Microstructural and optical properties of potassium niobate thin films

A potassium niobate thin film waveguide is an ideal candidate for producing a compact blue laser source by second harmonic generation. However, good epitaxial quality films are difficult to produce and high optical losses are a continuing problem. A report is presented in this paper on the investigations of the microstructural and optical properties of KNbO₃ thin films to better understand the origin of optical waveguide losses. Epitaxial, dense KNbO₃ thin films have been grown on MgO, MgAl₂O₄, and KTaO₃ substrates by ion-beam sputter deposition. X-ray diffraction, rocking curves, Rutherford backscattering spectroscopy, ion-channelling, field emission scanning electron microscopy, and atomic force microscopy were used to analyze the orientation, epitaxial quality, grain size, and surface roughness of the films. Optical properties including refractive index and optical scattering losses have been characterized by prism-coupling and an optical fiber loss measurement method. The dominant loss mechanism in these film waveguides is discussed. Green light by second harmonic generation has been produced in the transverse and waveguide modes in KNbO₃ films