Vera Marinova

Light-induced properties of Ru-doped Bi12TiO20 crystals

Absorption, light-induced absorption, photoconductivity and dark conductivity measurements are performed on Bi12TiO20 (BTO) crystals doped with different concentrations of ruthenium. It is found that a higher ruthenium addition shifts the optical absorption to the near-IR spectral region in comparison with non-doped BTO crystals and the absorption changes increase with increasing Ru content. The dark conductivity follows the Arrhenius law with an activation energy of 0.89 eV. From the measured light-induced absorption and nonlinear photoconductivity the participation of a shallow level in the charge transport mechanism in Ru-doped BTO crystals is suggested. Highly Ru-doped BTO samples possess infrared sensitivity and holographic gratings are successfully recorded at 790 and 825 nm, respectively.

Effect of ruthenium doping on the optical and photorefractive properties of Bi12TiO20 single crystals

Bi12TiO20 (BTO) single crystals nominally pure and doped with ruthenium are grown by top-seeded solution growth method. The effect of ruthenium concentration on optical and photorefractive properties is studied. Strong influence of doping on these properties is observed. It is shown that optical transmission of crystal samples with higher ruthenium content is shifted to the near IR spectral region, while the absorption coefficient is considerably increased. The optical activity of Ru doped crystals is further reduced in comparison with undoped BTO. Photochromic effect is observed in strongly doped crystal. Photorefractive properties are experimentally investigated using two-beam coupling. Response time and diffraction efficiency are changed with ruthenium content. � 2002 Elsevier Science B.V. All rights reserved.

Transformation processes in relaxor ferroelectric PbSc0.5Ta 0.5O3 heavily doped with Nb and Sn

Abstract The effect of a third type of B-site cation on the temperature- and pressure-induced structural changes in ABO3-type relaxors has been studied by in-situ Raman scattering and X-ray diffraction on 0.72PbSc0.5Ta0.5O3—0.28PbSc0.5Nb0.5O3 (PSTN) and 0.78PbSc0.5Ta0.5O3—0.22PbSnO3 (PSTS). The incorporation of Nb into the PST matrix is an isovalent substitution for Ta5+ on the ferroelectrically active B-site, whereas the incorporation of Sn4+ is a coupled aliovalent substitution for both types of B-cations in the host system. Both PSTN and PSTS exhibit the characteristic temperature T*, which for PSTS is shifted to slightly lower temperatures. The incorporation of Nb and Sn into the PST matrix smears the transformation processes near and below T* and suppresses the ferroelectric long-range order at low temperatures. Both PSTN and PSTS exhibit a continuous pressure-induced phase transition. The high-pressure phases have the same structural features as for pure PST: suppressed B-cation off-centre shifts, enhancement of coupled Pb—O ferroic species, and long-range order of anti-phase tilts of the BO6 octahedra. Nb-doping shifts the critical pressure pc from 1.9 GPa for pure PST to 2.5 GPa, whereas Sn-doping lowers pc to 1.3 GPa. The incorporation of both Nb and Sn decreases the degree of the overall pressure-induced structural distortion, with the effect of Sn being more pronounced than Nb. The dilution of the B-site cation system with Sn4+ results in local electric and elastic fields which reduce the coherence between nanoregions with anti-phase octahedral tilting and shifts the pressure at which long-range order of the tilts occurs to ∼4 GPa.

Real-time holography in ruthenium-doped bismuth sillenite crystals at 1064 nm.

Real-time holographic recording and an improvement of the response time in ruthenium (Ru)-doped Bi(12)SiO(20) (BSO) crystal at 1064 nm is obtained. Using green light pre-exposure, a significant operation speed of 60 ms is achieved. In addition, the ability for image reconstruction is demonstrated in Ru-doped BSO, supporting further applications as reversible media for real-time image processing at the near-IR spectral range.

Near-infrared properties of Rh-doped Bi12TiO20 crystals for photonic applications.

The effect of Rh doping in Bi(12)TiO(20) (BTO) crystals on the photosensitivity and recording speed at 1064 nm is reported. Response time of 0.1 s is measured during real-time holographic recording without any preliminary treatments. Once the crystal is pre-excited with a green light, the detected response time becomes 0.02 s. A possibility to implement BTO:Rh crystal plate with liquid crystals into a hybrid organic/inorganic device is demonstrated, which opens perspectives for further near-infrared applications.

DC and AC Conductivity Measurements of Bi12TiO20Photorefractive Crystals Doped with Ag, P, Cu and Cd

Bi12TiO20 single crystals nominally pure and doped with silver, phosphor, copper and cadmium are grown using the top seeded solution growth method (TSSG). The DC conductivity followed the Arrhenius law, with activation energies 0.4–0.82 eV for investigated crystals. The real and imaginary parts of AC conductivity are measured in the temperature range 300–550 K and frequency range 10 kHz–1 MHz. The imaginary part possesses linear frequency dependence in whole temperature range for all samples and is one to two orders of magnitude larger in comparison to the real part. According to the frequency and temperature analysis of conductivity it is established that transport mechanism in doped BTO crystals is due to charge carriers hopping between defect sites.

Light-induced properties of ruthenium-doped Bi4Ge3O12 crystals

The influence of ruthenium addition at different doping concentrations on the optical properties and holographic behavior of Bi4Ge3O12 crystals is investigated. Doped Bi4Ge3O12 single crystals are grown by using the Czochralski technique. The distribution coefficient of each crystal is measured to determine the concentration of a doped ruthenium element. Holographic experiments are performed. Ruthenium is found to make Bi4Ge3O12 sensitive in the red spectral range. Thermal annealing and ultraviolet exposure have been used to modify the defect structure and corresponding absorption band of each crystal such that the holographic recording has been influenced. The diffraction efficiency increases with increasing of ruthenium content, especially after irradiation by ultraviolet light.

Near-infrared sensitive photorefractive device using polymer dispersed liquid crystal and BSO: Ru hybrid structure

A near-infrared sensitive hybrid device, based on a Ru-doped BSO photorefractive substrate and polymer dispersed liquid crystal (PDLC) layer, is reported. It is found that the photoexcited charge carriers generated in the BSO:Ru substrate create an optically induced space charge field, sufficient to penetrate into the PDLC layer and to re-orient the LC molecules inside the droplets. Beam-coupling measurements at the Bragg regime are performed showing prospective amplification values and high spatial resolution. The proposed structure does not require indium tin oxide (ITO) contacts and alignment layers. Such a device allows all the processes to be controlled by light, thus opening further potential for real-time image processing at the near-infrared range.

Optical and holographic properties of Bi4Ge3O12 crystals doped with ruthenium

Bi4Ge3O12 single crystals doped with different concentration of Ru were grown by the Czochralski technique. Optical absorption bands were induced by illumination with ultraviolet light at room temperature. The absorption coefficient values increased with increasing ruthenium concentration. The induced photochromic effect was completely reversible: all samples recovered their transparency after thermal treatment. Preliminary holographic recording experiments showed a fast response time and a relatively slow erase time during the reading process.

Graphene oxide doped PDLC films for all optically controlled light valve structures

We study the effect of graphene oxide (GrO) on the switching voltage of polymer dispersed liquid crystal (PDLC). The threshold voltage decreases with addition of GrO nanoparticles. Scanning electron microscopy (SEM) supported that the size of LC droplets in GrO-doped PDLC increase in comparison with non-doped one. PDLC:GrO layer was combined with Bi12SiO20 (BSO) inorganic substrate into a hybrid structure and based on the surface activated photorefractive phenomena typical for BSO, it allows the opaque-transparent states to be all optically controlled, operating faster and requiring less intensity due to the GrO addition in PDLC.