The study of spontaneous domain nucleation in the interelectrode gap of phase modulator based on titanium indiffused waveguides in lithium niobate crystals

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The paper presents the analysis of nucleating kinetics and growing of switched domains in the surface layer of monodomain lithium niobate X-cut crystal in the interelectrode gap of integrated optical phase modulators. The work proposes the morphology model of domains growing along the boundary of surface electrodes in X-cut phase modulators. The mechanism of spontaneous needle-like domain growing as a result of the electric field induced by the pyroelectric effect at temperature changing of the crystal was theoretically substantiated. The Comsol Multiphysics cross-platform was used for the numerical estimation of the pyroelectric field in the interelectrode gap. The needle-like domain structures were studied experimentally at industrial samples of integrated optical phase modulators based on Ti:LiNbO3 waveguides. The experimental research of the form and size of domains was performed with the anisotropic etching method by HF solution and followed by visual analysis. For non-destructive testing, the authors used scanning electron microscopy and piezo-response force microscopy. For the first time, the morphology of needle-like domains occurring in the interelectrode gap of phase modulators based on lithium niobate was experimentally studied. The results showed the theoretical and numerical model of domain growing that involves the pyroelectric nature of the electric field. It was demonstrated that along the electrode boundary, the needle-like domains grow up to 20 μm long at normal conditions and achieve 30 μm after the thermal shock by cooling at ∆T = – 125 °С. The discovered switched domains in the interelectrode gap can affect electro-optical characteristics of integrated optical phase modulators with the lithium niobate base and should be taken into account in the future design of electrode topology and modulator usage.