Bi12TiO20 Crystals Research Articles

780 nm near-infrared photorefractive holograms recorded in undoped Bi12TiO20 crystal under the action of external electric field

In this paper, we studied recording and erasure of photorefractive holographic gratings recorded at 780 nm in an undoped Bi12TiO20 (BTO) crystal sample under the action of an externally applied electric field. The diffraction efficiency was measured by directly chopping the diffracted beams by the gratings recorded in a two-wave mixing setup without using of any phase-modulation technique of the recording beams or feedback mechanism. An enhancement of 3.5-fold in the diffraction efficiency was observed when the applied electric field increased from 0 to 6 kV/cm. The theoretical dependence of the diffraction efficiency on the external electric field considering the effect of screening of charges near the electrodes was investigated, and the result is supported by good fit to the experimental data, allowing us to compute characteristic parameters of the material, including the values of photoactive donor concentrations (ND1)eff ≈ 0.12 × 1016 cm−3 and (ND2)eff ≈ 0.8 × 1015 cm−3, respectively, for the levels placed at 1.6 eV and 1.3 eV below the conduction band responsible for photorefractive recording in the BTO sample.

Hole-electron competition investigated by holographic recording in Bi12TiO20 crystal under applied electric field

Hole-electron competition was induced in an undoped Bi12TiO20 crystal using coherent pre-illumination at 532 nm. Photorefractive recording and erasure at 639.7 nm under the action of external electric field E0 were used to theoretically and experimentally investigate the competition within the sample band-gap. The good agreement between theory and experimental data allowed to obtain, in both regimes without and with pre-illumination, the saturation space-charge fields Eq and holes and electrons effective donors concentrations (ND)eff's. The phase shift between the electronic gratings and the hole grating was also computed and revealed a competition behavior between the applied electric field effect of increasing the characteristic lengths of electrons and holes moving in opposite directions in the conduction and valence bands, respectively, and the intensity of decreasing such lengths. The computed holes donor density indicated a strong contribution of the coherent pre-illumination to the hole-electron competition mechanism.

INFLUENCE OF INVERSE PIEZOELECTRIC AND PHOTOELASTIC EFFECTS ON THE INDEX SURFACE OF THE NORMAL COMPONENT INVERSE TENSOR DIELECTRIC PERMEABILITY OF THE Bi12TiO20 CRYSTAL

The index surface of the normal component of variation of the inverse tensor dielectric constant of the photorefractive Bi12TiO20 crystal of 23 symmetry class is constructed and analyzed. The extremal values of the index surface of the normal component for Bi12TiO20 crystal with formed holographic grating are determined. It is shown that the combined action of the photoelastic and inverse piezoelectric effects leads to a change in the maximum and minimum values of the normal component of variation of the inverse tensor dielectric constant of the crystal with the wave vector of the holographic grating oriented along the crystallographic direction.

Determination of mechanical properties of Bi12TiO20 crystals by nanoindentation

Bi12TiO20 (BTO) single crystal was grown by Czochralski method and investigated mechanically by nanoindentation measurements. X-ray diffraction pattern of the crystal presented one intensive peak around 37.95° associated with (330) plane of cubic crystalline structure. Nanoindentation experiments were performed at various loads between 5 and 100 mN. Hardness and Young's modulus of the crystal were determined by Oliver-Pharr method. The hardness-load dependency exhibited behavior of indentation size effect. True hardness value of BTO crystal was revealed as 4.4 GPa. Young's modulus decreased with increase of load and load-independent Young's modulus was found around 93 GPa at high loads. The load-dependent elastic and plastic deformation components were calculated and it was observed that the dominant component in BTO single crystal is plastic deformation at the applied loads. The present paper reports for the first time the mechanical characteristics of the BTO single crystal by carrying out nanoindentation experiments.

High efficient Cefixime removal from water by the sillenite Bi12TiO20: Photocatalytic mechanism and degradation pathway

Water pollution is a significant concern that affects the environment and human health. Antibiotics are among the most dangerous types of pollutants spreading in our time because of their harmful effects, such as generating antibiotic-resistant bacteria. Therefore, it is necessary to attain an eco-friendly and efficient method for treating those hazardous compounds. This study proposes a clean approach for removing antibiotics from pharmaceutical wastewater, using an efficient photocatalyst Bi12TiO20 (BTO) synthesized by the sol-gel method. Several characterizations were carried out to identify the obtained catalyst, such as XRD, BET, Raman, FESEM, EDX with elemental mapping, TEM, UV–Vis, and PL, in which the space group of BTO crystals was discovered to be I23, with a bandgap of 2.9 eV. To evaluate the photocatalytic properties of the catalyst BTO, Cefixime (CFX) was chosen as a pollutant example. The photocatalytic efficiency was optimized using an artificial neural network (ANN) method with a deep learning technique. The ANN network was trained using experimental data with various BTO dosages and CFX initial concentrations at varying pH. The results have shown that the BTO catalyst can lead to 94.93% CFX degradation and 87.66% mineralization within only 3 h, this efficiency was very high compared to other catalysts used in previous studies. The relative importance of different photocatalytic parameters was estimated using ANN data, the highest effective parameter was the initial CFX concentration. Then, the by-products were analyzed using GC-MS, and a pathway for CFX degradation was suggested. The mechanism of the degradation was also investigated in the presence of scavenger agents. The results showed that the CFX molecules had been degraded totally into tiny molecules, proving the efficient performance of this catalyst. These findings make this sillenite an effective catalyst for removing antibiotics from the aquatic environment.

Lloyd’s mirror interferometer for holographic recording in photorefractive crystals

We report the use of Lloyd’s mirror interferometer for holographic recording in photorefractive material. The experimental results show that perturbations in the diffraction efficiency considerably decrease due the feature of that system; and, as a consequence, fast (electron-based) and slow (hole-based) gratings were recorded in a nominally undoped photorefractive Bi12TiO20 crystal. Measurement of the response times were determined as a function oflightintensity for the 0-1.2 mW/cm2 range using the 532 nm laser wavelength. Parameters that characterize the crystal were also determined and compared with available data in the literature in order to assess the reliability of this technique.

Degenerate Four-Wave Mixing by Transmission Holographic Gratings in a Bi12TiO20 Crystal of the (110)-Cut

Degenerate Four-Wave Mixing by Transmission Holographic Gratings in a Bi12TiO20 Crystal of the (110)-Cut

Вырожденное четырехволновое взаимодействие на пропускающих голографических решетках в кристалле Bi-=SUB=-12-=/SUB=-TiO-=SUB=-20-=/SUB=- среза (110)

The regularities of the stationary degenerate four-wave mixing on transmission holographic gratings formed in the Bi12TiO20 crystal of the (110)-cut are analyzed. The system of differential equations has been obtained; this system that can be used to find the components of the vector amplitudes of linearly polarized light waves in the case of four-wave mixing on phase and phase-amplitude holographic gratings. The theoretical model take into account the linear electrooptic, the photoelastic and the inverse piezoelectric effects, as well as natural optical activity, circular dichroism and crystal absorption. The values of the orientation angle and the crystal thickness where the reflection coefficient can reach maximum values are determined. It was experimentally found that the reflection coefficient can reach 2.4 with an optimal choice of the orientation angle in the Bi12TiO20 crystal of (110)-cut with the thickness of 7.7 mm. It is shown that the best agreement between theoretical and experimental data is achieved if the phase-amplitude structure of transmission holographic gratings formed in the Bi12TiO20 crystal is taken into account in the mathematical model of diffraction.

Near-infrared holographic photorefractive recording under applied electric field in undoped Bi12TiO20 sillenite crystal

Abstract Direct holographic recording in undoped Bi12TiO20 crystal at 1064 nm is investigated aiming the characterization of diffraction efficiency under action of applied dc electric field (E0). An enhancement of 12-fold in the diffraction efficiency was revealed when E0 increased from 0.0 to 4.2 kV/cm. The theoretical dependence of the diffraction efficiency and the holographic erasure time upon E0 was investigated using the standard model of Kukhtarev for photorefractivity and the results showed a good experimental data fitting in both cases, allowing the computation of the effective trap concentration (ND)eff ≈ 5.62 × 1015 cm−3 which is responsible by the recording mechanism into the crystal sample. In particular, the theoretical-experimental investigation using the holographic erasure time also allowed to determine others relevant parameters for Bi12TiO20 such as the diffusion transport length LD, the Maxwell relaxation time τ M , and the quantum efficiency Φ that can explain the relatively slow dynamics of photorefractive recording and erasure in near-infrared region. The type of charge carrier involved in the recording mechanism in the near-infrared region is also discussed.

Diffraction Efficiency of Mixed Transmission Holograms in a Photorefractive Optically Active Bi12TiO20 Piezoelectric Crystal of Arbitrary Cut

The process of reading mixed transmission holograms formed in a photorefractive Bi12TiO20 piezoelectric crystal (sillenite family) of arbitrary cut has been theoretically optimized. The optimal crystal cuts, which provide the highest diffraction efficiency of holograms, are determined. The results of the theoretical analysis are based on the previously obtained experimental data; they are in agreement with the crystal point symmetry group and may serve as the basis for increasing the efficiency of dynamic holography devices with Bi12TiO20 crystals.

Recording and erasure of photorefractive holograms in undoped BTO crystal at moderate to high intensities of 639.7 nm laser under action of 532 nm laser pre-illumination.

We investigate recording and erasure of photorefractive holographic gratings in an undoped Bi12TiO20 crystal in a moderate to high intensity regime of the recording beams at 639.7nm without and with the action of laser pre-illumination at 532nm. The detected hologram without pre-illumination indicates the participation of two photorefractive electronic gratings in its recording process, and the diffracted signal by itself exhibits a fivefold enhancement when the total intensity increases from 38.4 to 214.5 mW/cm2. The dependence of the measured total diffraction efficiency on intensity was investigated and showed linear behavior. At least three gratings are present in the regime of pre-illumination and participate in the writing and erasure of holographic mechanisms. Two of them are electronic, and one is hole-based, with a phase difference of Δϕ between them. The theoretical approach used to analyze the total diffraction efficiency based upon the photorefractivity standard model, and considering the presence of the three gratings, showed good agreement with the holographic erasure experimental data and permitted us to compute Δϕ, which exhibited strong and unusual dependence on the total intensity.

Crystal growth and optical characterization of terbium and niobium doped BTO single crystals

In this study, single crystals of Bi12TiO20 doped with niobium and terbium were grown by Top Seeded Solution Growth technique (TSSG). Samples with good optical quality were obtained and were investigated concerning optical absorption, optical activity, electro-optic effect, and photoconductivity. It was verified that doping with terbium and niobium changes the defects structure characteristic of sillenites, affecting in different ways the optical behavior of crystals.

Dielectric response of doped Bi12TiO20: Ru crystals in an alternating electric field

The results of examination of AC dependences of capacitance and dielectric loss tangent of sillenite Bi12TiO20 crystals doped with ruthenium on frequency are presented. Non-Debye dispersion of dielectric coefficients is found in the frequency interval of 5 × 102–105 Hz, and a resonance phenomenon is observed. Polarization processes in the studied samples are attributed to relaxators associated with metal–oxygen vacancies and structural elements incorporating 6s 2 lone-pair electrons.

Near-infrared sensitive organic–inorganic photorefractive device

Organic–inorganic hybrid structure, assembled by Rh-doped Bi12TiO20 crystal and liquid crystal (LC) layer, operating at near-infrared range is proposed and demonstrated. Due to the photorefractive properties of inorganic substrate, light illumination caused a space charge field which acts as a driving force for LC molecules re-alignment and subsequent refractive index modulation. All optically controlled phase retardation ability has been demonstrated supporting possibilities for further infrared applications.

Photorefractive and computational holography in the experimental generation of Airy beams

In this paper, we present the experimental generation of Airy beams via computational and photorefractive holography. Experimental generation of Airy beams using conventional optical components presents several difficulties and a practically infeasible. Thus, the optical generation of Airy beams has been made from the optical reconstruction of a computer generated hologram implemented by a spatial light modulator. In the photorefractive holography technique, being used for the first time to our knowledge, the hologram of an Airy beam is constructed (recorded) and reconstructed (read) optically in a nonlinear photorefractive medium. The Airy beam experimental realization was made by a setup of computational and photorefractive holography using a photorefractive Bi12TiO20 crystal as holographic recording medium. Airy beams and Airy beam arrays were obtained experimentally in accordance with the predicted theory; with excellent prospects for applications in optical trapping and optical communications systems.

Slowing down of light pulses using backward-wave four-wave mixing with local response

The slowing down of light pulses is achieved using backward-wave four-wave mixing in a medium with local response. A Bi12TiO20 crystal with an external dc field is used in the experiment as a proof-of-concept material. The delay and shape transformation of output pulses are studied and compared for the transmitted and phase conjugate channels. It is shown that the phase conjugate pulse achieves a longer delay under typical experimental conditions with equal intensities of the pump beams. This advantage of the phase conjugate beam is especially pronounced for short pulses with half-widths smaller than the response time of the medium. The agreement of the experimental results with numerical calculations confirms the validity of the previously developed theoretical model. Analytical solutions are derived for input pulses with triangular shapes.

Sign of the dominant charge carriers in photorefractive crystals determined by a phase-locked holographic technique

This work presents a holographic method based on active feedback techniques for determining the sign of the dominant charge carriers in photorefractive materials. A two-step procedure is proposed: first off a stationary phase-locked hologram is recorded; an electric field normal to the grating layers is then applied to the material, thus producing a running hologram. The sign of the charge carriers is determined by comparing the direction of the applied field with the direction of the hologram movement, which is known through the automatically attached light pattern. The method can be applied from highly photoconductive to highly insulating materials. Furthermore, no information on any material parameter is required. The method is validated by a set of holographic experiments using a Bi12TiO20 crystal that has electrons as the majority photocarriers.

Light-induced relaxation dynamics in Rh-doped Bi12TiO20 crystals

The lifetime of the excited charge carriers in Rh-doped BTO crystals is characterized by measuring the time-resolved photoinduced absorption (PIA) after nanosecond pulse excitation from a frequency doubled Nd:YAG laser (λ=532 nm). It was found that the Rh-addition in the BTO structure slows down the relaxation decay in comparison with non- doped BTO, which is attributed to additional trapping centers related to the rhodium dopant. The experimental curve is well fitted by a double-exponential decay which is ascribed to the presence of two different shallow traps contributing to the charge transport and recombination mechanisms in Rh-doped BTO crystal.

Influence of W5+ ions on the optical properties of doped Bi12TiO20

There have been studied single crystals of undoped and doped Bi12TiO20 with two concentrations of W5+ (2.62×1017cm−3 and 2.62×1018cm−3). There have been obtained absorption spectra in the energy range of 10,482–15,408cm−1 by classical measurements. There have been determined the cross-section (σa) of the impurity absorption and the oscillator strength of d–d transitions. There have been calculated the refractive index of doped crystals and the concentration of Ti3+ ions in an undoped sample through an experiment.

Orientation and Temperature Dependence of Piezoelectric Properties for Sillenite-Type Bi12TiO20 and Bi12SiO20 Single Crystals

The full matrix of electro-elastic constants of sillenite-type crystals Bi12TiO20 (BTO) and Bi12SiO20 (BSO) were determined by the resonance method, with d14 and k14 being on the order of 40–48 pC/N and 31%–36%, respectively. In addition, double-rotated orientation dependence of d33 was investigated, with the maximum values of 25–28 pC/N being achieved in ZXtl45°/54°-cut samples. The electrical resistivity of BSO was found to be two orders higher than that of BTO, being on the order of 7 × 105 Ω cm at 500 °C. The temperature dependence of dielectric and piezoelectric properties were investigated. BSO exhibited a high thermal stability in the temperature range of 25–500 °C, while BTO showed a variation of ~3% in the range of 25–350 °C. The high values of d14 and k14, together with the good thermal stability, make BTO and BSO crystals potential candidates for electromechanical applications in medium temperature range.