Dielectric and electrooptic properties of PMN-PT single crystals and thin films

Abstract

Dielectric and electro-optical properties of ferroelectric lead magnesium niobate-lead titanate Pb(Mg1/3Nb2/3)O3-PbTiO3 (or PMN-PT) single crystals and thin films were investigated to assess their potential for optical waveguide applications. Waveguide properties and optical loss of PMN-PT thin films on MgO substrates were examined at a wavelength of 1550 nm. Highly epitaxial PMN-PT thin films were deposited on MgO substrates using pulsed laser deposition. With the structural characterization of the films studied by X-ray diffraction, optimized deposition conditions to obtain high quality thin films in terms of substrate temperature, oxygen pressure, and film thickness were obtained. The optic axes of the thin films on various substrates related to their unit cell structures were determined to be all along the longer c-axis preferred orientation perpendicular to the film surface. This result was confirmed by the prism coupling method in which the natural birefringences for the polarized TE and TM modes were measured. The surface roughnesses of the PMN-PT thin films relating to the microstructural contributions to optical loss by scattering were examined with atomic force microscopy. The surface roughness was dependent on the film thickness and oxygen pressure provided during the film deposition. The film with a thickness for single mode coupling (- 420 nm for 1550 nm wavelength) had a root mean square (RMS) surface roughness of 4.6 nm, which was sufficiently small for waveguides with relatively low optical losses. Refractive indices, thickness, anisotropy and in-depth index profile of the PMN-PT thin films on MgO substrates were analyzed using the prism coupling method. Based on a mode dispersion modelling, the appropriate thickness for single mode propagation was determined. It was found that no = 2.589 and ne= 2.586 for a calculated thickness of about 422 nm. Real step-index waveguide structure was observed indicating that the films had grown homogeneously in the direction perpendicular to the substrate. With the value of no along the in-plane direction larger than that of the ne normal to the substrate, optical anisotropies of the films on various substrates were observed. Dielectric properties of the PMN-PT films with co-planar electrodes were investigated as a function of frequency. It was found that the relative permittivity depended on the film thickness. A dielectric relaxation was observed at around 4 GHz. Ferroelectric and relaxor behaviours were characterized as functions of the d.c. bias and temperature response of the films. Dielectric tunability of the film was determined as a function of frequency. Electro-optical properties of PMN-PT single crystals and thin films on MgO substrate were determined. Large linear electro-optical coefficient was observed for single crystals close to the morphotropic phase boundary (MPB) whereas PMN-PT films exhibited quadratic electro-optical properties only. Variation of the refractive index of PMN-PT thin films as a function of temperature at 弇=632.8 nm and 1550 nm was characterized. Thermo-optical properties of the film with and without corona poling were investigated. It was found that the thermo-optical coefficient is large compared with that of polymer commonly used for the thermo-optical device applications. PMN-PT ridge waveguide devices were fabricated using a process of photolithography. Geometries for the waveguide in single mode propagation at a wavelength of 1550 nm were optimized with effective index method. By using an end-firing method, the waveguide properties of the films were characterized and the propagation loss was determined to be 3.01 dB/cm which lies within the acceptable attenuation range (< 5dB/cm) suitable for optical guided wave applications.