Dispersion of electro-optic coefficients in sillenite crystals

The photorefractive crystals of the sillenite family (Bi₁₂SiO₂₀ or BSO, Bi₁₂GeO₂₀ or BGO, and Bi₁₂TiO₂₀ or BTO) that belong to the cubic 23 point group are optically active, and exhibit the piezoelectric, elasto-optic, electro-optic and electrogyration effect. In this paper we measure the dispersion of the electro-optic coefficient for all the crystals of the sillenite family in the visible spectral range (500–800 nm). For this we measure by ellipsometry the polarization properties of a beam transmitted in the transverse configuration under the action of an externally applied field. The experimental data are fitted with an analytical expression of the beam polarization to find the electro-optic coefficient. The results show a normal dispersion of the electro-optic coefficient in all three sillenite crystals, similarly to other electro-optic crystals.     

Index of refraction, optical activity and electro-optic coefficient of bismuth titanium oxide (Bi12TiO20)

12 TiO₂₀. The first two can be measured by simple and classical methods, but the coexistence of optical activity, electrogyration, field-induced linear birefringence, and piezoelectric and photoelastic effects in photorefractive materials such as Bi₁₂TiO₂₀ complicate the measurement of the electro-optic coefficient. For normal incidence of linearly polarized light we derive analytic expressions for the polarization of light that has passed through the crystal. The ellipticity of the polarization is a function of the electric-field-induced linear birefringence and hence of the electro-optic coefficient of the crystal. Therefore measurement of the ellipticity as a function of an electric field externally applied to the crystal leads to an electro-optic coefficient r₄₁ of 5.3±0.1 pm/V. Received: 22 December 1996/Revised version: 21 March 1997     

The magnetoelectric coupling in rhombohedral–tetragonal phases coexisted Bi0.84Ba0.20FeO3

Ba doped Bi_1.04−xBa_xFeO₃ ceramics with x up to 0.30 have been prepared by the tartaric acid modified sol–gel method. The X ray diffraction patterns show that the structure transforms from rhombohedral to tetragonal with increasing the Ba substitution concentration from 10% to 30% and the coexistence of distorted rhombohedral and tetragonal phases in 20% Ba substituted BiFeO₃, which was further confirmed by the Raman spectra. Bi_0.84Ba_0.20FeO₃ exhibits the highest magnetization (1.6 emu/g under magnetic field of 12 kOe) compared with the other samples of different Ba substitution concentration. Significant enhancement of the ferroelectricity has been observed in 20% and 30% Ba substituted BiFeO₃ with saturate polarization close to 6.6 μC/cm² for Bi_0.74Ba_0.30FeO₃. The magnetoelectric coupling of Bi_0.84Ba_0.20FeO₃ has been measured and the maximum decrease of magnetization under magnetic field of 9.8 kOe was about 0.06 emu/g with increasing applied electric field to 11 kV/cm, and the magnetoelectric coefficient is 1.5×10⁻¹² s/m.     

Highly efficient hologram enhancement in Bi12TiO20 fibre-type crystal

Experimental results for two-wave mixing experiments in a single Bi₁₂TiO₂₀ fibre-type crystal at a wavelength of 632.8 nm under transverse a.c. electric field are reported. An exponential gain factor =4.7 cm^-1, which is much higher than the absorption coefficient (0.6 cm^-1), is obtained. It is observed that the signal beam enhancement is in fact signal beam depletion for spatial frequencies higher than F _B, which is determined by fibre geometry. An unusually strong dependence of response time on the recording beams intensities ratio reflects the nonlinearities of the multi-two-wave mixing process.     

Linear photogalvanic current excited in sillenite crystals by femtosecond laser pulses

We observe electric pulses generated in sillenite crystals (Bi₁₂SiO₂₀ and Bi₁₂TiO₂₀) by 100-fs laser pulses at the wavelength of 400 nm (below the band gaps of both crystals). The peak value of the current pulses scales linearly with the intensity of laser pulses up to ∼45 GW/cm². The direction of the induced current depends on the polarization state of the laser pulse. This polarization dependence and features of the current detection via charge accumulation at the sample electrodes allow us to conclude that the electric pulses are generated due to the linear photogalvanic effect.     

Faraday rotation in single crystal erbium iron garnet

Measurements of the Faraday rotation of ErIG, Er₃Fe₅O₁₂, have been performed in the 4.2–300 K temperature range in magnetic field up to 20 kOe applied along the [111] direction and at 1.15 μm wavelength. The results are analysed under the assumption that the contribution of the Fe³⁺ ions to the total Faraday rotation is the same as that of YIG, Y₃Fe₅O₁₂. The temperature and field dependences of the contribution of the Er³⁺ ions are deduced. Both magnetic and electric dipole contributions of the Er³⁺ ions are calculated; the electric dipole coefficient C_e is found to present a linear temperature dependence between 30–300 K. The temperature dependence of the Faraday rotation susceptibility differs strongly from that of the magnetic susceptibility.     

Light Dispersion in Bismuth Germanate Crystals and Bismuth Oxide Films

The dispersion of light in Bi₄Ge₃O₁₂ and Bi₁₂GeO₂₀ single crystals and thin Bi₂O₃ films with a monoclinic structure was investigated in the visible spectral region. The parameters of a single-oscillator approximation have been found. It is established that in Bi₄Ge₃O₁₂ crystals the absorption band caused by the O2p–Bi6p transitions makes the main contribution to the dispersion curve in the visible region, whereas in Bi₁₂GeO₂₀ crystals this is made by transitions from the hybrid O2p–Bi6p states to the conduction band. The dispersion energy, the degree of the ionicity of binding, and the coordination number of the first coordination sphere of the Bi³⁺ cation have been determined.     

Transient energy transfer between writing beams during hologram formation in Bi12GeO20

A large transient energy transfer between writing beams during volume phase-hologram formation in Bi₁₂GeO₂₀ has been observed. The variation of the energy transfer by applying an external field and by changing the fringe spacing of the elementary holograms have been measured and described in the framework of Kukhtarevs nonlinear theory of transient self-diffraction. The results indicate that the transient energy transfer in Bi₁₂GeO₂₀ is due to a non-stationary phase mismatch between the intensity- and refractive index grating caused by photoconductivity and that the transient energy transfer increases with increasing fringe spacing and increasing applied electric field. Up to ten-fold amplification of coherent beams has been measured, corresponding to an exponential gain of Г = 2 cm^-1. A continuous energy transfer has been observed when the phase between recording beams was changed linearly with time during recording. It is shown that light is coherently amplified for electric field strengths larger than 2.3 kV/cm and that a peak amplification factor of Г₁ = 2 cm^-1 has been reached for E₀ = 11 kV/cm.     

Determination of the Space-Charge Field Amplitude by the Phase Modulation Method

Results of experimental and theoretical investigations into the conversion of the phase modulation of light into its intensity modulation in photorefractive crystals are presented. Based on the vector diffraction theory, an analytical expression describing a phase demodulation in crystals in an external electric field applied in an arbitrary direction is derived. The phase demodulation is used to determine the space-charge field amplitude in cubic crystals. It is demonstrated that the space-charge magnitude measured in GaP and Bi₁₂TiO₂₀ crystals is less than that predicted by the Kukhtarev single-level model.     

Wavelength and temperature characteristics of BiYbIG film/YIG crystal composite structure for magneto-optical applications

We report the wavelength and temperature characteristics of novel Bi-substituted rare-earth iron garnet films grown on a YIG substrate by a modified liquid phase epitaxy (LPE) technique. The Faraday-rotation spectrum was measured by the magneto-optically modulated dual-frequency technique with the wavelength varied from 800 nm to 1700 nm. The resultant Bi_0.37Yb_2.63Fe₅O₁₂ (BiYbIG) LPE film/YIG crystal structure showed an increased Faraday-rotation coefficient due to Bi³⁺-ion doping on the dodecahedral sites of the iron garnet without increasing absorption loss; therefore, a good magneto-optical figure of merit, defined by the ratio of Faraday rotation and optical absorption loss, has been achieved (21.5 deg/dB and 30.2 deg/dB at 1300-nm and 1550-nm wavelengths, respectively, at room temperature). In addition, since the Yb³⁺ and Y³⁺ ions provide opposite contributions to the wavelength and temperature characteristics of the Faraday rotation, the resultant BiYbIG LPE film/YIG crystal structure showed a flat Faraday-rotation curve versus wavelength and temperature. The Faraday-rotation wavelength coefficient was reduced to 0.06 %/nm at 1550-nm wavelength. The Faraday-rotation temperature derivative was reduced to 0.006 deg/°C at 1300-nm wavelength and 0.007 deg/°C at 1550-nm wavelength, respectively. PACS 78.20.Ls; 81.15.Lm; 75.50.Gg     

Local ordered magnetic fields in bismuth dielectrics. 209Bi NQR in Bi4Ge3O12 single crystal

The results of the ²⁰⁹Bi NQR experiments carried out on α-Bi₂O₃, Bi₃O₄Br, Bi₂M₄O₉ (M=Al, Ga), Bi₂Ge₃O₉, and Bi₄Ge₃O₁₂ showed that these compounds are not diamagnets in the conventional sense. The Zeeman perturbed zero-field ²⁰⁹Bi NQR spectra gave an indication that local ordered magnetic fields of the order of 30–200 G exist in these substances comprising neither transition nor rare earth elements. Further aspects of this principally new phenomenon of yet unknown nature were studied by the ²⁰⁹Bi NQR experiments on the Bi₄Ge₃O₁₂ single crystal in external magnetic fields below 500 Oe, applied at various orientations with respect to the crystal axes. The spectral patterns of dramatically increased intensity and multiplicity, caused by the unknown magnetism of the compound, were observed in applied fields and modeled. Based on the results of the modeling, the conclusions were made that at least four magnetically non-equivalent Bi sites characterized by antiferromagnetically ordered local fields of the order of 30 G are present in the Bi₄Ge₃O₁₂ crystal; the intensity increase was interpreted to arise from the change in orientation of the electric field gradient axes at the Bi site upon applying an external magnetic field.     

中子非弹性散射对Bi12GeO20和Bi12SiO20旋声性的研究

通过用中子非弹性散射测量旋声晶体高对称方向的横声学支声子色散曲线的劈裂,从三个方面研究了同构异质晶体Bi₁₂GeO₂₀和Bi₁₂SiO₂₀的旋声特性。值得注意的是,沿〈111〉方向传播的左旋、右旋圆偏振声子具有不同的衰减。其中传播速度快的圆偏振声子衰减较大,该结论为这两种晶体所证实,这是在晶体旋声性研究中首次观察到的现象。实验还直接确定了衰减较大的这支声子的寿命约为2×10^-11S。 关键词：     

Photorefractive effect in sillenite crystals with shallow traps in a sign-alternating electric field

The results of a theoretical analysis of the photorefractive response in crystals with shallow traps to in a sign-alternating, square-wave electric field are presented. The numerical analysis method developed imposes no restrictions on the frequency of the external field and the period of the photorefractive grating. The parameters characterizing deep donor and shallow trap centers are estimated on the basis of investigations of two-beam interaction in a Bi₁₂SiO₂₀:Cd crystal with the application of a sign-alternating, square-wave electric field. Fiz. Tverd. Tela (St. Petersburg) 40, 2037–2043 (November 1998)     

Ferroelectric properties of Pr6O11-doped Bi4Ti3O12

Praseodymium doped Bi₄Ti₃O₁₂ (BIT) ceramics with composition Bi_2.9Pr_0.9Ti₃O₁₂ (BPT) were prepared by solid state reaction. These samples have polycrystalline Bi-layered perovskite structure without preferred orientation, and consist of well-developed plate-like grains with random orientation. Pr doping into BIT causes a large shift of the Curie temperature (T_C) of the BIT from 675 to 398 °C. At an electric field of 87 kV/cm, the remanent polarization and the coercive field of the BPT ceramics are 30 μC/cm² and 52 kV/cm, respectively. Furthermore, the dielectric permittivity and the dissipation factor of the BPT ceramics are 300 and 0.003 at 1 MHz, 1 V, and room temperature. Ferroelectric properties of the BPT ceramics are superior to V-doped Bi₄Ti₃O₁₂ (∼20 μC/cm² and 80 kV/cm) and (Sr, Ta)-doped Bi₄Ti₃O₁₂ (∼12 μC/cm² and 71 kV/cm) ceramics. In addition, the dense ceramics of praseodymium-doped B₄Ti₃O₁₂ were obtained by sintering at 1100 °C, about 100-200 °C lower than those of the SrBi₂Ta₂O₉ system.     

Magneto-electrogyration in cubic Sr(NO3)2, Ba(NO3)2, and Pb(NO3)2

The simultaneous application of electric and magnetic fields on single crystals of Sr-, Ba-and Pb-dinitrate yields an additional term to the pure electrogyration and Faraday effect. The symmetric part of the fourth-rank tensor of this magneto-electrogyration has been completely determined with the aid of a high-resolution computer-aided polarimetric device. In Pb(NO₃)₂ a maximum magneto-electrogyration of about 10% of the magnitude of electrogyration is observed when a magnetic field of 1000 kA/m (1.2 Tesla) is applied along [111]. The effects in Sr(NO₃)₂ and Ba(NO₃)₂ are much smaller. Index of refraction, electrogyration, Faraday effect, and the new magneto-electrogyration obey the same sequence: Pb(NO₃)₂ > Ba(NO₃)₂ > Sr(NO₃)₂.     

Interaction of running gratings of the space charge and conductivity in photorefractive Bi<Subscript>12</Subscript>Si(Ti)O<Subscript>20</Subscript> crystals

The simultaneous excitation and nonlinear interaction of the space-charge and photoconductivity gratings are studied experimentally in photorefractive Bi₁₂SiO₂₀ and Bi₁₂TiO₂₀. The measurements are performed using the diffraction technique, which implies the illumination of the crystal by an oscillating interference pattern (_r=532 nm) along with the application of dc and ac electric fields and detection of the diffracted probe beam (_p=650 nm). Such illumination excites the running photoconductivity grating, which interacts with the ac component of the applied field giving rise to the space-charge wave. Being the eigenmode of the space-charge oscillations, this wave reveals itself as a low-frequency resonant maximum at the frequency-transfer function of the detected signal. The drift mobilities of electrons are estimated using the developed technique: =(1.1–1.4)×10^-2 cm²/Vs (Bi₁₂SiO₂₀, T=296–298 K) and =2.8×10^-3 cm²/Vs (Bi₁₂TiO₂₀, T=293 K). PACS 42.65.Sf; 42.70.Nq     

Growth and characterization of Nd:Bi12SiO20 single crystal

A Nd:Bi₁₂SiO₂₀ crystal was grown by the Czochralski method. The thermal properties of the crystal were systematically studied. The thermal expansion coefficient was measured to be α=11.42×10^?6 K^?1 over the temperature range of 295–775 K, and the specific heat and thermal diffusion coefficient were measured to be 0.243 Jg^?1 k^?1 and 0.584 mm²/s, respectively at 302 K. The density was measured to be 9.361 g/cm³ by the buoyancy method. The thermal conductivity of Nd:Bi₁₂SiO₂₀ was calculated to be 1.328 Wm^?1 K^?1 at room temperature (302 K). The refractive index of Nd:Bi₁₂SiO₂₀ was measured at room temperature at eight different wavelengths. The absorption and emission spectra were also measured at room temperature. Continuous-wave (CW) laser output of a Nd:Bi₁₂SiO₂₀ crystal pumped by a laser diode (LD) at 1071.5 nm was achieved with an output power of 65 mW. To our knowledge, this is the first time LD pumped laser output in this crystal has been obtained. These results show that Nd:Bi₁₂SiO₂₀ can serve as a laser crystal.     

NQR indications of unconventional magnetism in some bismuth-based diamagnets

An evidence that local ordered magnetic fields from 30 to 200 G exist in diamagnets α-Bi₂O₃, Bi₃O₄Br, Bi₄Ge₃O₁₂, Bi₂Al₄O₉ which comprise neither d- nor f-elements was earlier given by the zerofield and Zeeman-perturbed²⁰⁹Bi nuclear quadrupole resonance (NQR) spectra. With a view to find similar spectroscopic effects in other compounds, we examined the²⁰⁹Bi NQR Zeeman-perturbed spectra of the Bi₃B₅O₁₂ oriented single crystal as well as the zero-field spin echo envelopes in Bi₃B₅O₁₂ and Bi₂Ge₃O₉. Distinctive modulations were displayed by the zero-field²⁰⁹Bi spin echo envelopes in Bi₂Ge₃O₉ powder. The modeling of the spin echo envelopes within the density matrix formalism explained the observed effect by the presence of local ordered magnetic field of the order of 65 G at the Bi atoms. The zero-field modulations of the²⁰⁹Bi spin echo envelopes were also observed in Bi₃B₅O₁₂ indicating the presence of the internal source of line splitting. This finding and considerable deviation of the resonance intensity ratios from that in a pure NQR, found in the zero-field²⁰⁹Bi spectrum of the single crystal, were understood as indirect evidences that a local ordered magnetic field exists also in Bi₃B₅O₁₂. The zero-field²⁰⁹Bi spin echo envelopes assigned to the lines split by the local magnetic fields in α-Bi₂O₃ and Bi₄Ge₃O₁₂ were observed to display modulations on the appropriate curves.     

Estimation of trap concentration in photorefractive crystals by a technique of adaptive holographic interferometry

12 GeO₂₀, Bi₁₂SiO₂₀, Bi₁₂TiO₂₀). Received: 17 June 1998/Revised version: 3 August 1998     

Anomalous magnetism and 209Bi nuclear spin relaxation in Bi4Ge3O12 crystals

The quadrupole ²⁰⁹Bi spin–spin and spin–lattice relaxation were studied within 4.2–300 K for pure and doped Bi₄Ge₃O₁₂ single crystals which exhibit, as was previously found, anomalous magnetic properties. The results revealed an unexpectedly strong influence of minor amounts of paramagnetic dopants (0.015–0.5 mol.%) on the relaxation processes. Various mechanisms (quadrupole, crystal electric field, electron spin fluctuations) govern the spin–lattice relaxation time T ₁ in pure and doped samples. Unlike T ₁, the spin–spin relaxation time T ₂ for pure and Nd-doped samples was weakly dependent on temperature within 4.2–300 K. Doping Bi₄Ge₃O₁₂ with paramagnetic atoms strongly elongated T ₂. The elongation, although not so strong, was also observed for pure and doped crystals under the influence of weak (～30 Oe) external magnetic fields. To confirm the conclusion about strong influence of crystal field effects on the temperature dependence of T ₁ in the temperature range 4.2–77 K, the magnetization vs. temperature and magnetic field was measured for Nd- and Gd-doped Bi₄Ge₃O₁₂ crystals using a SQUID magnetometer. The temperature behavior of magnetic susceptibility for the Nd-doped crystal was consistent with the presence of the crystal electric field effects. For the Gd-doped crystal, the Brillouin formula perfectly fitted the curve of magnetization vs. magnetic field, which pointed to the absence of the crystal electric field contribution into the spin–lattice relaxation process in this sample.