Photorefractive Bi12TiO20 Crystal Research Articles

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Self-calibrating speckle photo-electromotive force for large vibration amplitude measurement

We report the use of the photo-electromotive-force effect in a speckled pattern of light on a photorefractive Bi12TiO20 crystal to measure the amplitude of transverse vibrations in a self-calibrating mode. Our theoretical model shows that, for vibrations much faster than the photorefractive response time, the photo-emf signal exhibits a maximum at a characteristic vibration amplitude value that may be used to calibrate the photo-emf sensor. Small differences between experiment and theory are still under research.

Waveguide properties of arrays of dark spatial solitons in a Bi12TiO20photorefractive crystal

The goal of this work is to present a study of waveguide properties of arrays of dark spatial solitons in Bi12TiO20 (BTO) crystal. We simulated arrays of dark spatial solitons by solving the nonlinear Schrödinger equation using first a tanh function and then a periodic function as initial conditions. When the periodic function was used, the output intensity distribution gives rise to an array of dark spatial solitons which, depending on the period, were separated by dark fringes. Nevertheless, the guiding ability of the arrays depends on the induced nonlinearity and the input angle of the probe beam. The guiding ability does not depend on the number of solitons in the array. We present experimental results for parallel propagation of one probe beam and the array of dark solitons generated by interference of two coherent laser beams in a photorefractive BTO crystal.

Photorefractive fringe-locked running hologram analysis in three-dimensional space

We report a new procedure allowing one to use fringe-locked running hologram experiments to characterize photorefractive materials in a very simple, reliable and direct way. This procedure is based on the representation of data in a three-dimensional space (diffraction efficiency, frequency detuning and applied electric field) that considerably facilitates data fitting. The latter procedure is evaluated by means of a mathematical simulation. The technique is applied to a photorefractive Bi12TiO20 crystal sample in order to verify its adequacy for computing some of the more relevant material parameters: diffusion length, Debye screening length and quantum efficiency for photoelectron generation. The effectively applied electric field inside the sample is obtained at the same time. The results are in good agreement with data already available for this sample.

Polarization properties of light-induced scattering in Bi_12TiO_20 crystals: theory and experiment for diagonal geometry

Illumination of ac-biased photorefractive Bi12TiO20 crystals with a coherent light beam results in the development of strong nonlinear scattering. Theoretically and experimentally we investigate the angular and polarization characteristics of the scattered light for the diagonal ([1¯1¯1]) optical configuration and different polarization states of the pump. A satisfactory understanding of the observed scattering properties is achieved for most of the cases investigated.

Enhanced photocurrent in photo-emf experiments in pure and doped absorbing photorefractive crystals

We report a mathematical formulation that successfully describes the holographic photocurrent that is produced, in strongly absorbing photorefractive materials, by the action of a pattern of interference fringes of light vibrating sinusoidally with large amplitude. The large vibrating amplitude produces a sensible enhancement of the photocurrent signal and in this way facilitates measurements. We also show that taking account of the bulk light absorption of the sample is essential for adequately describing the experiment. We measure the first temporal harmonic of the photocurrent, without an externally applied field, as a function of the amplitude and the temporal frequency of the vibrating pattern of fringes and show that these data fit our theoretical model well. From this fit we are able to determine some material parameters for pure and doped photorefractive Bi12TiO20 crystals.

Dark conductivity, photoconductivity, and light-induced absorption in photorefractive sillenite crystals

We measure the bulk and light-induced absorption as well as the dark- and photoconductivity in doped and undoped photorefractive Bi12TiO20 crystals and compare the results obtained using different techniques. From these data we compute the quantum efficiency for the photogeneration of charge carriers and the carriers mobility–lifetime product, and characterize the suitability of these crystals for holographic recording. A two-center model theory is shown to adequately describe absorption and photoconduction in these materials.

Speed and diffraction efficiency in feedback-controlled running holograms for photorefractive crystal characterization

We report the measurement of the diffusion length, the Debye screening length and the quantum efficiency of photoelectron generation in strongly light absorbing photorefractive Bi12TiO20 crystals, using fringe-locked running hologram experiments. The effective applied electric field inside the sample is also computed and self-diffraction is considered. The novelty here, as compared to formerly reported experiments, is that the diffraction efficiency is now measured simultaneously with the hologram speed v. From these data the above referred to photorefractive and experimental parameters are obtained without the need for additional experiments. The method is used to analyze two photorefractive Bi12TiO20 crystal samples, in different experimental conditions, using the 514.5 nm wavelength. The computed parameters are in good agreement with the available information about these samples.

Combined formation of a self-pumped phase-conjugate mirror and spatial subharmonics in photorefractive sillenites

We found that the formation of self-pumped phase-conjugate mirrors (SPPCM) in photorefractive Bi12TiO20 crystals under external alternating electric field is accompanied by strong enhancement of spatial subharmonics. This combined formation of the fundamental grating and its spatial subharmonics results in an expansion of the spatial-frequency band allowed for SPPCM recording.

Self-focusing of a single laser pulse in a photorefractive medium

An original experimental and theoretical time-resolved study of a single laser pulse self-focusing in a nonlinear photorefractive medium is reported. The behavior of the self-focusing process is experimentally observed in a photorefractive Bi12TiO20 crystal during the 5 ns pulse duration of a doubled Nd:YAG (yttrium aluminum garnet) laser. A theoretical interpretation is provided, based on a simple model of photorefraction on the nanosecond time scale.