Gain optimization with respect to the thickness of a sillenite crystal

Simple analytical expressions for the dependence of the maximum gain on the polarization angle of the interacting light waves and the crystal thickness are obtained for two-wave mixing in a cubic optically active piezoelectric crystal. By applying the theory to Bi₁₂SiO₂₀ and Bi₁₂TiO₂₀ crystals, the optimum gain is found also with respect to the grating orientation. The r?le of the (inverse) piezoelectric effect is investigated. Received: 23 November 1998 / Revised version: 8 January 1999 / Published online: 7 April 1999     

Photorefractive effects in long, narrow BSO crystals with applied electric field

In a photorefractive Bi₁₂SiO₂₀ crystal with high applied electric ac field of square-wave shape a fast two-wave coupling response (less than 1 s) and a slow hologram readout decay (minutes) was found for a wavelength of 633 nm. This can be explained by electron–hole transport with two trap levels. An intensity dependence of the slower complementary grating was found. Illuminating with the readout wave without applied electric field leads to a very slow grating decay (many hours). Received: 18 November 1998 / Revised version: 3 February 1999 / Published online: 7 April 1999     

Longitudinal parametric oscillation in photorefractive sillenites: comparison between theory and experiment

By using an alternative setup for photorefractive parametric oscillation in which wave mixing between the recording beams is avoided it has become possible to make more detailed comparisons with the space-charge wave theory. In the present paper we compare the experimental features of longitudinal parametric oscillation observed in a crystal of Bi₁₂SiO₂₀ with the theoretical predictions. Received: 24 November 1998 / Revised version: 13 January 1999 / Published online: 7 April 1999     

Influence of the optical activity on hologram formation in photorefractive crystals

It is shown that dynamical hologram formation in photorefractive, optically active crystals is accompanied by polarization and energy exchange. These effects may be used like new effective holographic methods for determinations of crystal parameters and optical signal processing in real time. In self-diffraction a polarization rotation the order of degrees/cm for Bi₁₂TiO₂₀ and Bi₁₂SiO₂₀ is predicted.     

Nonlinear interactions and scattering of space charge waves in sillenites

For the first time the effects of overall doubling, spatial doubling, and overall rectification of space charge waves in a photorefractive crystal (Bi₁₂GeO₂₀, BGO) have been detected and theoretically studied. The effects of overall doubling and overall rectification resemble second-harmonic generation and rectification of light in nonlinear optics. A good agreement between theory and experiment is found for the effect of spatial doubling, while for overall doubling and overall rectification effects only a qualitative agreement between theory and experiment has been established until now. Received: 13 July 2001 / Published online: 10 October 2001     

Photorefractive properties of (1-10) and (111)-cut sillenite crystals when external electric field is applied along the direction of the optimum diffraction efficiency

We study the influence of the application of an external electric field on the grating formed in (1-10) and (111)-cut photorefractive sillenite crystals. Optimum conditions for application of the bias field are examined, taking into account the shift of the grating orientation for maximum diffraction efficiency along the crystal depth. The contribution of the piezoelectric effect is considered both in the electro-optic tensor and space charge field calculations. An analysis on the diffractive properties under these optimum conditions shows an increase of 30% in maximum diffraction efficiency in Bi₁₂GeO₂₀, and experimental verification for this is provided. Received: 5 November 2001 / Revised version: 18 April 2002 / Published online: 8 August 2002     

Denisiuk-type reflection holography display with sillenite crystals for imaging and interferometry of small objects

A novel setup for imaging and interferometry through reflection holography with Bi₁₂TiO₂₀(BTO) sillenite photorefractive crystals is proposed. A variation of the lensless Denisiuk arrangement was developed resulting in a compact, robust and simple interferometer. A red He-Ne laser was used as light source and the holographic recording occurred by diffusion with the grating vector parallel to the crystal [0 0 1]-axis. In order to enhance the holographic image quality and reduce noise a polarizing beam splitter (PBS) was positioned at the BTO input and the crystal was tilted around the [0 0 1]-axis. This enabled the orthogonally polarized transmission and diffracted beams to be separated by the PBS, providing the holographic image only. The possibility of performing deformation and strain analysis as well as vibration measurement of small objects was demonstrated.     

Speed and diffraction efficiency in feedback-controlled running holograms for photorefractive crystal characterization

We report the measurement of the diffusion length, the Debye screening length and the quantum efficiency of photoelectron generation in strongly light absorbing photorefractive Bi₁₂TiO₂₀ crystals, using fringe-locked running hologram experiments. The effective applied electric field inside the sample is also computed and self-diffraction is considered. The novelty here, as compared to formerly reported experiments, is that the diffraction efficiency is now measured simultaneously with the hologram speed v. From these data the above referred to photorefractive and experimental parameters are obtained without the need for additional experiments. The method is used to analyze two photorefractive Bi₁₂TiO₂₀ crystal samples, in different experimental conditions, using the 514.5 nm wavelength. The computed parameters are in good agreement with the available information about these samples. Received: 23 November 2000 / Published online: 21 March 2001     

Photorefractive waveguides in oxide crystals: fabrication, properties, and applications

3 , LiTaO₃, BaTiO₃, KNbO₃, Sr_xBa_1-xNb₂O₆ (0.25≤x≤0.75, SBN), and Bi₁₂(Si,Ti,Ge)O₂₀ (BSO, BTO, BGO) waveguides are discussed. Furthermore, the suitability of photorefractive waveguides for nonlinear optical components is demonstrated in some examples. Received: 21 April 1998/Revised version: 28 May 1998     

Diffraction efficiency and energy transfer in two-wave mixing experiments with Bi12SiO20 crystals

The diffraction efficiency and energy transfer are investigated in photorefractive Bi₁₂SiO₂₀ crystals (BSO). Dependence on fringe spacing, electric fields, light intensity and rise-time constants are reported. The optimum crystallographic orientation for each effect is determined for vertical polarization of the recording beams. It is shown that beam coupling is a very sensitive phenomenon in BSO crystals where charge transport lengths are equivalent to usual fringe spacings. Experimental results are interpreted on the basis of the nonlinear theory of self-diffraction developed by Kukhtarev et al. [10].     

Drift mobility of photogenerated current carriers in Bi12GeO20 crystals

The drift mobility of photogenerated electrons and holes in Bi₁₂GeO₂₀ crystals with different degrees of doping is investigated experimentally. Fiz. Tverd. Tela (St. Petersburg) 40, 2054–2055 (November 1998)     

Multi-wavelength holography in Bi12TiO20 crystals: Applications in refractometry

We present a recent development in holography with multimode, large free-spectral range (FSR) diode lasers in photorefractive sillenite crystals. A novel refractometry method based on this type of holographic recording in Bi₁₂TiO₂₀ (BTO) crystals is proposed. The holographic image of a prism-shaped transparent sample appears covered of interference fringes, and as the sample is properly translated, the fringes run along the holographic image. An expression providing the refractive index of the medium as a function of the sample displacement and the correspondent number of running fringes was derived. The refractive indexes of optical (BK7) glass, ethanol, hexan, cumene and aqueous solution of NaCl with different concentrations were measured in order to test the method. The obtained results are in good agreement with the ones reported in literature or measured by us using a commercial Abbe refractometre.     

Growth of Bi-Sb alloy thin films and their characterization by TEM, PIXE and RBS

Polycrystalline bulk materials of Bi₉₃Sb₇ Bi₈₈Sb₁₂, Bi₈₅Sb₁₅ and Bi₈₀Sb₂₀ were synthesized by melt-quench technique starting from the stoichiometric mixture of constituent elements. The phase purity and compositional uniformity of bulk materials were investigated using powder X-ray diffraction (XRD) and proton induced X-ray emission (PIXE) experiments. The single phase formation and the compositional analysis of thin films were confirmed by transmission electron microscopy (TEM) and Rutherford backscattering spectroscopy (RBS). X-ray diffraction studies confirmed the phase homogeneity of the materials. Atomic concentration ratio of constituent elements (Bi and Sb) determined by PIXE and RBS revealed that near-stoichiometric composition is nearly the same in the bulk as well as in thin film forms.     

Combinatorial exploration of flux material for Bi4Ti3O12 single crystal film growth

The combinatorial approach to materials synthesis was employed for the quick screening of a flux material for liquid phase-mediated epitaxy of Bi₄Ti₃O₁₂ single crystal film. A series of ternary flux libraries composed of two self-fluxes (Bi₂O₃ and Bi₄Ti₃O₁₂) and an impurity flux (VO_x, WO_x, CuO_x, BiPO_x, BaO, MoO_x) were fabricated on the SrTiO₃ (0 0 1) substrates. Then, stoichiometric Bi₄Ti₃O₁₂ was grown on each one of these flux libraries at a temperature presumed to melt the flux. High-throughput characterization with the concurrent X-ray diffraction (XRD) method resulted in the discovery of a novel flux material, CuO, containing Bi₂O₃, for Bi₄Ti₃O₁₂ single crystal film.     

Quasipermanent hole-photorefractive and photochromic effects in Bi12TiO20 crystals

We report the presence of quasipermanent effects of photochromic and hole-photorefractive nature in Bi₁₂TiO₂₀ (BTO) at low irradiance at 514.5 nm. A new selective two-wave mixing (S2WM) detection technique was used that allows to separately detect phase and amplitude gratings. S2WM is used in this work for operating a self-stabilized recording setup which allows to produce unusually large quasipermanent effects. We also show that stabilized recording is necessary to obtain reliable information from hologram erasure experiments. Results reported in this paper show the complex nature of recording in BTO and the capabilities of stabilized recording and S2WM for fundamental and applied research on photorefractive crystals.     

Induced impurity photoconductivity in crystals of Si-and Ge-sillenites

The spectral distribution of the uv-induced photoconductivity intensity and relaxation in Bi₁₂SiO₂₀ and Bi₁₂GeO₂₀ crystals both undoped and doped with Al, Ga, Cr, Cu, Mn, and V is investigated in the optical range 0.5–3.5 eV in the temperature ranges 85–95 K and 285–295 K. It is shown that in the short-wavelength region 2.2–3.5 eV it is controlled by multicenter recombination in which both “fast” and “slow” recombination participate. Fiz. Tverd. Tela (St. Petersburg) 40, 1027–1029 (June 1998)     

Influence of shallow traps on holographic recording in Bi12SiO20 in the temperature range 200–350 K

The photoconductivity response time in Bi₁₂SiO₂₀ grows 3 orders of magnitude when the temperature changes from 350 to 200 K. We attribute this to the influence of shallow traps with activation energy 0.4 eV. For low temperatures, the time of holographic grating formation is determined by the time of shallow-level filling. Received: 29 October 1998 / Revised version: 18 December 1998 / Published online: 12 April 1999     

Thermooptical investigation of deep levels in doped Bi12SiO20 crystals

The temperature dependence of the impurity optical absorption of nominally pure Bi₁₂SiO₂₀ (BSO) crystals and BSO crystals doped with Ga, Cr, Mn, and Ag ions is investigated. A method is proposed for determining the thermal activation energy, and certain properties of deep levels are found. Fiz. Tverd. Tela (St. Petersburg) 40, 452–457 (March 1998)     

Effects of the homogeneous Bi doping process on photoluminescence properties of YVO4:Bi,Eu nanophosphor

YVO₄:Bi³⁺,Eu³⁺ nanophosphors at a high Bi³⁺ concentration of 15 at% are synthesized from a Bi³⁺ source, nitrates of yttrium and europium(III), and sodium orthovanadate(V) by a low-temperature aqueous precipitation in the presence of citrate ions. When an ethylene glycol solution of bismuth(III) nitrate is used as a Bi³⁺ source, YVO₄:Bi³⁺,Eu³⁺ nanophosphors of ∼20 nm in size crystallize during aging at 85 °C without any by-products where the contents of Bi³⁺ and Eu³⁺ incorporated into crystalline YVO₄ are close to the respective nominal contents, as confirmed by transmission electron microscopy, X-ray diffractometry and X-ray fluorescent analysis. These nanophosphors show red emission corresponding to the f-f transition of Eu³⁺ under the excitation of Bi³⁺-V⁵⁺ charge transfer. When aging is continued after the completion of the crystallization, the photoluminescence intensity of nanophosphors reaches the constant value. This is the improved behavior in comparison to our previous work, where the photoluminescence intensity decreases after the prolonged aging because of the inhomogeneous doping of Bi³⁺ ions, and hence the concentration quenching.     

Dispersion of electrogyration in sillenite crystals

In this paper we report on the dispersion of the electrogyration of the sillenite crystals Bi₁₂SiO₂₀, Bi₁₂GeO₂₀, and Bi₁₂TiO₂₀ over the visible spectrum. We measure the electrogyration coefficient from the rotation of the polarization plane of the transmitted beam due to the externally applied electric field. Both light transmission and electric field direction are parallel to the [111] crystallographic direction in which the influence of the electrooptic effect is diminished. In all cases the electrogyratory coefficient is found to be between 0.5–3.75×10^-13 m/V and has the same dispersion pattern like optical activity and refractive index. PACS 78.20.Ek; 78.20.Jq; 42.70.Nq