|Title:||Barium Strontium Titanate ferroelectric tunable photonic and phononic crystals|
|Subject:||Hong Kong Polytechnic University -- Dissertations.|
Ferroelectric thin films.
|Department:||Department of Applied Physics|
|Pages:||xxvi, 175 leaves : ill. (some col.) ; 30 cm.|
|Abstract:||This thesis presents the results of theoretical simulations and experimental investigations on developing electro-optically tunable photonic crystals and thermally tunable phononic crystals based on the ferroelectric materials, Barium Strontium Titanate (Ba0.7Sr0.3TiO3, BST). One-dimensional photonic crystal (PC) consists of five periods, with each period consisting of a 90 nm thick Ba0.7Sr0.3TiO3 layer and a 10 nm thick MgO layer, was fabricated using pulsed laser deposition. X-ray diffraction study confirmed the epitaxial growth of the Ba0.7Sr0.3TiO3 layers with good crystalline quality. A photonic bandgap (transmission dip) with a centre wavelength at -464 nm has been observed in the transmission measurement which is consistent with simulation using the plane wave expansion (PWE) method and the transfer matrix method. A 2-nm shift towards the longer wavelength is observed when a dc voltage of 240 V (corresponding to an electric field of about 12 MV/m) has been applied across the coplanar electrodes on the film surface. The experimental result suggests that the electric field induced change in the refractive index of Ba0.7Sr0.3TiO3 is about 0.5 %. Photonic bandstructures and photonic bandgap maps of two-dimensional (2D) Ba0.7Sr0.3TiO3-based photonic crystals with different cavity geometries (square or circular air rods) in square lattice were calculated using the PWE method. Simulation results suggested comparable bandstructures and bandgap maps for square or circular air rod photonic crystals, if (1) the dimension of the air rod is small compared to the electromagnetic wavelengths inside the PC being considered, or (2) the frequencies of the electromagnetic waves are less than Q.35(2nc/a). The photonic bandgap maps of two types of 2D Ba0.7Sr0.3TiO3-based PC, namely, the air-hole-in-BST type and the BST-rod-in-air type both in square lattice and in triangular lattice were calculated. It is found that PCs in triangular lattice contain richer bandgap feature in general. Bandgap features along different symmetry directions have also been compared. The refractive indices of the Ba0.7Sr0.3TiO3 thin film on a MgO (001) substrate was measured using the prism coupling technique. The appropriate geometry of a single-mode rib waveguide based on Ba0.7Sr0.3TiO3 thin film was determined by applying the effective index method. A photonic crystal cavity embedded Ba0.7Sr0.3TiO3 rib waveguide which functions as a tunable filter (on-off switch) for A, = 1550 nm was designed with the help of the finite-difference time-domain (FDTD) simulation. The required PC cavity is composed of two 5-row-4 PC mirrors, which is formed by air holes arranged in triangular lattice in the Ba0.7Sr0.3TiO3 matrix, with cavity length of 800 nm. The radius of the air holes is 250 nm and the periodicity is 625 nm. A 6-nm shift in the resonant peak for a 0.5% change in the refractive index of the Ba0.7Sr0.3TiO3 was illustrated in the simulation. Photonic crystal cavities were fabricated on a rib waveguide by focused ion beam etching with satisfactory results. The shear and transverse wave velocities of BaojSro.sTiOs ceramic as a function of temperature were determined using the ultrasonic through-transmission technique. A drastic variation in the wave velocities was observed across the Curie temperature of BaojSrojTiOs. Phononic crystal composed of Ba0.7Sr0.3TiO3 square rods in a matrix of epoxy were fabricated using the dice-and-fill method. The width of the Ba0.7Sr0.3TiO3 rods is 200 um with periodicity of 265 um. The temperature dependence of the phononic bandgaps of the phononic crystal was characterized by the reflection spectra which were obtained using the ultrasonic pulse-echo technique. Thermal tuning of the phononic bandgap was observed and the results were in good agreement with the phononic bandstructure calculation by the plane wave expansion method.|
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