L. Arizmendi

Determination of H concentration in LiNbO3 by photorefractive fixing

A new method to determine the proton concentration in LiNbO3 is presented. The method is based on the measurement of the diffraction efficiency of a photorefractive grating in two situations. It is first measured after recording at room temperature, and second after saturation of the fixing process at a given temperature (about 150 °C). From only these two experimental data, the value obtained for the proton concentration in our sample is H0=(4.4±0.7)×1018 cm−3. This value agrees, within the experimental error, with that obtained from the infrared absorption arising from the OH− stretching bond.

Study of developing thermal fixed holograms in lithium niobate

Experimental results on the developing kinetics and final diffraction efficiency of fixed holograms in iron-doped lithium niobate are presented. Samples with two different oxidation states are studied. The developing kinetics of well-oxidized samples show oscillations superposed to a saturation dependence, whereas they are not present for the less oxidized sample. The final developed ratio is found to depend on the grating spacing and the oxidation state of the samples. All these features are well explained with the charge-transport theory and are found to be dependent on the photovoltaic properties of the samples, doping, and oxidation state. From the analysis of the experimental data, the photovoltaic field amplitude of the samples is obtained.

Luminescence spectra of lead-doped NaCl, KCl and KBr

Abstract The luminescence spectra of lead-doped NaCl, KCl and KBr have been systematically investigated. Special attention has been paid to the effects of concentration and thermal history of the crystals. In the three systems, the emission spectra for A and C band excitation consists mainly of two well-defined emission bands whose energy separation is ∼0.7 eV. It has been concluded that none of the bands can be attributed to a single type of lead center but are both typical of Pb 2+ luminescence. In fact, their behavior can be correlated with that found for most monovalent ions and interpreted in a similar way. The excitation spectra for the two emissions have shown that the A-band is complex. One of the components appearing in very low doped and quenched samples is ascribed to dipoles, whereas additional side bands are attributed to complexes or small aggregates involving Pb 2+ ions.

Lifetimes of thermally fixed holograms in LiNbO 3 :Fe crystals

The thermal erasure decay at different temperatures of holographic gratings thermally fixed in iron-doped lithium niobate has been exhaustively studied. An activation energy of 0.94 eV was found to be independent of the grating fringe spacing. At a given temperature the decay time is strongly dependent on the grating spacing. This dependence is well fitted to a square fringe spacing law, as predicted theoretically. From the experimental data the room-temperature hologram lifetime under illumination is extrapolated, giving as a result t(years)=(5.2+/-0.2)?(2) , with the grating spacing ? in micrometers.

Erasure kinetics and spectral dependence of the photorefractive effect in Fe:LiNbO 3

The erasure kinetics of holographic gratings has been studied as a function of the erasing wavelength in Fe:LiNbO3. Sample self-absorption has been rigorously taken into account and is a relevant point. After such a correction, even those apparently nonexponential decays can be nicely fitted to simple exponential ones. This correction also results in a more reliable spectral dependence of the erasing efficient transitions. The photorefractive writing spectrum has been obtained by the compensator technique. Writing and erasing show different spectral behaviors, which are briefly discussed.

Nephelauxetic effect in luminescence of Cr3 + -doped lithium niobate and garnets

Abstract The model based on Harrison theory of bonding is used to explain quantitatively the nephelauxetic effect in LiNbO 3 and Y 3 Al 5 O 12 crystals doped with chromium, studied by high-hydrostatic pressure spectroscopy. The model uses only one adjustable parameter and can also be used for other dopant–ligands systems in different compounds. We show that not only ligands, but also the second neighbors of the dopant, influence the nephelauxetic effect.

Experimental effects of light intensity modulation on the recording and erasure of holographic gratins in BSO crystals

Experimental data of the influence of light modulation amplitude m on the photorefractive effect in Bi12SiO20 (BSO) are reported. In these experiments charge diffusion was the unique transport mechanism. The recording and erasure kinetics are studied in detail for the three first harmonics of the refractive gratings. Differences between linear (m⪡1) and nonlinear regimes were clearly observed. A superlinear dependence of the refractive index change on the light modulation (for large m) has been observed, confirming theoretical predictions.

Nephelauxetic effect in LiNbO3:Cr3+ crystals

A model, based on the Harrison theory of bonding is developed for quantitative explanation of the nephelauxetic effect, i.e., a reduction of interelectronic crystal field Racah repulsion parameters B and C due to the covalency of bonds. The results are used for a description of the effect in chromium-doped lithium niobate crystals. The model can be also applied to other systems.

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.