Vapor transport equilibration was used to prepare lithium niobate crystals of a variety of controlled off‐congruent compositions. Crystals were characterized through measurement of the ferroelectric Curie temperature Tc and measurement of the temperature for noncritical phase matching TPM of second‐harmonic generation from both 1.064‐ and 1.32‐μm laser sources. Across the majority of the single‐phase region, both Tc and TPM were observed to vary nearly linearly with Li/Nb ratio. The variation in TPM with Li/Nb ratio was observed to change direction within the single‐phase region at some small but finite compositional interval away from the Li‐rich phase boundary. Crystals equilibrated to the Li‐rich phase boundary had excellent optical homogeneity. Preparation of Li‐poor crystals was hampered by extremely slow lithium diffusivity and problems with second‐phase precipitation.

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Preparation and characterization of off-congruent lithium niobate crystals

The state of the art of optical waveguide fabrication in lithium niobate is reviewed, with particular emphasis on new technologies and recent applications. The attention is mainly devoted to recently developed fabrication methods, such as femtosecond laser writing, ion implantation, and smart cut waveguides as well as to the realization of waveguides with tailored functionalities, such as photorefractive or domain engineered structures. More exotic systems, such as reconfigurable and photorefractive soliton waveguides, are also considered. Classical techniques, such as Ti in-diffusion and proton exchange, are cited and briefly reviewed as a reference standpoint to highlight the recent developments. In all cases, the application-oriented point of view is preferred, in order to provide the reader with an up-to date panorama of the vast possibilities offered by lithium niobate to integrated photonics.

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Optical waveguides in lithium niobate: Recent developments and applications

Z -cut X -propagating, LiNbO 3 :Ti planar waveguides have been characterized over a wide range of diffusion conditions. Measurements of Ti concentration and refractive indexes have revealed Gaussian profiles. Their dependence with diffusion conditions have been studied. The relation, established at several wavelengths ( \lambda = 0.6328 \mu m, 1.1523 μm, 1.523 μm), between extraordinary and ordinary refractive index change \Deltan_{e,0}(z) and titanium concentration C(z) is found to be nonlinear and of the form: \Deltan_{e,0}(z) = A_{e,0}(C_{0},\lambda) (C(z))^{\alpha_{e,0}} While \alpha_{e,0} is a constant, the proportionality coefficient A_{e,0} depends not only on the wavelength λ, but also on the diffusion conditions, characterized by C 0 , the Ti surface concentration of the diffused guide. Such a nonlinearity of Ti diffusion in LiNbO 3 has been revealed thanks to a complete and wide range investigation on both indiffused Ti and index profiles.

Wavelength dispersion of Ti induced refractive index change in LiNbO 3 as a function of diffusion parameters

The congruent composition of LiNbO/sub 3/ was determined precisely by correlating crystal growth compositions with the Curie temperatures of samples quenched from a melt before and after crystal growth and of various sections of crystals grown from that melt. The initial melt composition was determined by control of Li/sub 2/CO/sub 3/ and Nb/sub 2/O/sub 5/ contents. The melt compositions were varied from 47 to 49 mol% Li/sub 2/O. The variation of T /SUB c/ with melt composition was found to follow T /SUB c/ = 9095.2 - 369.05C + 4.228C/sup 2/, where C is mol% Li/sub 2/O. High-temperature DTA was used to determine T /SUB c/ with a precision of + or - 2/sup 0/C. Above 1000/sup 0/C the Li-rich phase boundary was found to be a function of temperature by determining the weight gain after complete lithiation of congruently grown crystals. This boundary curves slightly toward lower Li/sub 2/O content as temperature increases. The congruent composition of LiNbO/sub 3/ contains 48.45 mol% Li/sub 2/O and has a measured Curie temperature of 1138/sup 0/ + or - 2/sup 0/C.

Congruent Composition and Li-Rich Phase Boundary of LiNbO3

Homogeneous, crack-free, thin films of crystalline LiNbO3 were synthesized above 250°C on Si(100) substrates by the dip-coating method using a double alkoxide solution. The coating solution, which was prepared by the controlled partial hydrolysis of the double alkoxide, gave stoichiometric LiNbO3 crystalline films at temperatures as low as 250°C. The concentration of the alkoxide solution influenced both of thickness and quality of films. Crystallinity of thin film top-coated directly on the substrate affected the crystallization state of films coated on the film remarkably. Films crystallized on α-Al2O3(0001) showed preferred orientation along the c-axis, while the preferred orientation could not be observed on Si(100) substrates.

Formation of LiNbO3 films by hydrolysis of metal alkoxides

The diffusivity of Ti into LiNbO3 has been measured as a function of crystal orientation and Li/Nb ratio in the temperature range 980–1112 °C. The Li/Nb ratios of single crystal samples were adjusted by vapor phase equilibration with fixed Li2O activities. No anisotropy of diffusivity with crystal orientation could be detected. The diffusion constants for Ti in LiNbO3 decrease with increasing Li2O content. At 1050 °C, the values decrease from 2.60×10−12 cm2/s at the Li2O‐deficient phase boundary (48.1 mol % Li2O) to 1.06×10−12 cm2/s at the congruently melting composition at (48.6 mol  % Li2O) to 0.4×10−12 cm2/s at the Li2O‐rich phase boundary (50.0 mol % Li2O). It is suggested that enhanced lateral diffusion observed for stripe geometry Ti sources results from increased diffusivity in a surface layer formed by Li2O outdiffusion.

D. M. Smyth

Papers

Titanium diffusion into LiNbO3 as a function of stoichiometry