Faraday rotation measurement with photorefractive Bi12TiO20: comment

In a recent paper (Opt. Laser Technol. 34 (2002) 605) Anand and Narayanamurthy described a method for measurement of Faraday rotations using a photorefractive Bi₁₂TiO₂₀. The effects they observed have nothing to do with the source intensity nor with the photorefractive nature of the crystal. The same effect could be obtained without the use of a photorefractive crystal by simple rotation of the analyzer.     

Leaky photorefractive surface waves in Bi12TiO20 and Bi12SiO20 crystals

Consistent experimental and numerical simulation studies of photorefractive surface wave propagation in Bi₁₂TiO₂₀ and Bi₁₂SiO₂₀ crystals are presented. We induce the photorefractive surface wave with a Gaussian beam of small diameter and show that light leakage from the surface wave takes the form of multiple beam reflection from the crystal surface. The efficiency of surface wave excitation is proved to be much higher for the divergent Gaussian beam than for the convergent one.     

Interferometric generation of dark spatial solitons in a photorefractive Bi12TiO20 crystal

We present an interferometric way to generate dark spatial solitons by propagating two coherent beams in a photorefractive Bi₁₂TiO₂₀ crystal under application of an external electric field. Our results shown that the initial width, initial separation, and relative phase between the beams allow to control the type of dark soliton generated.     

Rapid microwave-assisted hydrothermal synthesis of Bi12TiO20 hierarchical architecture with enhanced visible-light photocatalytic activities

Flower-like Bi₁₂TiO₂₀ hierarchical nanostructures composed of numerous nanobelts were synthesized at 180 °C within 1 h by a microwave-assisted hydrothermal method in the presence of cetyltrimethylammonium bromide (CTAB) for the first time. The as-prepared products were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and ultraviolet–visible (UV–vis) absorption spectroscopy. Furthermore, the hierarchical Bi₁₂TiO₂₀ nanostructures exhibited higher photocatalytic activities in the degradation of Rhodamine B under visible-light irradiation than that of the samples prepared without CTAB. In addition, the role of CTAB cationic surfactant has been investigated thoroughly and a possible mechanism is proposed.     

Crystal chirality and optical rotation sense in isomorphous Bi12SiO20 and Bi12GeO20

Laevorotatory crystals of isomorphous Bi₁₂SiO₂₀ and Bi₁₂GeO₂₀ are shown, by use of the anomalous X-ray scattering component in the structure amplitudes, to have identical chirality (handedness of atomic arrangement) in contrast with the case of NaC10₃ and NaBrO₃, in which crystals of the same chirality rotate plane polarized light in opposite senses.     

IR-induced nonlinear optics in Ge-doped Bi12TiO20 large-sized nanocrystallites

Photoinduced non-linear optical effects in large-sized (up to 25 nm) nanocrystallites (NC) of Ge-doped Bi₁₂TiO₂₀ (BTO:Ge) incorporated within olygoether photopolymer matrix have been studied. Photoinduced second harmonic generation (PISHG) was measured. Nd:YAG pulsed laser (λ=1.06 μm) was used as a source of photoinducing light. As a fundamental light source for the SHG and two-photon absorption, Er:LiYF₄ laser (λ=2.065 μm) was used. We have found that with increasing IR pump power density, the output doubled frequency SHG signal (λ=1.03 μm) increases and achieves its maximum value at the pump power density about 0.45 GW/cm² and NC size about 12 nm.The values of second-order optical susceptibilities were almost 20% larger than for the pure BTO NC single crystals. With decreasing temperature below 60 K, the SHG signal increases achieving maximal value at LHeT.     

Investigation of photo-induced absorption in a Bi12TiO20 crystal

A photorefractive Bi₁₂TiO₂₀ (BTO) crystal is exposed with high intensity pulsed-laser beams 532 nm (the average intensity of the laser was 110 mW/cm², the average intensity per pulse was 2 MW/cm²), and the photo-induced dynamics of the absorption between 480 and 900 nm are studied and explained by two long-lived energy levels in the forbidden band. The relaxation times of the long-lived energy levels are experimentally found to be 10⁴ and 10⁵ s. The hysteresis character of the absorption coefficient is discussed. PACS  42.70.Mp; 42.70.Gi     

Novel procedure for the simultaneous determination of the Debye length and electro-optic coefficient for an optically active photorefractive Bi12SiO20 crystal

We present a novel procedure based on an improved physical model and a versatile numerical fitting technique, to simultaneously determinate the Debye screening length and electro-optic coefficient using a thick sample of an optically active photorefractive crystal Bi₁₂SiO₂₀ (BSO). For the computation, experimental data of gain vs. grating spacing were obtained by a two-beam coupling arrangement. Unlike former calculation models, in our work, the general expression for the diffraction efficiency in the presence of self-diffraction is considered, and the influence of the optical activity in the coupling parameters is had into account for the calculation of the gain values. The fitting of the experimental data to the predicted theoretical behavior by our model is achieved by finding the closest theoretical curve to a set of data sampled from a spline-smoothed curve of the experimental data. Both, the Debye screening length l_s and the electro-optic coefficient r₄₁ are used as fitting parameters by searching in a rather wide range for each one of the parameters, so that, the estimation of their values is obtained in a more reliable and direct way from the same experiment. The calculations are performed in diffusion regimen and the procedure leads to l_s = 0.22 μm and r₄₁ = 4.5 × 10 ^− 12m/V. Because the optical activity can alter the maximum gain and self-diffraction effects influence the energy exchange, the procedure reveals to be physically appropriated for the simultaneous determination of these physical parameters when thick photorefractive crystals with high optical activity are considered.     

A magneto-optically modulated CH3OH laser for Faraday rotation measurements in Tokamaks

Distortion-free intracavity polarization modulation of an optically pumped CH₃OH laser is shown to be viable. The possible use of this modulation technique to make a multichannel Faraday rotation measurement on a Tokamak device is discussed. In addition, the CdTe Faraday modulator employed in this study is shown to have an anomalously large Verdet constant.     

“Running” holograms in photorefractive Bi12SiO20 crystals

It is shown that under uniform illumination and application of an external electric field, a phase volume hologram in a photorefractive crystal exists in the form of relaxing running wave. Effective recording of such a running hologram is possible only by an interference pattern moving synchronously with the recorded grating. The physical nature of this phenomenon, the possible enhancement of the hologram diffraction efficiency upon recording of a running pattern and the main factors limiting its value are discussed for the case of photorefractive Bi₁₂SiO₂₀ crystals.     

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.     

Real-time image division in four-wave mixing with photorefractive Bi12GeO20 crystals

Real-time image division is obtained by using four-wave mixing of a reflection hologram type with photorefractive Bi₁₂GeO₂₀ crystals. This method utilizes the nonlinear characteristic of four-wave mixing which reverses the phase of the incident wave.     

Real-time image subtraction in four-wave mixing with photorefractive Bi12GeO20 crystals

Based on a classical two-exposure holographic technique, real-time image subtraction is obtained in four-wave mixing of a reflection hologram type with highly sensitive photorefractive Bi₁₂GeO₂₀ crystals. This method takes advantage of the relatively slow response and erasure times of holographic recording in Bi₁₂GeO₂₀ crystals.     

Preparation and Faraday rotation of Bi-YIG/PMMA nanocomposite

Bismuth-substituted yttrium iron garnet (Bi-YIG) nanoparticles (NPs) were prepared by coprecipitation and subsequent heating treatment. Thermal gravity-differential thermal analysis was performed to investigate the thermal behavior of the Bi-YIG precursors and to decide the best annealing temperature. Phase formation of garnet NPs was investigated by X-ray powder diffraction. The size of Bi-YIG NPs was investigated by transmission electron microscopy, and the magnetic properties of Bi-YIG NPs were measured using a vibrating sample magnetometer. The results show that the temperature needed for the transformation of Bi-YIG from the amorphous phase to the garnet phase decreases with increasing Bi content, and Bi-YIG NPs with sizes of 28–78 nm are obtained after heating treatment at 650–1000 °C. The saturation magnetization of Bi-YIG NPs increases as the Bi content increases. Moreover, the Faraday rotation of polymethyl methacrylate (PMMA) slices doped with Bi-YIG NPs was investigated. The results indicate that the angle of Faraday rotation increases with increasing Bi content in PMMA composites, and the maximum value of the figure of merit is 1.46°, which is comparable to the value of a sputtered film. The Bi-YIG NPs-doped PMMA slices are new promising materials for magneto-optical devices.     

87Rb D1线法拉第反常色散光学滤波

为了得到在高背景噪音下对弱信号光的提取,实验研究了基于⁸⁷Rb D₁线5S_1/2F=2→5P_1/2 F'=1跃迁的795 nm法拉第反常色散光学滤波器.充铷的样品池所含⁸⁷Rb的比例高于自然铷,样品池处在均匀的磁场中并且夹在两个相互正交的偏振片之间.入射的探测光通过样品池,与原子相互作用,由于法拉第旋转效应实现滤波功能.改变实验条件,透射结果随之明显变化.当温度从340 K升高到360 K,透射谱的变化情况被细致记录,并且分析了导致透射情况变化的原因.在适当的工作温度以及磁场条件下,得到线宽为约220 MHz的超窄带透射谱线,谱线透过率约为48%.⁸⁷Rb D₁线的实验结果优于⁸⁵Rb的吸收线.     

Coherent signal beam amplification in two-wave mixing experiments with photorefractive Bi12SiO20 crystals

The amplification of the input signal beam in two-wave mixing experiments with photorefractive Bi₁₂SiO₂₀ crystals is achieved when an additional phase shift is established between the photoinduced index modulation (phase volume hologram) and the incident fringe pattern. This stationary phase shift is introduced by either moving the crystal or the interference fringes at a constant speed. The transferred intensity is measured versus the applied electric field, fringe spacing and crystal velocity. The crystallographic orientation and the relative displacement with respect to the applied electric field polarity determine the amplitude of the energy transfer. For the first time in this crystal, signal beam amplification is reached for an applied field E₀ > 8 kV cm^?1 and a crystal or fringe displacement speed around 5 μm s^?1 at the green line (λ = 514 nm) of an argon laser.     

Mapping of vibration amplitudes by time average holography in Bi12SiO20 crystals

We report the performance of a holographic set-up with a Bi₁₂SiO₂₀ (BSO) photorefractive crystal for measuring vibration amplitudes in a time average holography system. The volume hologram is recorded with the 514.5 nm wavelength of an Ar⁺ laser. In this set-up the reference beam is sinusoidally phase-modulated with the same frequency of the vibrating object. By increasing the value of the phase modulation, the fringes of the time average pattern run along the object surface, so that the vibration amplitude mapping is carried out. This technique allows the improvement of the measurement sensitivity compared with conventional time average holography, even for interferograms with poor visibility.     

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：     

Light-induced charge transfer processes in Mn-doped Bi12GeO20 and Bi12SiO20 single crystals

The optical absorption and ESR spectra of Bi₁₂GeO₂₀ and B₁₂SiO₂₀ doped with Mn have been measured before and after illumination with visible light. Uniaxial stress measurements on a sharp line observed at 8026 cm^?1 were performed. The observed ESR spectrum is a superposition of six lines resulting from the hyperfine interaction of manganese ions in tetrahedral positions. The g-factor and hyperfine constant are g = 1.999 ± 0.003 and A = 78 Gs. Analysis of the light-induced absorption spectrum leads to the conclusion that a small hole polaron bound to an Mn impurity at a tetrahedral site is responsible for the very broad absorption band which appears after illumination. The sharp line is interpreted as due to a transition inside the Mn⁺ center in tetrahedral coordination. Bands in the region 10,000–16,000 cm^?1 are due to Mn³⁺ centers in interstitial positions, whose symmetry can be treated to a first approximation as tetragonal. The following crystal field parameters for this center were found: B = 565 cm^?1, Dq = 1400 cm^?1, Dt = ?330 cm^?1, Ds = 4170 cm^?1 and C = 2260 cm^?1. The illumination conditions which are needed for homogeneous coloration of the sample are also discussed.     

Optimal multi-photon entanglement concentration with the photonic Faraday rotation

We put forward an optimal entanglement concentration protocol(ECP) for recovering an arbitrary less-entangled multi-photon Greenberger–Horne–Zeilinger(GHZ) state into the maximally entangled GHZ state based on the photonic Faraday rotation in low-quality(Q) cavity. In the ECP, only one pair of less-entangled multi-photon GHZ state and one auxiliary photon are required, and the concentration task can be realized by local operations. Moreover, our ECP can be used repeatedly to further concentrate the discarded items of conventional ECPs, which can increase its success probability largely. Under the practical imperfect detection condition, our protocol can still work with relatively high success probability. This ECP has application potential in current and future quantum communication.