Optical waveguides in LiNbO<Subscript>3</Subscript> and stoichiometric LiNbO<Subscript>3</Subscript> crystals by proton exchange

The formation of optical planar waveguides in LiNbO₃ and stoichiometric LiNbO₃ crystals by proton exchange was reported. The prism-coupling method was used to characterize the dark-line spectroscopy at the wavelength of 633 and 1539 nm, respectively. The mode optical near-field outputs from proton-exchanged LiNbO₃ and SLN waveguides at 633 nm were presented. The mode field from stoichiometric LiNbO₃ (SLN) waveguide is lighter and more uniform than that from LiNbO₃ waveguide, which means the quality of the waveguide in SLN crystal is better than that of the LiNbO₃ waveguide. For proton-exchanged LiNbO₃ waveguides, the evolution of the refractive index profile with annealing was presented. The disorder profiles of Nb atoms in proton-exchanged LiNbO₃ waveguides were obtained by Rutherford backscattering/channeling technique. It is shown that the longer the exchange time, the larger the displacement of Nb atoms. Supported by the National Natural Science Foundation of China (Grant No. 10475052) and the Scientific Research Start-up Financing of Qufu Normal University     

Light scattering in single crystals of nonlinear-optical photorefractive LiNbO<Subscript>3</Subscript>:Cu and LiNbO<Subscript>3</Subscript>:Zn

Comparative studies are made of the photorefractive scattering of light in nonlinear single crystals of lithium niobate with congruent compositions (LiNbO₃) doped with "photorefractive" Cu [0.015 mass %] and "nonphotorefractive" Zn [0.5 mass %] cations. For the first time it is found that single crystals doped with "photorefractive" and "nonphotorefractive" cations have different indicatrices for photorefractive light scattering. The aperture angle for photorefractive scattering reaches its steady state value more rapidly with high laser powers than with low. However, at high powers laser induced heating of the crystal is greater, and this leads to a narrowing of the scattering indicatrix. It is also found that photorefractive scattering in these single crystals depends on the region of the boule from which a sample has been cut. This indicates that there is a nonuniform distribution over the boule of the imperfections with localized electrons which determine the magnitude of the photorefractive effect.     

Selective photorefractive scattering in LiNbO<Subscript>3</Subscript>:Rh crystals

Photorefractive scattering occurs in rhodium-doped lithium niobate crystals irradiated by coherent light. The photorefractive scattering has both wide-angle and selective components. The results of experimental investigation of selective photorefractive scattering in LiNbO₃:Rh crystals and calculation of the spatial structure of scattering are reported. The selective scattering is regarded as a kind of a frequency-degenerate fourwave vector interaction.     

Controlling optical inhomogeneity of LiNbO<Subscript>3</Subscript> single crystals

Different ways of inducing changes in the optical inhomogeneity of single crystals of lithium niobate are described. These include the application of a conductive coating in combination with the heating of samples and exposure to the field of a corona discharge, to UV radiation from a Nd:YAG laser, and to the electron beam of an electron microscope. All such effects can be used to record images.     

Photoconductivity in LiNbO<Subscript>3</Subscript> crystals codoped with MgO and Cr<Subscript>2</Subscript>O<Subscript>3</Subscript>

Polymer stabilized cholesteric texture cells with various chiral and monomer concentrations were fabricated. Electro-optical measurement indicates that the chiral concentration and the monomer concentration significantly influence the hysteresis, threshold voltage and field-on and field-off response times of the cell. The chiral dopant is found to increase the hysteresis width and to decrease the field-off response time of the cell. The monomer is found to decrease the hysteresis width and to increase the field-off response time of the cell. The field-on response time is independent of the monomer concentration and the chiral concentration of the cell. A thinner cell has a smaller hysteresis width. PACS 61.30.-v; 42.70.Df; 42.79.Kr     

Reduction-induced polarons and optical response of Mg-doped LiNbO<Subscript>3</Subscript> crystals

We studied the visible and IR dispersion of absorption coefficient and refractive index for congruent LiNbO₃ and Mg:LiNbO₃ crystals before and after chemical reduction at different annealing temperatures. The concentration of Mg in Mg:LiNbO₃ samples was just below or above the photorefractive threshold. The reduction-induced changes in the absorption coefficient reveal the formation of polarons typical for doped LiNbO₃ crystals. It was shown that the polaron concentration is maximal when the Mg concentration is just below the photorefractive threshold and the annealing temperature is near 500 °C. This temperature is optimal for the most efficient polaron formation at all considered concentrations of Mg. The fitting of the experimental absorption dispersion curves indicates that intermediate polarons are formed in LiNbO₃:Mg crystals preferably. The spectral dependence of transmission for samples of lithium niobate of various thicknesses was studied. The results indicate that there are spatial regions with much greater absorption than that of bulk crystals. We assume that, in general, polarons are localized in thin near-surface regions. The spectral dependence of the refractive index in the vicinity of the phonon absorption edge indicates some essential changes of the phonon subsystem taking place after reduction. The infrared contribution into the dispersion of the dielectric function real part increases considerably after reduction. PACS 71.38.Ht; 71.38.-k; 78.20.Ci     

LiNbO<Subscript>3</Subscript>:Yb<Superscript>3+</Superscript> crystals for optical temperature sensors

Possibilities of using lithium niobate crystals, doped with ytterbium (LN:Yb³⁺) as materials for optical temperature sensor (OTS), are discussed. We consider both the radiative and absorptive characteristics of the crystals, assuming that their temperature dependences are caused by the Boltzmann factor of the initial population of Stark sublevels of the ion. It is shown that the crystals can be used as materials for OTS in the temperature range 150–400 K.     

ZnO-doped LiNbO<Subscript>3</Subscript> single crystals studied by X-ray and density measurements

Energy Dispersive X-ray Fluorescence Spectroscopy, X-ray diffraction and density measurements were conducted on undoped and ZnO-doped congruent LiNbO₃ single crystals grown by the Czochralski method. Based on the experimental results, an intrinsic defect evolution model was proposed. When ZnO was doped into the congruent LN crystals, the Zn ions replaced first the Li ions and increased the density. Then, the Zn ions simultaneously replaced the Li ions and the antisite Nb_Lis until all Nb_Li ions were replaced, which increased the density further. After that, the Zn ions substituted Nb ions in the Nb-sublattice sites with the reduction of the Li vacancies as self-compensation and thus reduced the density. When the Li vacancies disappeared completely, the Zn ions substituted simultaneously both Li ions in the Li-sublattice sites and Nb ions in the Nb-sublattice sites. The simultaneous substitution might finally lead to the generation of oxygen vacancies and decreased the density further. PACS 61.72.-y; 06.30.Dr; 61.10.-i     

Spectroscopic properties of Er<Superscript>3+</Superscript> ions in LiNbO<Subscript>3</Subscript> crystals codoped with HfO<Subscript>2</Subscript>

The results of the spectroscopic analysis of transition strengths for Er³⁺ ions in a series of Hf:Er:LiNbO₃ crystals with variable Hf content and fixed Er content are reported. Unpolarized UV-VIS-NIR absorption spectra, upconversion fluorescence spectra excited at 800 nm, and microsecond time-resolved spectra excited at 400 nm and 800 nm by 800 nm femtosecond laser were measured at room temperature. The HfO₂ incorporation has influence on Er³⁺ radiative lifetimes, and fluorescence branching ratios. For Hf(4 mol %):Er(1 mol %):LiNbO₃, Ω₂=2.63×10^-20 cm², Ω₄=2.86×10^-20 cm², and Ω₆=0.72×10^-20 cm². Ω₂<Ω₄ is contrary to the Er³⁺ general trend of Ω₂>Ω₄>Ω₆ when the Hf content is below its threshold concentration. In addition, the sum of Ω increases with the Hf content when the HfO₂ content below 6 mol % is unfamiliar. The upconversion mechanism is discussed in this work. PACS 71.20.Eh; 77.84.Dy; 42.62.Fi; 42.65.Ky     

Investigation of the stability of electrical properties of reduced LiNbO<Subscript>3</Subscript> crystals

The instability of the electrical properties of lithium niobate single crystals of congruent composition subjected to reducing thermochemical treatment has been investigated by impedance spectroscopy. It has been shown that the subsequent heating of the reduced lithium niobate samples in dry air up to 380 K or higher is accompanied by the progressive increase in their electric resistance, which is due to the oxidization of the crystal surface layers.     

Anomalies in the pyroelectric properties of LiNbO<Subscript>3</Subscript> crystals of the congruent composition

A number of anomalies are revealed in the temperature dependence of the primary pyroelectric coefficient for single-domain crystals of lithium niobate LiNbO₃ in the temperature range 310–330 K. It is shown that these anomalies are associated with the transformation of the domain structure of the lithium niobate crystals. The problem regarding the correct performance of the pyroelectric measurements and the possible influence of the observed effects on the optical properties of lithium niobate crystals and on the recording of optical phase holograms are discussed.     

Periodic domain structures obtained by growth of LiNbO<Subscript>3</Subscript> crystals doped with gadolinium

The growth of a periodic domain structure in LiNbO₃ crystals doped with gadolinium (Gd, 0.44 wt %) was investigated by special electron beam surface charging in a scanning electron microscope and on a atomic force microscope. Ferroelectric domain boundaries with “tail-to-tail” and “head-to-head” P _s orientations were comparatively analyzed. The domain boundaries had different forms and differed by their physical properties. Micron-size surface crystal regions close to the “tail-to-tail” boundaries were charged slower by electron irradiation and had modified elastic properties. This region was detected by using the lateral force mode of atomic force microscopy. The observed morphology and property features were supposed to be due to different concentration gradients of the Gd impurity and different width of its distribution close to the domain boundaries of different types.     

Origins of waveguiding in femtosecond laser-structured LiNbO<Subscript>3</Subscript>

Femtosecond laser-induced structural changes in LiNbO₃ are studied. Depending on the laser processing parameters two different types of modification are identified and their origin is discussed. Both types of modification can be described within the framework of induced lattice defects. For strong material damage a refractive index increase can be obtained due to the induced stress field. By appropriate tailoring of this stress field thermally stable and highly symmetric waveguides can be obtained well suited for nonlinear integrated-optical applications. PACS 61.80.Ba; 77.84.Dy; 42.65.Re; 42.82.Et     

Properties of thermally fixed holograms in photorefractive LiNbO<Subscript>3</Subscript>:Zn:Fe crystals

We have investigated the properties of thermally fixed holograms in LiNbO₃ crystals doped with the optical damage inhibitor Zn as well as the photorefractive Fe dopants. Time decays of fixed holograms at different temperatures showed a single thermally activated process with an activation energy of ∼1.08 eV. We have also studied the effect of an external electric field on the diffraction efficiency of these holograms. Results analysis has provided a new method to determine the photovoltaic field of the samples as well as the effective concentration of photorefractive traps.     

Superlinear photovoltaic currents in proton-exchanged LiNbO<Subscript>3</Subscript> waveguides

Photovoltaic currents along the c axis have been measured in α-phase LiNbO₃ proton-exchanged waveguides at several visible wavelengths for a guided-beam configuration. The light-intensity dependence is superlinear and all experimental curves are very well fitted by computer simulations using a two-centre model, with Fe²⁺/Fe³⁺ as primary and Nb_Li ⁴⁺/Nb_Li ⁵⁺ as secondary photovoltaic centres. The superlinear behaviour arises from a much higher effective photovoltaic length of Nb_Li ⁴⁺ (small polaron) compared with that of Fe²⁺. In β₁-phase guides, the photocurrents are much smaller than in α-phase guides and apparently do not show superlinear behaviour. Received: 22 October 2002 / Revised version: 6 January 2003 / Published online: 12 May 2003 RID="*" ID="*"Corresponding author. Fax: +34-91/3978-579, E-mail: m.carrascosa@uam.es     

Luminescence and electronic excitations in KBe<Subscript>2</Subscript>BO<Subscript>3</Subscript>F<Subscript>2</Subscript> crystals

This paper reports on a study of the dynamics of electronic excitations in KBe₂BO₃F₂ (KBBF) crystals by low-temperature luminescent vacuum ultraviolet spectroscopy with nanosecond time resolution under photoexcitation by synchrotron radiation. The first data have been obtained on the kinetics of photoluminescence (PL) decay, time-resolved PL spectra, time-resolved PL excitation spectra, and reflection spectra at 7 K; the estimation has been performed for the band gap E _g = 10.6−11.0 eV; the predominantly excitonic mechanism for PL excitation at 3.88 eV has been identified; and defect luminescence bands at 3.03 and 4.30 eV have been revealed. The channels of generation and decay of electronic excitations in KBBF crystals have been discussed.     

Temperature effects in proton exchanged LiNbO<Subscript>3</Subscript> waveguides

Temperature effects on single, homogeneous phases of proton exchanged LiNbO₃ waveguides have been studied. The refractive index of -phase guides on z-cut and x-cut congruent substrates monotonically increases with temperature, the thermo-optical coefficient being similar to that of the substrate. In ₂-phases on z-cut congruent substrates and in ₁-phases on z-cut and x-cut congruent and quasi-stoichiometric substrates, the thermo-optical coefficient is smaller than that of the substrate below 60 °C and higher above this temperature. A rather similar effect has been observed in ₁-phases on z-cut and x-cut congruent substrates in the linear expansion coefficient. The microscopic origin of the phase transition is not known for the moment. PACS Provide PACS codes please.     

Study of Oxygen Diffusion in Reduced LiNbO<Subscript>3</Subscript> Crystals

Using the method of impedance spectroscopy and optical density measurements, the diffusion of oxygen in single crystals of lithium niobate of the congruent composition after the reductive thermochemical processing is studied. The parameters describing the diffusion of oxygen in the temperature range 493–693 K are established.