Author: Liu, Wenchao
Title: Raman and optical characterization of tungsten bronze niobates thin films and nanocrystals
Degree: Ph.D.
Year: 2009
Subject: Hong Kong Polytechnic University -- Dissertations.
Raman spectroscopy.
Tungsten bronze.
Ferroelectric crystals.
Department: Department of Applied Physics
Pages: xxi, 230 p. : ill. ; 30 cm.
Language: English
Abstract: Lead-free ferroelectric niobates with tetragonal tungsten-bronze (TTB) structure have large spontaneous polarization, significant piezoelectric properties, enhanced photorefractive and nonlinear optic properties. Among various TTB niobates, calcium-modified strontium sodium niobate, Sr2-xCaxNaNb5O15 (SCNN, 0.05 <= x <= 0.35 ) and Sr1-xBaxNb2O6 (SBN, 0.25<x<0.75) are two typical materials. With similar structure, SCNN presents similar electro-optic, pyroelectric and piezoelectric with SBN. By adding Na into the void site, the Curie temperature of SCNN is substantially increased. The planar waveguide structures have been proposed and demonstrated for many applications, including nonlinear optical devices, electro-optic waveguide modulators and passive integrated optical circuits. The fabrication of SCNN waveguides is very attractive because the optical confinement inherent in the waveguide structure allows a significant improvement in the efficiency of the electro-optic and nonlinear optical applications. Optical waveguides can be achieved by depositing the SCNN films on a substrate of lower refractive index such as MgO and fused quartz. We focus on studying the relationship between microstructures and optical waveguide properties (i.e., refractive index, birefringence, transmittance and propagation loss). We have investigated the optical properties of the planar waveguides using the prism coupler technique. The ordinary (n0) and the extraordinary (ne ) refractive indices of the SCNN/MgO films are determined to be 2.278 and 2.183, respectively. The relatively large index difference of 0.095 due to uniaxial birefringence is the resulted of anisotropic crystal microstructure. An optical propagation loss of 0.90 dB/cm for the TE0 mode of the SCNN/MgO waveguide was measured. Thermal and electric field dependence of the refractive indices of SCNN and SBN thin films has been discussed. Raman spectroscopy is a powerful and sensitive technique used to study the crystalline structures based on the vibrational bands. We investigated the temperature induced ferroelectric to paraelectric phase transition of epitaxial SCNN and SBN thin films using thermo-Raman spectroscopy. The temperature dependence of peak position and FWHM of the Raman bands around 238 cm-1 present obvious softening (shift to lower wavenumber) and clear anomalies near the phase transition. Temperature dependence of Brillouin light scattering (BLS) was employed to investigate the phase transition and dynamics of polar nanoregions of the SCNN film on Si substrate. The LA and TA mode frequency shifts change continuously with temperature. A broad increase in the LA and TA shifts are found near 270 oC in the heating process. Such a continuous change is typical for relaxor ferroelectrics. A broad clear central peaks (CPs) was observed, we believe that the CPs is related to polarization fluctuations along the c axis. Magnetoelectric (ME) materials have attracted wide and increasing attention due to their attractive physical properties and potential applications in actuators, transducers, field sensors, and data storage devices. We selected SCNN Sr1.9Ca0.1NaNb5O15 (SCNN) and Co-ferrite CoFe2O4 (CFO) as the ferroelectric and ferrimagnetic components due to a high Curie point for the ferroelectric phase, a high Neel temperature for the ferrimagnetic phase, and large piezoelectric as well as magnetostrictive coefficients. A remarkable dielectric constant dispersion spectrum with Curie temperature of 270 oC was observed. A well-defined ferroelectric hysteresis loop and a ferrimagnetic hysteresis loop were obtained for the same composite ceramic. A maximum ME coupling coefficient aE of 58 mV/cm Oe was obtained under zero dc bias and an ac magnetic field of 10 Oe at 88 kHz.
Access: open access

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