Ivan de Oliveira

Dielectric relaxation time measurement in absorbing photorefractive materials

We show that absorbing photorefractive materials characterized by a space-charge exponential relaxation time law exhibit an overall hologram optical erasure that is described by the so-called exponential integral function, as far as self-diffraction can be neglected. This fact is due to the bulk absorption producing an exponentially decreasing distribution of the erasure beam irradiance along the sample thickness that results in a correspondingly increasing dielectric relaxation time. The theoretical development in this paper is experimentally verified by the analysis of the holographic erasure in a nominally undoped Bi TiO photorefractive crystal using the 514.5 nm laser wavelength where this material exhibits a relatively 12 20

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.

Holographic phase-shift measurement during development of a fixed grating in lithium niobate crystals

We report what is believed to be the first direct measurement of the grating phase-shift evolution during white-light illumination for the development of a fixed grating in an Fe-doped lithium niobate crystal. Stabilized holographic recording is shown to be essential for such measurements. Experimental data are in good agreement with theory and allow computation of the relevant material parameters for the sample under analysis. The results are of the utmost relevance for understanding the advantageous behavior of oxidized samples in hologram fixing.

Fixed holograms in iron-doped lithium niobate: simultaneous self-stabilized recording and compensation

We analyze the mechanisms leading to a highly diffractive fixed hologram in photorefractive Fe-doped lithium niobate crystals by simultaneous self-stabilized holographic recording and compensation at moderately high temperatures. We show that a partially compensated running hologram is produced during recording under this condition and discuss the performance of the process in terms of the operating temperature, the degree of oxidation ([Fe(3+)]/[Fe(2+)] ratio) of the sample, and the effect of the absorption grating arising from the spatial modulation of the Fe(2+) concentration produced during photorefractive recording. We experimentally measure the evolution of the uncompensated remaining hologram during recording and the evolution of the diffraction efficiency of the fixed hologram during white-light development and show that the maximum fixed grating modulation to be achieved is roughly limited by Fe-dopant saturation. A reproducible eta approximately 66% efficiency fixed grating was obtained on a sample exhibiting an otherwise maximum fixed eta approximately 3% when using the classical three-step (recording at room temperature--compensating at high temperature--developing at room temperature) process.

Campomanesia adamantium Peel Extract in Antidiarrheal Activity: The Ability of Inhibition of Heat-Stable Enterotoxin by Polyphenols

Campomanesia adamantium (Myrtaceae) is a medicinal plant distributed in Brazilian Cerrado. Different parts of this plant are used in popular medicine for treatment of several diseases like fever, diarrhea, hypercholesterolemia and rheumatism. The aim of this work was to evaluate the inhibition of heat-stable enterotoxin type A (STa) by gallic acid present in the peel of C. adamantium fruit and assays to assess the antidiarrheal activity, anti-inflammatory and cytotoxic properties of peel extract using the T84 cell line model. The possible inhibition exerted by the gallic acid of the peel extract on the STa peptide was inferred by molecular dynamics simulations. The antidiarrheal effects were investigated measuring cGMP accumulation in cells after stimulation by STa toxin and antibacterial activity was assessed. The anti-inflammatory activity was assessed by inhibition of COX-1 and COX-2. MTT and LDH assays were used to evaluate any possible cytotoxic action while the CyQUANT test was used to investigate the effect on cell proliferation. A representation showing how the possible interactions between STa and the gallic acid of the extract might reduce the action of the enterotoxin is presented. C. adamantium peel extract significantly decreased the levels of cGMP in T84 cells. However, no effect on the species of microorganisms was observed. The extract also inhibited COX-1 (IC50 255.70 ± 0.04 ng/mL) and COX-2 (IC50 569.50 ± 0.11 ng/mL) enzymes. Cytotoxicity assay have shown significant changes in cells treated with the extract, which inhibited the cell proliferation until 72 hours of treatment. Direct interactions of phenolic compounds present in the extract with the STa toxin may limit its activity. Curative effect in the diarrhea treatment and its anti-inflammatory action is based on the pharmacological properties, mechanism of action of the C. adamantium peel extract, and no toxic effects of the peel extract presented on this work.

Use of specklegrams background terms for speckle photography combined with phase-shifting electronic speckle pattern interferometry

Abstract. Electronic speckle pattern interferometry (ESPI) is combined with digital speckle photography (DSP) to measure out-of-plane deformation in the presence of large in-plane translation or rotation. ESPI is used to measure out-of-plane displacements smaller than the speckle diameter. In-plane displacements larger than the speckle size are obtained by DSP using artifacts images computed from the phase-stepped specklegrams. Previous works use the specklegram modulation for that purpose, but we show that this can lead to errors in the case of low modulation. In order to avoid this, a simple averaging of phase-stepped specklegrams allows obtaining the average irradiance, which contains information on the speckled object image. The latter can be used more efficiently than the modulation in DSP and is simpler to compute. We also perform a numerical simulation of specklegrams, which show that the use of background terms is much more stable against some error sources as compared to modulation. We show experimental evidence of this in various experiments combining out-of-plane ESPI measurements with in-plane translations or rotations obtained by our DSP method. The latter has been used efficiently to restore phase loss in out-of-plane ESPI measurements due to large in-plane displacements.

Fixed photorefractive holograms with maximum index-of-refraction modulation in LiNbO3:Fe

We report the recording of a fixed good quality transmission hologram in LiNbO3:Fe with maximum index-of-refraction modulation using the simultaneous recording/compensation process at 120 °C in a specially designed setup with λ=514.5 nm. This process was shown to be reproducible and in good agreement with an already reported theoretical model. The analysis of this recording process showed that material saturation was reached so that the maximum possible fixed index-of-refraction modulation was achieved. From the comparison of theoretical and experimental recording/compensation process data some material parameters (dielectric relaxation time τMe≈15 min, saturation space-charge field Eq=18.8 kV/cm, and photovoltaic-to-saturation field ratio Eph/Eq=0.80) were determined. The diffraction efficiency of this grating was measured using a λ=633 nm probe laser beam in an independent setup and its actual value computed, taking into account the angular divergence of the probe beam. The good grating performance as a...

Self-stabilized recording of fixed gratings at high temperature in LiNbO3:Fe.

A high quality fixed holographic grating was recorded in a photorefractive LiNbO(3):Fe crystal at about 100 degrees C in a homemade temperature-controlled vacuum chamber. The recording was carried out using self-stabilization techniques with lambda=532 nm beams guided onto the crystal by polarization maintaining fibers. The diffraction efficiency of the fixed grating was eta=0.44 when measured in the recording setup using the same lambda=532 nm recording beams. A compatible eta was measured with lambda=633 nm in an auxiliary setup, and a 1 mrad angular Bragg selectivity at FWMH was estimated, thus demonstrating the uniformity and good quality of the fixed grating.

Detection of resonance space-charge wave peaks for holes and electrons in photorefractive crystals

We report nonstationary photorefractive holograms in absorbing Bi12TiO20 samples with different degrees of hole-electron competition exhibiting resonance peaks due to electron and hole charge carriers. One sample with moderate hole-electron competition and another with a much larger effect were studied. Experimental data from these samples were analyzed using a theoretical model accounting for electrical hole-electron coupling, wave coupling, and response-time variation along the sample thickness due to bulk light absorption. Comparing experimental data and theoretical results allows finding out material parameters adequately describing hole and electron photoactive centers in these samples.

Holographic recording and characterization of photorefractive Bi2TeO5 crystals at 633 nm wavelength light

We report on the holographic recording on photorefractive Bi2TeO5 crystals using λ=633 nm wavelength light. We studied the behavior of this material under the action of this low photonic energy light and found out the presence of a fast and a slow hologram, both of photorefractive nature and exhibiting rather high diffraction efficiencies. The faster and the slower holograms are based on the excitation and diffusion of oppositely charged carriers (likely electrons and holes). Relevant parameters for the photoactive centers responsible for both kind of holograms were characterized using purely holographic techniques. No evidences of non-photosensitive ionic charge carriers being involved in the recording process at room temperature nor self-fixing effects were found.