The polarizing effect in Bi12GeO20 crystals

The results of the measurement of the d-c electric field effect on the resonant frequency of the longitudinally vibrated bars from Bi₁₂GeO₂₀ crystals parallel to the planes (001) and (111) are given. The polarizing correction terms of the elastic couplings are calculated from the change of the frequency.     

Thermal ionization of impurity centers in Fe-doped Bi12SiO20 and Bi12GeO20 crystals

The spectral and temperature dependence of the optical absorption and thermally stimulated depolarization currents in Fe-doped Bi₁₂SiO₂₀ and Bi₁₂GeO₂₀ crystals are investigated in the photon energy range 1.36–3.46 eV and temperature 85–750 K. The results show thermally induced electron redistribution between donor and acceptor levels and defect association-dissociation processes and are discussed using the configuration-coordinate model. Fiz. Tverd. Tela (St. Petersburg) 41, 1006–1011 (June 1999)     

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)     

Degenerate four-wave mixing in photorefractive Bi12GeO20 crystals involving internal reflections

Degenerate four-wave mixing in photorefractive Bi₁₂GeO₂₀ crystals involving internal reflections is studied theoretically and experimentally. Two sets of four-wave mixing exist simultaneously. One set is at the Bragg angle while the other is off the Bragg angle. Theoretical and experimental relations between the wavefront reflectivity and the external incident angle of the reading beam, as well as the reference-to-object beam ratio, are investigated.     

Determinations of the photorefractive parameters of Bi12GeO20 crystals using transient grating analysis

A method to characterize the photorefractive parameters of Bi₁₂GeO₂₀ crystal is described. An analysis of the transient photoinduced grating under an external applied electric field allows to determine both the diffusion length and the Debye's screening length which are the important parameters for device applications.     

Electromechanical properties of bismuth germanium oxide (Bi12GeO20)

The elastic and piezoelectric constants and their temperature coefficients of bismuth germanium oxide have been determined. All of the elastic and piezoelectric constants were obtained from dynamic measurements of piezoelectrically driven narrow bars and thin plates at the temperature 22 °C. The temperature coefficients were measured on the same resonators in the temperature range from 16 °C to 70 °C. The thermal expansion measured in the temperature range from 50 °C to 200 °C is also given.The author wishes to acknowledge the help rendered by Professor Ing. Dr.Jaroslav Stank in connection with the measurement of the thermal expansion coefficient and by R.Trepera in the course of the preparation of the measured resonators.     

Optical evidence of a photorefractive effect due to holes in Bi12GeO20 crystals

We report on the first observation to our knowledge of the photorefractive effect due to hole photoconduction in Bi₁₂GeO₂₀ (BGO) crystals. In cubic crystals such as BGO, the study of energy redistribution in two beam coupling experiments does not allow by itself the determination of the sign of the photocarriers. Additional experiments such as ellipsometric measurements have to be performed to give the sign of the effective electrooptic coefficient. Such experiments points out that either holes or electrons govern the photorefractive effect this behavior is demonstrated in both undoped and iron-doped BGO crystals.     

Dielectric relaxation in Bi12SiO20 crystals

Anomalies of the dielectric properties of undoped and aluminum-and gallium-doped crystals of Bi₁₂SiO₂₀ are investigated in the frequency and temperature range ν=10²–10⁸ Hz and T=300–800 K. They are shown to be due to Debye relaxation processes and determined by the relaxor parameters. The mechanism of electron thermal polarization is discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 1223–1229 (July 1997)     

Dielectric relaxation in Bi12SiO20: Cr crystals

A study is reported of the temperature and frequency dependences of the permittivity and losses in Cr-doped Bi₁₂SiO₂₀ crystals at sonic frequencies and in the range 300–800 K. A number of dielectric anomalies and a close-to-linear Cole-Cole diagram have been observed. The results are discussed by invoking the concepts of electron hopping and screening of the induced polarization through the relaxation of local lattice distortions.     

La掺杂对Bi4Ti3O12薄膜铁电性能的影响

利用Sol-Gel法在Pt/Ti/SiO2/Si衬底上制备出Bi4Ti3O12和Bi3.25La0.75Ti3O12薄膜,研究了La掺杂对Bi4Ti3O12薄膜的晶体结构、铁电性能和疲劳特性的影响,发现La掺杂没有改变Bi4Ti3O12薄膜的基本晶体结构,并且提高了Bi4Ti3O12铁电薄膜的剩余极化值和抗疲劳性能,对La掺杂改善Bi4Ti3O12铁电薄膜性能的机理进行了讨论.     

Combined Optical/MCD/ODMR investigations of photochromism in doubly-doped Bi 12 GeO 20

Electron paramagnetic resonance is detected optically via the change of magnetic circular dichroism under microwaves at 35 v GHz. The technique is applied to Bi 12 GeO 20 samples co-doped with vanadium and a second transition metal (Cr, Mn, Co, Cu). The optical and magnetic properties of several paramagnetic defects (V Ge 4+ and Cr Ge 4+ ) are directly correlated. The basic photochromic processes occuring in samples doped with V, Mn, and Mn+V are explained. The V Ge 4+/5+ level is positioned roughly 2.2 v eV above the valence band.     

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.     

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)     

Thermooptic investigation of impurity centers in Cu-doped Bi12SiO20 crystals

Temperature dependences of the optical absorption of BSO:Cu crystals were investigated at photon energies between 1.36 and 3.46 eV and temperatures between 85 and 700 K. These dependences are analyzed for the energy model of a partially compensated p-type semiconductor taking into account the temperature dependence of the concentration of shallow and deep ionized acceptors. Fiz. Tverd. Tela (St. Petersburg) 40, 1246–1251 (July 1998)     

锗酸铋晶体二波耦合效应的研究

本文叙述了锗酸铋晶体的生长。采用二波耦合光路测试BGO晶体的衍射效率和增益系数,研究BGO晶体的二波耦合中光波的位相变化。     

Beam coupling and decoupling in degenerate two-wave mixing in a reflection geometry with photorefractive Bi2GeO20 crystals

The dependence of the effective gain on the incident angle of the beams and on the initial beam intensity ratio is studied in the experiment of degenerate two-wave mixing (TWM) in a reflection geometry with photorefractive Bi₁₂GeO₂₀ (BGO) crystals. A saturation value of the effective coupling constant ¦g¦-0.4 cm^–1 is obtained. In contrast to TWM operated in the drift mode (i.e. with a nonzero electric field applied to the crystal), for TWM operated in the diffusion mode (zero external electric field) as is our case, beam coupling is reduced by moving the crystal or the interference fringes at a constant speed. At high moving speeds, complete beam decoupling can be reached. A comparison between the theoretical and measured dependence of the effective gain on the moving speed is also made. Using this technique, complete isolation of two intersecting coherent beams inside a nonlinear medium can be achieved.