured profiles, characteristic diffusion parameters such as mean Mg2+ and Ti4+ diffusivities and surface Ti4+-concentration are evaluated. The diffusivity is discussed in comparison with the previously reported results on single Mg2+, Ti4+ diffusion and Mg2+/Ti4+ co-diffusion in pure (or bulk MgO-doped), congruent (or NS) LiNbO3 crystal. Finally, the keys to the succeeding in fabrication are highlighted. [Copyright &y& Elsevier]

Abstract Gallium-diffused lithium niobate (LiNbO 3) channel optical thin-film was fabricated by diffusion of photolithographically patterned gallium-oxide stripes on a Z-cut congruent LiNbO 3 substrate. Composition, optical properties of the channel thin-film and gallium diffusion profiles were characterized. The results show that the thin-film is in congruent composition environment, confines single-mode ordinary and extraordinary waves with a loss <0.5 dB/cm at 1.5 μm wavelength. The confined mode field follows Gaussian/Hermite-Gaussian profile in the width/depth direction of the channel thin-film, and shows definite anisotropy. The diffused gallium ions follow a sum of two error functions in the width and a Gaussian function in the depth direction of the channel optical thin-film. Two-dimensional gallium concentration and refractive index profiles in the channel thin-film are modeled. [ABSTRACT FROM AUTHOR]