Dynamics of the proton exchange process in benzoic acid, interacting with the substrate of lithium niobate crystal

Demin V.A., Petukhov M.I., Ponomarev R.S., Topova A.V.

Direct numerical simulation of a process of hydrogen diffusive intrusion into the lithium niobate monocrystal is fulfilled with reference to the manufacturing two-channel system of waveguides. The calculations were carried out according to the technology of waveguides production, taking into account the presence of several stages, which at first include the material saturation with the protons by treating the working surface with the melt of benzoic acid, and then annealing the sample. The contribution of the nonlinear diffusion to the process of the waveguide shaping is analyzed. It is shown that the formation of a stepped waveguide boundary is significantly influenced by the procedure of monocrystal annealing. Heretofore, the annealing stage was not quantitatively investigated. It can be emphasized that the attention has not been paid to the possible role of the annealing on formation of the sharper boundary between the waveguide and its mother substrate. A theoretical model of anisotropic diffusion in a solid material is constructed on the basis of experimental data, which indicate the presence of a transitional surface layer with pronounced regular mesostructural directions in the polished lithium niobate monocrystal. Based on the derived equations, the waveguides shape in a cross-section was simulated numerically for different values of inclination angle of the main axes with respect to the cut lines of the crystal. It is demonstrated that in the region of the waveguide bifurcation, when at the stage of protons intrusion the interaction of diffusive fluxes is possible, the diffusion anisotropy can lead to a breakdown of the waveguides symmetry, which can affect their optical properties.

Volyntsev A.B., Malinina L.N., Kichigin V.I., Permyakova M.A., Myshinsky S.S., Petukhov I.V.

С использованием методов оптической микроскопии в поляризованном свете, модовой спектроскопии, ИК-спектроскопии и рентгеноструктурного анализа подтверждена последовательность фазовых превращений, происходящих при отжиге протонообменных слоев на Z-срезе кристалла ниобата лития: b2-, b1-фазы ® k2-фаза ® k1-фаза ® α-фаза. Установлено, что в процессе отжига в ходе фазовых превращений когерентность слоев, образованных из различных фаз, не нарушается, что обусловлено меньшей величиной деформаций (напряжений) формирующихся протонообменных фаз по сравнению с деформациями (напряжениями) на X-срезе кристалла.  Работа выполнена при финансовой поддержке гранта РФФИ (проект № 17-43-590309 р_а).

Mushinsky S.S., Minkin A.M., Petukhov I.V., Kichigin V.I., Shevtsov D.I., Malinina L.N., Volyntsev A.B., Neradovskiy M.M., Shur V.Y.

The influence of controllable amount of water admixture (up to 0.5 w.%) in benzoic acid on the proton exchange process of X-cut congruent lithium niobate was studied in this paper by means of mode spectroscopy, optical microscopy, optical profilometry, atomic force microscopy, XRD, and chemical etching. It has been determined that addition of 0.5 w.% Н2О to benzoic acid increases proton exchange rate and changes phase composition and number of defects of proton exchanged layer in lithium niobate.

Mushinsky S.S., Kichigin V.I., Petukhov I.V., Minkin A.M., Skachkov A.P., Salgaeva U.O., Malinina L.N., Shevtsov D.I., Volyntsev A.B.

The phase structure of planar and channel, un-annealed and annealed proton exchanged waveguides made in benzoic acid and benzoic acid + lithium benzoate melts on Z cut of congruent lithium niobate crystal was investigated using mode spectroscopy, IR spectroscopy, XRD, SEM, AFM, microindentation, and wet etching techniques. In un-annealed waveguides, three layers (probably β2, β1, and κ2 phases) can be distinguished. At least two different structural layers are present in annealed channel waveguides. Proton exchange (225oC, 28 h) in benzoic acid + 3 mol.% of lithium benzoate leads to formation of κ2 phase and a thin layer of α phase.

Petukhov I.V., Kichigin V.I., Skachkov A.P., Mushinsky S.S., Shevtsov D.I., Volyntsev A.B.

Microindentation of the proton exchange layers on X cut of lithium niobate single crystals was performed. The results were interpreted in the framework of the proportional specimen resistance model. It was found that proton exchange causes increase in the load-independent (“true”) hardness and decrease in the crystal elasticity. The elastic properties of proton exchange layers improve due to annealing while their load-independent hardness decreases. However, in all annealing conditions both hardness and elasticity cannot restore to the corresponding original values of the virgin lithium niobate.

Kichigin V.I., Petukhov I.V., Mushinsky S.S., Oborin V.A., Minkin A.M., Malinina L.N., Shevtsov D.I., Volyntsev A.B.

Methods of chemical etching, non-contact profilometry, X-ray analysis and mode spectroscopy enabled to find out that proton exchange channel waveguides formed on Z cut of lithium niobate crystal in the molten benzoic acid with the addition of 3 mol.% of lithium benzoate at the temperature of 225°C were composed of κ 2 -phase and a thin layer of α-phase. Proton exchange on +Z and -Z faces of the lithium niobate crystal in benzoic acid (with or without the addition of lithium benzoate) occurs at the same rate.

Zhakin A.I.

Даётся обзор основных положений электрогидродинамики (ЭГД). Обсуждается основная система ЭГД-уравнений и граничных условий. Рассматриваются условия, при которых справедливы различные ЭГД-модели, а также усреднённые уравнения при переменных внешних полях. Даются безразмерные критерии, определяющие основные закономерности электрических и ЭГД-процессов. Приводятся экспериментальные данные и соответствующие теоретические расчёты основных ЭГД-явлений: переходных процессов, звуковых волн в электрическом поле, ЭГД-неустойчивостей, ЭГД-течений и ЭГД-теплообмена.

Zhakin A.I.

Дается сравнительный анализ кинетики электронных переходов между электродом и электроноакцептором в электролитах, жидких диэлектриках и плотных газах в области слабых полей. Представлены результаты исследования структуры распределения зарядов в приэлектродной области жидкого диэлектрика (плотного слабопроводящего газа), находящегося во внешнем высоковольтном поле. Показано, что квадратичность вольтамперных характеристик связана с характером электронных переходов из поверхностных состояний. Проведен анализ переходных процессов, приведены вычисленные характерные времена формирования стационарных приэлектродных структур.

Korkishko Y.N., Fedorov V.A., Kostritskii S.M., Alkaev A.N., Maslennikov E.I., Paderin E.M., Apraksin D.V., Laurell F.

We show that proton exchanged LiNbO3 waveguides exhibit very complex structural chemistry. Seven HxLi1-x NbO3 and six HxLi1-xTaO3 crystallographic phases have been identified in PE LiNbO3 and LiTaO3 waveguides, respectively. A correlation is done between the electrooptical, nonlinear and photorefractive properties, the processing conditions and the refractive index changes of the waveguides. Some integrated optical devices have been realized.

Korkishko Y.N., Fedorov V.A.

We show that proton exchanged H x /Li 1-x NbO 3 single-crystalline solid solutions exhibit very complex structural chemistry, which is different from those known for powders. Seven crystallographic phases have been identified in H x /Li 1-x NbO 3 layers. Correlation between the crystal structure and the ordinary and extraordinary refractive indices has been experimentally determined which allows us to explain some of the observed optical phenomena and to predict the characteristics of the great variety of proton exchanged waveguides.

Pontiga F., Castellanos A.

The linear stability of a plane layer of dielectric liquid subjected to an electric field and a thermal gradient is studied. The liquid is supposed to have a non-negligible residual conductivity and to exhibit an injection of charge from one of the electrodes. Both effects are due to the dissolution of a given salt in the liquid. The ionic mobility and the dielectric constant are temperature dependent. The stability of this configuration is studied with the help of an heuristic model and a precise discussion of the physical mechanisms of instability is made. Complete sets of the stability maps corresponding to different cases are presented.  

Vohra S.T., Mickelson A.R., Asher S.E.

A discussion of the modeling of the proton exchange fabrication process is given, including a brief review of results of analytical measurements of concentration and concentration/index relation, as well as some presentation of the speculations made as to mechanisms of resulting index profiles. Discussion is then given to modeling of the exchange process itself via the ion exchange equations. The concentration profiles of protons and lithium in proton-exchanged LiNbO3 crystals measured by secondary ion mass spectroscopy are then presented. The proton concentration profiles are found to be nearly rectangular in shape. The diffusion characteristics of the ion exchange process are empirically modeled by solving the ion exchange equations with concentration dependent self-diffusion coefficients. Secondary ion mass spectroscopy (SIMS) measurements on annealed H+:LiNbO3 samples show proton and lithium concentration profiles to be Gaussian in nature. The proton and lithium concentration profiles of annealed H+:LiNbO3 samples are fit to the solutions of diffusion equations obtained from standard thermal diffusion theory. The fit has resulted in values of diffusion coefficient for protons during annealing (Dp=0.77 μm2/h). Index profiles are then calculated from these equations and are found to agree with the profiles reported elsewhere. Optical mode measurements on channel waveguides fabricated under various diffusion conditions and annealed for various times, are carried out. The mode-tailoring ability of the proton exchange process is demonstrated by fabricating low-loss single-mode channel guides (λ=0.83 μm) having channel widths as large as 6 and as small as 2.5 μm, simply by changing the diffusion conditions. Some anomalous results obtained in waveguides fabricated under extreme conditions are discussed.

Goto N., Yip G.L.

Low-loss optical waveguides in Z-cut LiNbO3 made by proton-exchange with pyrophosphoric acid are investigated. The index increase of 0.131 can be reduced by thermal annealing, which provides a low mismatch of optical fields between a Ti-diffused waveguide and a proton-exchange one. Fabrication conditions of single-mode waveguides are presented. Single-mode slab waveguides without and with annealing show low propagation losses at 0.25 and 0.15 dB/cm with an effective index of 2.2932 and 2.2068, respectively. These waveguides are designed to be just below the cutoff of the next higher-order mode. The diffusion coefficient resulting from both proton-exchange and annealing is also discussed. The fabrication conditions and the prescribed parameters give useful information for designing and fabricating waveguides by proton-exchange and annealing.

Rice C.E.

The properties of Li 1− x H x NbO 3 have been studied as a function of x , temperature, and stoichiometry of the LiNbO 3 used for its preparation. X-ray diffraction, thermal analysis measurements, and infrared spectroscopy have been used. The intent of this study was to gain a deeper understanding of the basic properties of this material, which has potential importance as a waveguide material in LiNbO 3 optical devices. An approximate phase diagram was constructed for the stoichiometric LiNbO 3 HNbO 3 system. In one concentration range (0.56 ≤ x ≤ 0.75), particularly complex structural behavior was found: depending on x , samples undergo one, two, or three phase transitions with temperature, and the system appears to exhibit critical behavior. Samples made by proton exchange of congruent LiNbO 3 ([Li 2 O] 0.486 [Nb 2 O 5 ] 0.514 ) show generally similar structural chemistry, with one exception: a new monoclinic phase, isomorphous with MnF 3 , was found for 0.75 ≤ x ≤ 0.77. Possible reasons for the refractive index changes caused by proton exchange are discussed.

De Micheli M., Papuchon M., Botineau J., Neveu S., Sibillot P., Ostrowsky D.B.

We demonstrate the possibility of controlling, practically independently, the form and indices of proton-exchanged lithium niobate guides by means of guide annealing and proton exchange in lithium-rich solutions. Experimental results are presented that indicate how one can realize specific guide designs.