Josué Carvalho

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.

Study of the interaction between indole-based compounds and biologically relevant G-quadruplexes

G-quadruplexes (G4s) are high-order secondary structures that modulate several key cell processes such as telomere function, gene expression and DNA replication. In the past years G4s have emerged as promising targets for drug development due to the discovery of small molecules which bind and stabilize these structures. In this work, we report the synthesis of indole-based compounds and the study of their interaction with the biological relevant G4s c-MYC and human telomeric repeat 22AG using several biophysical techniques. The ligands are G4 specific and they can discriminate different G4 structures namely parallel and hybrid-1 topologies. The NMR study of interaction between the most promising indole-derivative (compound 3) and c-MYC quadruplex suggests that the ligand binds on the external tetrads with additional actions in the loops/grooves in a 2:1 ratio. The molecular docking calculations of compound 3 to c-MYC quadruplex corroborates with the 1H and NOESY NMR studies. Overall, the results suggest that indole-based ligands are promising candidates for future lead optimizations in drug development.

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.

Characterization of Photorefractive Materials Using Holographic and Photoconductivity Techniques

Photorefractive materials exhibit photoconductive and electro‐optic properties. The first step in the optical recording process in these materials is the building up of a spatial modulation of electric charges via charge carriers excitation from localized to extended states where they diffuse and/or are drifted and then retrapped in a localized state. In this way the pattern of light onto the sample is converted into a spatial modulation of charges and its corresponding space‐charge electric field produces a volume index‐of‐refraction modulation via electro‐optic effect. The latter is the resulting volume phase hologram. The position of the photoconductive centers in the localized states in the material band gap as well as the density and properties of the donor/acceptor centers and charge carriers transport properties are therefore important parameters characterizing the materials and determining their performance as far as optical recording is concerned. For determining these parameters we use optical, ...