Bi12TiO20 Crystals Research Articles

Self-diffraction of frequency-modulated light in the Bi12TiO20 crystal

The self-diffraction of frequency-modulated light in the photorefractive B12TiO20 crystal was studied experimentally. To observe the effect, the crystal was illuminated by two light beams with the relative frequency shift Δf(t). In the experiments, linear frequency modulation was used: Δf(t) = At. As a result of the light self-diffraction on a hologram moving with a constant acceleration, the power of the light beams at the crystal output changed in the form of a chirp pulse. It was found that the pulse appears at the instant of stopping the interference pattern, and its duration is determined by the rate of frequency change A and the hologram recording time τsc.

Non-steady-state photoelectromotive force and two-wave mixing in photorefractive crystals under frequency modulated illumination

We report the excitation of non-steady-state photoelectromotive force and two-wave mixing signals using uniformly accelerated motion of the recording light pattern. Such illumination is created by linear frequency modulation of the interfering light beams. The pulse response is predicted theoretically and observed experimentally in GaAs and Bi12TiO20 crystals at λ=633  nm. The evolution of the pulse shape versus sweep rate is demonstrated and explained in the frames of the developed theory. The application of the effects in laser Doppler velocimeters and accelerometers is discussed as well.

Photorefractive digital holographic microscopy: an application in the microdevices surfaces

In this work, we present a Photorefractive Digital Holographic Microscopy (PRDHM) technique based on the writing-reading holographic process in photorefractive Bi12TiO20 (BTO) crystal using the numerical reconstruction of the phase and amplitude image performed by a Digital Holographic Microscopy (DHM) method. We demonstrate that a holographic reconstructed image by diffraction process in photorefractive BTO crystal can be combined with a second reference beam to form a hologram in CCD plane in a DHM setup. The experimental results in a glass slide resolution target and thin film structure was obtained, where, amplitude, phase and 3D phase images are calculated performing digital reconstruction. This technique presents potentials possibilities to obtain 3D phase image of the microdevices for dynamic holography applications.

Efficiency of electrooptic spin-to-orbital angular momentum conversion in crystals

We introduce a figure of merit for the photon spin-to-orbital angular momentum conversion via electrooptic (EO) Pockels effect in single crystals. Relations for the effective EO coefficients are derived for different symmetry groups. We show that Bi12TiO20 crystals reveal the highest figure of merit among well studied crystalline EO materials. The efficiency of the spin-to-orbital angular momentum conversion is measured experimentally for a number of single crystals.

Resonance running hologram velocity nonlinearity dependence upon light intensity in photorefractive crystals

We report on the nonlinear relation between the resonance hologram velocity and the recording light irradiance in fringe-locked running hologram experiments carried out on a nominally undoped photorefractive Bi12TiO20 crystal using 532 nm wavelength laser light. Such nonlinearity is due to the dependence of the material diffusion length on the light irradiance. Experimental data show a good agreement with the theoretical equations thus supporting the mathematical model here reported and allowing to compute the quantum efficiency for photoelectron generation, the dependence of the diffusion length upon the recorded light irradiance, and the far from saturation diffusion length.

Experimental determination of effective electro-optic coefficient and electric screening field factor in the electrically induced birefringent Bi12TiO20 crystal by using an oblique incidence setup

We present an alternative procedure to determine the effective electro-optic coefficient (reff) and the electric screening field factor by measuring the optical intensity variation induced by an applied electric field (EApp), which is transmitted through an undoped photorefractive Bi12TiO20 (BTO) crystal in an oblique incidence setup. The transmitted intensity variation (TIV) was modeled taking into account the transmission coefficients for the polarization plane parallel and perpendicular to the incidence plane, as well as the birefringence induced by the EApp, which changes the components of the polarization vector of the light beam. The measurements were performed with infrared radiation at 780nm and provided reff=5.5±0.2pm/V. It allowed us to conclude, with support of results from the literature, that the region without dispersion of the electro-optic coefficient in the BTO crystal ranges from 510nm to at least 780nm. The procedure proposed here can be used for distinct wavelengths and seems to be suitable for other electrically induced birefringent materials.

Template-free and morphology-controlled hydrothermal growth of single-crystalline Bi12TiO20 with excellent simulated sunlight photocatalytic activity

Single-crystalline bismuth titanates (Bi12TiO20) with the cubic phase and various morphologies (flower-, belt-, tetrahedral-like) were controllably fabricated by using a mild template-free hydrothermal process. The samples were fully characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution TEM, nitrogen porosimetry measurement, UV-vis diffuse reflectance spectroscopy (DRS), and X-ray photoelectron spectroscopy (XPS). The influence of the solvent and the concentration of NO3− in the reaction system on the crystallinity and morphology of the Bi12TiO20 samples was investigated in detail. Bi12TiO20 crystals were used as the sunlight-driven photocatalysts to degrade aqueous dye methyl orange (MO) and p-nitrophenol (PNP), and the influence of Bi12TiO20 morphology on the photocatalytic activity was discussed. Finally, the recyclability of Bi12TiO20 was evaluated through three consecutive runs.

Photorefractive digital holographic microscopy applied in microstructures analysis

In this work, we present a Photorefractive Digital Holographic Microscopy (PRDHM) technique based on the writing–reading holographic process in photorefractive Bi12TiO20 (BTO) crystal and the obtainment of phase and amplitude of the object wave were performed by a digital holographic method. We demonstrate that the wave diffracted by a photorefractive hologram recorded in a BTO crystal can be combined with a reference wave to record a second hologram in a CCD sensor in a configuration of digital holographic microscopy. The experimental measurements were performed on samples like red blood cells and a thin film structure, and were obtained quantitative values of amplitude and phase of the object wave, as well as 3D graphs, of the analyzed samples by the digital reconstruction holographic method. This technique presents a new method in replacement of the usual methods for reconstruction of holograms recorded in a photorefractive crystal and also presents the possibilities to obtain 3D phase images for surfaces characterization and applications in dynamic holography.

Photorefractive Fourier transform profilometer for the measurement of 3-D object shapes

In the present work dynamic moiré-like fringe patterns, produced by photorefraction with low spatial frequencies, applied for profile determination of small objects is proposed. Basically, a Fourier transform profilometry technique is developed for an automated profile determination. This means, as far as we know, a new experimental procedure that exploits the real time holographic two wave mixing in Bi12TiO20 crystal sample. Besides, the mainly advantages of this procedure are, comparatively to the classical fringe projection method using the Michelson interferometer, best fringe pattern contrast, less speckle noise, absence of noises produced by spurious reflections and, the most significant, a pure sinusoidal shape obtained by dynamic holographic moiré-like process using photorefraction.

Amplitude autocorrelation of femtosecond laser pulses using linear photogalvanic effect in sillenite crystals

We demonstrate excitation of the linear photogalvanic current in a Bi12TiO20 crystal by two orthogonally polarized femtosecond laser pulses with detecting the electrical current via charge accumulation on the sample electrodes. Such a setup was used to implement an interferometric autocorrelation technique for characterization of ultrashort light pulses. Integration of the detected current in femtosecond time domain leads to vanishing of a bipolar component of the photogalvanic current which arises due to a pulse chirping. The advantage of the proposed technique is that it produces the electric field correlation function directly without the need for data processing using a compact, robust, and non-expensive detector in the form of a photoconductive cell of a non-centrosymmetric crystal.

Optical recording mechanisms in undoped titanosillenite crystals

We show that optical recording in undoped photorefractive titanosillenite Bi12TiO20 (BTO) crystals involve electrons, holes and ions, depending on the experimental conditions. Holographic recording and erasure at higher than room temperature was carried out on an undoped crystal sample, showing the presence of a fast photosensitive electron-based hologram and a slower compensating one of nonphotosensitive nature that is responsible for hologram fixing in BTO and is likely to be based on H+ ions. The fixed grating showed a diffraction eficiency η≈0.002 and a characteristic activation energy of 0.85±0.05 eV. A strong light-induced (darkening) photochromic effect was also detected, that had to be accounted on for diffraction efficiency measurement. Holes are shown to participate in the optical recording process when selectively photoexcited with infrared light.

Measurement of low-derivative surface lenses by two-laser holography with Bi_12TiO_20 crystals

Refractive and profilometric analyses of lenses with large radii of curvature and/or large focal distance were performed through photorefractive holography using a Bi12TiO20 crystal as the recording medium and two red diode lasers as light sources. Both lasers were properly aligned and tuned in order to provide submillimetric synthetic wavelengths providing real-time interferograms in a two-color holography experiment. The resulting contour interferogram describes the form of the wavefront after the beam traveled back and forth through the lens. The fringe quantitative evaluation was carried out through the four-stepping technique, and the resulting phase map and the branch-cut method were employed for phase unwrapping. Exact ray tracing calculation was performed in order to establish a relation between the output wavefront geometry and the lens parameters such as radii of curvature, thickness, and refractive index. By quantitatively comparing the theoretically calculated wavefront geometry with the experimental results, errors below 1% for both refractive index and focal length were obtained.

Two-wave mixing on reflection dynamic gratings in sillenite crystals under phase modulation of signal beam

The counter mixing of a strong circularly polarized wave of optical pump with a weak phase-modulated signal wave on the reflection grating formed by them in (111)-, (110)-, and (100)-cut Bi12TiO20 and Bi12SiO20 phototrefractive crystals was theoretically and experimentally studied.

Photorefractive two-wave mixing phase coupling measurement in a self-stabilized recording regime

The measurement of the phase coupling between the two-wave mixed transmitted and diffracted beams behind a photorefractive material is directly obtained using a self-stabilized recording technique. These phase coupling data are used to compute the Debye screening length on an undoped Bi12TiO20 crystal. The diffraction efficiency, which is obtained in two different ways from the same self-stabilized experimental run, is similarly processed to compute the Debye length too. The Debye length is also measured for Pb- and Zr-doped Bi12TiO20 crystals, and from this parameter the corresponding effective density of traps is computed and shown to be lower than for the undoped sample.

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.

Spectral dependence of absorption photoinduced in a Bi12TiO20 crystal by 532-nm laser pulses

The spectral dependences of absorption photoinduced in a pure bismuth titanium oxide crystal by 532-nm laser pulses are studied. It is shown that optical absorption in the crystal in the range from 492 to 840 nm increases with increasing exposure. The photoinduced absorption relaxes in the dark for more than 60 hours. A model of photoinduced absorption is proposed which assumes the population of two trap centres with the normal energy distribution law for the concentrations of electrons photoexcited from donors to the conduction band. This model well describes the spectral dependences of photoinduced absorption by using the average ionisation energies of the traps E1 = 1.60 eV and E2 = 2.57 eV. The model is used to estimate the increase in the photorefractive sensitivity of a bismuth titanium oxide crystal in the near IR region, which was earlier observed after exposing the crystal to visible radiation. It is predicted that the speed of response of dynamic holography devices based on BTO crystals exposed to green light can be increased.

Density of states in Bi12TiO20 from time-of-flight measurements

Two sillenite crystals of Bi12TiO20, one intrinsic and one lead doped, have been studied by means of the time-of-flight (TOF)experiment. After a short presentation of the technique, we return to the different methodsproposed to extract the transit time from the current transients recorded with theTOF experiment. Further, we illustrate the possibility of extracting the densityof states from these transients by means of a Laplace transform applied to thevariation with time of the transient currents. Then we present some experimentalresults. The room temperature drift mobilities we measured were equal to 2 and0.2 cm2 V−1 s−1 for the lead doped and intrinsic sample, respectively. Finally we present the densities ofstates found for each crystal and compare them to the results obtained from modulatedphotocurrent experiments applied to the same samples.

Polarization effects at two-beam interaction on reflection holographic gratings in sillenite crystals

The polarization structure of the signal wave that interacts with a circularly polarized pump wave on the reflection holographic gratings in the (112)-, (111)-, and (110)-cut Bi12TiO20 (BTO) crystals is experimentally and theoretically analyzed. An analytical expression for the vector field of the signal wave that is valid for an arbitrary orientation of the cubic refractive crystal is derived using approximation of the nondepleting circularly polarized pump wave. The contribution of the absorption component of the reflection hologram into the counter interaction is estimated for the crystals under study using the approximation of the experimental data with theoretical curves. Based on the experiments with incoherent quasi-monochromatic radiation, it is demonstrated that the real BTO crystals exhibit weak induced linear birefringence.

Phenomenological characterization of photoactive centers in Bi12TiO20 crystals

We report optical and electrical measurements contributing for a better characterization of the relevant photoactive center levels in undoped photorefractive Bi12TiO20 (BTO) crystals grown in Brazil. Comparative results for Pb-doped BTO and Bi12GaO20 are also reported. A center responsible for photochromism was identified at 0.42–0.44eV, probably below the conduction band (CB). The main electron and hole donor center is detected at 2.2eV from the CB and the equilibrium Fermi level is pinned at this level. Other localized centers were identified at different positions in the band gap and their relation with the behavior of BTO under different wavelengths and operating conditions is discussed with particular attention to holographic recording.

Doping effect on the optical, electro-optic, and photoconductive properties of Bi12MO20 (M = Ge, Si, Ti)

The doping effect on the optical, electro-optic, and photoconductive properties of Czochralski-grown Bi12MO20 (M = Ge, Si, Ti) sillenite-structure single crystals was studied. The dopants were introduced into the growth charge in the form of oxides. Bi12TiO20 crystals were doped with V, Zn, Cu, P, and Nb; and Bi12SiO20 crystals were doped with Cd and Mo. The results indicate that the doping level has a significant effect on the photoconductivity of the Bi12TiO20〈Zn〉, Bi12TiO20〈Cu〉, Bi12SiO20〈Cd〉, and Bi12SiO20〈Bi24CdMoO40〉 crystals, which may exceed that of undoped crystals at low doping levels and may be substantially lower than it, down to zero, at increased dopant concentrations. Niobium doping of bismuth titanate has no effect on its photosensitivity and electro-optic properties. Phosphorus and vanadium enhance the photosensitivity of bismuth titanate over the entire composition range studied but have little effect on its electro-optic coefficient r41. A slight increase in r41 was only observed at high vanadium concentrations. The axial impurity distribution in the crystals is shown to be nonuniform, which reflects in their photoresponse: the photoconductivity of the copper-doped crystals near the seed end is 3 times that near the tail end.