STRUCTURAL PHASE TRANSITIONS IN PROTON EXCHANGED LAYERS OF LITHIUM NIOBATE CRYSTALS 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 X-cuts of lithium niobate crystals during annealing. In our research, the following methods were used: polarized light optical microscopy (bright fi eld and dark fi eld), mode spectroscopy (determination of the refractive index profi le), IR-spectroscopy, and XRD (θ/2θ curves). Congruent lithium niobate (Crystal Technology) was used as a material. Proton exchange was carried out in molten benzoic acid at 175 оС for 2 hours. The samples were then annealed at 1-hour intervals at 330 оС. The total duration of annealing was 12 hours. β1 and β2 phases formed during the proton exchange stage, transformed into the κ2-phase within the fi rst hour of annealing. After the fi rst annealing step (1 hour) small particles of the κ1-phase appeared in the matrix of the κ2-phase. The κ2-phase gradually transformed into the κ1-phase within the 2nd and 3rd hour of annealing. κ1-phase particles formed a modulated structure along certain crystallographic directions. The κ1-phase then slowly transformed into α-phase. Residues of the κ1-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 κ1-phase particles can be explained by the signifi cant difference in internal stresses of the κ2 and κ1 phases formed on X-cuts of lithium niobate. During the annealing, relaxation takes place due to the incoherence of the boundary between the κ2- and κ1-phases.

Keywords

• lithium niobate
• X-cut
• proton exchange
• annealing
• phase transformation.