Gábor Mandula

Holographic scattering as a technique to determine the activation energy for thermal fixing in photorefractive materials

We introduce holographic scattering as a technique to determine the activation energy for thermal fixing of refractive index patterns in photorefractive crystals. After recording a parasitic hologram at ambient temperature, we measured the time dependence of the transmitted intensity at the fixing temperature, to determine the time constant. The temperature dependence of the latter allowed us to evaluate the thermal activation energy. For comparison, we performed an equivalent experiment employing the standard two-wave mixing method. The values obtained using the two techniques agree very well.

Refractive index measurements on bismuth tellurium oxide (Bi2TeO5) single crystal

The refractive indices of the biaxial Bi2TeO5 single crystal have been measured for the first time at seven wavelengths in the range of 457.9–632.8 nm at room temperature with an error of about 0.0003 (at λ=632.8 nm, n1=2.3203, n2=2.3678 and n3=2.4022). The optical plane was found to be the (010) plane and the crystal is shown to be optically negative. The crossing angle of the optical axes was found to be between 79° and 86°. The parameters of Sellmeiers one- and two-term dispersion equations have been determined by the least squares method. Resonance wavelengths λ1=219 nm for the first and λ1=182 nm and λ2=318 nm for the second one were found. The conditions of phase-matched SHG are discussed.

Photorefractive damage resistance threshold in stoichiometric LiNbO 3 :Zr crystals

Several optical methods including ultraviolet absorption, infrared absorption of the hydroxyl ions, Raman spectroscopy, and the Z-scan method have been used to determine the damage resistance threshold in 0-0.72 mol. % Zr-containing, flux-grown, nearly stoichiometric LiNbO₃ single crystals. All spectroscopical methods used indicate that samples containing at least ≈0.085 mol. % Zr in the crystal are above the threshold while Z-scan data locate the photorefractive damage threshold between 0.085 and 0.314 mol. % Zr.

Kinetics of OH− ions in nearly stoichiometric LiNbO3 crystals

Abstract Kinetics of the complex OH− stretching vibrational band in a nearly stoichiometric LiNbO3 crystal (Li/Nb ≈ 0.988) has been studied in order to explore the relation between the thermal fixing process of holograms and the hydroxyl ion absorption. By a careful analysis of the spectra we have observed that the intensity of the absorption band components at a given temperature changes with time suggesting that protons migrate among the different sites until they reach a thermodynamic equilibrium. The time dependence of the intensities has been measured at different temperatures between 40–120 °C after a heat treatment at 250 °C. From the measured data the time constants and the corresponding activation energies (Eav ≈ 1.1 ± 0.1 eV) were determined and compared with those of the thermal fixing process. The results obtained confirm that the hologram fixing process is governed by proton migration in the crystal.

Thermal fixing of holographic gratings in nearly stoichiometric LiNbO3 crystals

The thermal decay of holographic gratings recorded using the conventional two-wave mixing technique has been studied in congruent and nearly stoichiometric LiNbO3 crystal doped with Mn. The activation energies of this process have been determined in the 70-130 degrees C range for congruent and 20-80 degrees C range for nearly stoichiometric crystals, the obtained values being 1.06 +/- 0.03 and 1.10 +/- 0.03 eV, respectively. The kinetics of the OH absorption spectrum has also been studied in undoped nearly stoichiometric LiNbO3 between 40-120 degrees C. The time dependence of the band intensities can be characterized by exponential time constants obeying the Arrhenius-law. The average activation energy, Ea equals 1.1 +/- 0.1 eV is in good agreement with those obtained from the thermal decay indicating that the hologram fixing process in nearly stoichiometric LiNbO3 is governed by proton migration.

Decay of photorefractive gratings in LiNbO3:Fe by neutron irradiation

The effect of neutron irradiation on photorefractive gratings in LiNbO3:Fe single crystals is studied experimentally. The observed phenomena result from the large effective cross section of Li6 for thermal and cold neutrons and from the large number of the electrons excited to the conduction band by the high kinetic energy that is released during the neutron generated fission of Li6 nuclei. The excited electrons erase the previously recorded holographic grating. The sensitivity threshold of the effect is better than 160mSv (1.2×1010cm−2 fluence) at neutron energy of 0.17eV. Potential applications of the phenomena are discussed.

Activation energy of thermal fixing in LiNbO3: a comparative study

The activation energy of thermal fixing is determined in congruent and nearly stoichiometric lithium niobate crystals doped with manganese or iron, respectively. Three different techniques were employed: two-wave mixing, holographic scattering and DC conductivity measurements. A comparison between the three techniques is made and the possible reasons for the discrepancy in the values of the activation energy are discussed. Holographic techniques have the advantage of being contactless methods by which problems coming from electrodes effects are ignored. The holographic scattering technique is much simpler than two-wave mixing technique and gives the same results at high density of the compensating ions. At low free ions concentration it is an ideally sensitive technique to detect the possible dependence of the compensation time constant on the spatial frequency and to determine the concentration of free ions that are responsible for thermal fixing.

Activation Energy of Proton Migration in Mn- and Fe-Doped Lithium Niobate Obtained by Holographic Methods

The activation energy of thermal fixing of photorefractive gratings is determined in congruent and nearly stoichiometric lithium niobate crystals, both doped with iron or manganese. The novel technique called holographic scattering method is compared with the standard two-wave mixing method. A measurement of the angular distribution of the self scattered intensity and its possible analytical function is presented. The mathematical problems of the holographic scattering method are discussed applying the angular distribution functions.

Real-time dynamic calibration of a tunable frequency laser source using a Fabry-Pérot interferometer

We report on a method for real-time dynamic calibration of a tunable external cavity diode laser by using a partially mode-matched plano-concave Fabry-Pérot interferometer in reflection geometry. Wide range laser frequency scanning is carried out by piezo-driven tilting of a diffractive grating playing the role of a frequency selective mirror in the laser cavity. The grating tilting system has a considerable mechanical inertness, so static laser frequency calibration leads to false results. The proposed real-time dynamic calibration based on the identification of primary- and Gouy-effect type secondary interference peaks with known frequency and temporal history can be used for a wide scanning range (from 0.2 GHz to more than 1 GHz). A concave spherical mirror with a radius of R = 100 cm and a plain 1% transmitting mirror was used as a Fabry-Pérot interferometer with various resonator lengths to investigate and demonstrate real-time calibration procedures for two kinds of laser frequency scanning functions.

A Method to Determine H+ Concentration in Dehydrated Iron Doped Lithium Niobate Using Photorefractive Beam Fanning Effect

We investigated the thermal behavior of the photorefractive fanning effect in dehydrated lithium niobate doped with iron. Thermal dark decay of backward scattering did not follow the law determined for thermal fixing of two-wave mixing gratings. It can be explained by a combination of reflection and small-angle fanning. The concentration of hydrogen in the sample was determined by comparing the decay of 90° scattering with that of the small-angle one. A calibration factor of A/N H = (2.5 ± 0.5) × 10−18 cm, and a cross section of (2.1 ± 0.4)× 10 − 19 cm 2 were also determined for the 3480 cm − 1 IR absorption band of OH– ions.