Jesiel F. Carvalho

Optical and magnetic characterization of pure and vanadium-doped Bi12TiO20 sillenite crystals

Abstract Visible and Infrared (IR) optical absorption, Raman scattering and Electron Paramagnetic Resonance (EPR) techniques were used for systematic characterization of sillenite type crystals: nominally pure Bi 12 TiO 20 (BTO) and doped with vanadium (BTO:V). IR and Raman scattering results showed that the most conspicuous difference between BTO and BTO:V was the appearance of a band, at 767 cm −1 and at 790 cm −1 , attributed to the (VO 4 ) −3 . EPR, at 20 K, led us to suggest that the valence state of vanadium ions in BTO is 5+. In pure BTO samples, anisotropy of the EPR spectra at 20 K may be related to Fe 3+ impurities coming from starting compounds plus the presence of the (Bi M 3+ + h O + ) intrinsic defect. Results obtained from these different techniques confirm the Oberschmid and Grabmaier model [R. Oberschmid, Phys. Stat. Sol. (a) 89 (1985) 263; B.C. Grabmaier, R. Oberschmid, Phys. Stat. Sol. (a) 96 (1986) 199].

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.

Electron spin resonance of Cu2+ impurities in L-arginine phosphate monohydrate single crystals

Abstract Using electron paramagnetic resonance spectroscopy we studied single crystals of l -arginine phosphate monohydrate doped with copper. The angular dependence of the spectrum in three orthogonal planes shows that the Cu 2+ ions occupy two symmetry related sites in the structure. The Zeeman and hyperfine coupling tensors were determined. The principal values are: g 1 =2.053, g 2 =2.054, g 3 =2.251, A 1 ≈0, A 2 =24×10 −4 cm −1 and A 3 =177×10 −4 cm −1 . Superhyperfine interactions with 14 N ligands, well resolved for certain orientations of the magnetic field, suggest that Cu impurities have three N ligands. In terms of the molecular structure and based in similar systems, our findings are discussed and suggestions are made.

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.

Photovoltaic effect in Bi2TeO5 photorefractive crystal

We report on the presence of a strong photovoltaic effect on nominally undoped photorefractive Bi2TeO5 crystals and estimated their Glass photovoltaic constant and photovoltaic field for λ = 532 nm illumination. We directly measured the photovoltaic-based photocurrent in this material under λ = 532 nm wavelength laser light illumination and compared its behavior with that of a well known photovoltaic Fe-doped Lithium Niobate crystal. We also show the photovoltaic current to strongly depend on the polarization direction of light. Holographic diffraction efficiency oscillation during recording and the behavior of fringe-locked running holograms in self-stabilized experiments are also demonstrated here as additional indirect proofs of the photovoltaic nature of this material.

Nonlinear light-induced absorption in Bi 2 TeO 5 photorefractive crystals

We report on the light-induced absorption in undoped photorefractive Bi2TeO5 (BTeO) crystal. The so-called “two-center” model is shown to adequately describe the present material behavior. We demonstrate that nonlinear light-induced absorption occurs for high light intensities. The theoretical model allowed us to find out some material parameters from experimental data. We also investigate the effect of intensity and wavelength on the light-induced absorption.

Holographic recording in photorefractive Bi2TeO5 crystals at high intensity

We report the recording of nearly 35% diffraction efficiency holograms of photorefractive nature in Bi2TeO5 crystals, using 633 nm wavelength laser beams. Holographic techniques showed that a slow and a fast holograms arise, the latter based on electrons and the former based on positively charged carriers. We also measured the quantum efficiency for photoelectron generation and the characteristic diffusion (LD) and Debye (ls) lengths of these holograms and verified that LD and ls are modified as the light intensity onto the crystal increases, in agreement with previously reported theoretical prediction and experimental results but on other photorefractive material.

Construção de uma fonte de corrente e de uma sonda para medida de condutividade pelo método da sonda de quatro pontas

This paper describes the construction of an eletrical current source and of a probe to be used in the measurement of eletrical conductivity through a four-point probe method. These pieces of equipments can be obtained at the low price of US

Photoelectric Conversion Effect in Non-Photovoltaic Photorefractive Materials

50.00 and are adequate for eletrical conductivity measurements in the semiconductor range, that is from 10-1 to 10-6 S cm-1.

Electron paramagnetic resonance study of ternary CuII compounds with glycine and phenanthroline

Abstract: We report on the photoelectric conversion at a non-photovoltaic photorefractive material sandwiched between transparent conductive electrodes. Its behavior is based on the light-induced Schottky effect. Its nature and performance was clearly established by wavelength-resolved photoconductivity. OCIS codes: Photoconductivity (040.5150); Photorefractive Materials (160.5320); Optoelectronics (230.0250)