ОСОБЕННОСТИ СТРУКТУРНО-ФАЗОВЫХ ПРЕВРАЩЕНИЙ ПРИ ОТЖИГЕ ПРОТОНООБМЕННЫХ СЛОЕВ В КРИСТАЛЛАХ НИОБАТА ЛИТИЯ

Sergey S. Myshinsky; Igor V. Petukhov; Mariya А. Permyakova; Vladimir I. Kichigin; Ljudmila N. Malinina; Anatoliy B. Volyntsev

doi:10.17308/kcmf.2017.19/236

Sergey S. Myshinsky Engineer, Perm State University, ph.: +7(342) 2396789, e-mail: sergey.mushinsky@gmail.com
Igor V. Petukhov Cand. Sci. (Chem.), Associate Professor, Department of Physical Chemistry, Perm State University; ph.: +7(342) 2396789, e-mail: petukhov-309@yandex.ru
Mariya А. Permyakova graduate student, Department of Physical Chemistry, Perm State University; ph.: +7(342) 2396789
Vladimir I. Kichigin Cand. Sci. (Chem.), Senior Researcher, Department of Physical Chemistry, Perm State University; ph.: +7(342) 2396452, e-mail: kichigin@psu.ru
Ljudmila N. Malinina Engineer of Solid State Physics Department, Perm State University; ph.: +7(342) 2396383
Anatoliy B. Volyntsev Dr. Sci. (Phys.-Math.), Professor, Head of Solid State Physics Department, Perm State University; ph.: +7(342) 2396410, e-mail: voland@psu.ru

DOI: https://doi.org/10.17308/kcmf.2017.19/236

Keywords: lithium niobate, X-cut, proton exchange, annealing, phase transformation

Abstract

Lithium niobate crystals are widely used in the manufacturing of integrated optical phase and intensity modulators. The production method is based on the formation of optical waveguides in the surface layers of LiNbO₃ by means of proton exchange. Subsequent annealing of proton-exchanged structures is usually performed in order to obtain stable and low-loss waveguides.

The aim of this paper is to study phase transitions in the surface layer of proton-exchanged waveguides on X-cuts of lithium niobate crystals during annealing. In our research, the following methods were used: polarized light optical microscopy (bright field and dark field), mode spectroscopy (determination of the refractive index profile), IR-spectroscopy, and XRD (q/2q curves). Congruent lithium niobate (Crystal Technology) was used as a material. Proton exchange was carried out in molten benzoic acid at 175 °С for 2 hours. The samples were then annealed at 1-hour intervals at 330 °С. The total duration of annealing was 12 hours.

β1 and β2 phases formed during the proton exchange stage, transformed into the k2-phase within the first hour of annealing. After the first annealing step (1 hour) small particles of the k1-phase appeared in the matrix of the k2-phase. The k2-phase gradually transformed into the k1-phase within the 2nd and 3rd hour of annealing. k1-phase particles formed a modulated structure along certain crystallographic directions. The k1-phase then slowly transformed into a-phase. Residues of the k1-phase were still detected by means of optical microscopy after up to 10 hours of annealing. The structure became fully homogeneous after 12 hours of annealing. The results obtained previously by various methods comply with this phase transition sequence.

The formation of k1-phase particles can be explained by the significant difference in internal stresses of the k2 and k1 phases formed on X-cuts of lithium niobate. During the annealing, relaxation takes place due to the incoherence of the boundary between the k2- and k1-phases.

ACKNOWLEDGEMENTS

The reported study was supported by the Russian Foundation for Basic Research (project No. 17-43-590309 r_a).

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STRUCTURAL PHASE TRANSITIONS IN PROTON EXCHANGED LAYERS OF LITHIUM NIOBATE CRYSTALS DURING ANNEALING

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