Annealed proton-exchange waveguides in mixed lithium niobate-tantalate solid solutions: defect structure and optical losses

New optical materials are one of the drivers of the development of integrated and nonlinear photonics. In this paper, we study the prospects for using mixed crystals of X- and Z-cut lithium niobate-tantalate (LNT) solid solution for the manufacturing of optical waveguides by the annealed proton exchange (APE) method. We have evaluated the defectiveness of the LNT samples by means of scanning electron microscopy visualization and optical profilometry after wet chemical etching and by X-ray diffraction analysis. The studies showed that the LNT structure has pores with depth up to 60 nm and surface density of 2 pcs/mm -2 . Also, it was established that LNT crystals possess higher density of etch pits or dislocations regardless of the crystallographic orientation as compared to conventional lithium niobate. We determined the geometric parameters of single-mode channel APE waveguides by OptiBPM simulation. We experimentally measured the optical losses of channel APE waveguides by fiber-to-fiber method. The optical losses were 6.2 dB (Z-cut) and 19.0 dB (X-cut), that is two times higher than those for APE LN under the same conditions. This effect was related to surface and bulk defects of LNT crystals. The study highlights the relationship between structural and optical characteristics of APE LNT waveguides and contributes to the further study and development of mixed LNT crystals as a material platform for photonics and optics.