Potassium-tantalate-niobate mixed crystal thin films for applications in nonlinear integrated optics

摘要

Summary form only given. Perovskite potassium tantalate-niobate mixed crystals (KTa_1-xNb_xO₃ with 0 ≤ x ≤1, KTN) undergo a phase transition from a paraelectric cubic to a ferroelectric tetragonal structure with decreasing temperature [1]. By adjusting the Ta/Nb content one can tune the phase-transition temperature of KTN and thus also its main properties at a given temperature. For example, the phase transition occurs around room temperature for x = 0.4, accompanied by changes in the dielectric and electro-optic (EO) properties [2]. This extremely promising material is of great interest because of its large EO effect and excellent nonlinear optical performance [2, 3]. However, due to the fact that the crystals grown have compositions being different from those of the molten ingredients, high-quality and homogeneous single-crystalline KTN is difficult to produce, which is limiting the application of this material [4, 5].In this contribution we report about the fabrication of smooth KTa0.5Nb0.5O3 thin films on single-crystal MgO(001) substrates employing pulsed laser deposition (PLD). The composition x = 0.5 is chosen because its corresponding Curie temperature (Tc | 100 °C) ensures the crystalline structure of KTN stays in the ferroelectric phase at room temperature. By optimizing the deposition conditions, such as laser parameters, ambient pressure, substrate temperature etc., as well as by installing a vane velocity filter to remove unwanted droplets formed during the ablation process [6], the as-deposited KTN films are highly oriented with only 00l peaks as revealed by X-ray diffraction analysis (as shown in Fig. 1a) and exhibit in-plane orientation, indicating epitaxial growth. To further smoothen the film and thus improve its optical properties, an in situ thermal post-annealing process and a subsequent optical polishing operation are conducted, resulting in an RMS surface roughness of | 1 nm, which is the lowest reported value so far for PLD-grown KTN thin films. Figure 1b illustrates the surface topographies of KTN films measured by atomic force microscopy. The waveguiding properties and refractive indices no and ne of KTN films are studied by using prism coupling method. The data presented here suggests promising qualities of PLD-grown KTa0.5Nb0.5O3 films for use in integrated-optical applications.