PECULIARITIES OF STRUCTURAL PHASE TRANSFORMATIONS IN PROTON EXCHANGED LAYERS OF Z-CUT LITHIUM NIOBATE CRYSTAL DURING ANNEALING

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 Z-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 (Sipat) was used as a material. Proton exchange was carried out in molten benzoic acid at 190 °С for 2 hours. The samples were then annealed at 1-hour intervals at 330 °С. The total duration of annealing was 18 hours.

The sequence of the phase transformations at annealing proton-exchange layers on Z-cut lithium niobate crystals is similar to that on X cut: b₁, b₂-phase ® k₂-phase ® k₁-phase ® α-phase. The difference is that k₁-phase particles are not formed during annealing and the coherence of the layers of different phases is not disturbed. This results from a lesser magnitude of strains in proton-exchange phases on Z cut LiNbO₃ as compared to X cut.

ACKNOWLEDGMENTS

This work was supported by Russian Foundation for Basic Research (project No. 17-43-590309 р_а).