Alice Liu

PHOTOREFRACTIVE PROPERTIES OF TUNGSTEN BRONZE FERROELECTRIC LEAD BARIUM NIOBATE (PB1-XBAXNB2O6) CRYSTALS

Lead barium niobate (Pb1−xBaxNb2O6) is a very promising material for photorefractive applications in that it has a high ferroelectric-to-paraelectric transition temperature (300–500 °C depending on composition) and simultaneously can have large electro-optic coefficients, because of the nearly vertical morphotropic phase boundary (MPB) near 1−x=0.63. Pb1−xBaxNb2O6 crystals have been grown by the vertical Bridgman method for near-MPB compositions (0.5<1−x<0.6), and their photorefractive properties were investigated by the two wave mixing experiments. Material properties such as composition and absorption coefficient varied along the growth direction because of the incongruent charge composition and the nonuniform distribution of impurities. A maximum diffraction efficiency of 50% at λ=514.5 nm was observed in a 3.2 mm thick nominally undoped as-grown crystal. The photorefractive sensitivity increased as the wavelength decreased. The gain coefficient also increased with decreasing wavelength, changing from ...

Observation and thermal fixing of holographic gratings in lead barium niobate crystal

We report what we believe to be the first observation of photorefractive gratings in lead barium niobate (Pb(1-x)Ba (x)Nb(2)O>( 6) ; PBN). A diffraction efficiency of 9% and a beam-coupling coefficient of 9 cm(-1) are obtained. After thermal fixing a grating of 0.5% is revealed at room temperature. We can improve this diffraction efficiency by optimizing fixing temperature, timing, and material parameters.

Resonant two-photon processes for nonvolatile holography in photorefractive crystals under continuous-wave illumination

We investigate the resonant two-photon (two-step) processes of photorefractive grating formation and identify the regimes in which nonvolatile holography is possible. We develop a charge-transport model to describe this behavior for a photorefractive crystal with a single active impurity species under continuous-wave illumination. For the cases that allow for nondestructive reconstruction of gratings, we evaluate the maximum refractive-index perturbation and the response rate with respect to illumination intensities and impurity characteristics. We evaluate the importance of the impurity intermediate-state occupancy. Holographic data storage system issues are also discussed. The present results are consistent with previously reported photorefractive behavior and predict additional properties that characterize these resonant processes. The analysis may be used further to study other related two-photon phenomena of interest in holographic storage systems.

Enhanced photorefractive effects of cerium-doped lead barium niobate crystals

Photorefractive properties of a 0.05-wt. % Ce2O3-doped Pb0.5Ba0.5Nb2O6 crystal are investigated at visible wavelengths. Compared with the performance of the undoped crystal, sensitivities as high as 3 cm/J represent an increase of almost two orders of magnitude, and a photorefractive gain of 25 cm-1 for extraordinary polarization exhibits a fourfold increase. The saturated diffraction efficiency varies with the intensity ratio of the incident beams owing to strong beam coupling. Photoconductivity at 514.5 nm and dark conductivity are also measured. The intensity dependence of these light-induced parameters suggests a nonsingle-photon behavior. We use a two-center model that involves both a shallow level and a deep level to explain the measured results.

Light-induced absorption of cerium-doped lead barium niobate crystals

Tetragonal lead barium niobate (Pb(1-x)Ba(x)Nb(2)O(6);1-x<0.63) crystals are promising photorefractive materials for applications such as holographic data storage. Measurements of light-induced absorption in Ce-doped lead barium niobate (Pb(0.5)Ba(0.5) Nb(2)O(6)) were performed. The induced absorption that was due to the presence of shallow traps was as large as 0.3cm(-1) at 633 nm and exhibited saturation behavior at green-pump intensity greater than 2W/cm(2) . The temporal profile of the dark decay was a double exponential, and the activation energies of the slow- and the fast-decaying components were determined to be 0.16 and 0.3 eV, respectively.

ELECTRO-OPTIC AND PHOTOREFRACTIVE TWO-BEAM COUPLING PROPERTIES OF LEAD BARIUM NIOBATE CRYSTALS

Lead barium niobate crystals (Pb1−xBaxNb2O6) are attractive photorefractive materials for holographic data storage. The electro-optic coefficients r33 and r13 are determined using a Mach–Zehnder interferometer setup, and are measured to be 160±20 and 32±4 pm/V, respectively. The angular dependence of the photorefractive two-beam coupling gain is also investigated, and the products of the electro-optic coefficients and the electron-hole competition factor rζ are determined and compared with the electro-optic coefficients obtained using the interferometric method. It is found that the electron-hole competition factor is at least 0.7, indicating that the photorefractive process is mostly due to electrons. The effective trap densities at several wavelengths are also determined from the gain dispersion measurements, and are found to be 1.3–2.2×1016 cm−3.