Photorefractive properties of paraelectric potassium lithium tantalate niobate crystal doped with iron

We report the successful growth of paraelectric potassium lithium tantalate niobate (KLTN) single crystal doped with iron. Detailed investigations have been made on the photorefractive properties of the as-grown crystal. The key parameters such as space-charge field, grating response time, photorefractive sensitivity and sign of the dominant charge carrier were obtained by two-wave mixing technique. 1.7 mm thick sample exhibits a high diffraction efficiency of 78% at the external field of 3.3 kV/cm and a sensitivity of 1.49 × 10⁻¹⁰E₀ cm²/J. The two-wave mixing gain coefficient increases linearly with external field, and reaches a large value of 19.4 cm⁻¹ at 4 kV/cm. Based on experimental results, iron is an effective dopant to KLTN which shows high diffraction efficiency and two-wave mixing gain coefficient.     

Charge transport in highly iron-doped oxidized lithium niobate single crystals

The photorefractive properties of highly iron-doped lithium niobate crystals in as-grown and completely oxidized states are investigated. Iron concentrations range from 0.5 to 3 wt. %. Measurements of light-induced refractive-index changes, two-beam coupling gain, bulk-photovoltaic current densities, and conductivities are performed with visible light. Absorption spectra and the bulk photovoltaic currents are used to determine the Fe²⁺ concentration. The dark conductivity confirms the tunneling model for charge-carrier migration. Beam-coupling experiments as well as photocurrent measurements with highly oxidized samples indicate that the one-center model is valid in such highly doped crystals, but that hole conductivity dominates the charge transport. PACS 72.20.Jv; 72.40.+w; 81.40.Tv     

Electric-field-controlled two-dimensional Raman-Nath diffraction from paraelectric potassium lithium tantalate niobate

Electric-field-controlled two-dimensional Raman-Nath diffraction has been realized using a photorefractive diffraction grating. The grating was produced by two-wave coupling (at a wavelength of 632.8 nm) at small incidence angles using a potassium lithium tantalate niobate single crystal. Results for the Raman-Nath diffraction from the g₄₄ grating are presented, in which the externally applied field is perpendicular to both the grating vector and the wave vector of the incident beam. Two pairs of coherent beams were used to record the grating for two-dimensional Raman-Nath diffraction. The wave vector and the polarization of one pair lay in the (x, z) plane, and those of the other pair lay in the (y, z) plane. The influence of the applied electric field was studied, and the results show that the intensity of the Raman-Nath diffraction could be controlled by the direction and intensity of the applied field.     

Polaron luminescence in iron-doped lithium niobate

Photoluminescence related to the bound polaron Nb_Li⁴⁺ is investigated as a function of temperature and incident light intensity in iron-doped lithium niobate crystals with various iron concentrations. Experiments are done under constant-wave (CW) and pulsed illumination. Its found that the decay time is always monoexponential. The radiative lifetime, the activation energy of the nonradiative lifetime and the quenching temperature are only weakly sensitive to iron concentration. On the other hand, the magnitude of the photoluminescence signal seems strongly correlated to the Fe²⁺ concentration, and the superlinear regime evidenced at low CW illumination definitely confirms that polaron excitation in lithium niobate is a two-step process.     

Reflection holograms in iron-doped lithium niobate

Received: 7 November 1996/Revised version: 16 December 1996     

One-dimensional steady-state photorefractive spatial solitons in centrosymmetric paraelectric potassium lithium tantalate niobate

We report the first observation of spatial one-dimensional photorefractive screening solitons in centrosymmetric media and compare the experimental results with recent theoretical predictions. We find good qualitative agreement with theory.     

Spectral studies on erbium doped potassium lithium tantalate niobate single crystal grown by the step-cooling technique

An erbium doped K_0.603Li_0.397Ta_0.428Nb_0.572O₃ single crystal was grown by the step-cooling technique. The crystal has a tetragonal tungsten bronze-type structure at room temperature with a Curie temperature of 303°C. There are Er ions characteristic absorption bands around 449, 485, 521, 550, and 652 nm in the visible absorption spectrum. Upconversion fluorescence spectra and power dependence centered at 527 nm, 548 nm, and 660 nm under 975 nm excitation were measured at room temperature. Decay lifetimes of the 548 nm and 660 nm emission bands are 281 μs and 420 μs, respectively. The lifetime of the 548 nm emission corresponding to the transition of ?⁴ S _3/2→⁴ I _15/2 is ten times the lifetime of the same transition of Er³⁺ in LiNbO₃ crystal and twice in KYb(WO₄)₂ crystal. The crystal might become a promising upconversion laser material. The upconversion mechanism of Er³⁺ in the sample was discussed based on decay curves and pump power dependence analyses in this work.     

Polar behaviour induced by lithium in potassium tantalate ceramics

Polar behaviour in K(1-x)Li(x)TaO(3) ceramics with x = 0:02, 0.05 and 0.10, processed by the conventional solid state method, is studied by Raman spectroscopy and thermally stimulated depolarization current (TSDC) techniques between 10 and 290 K. The TO1 mode of KTaO(3) is revealed to harden in the whole temperature range and to split in the low-temperature range by Li doping. One splitting is observed for x = 0:02 and two consequent splittings are detected for x = 0:05 and 0.10. The temperatures, where TO1 mode splitting occurs, are found to correspond to those of the peaks of TSDC, and hence to the onset of the electric polarization. Such behaviour provides evidence for the order-disorder ferroelectric phase transition induced in KTaO(3) by lithium doping, which emerges from deformations of the cubic phase developed on cooling either in one (for x = 0:02) or two steps (for x = 0:05 and 0.10).     

Field-induced light deflection in lithium niobate and lithium tantalate: the possible configurations

Field-induced light deflection by ferroelectric domain walls in lithium niobate (LN) and lithium tantalate (LT) is theoretically studied. The phenomenon can occur not only when both the incident wave vector k and the electric field E are parallel to the z-axis—as demonstrated by experiments made so far—but also when E and/or k are perpendicular to z. In particular, for E parallel to x and k parallel to y, the deflection phenomenon is predicted to have the same characteristics as in triclinic or monoclinic ferroelastics: the large deflection angle is related to the natural birefringence, whereas the deflection amplitude is proportional to the small tilt angle of the neutral lines, which is here induced by the electric field. In periodic domain structures, interference between deflected waves occurs, and the deflected intensity is expected to be largely enhanced when the Bragg condition is satisfied. This transverse configuration is thus specially attractive to characterize periodically-poled crystals.     

LiNbO3:Fe晶体二波耦合弱光放大的物理机理

对LiNbO₃: Fe晶体中二波耦合过程进行了动态观测.进一步探讨了LiNbO ₃:Fe晶 体中弱光放大的物理机理.发现LiNbO₃ : Fe晶体中二波耦合过程的弱光放大 对c轴指 向有明显的依赖关系，虽然光生伏打效应对光生载流子的迁移有主要贡献，但扩散机理的贡 献仍不可忽略.弱光最终得到放大是瞬态能量转移与扩散机理引起的能量转移的共同贡献.弱 光放大达到准稳态之后的下降过程为瞬态能量转移的时间指数衰减过程与光散射引起的能量 耗散的共同贡献. 关键词： 光折变效应 光放大 掺杂铌酸锂     

Growth,characterization and upconversion properties of erbium-doped potassium lithium tantalate niobate single crystals under 975 nm laser excitation

Potassium lithium tantalate niobate single crystals doped with erbium ions are grown by top-seeded solution growth method. The crystals are characterized by X-ray diffraction and differential thermal analysis. The refractive indices of the crystal are measured using ellipsometry method and fitted by Sellmeier equation. The as-grown crystals are tetragonal phase tungsten bronze-type structure with Curie temperature of 271.3 °C. Characteristic Er^3 + absorption bands are observed from 350 to 1100 nm in ultraviolet–visible-near infrared absorption spectra. These crystals emit brightly green and red upconversion fluorescence under 975 nm LD laser excitation, and the steady state upconversion spectra are obtained at room temperature. The red emission intensity increases as the erbium ions concentration increases in crystals. Processes of excited state absorption and energy transfer are responsible for upconversion luminescence. The emission intensities are quadratic dependences on pump power from pump power dependence analyses and deduction of transition rate equation model.     

Laser ablation of lithium and lithium/cadmium alloy studied by time-of-flight mass spectrometry

Ablation of solid lithium and lithium/cadmium alloy was performed by a 308-nm, nanosecond excimer laser. Analysis of the atomic and molecular composition of the plume in vacuum and in nitrogen atmosphere was performed by means of a linear time-of-flight mass spectrometer. Several ionic masses were observed and systematically studied with respect to the laser fluence, laser beam spot size, background pressure, and target composition. PACS 52.38.Mf; 52.50.Jm; 82.80.Rt     

Structure and refractive index dispersive behavior of potassium niobate tantalate films prepared by pulsed laser deposition

Pure perovskite phase and crack-free KTa_0.5Nb_0.5O₃ thin films were prepared on Pt/Ti/SiO₂/Si substrates by pulsed laser deposition. The structure and orientation were analyzed by X-ray diffraction. The optical properties were investigated by an ellipsometer. The relationship between the refractive index dispersive behavior and internal structure was analyzed by Sellmeier dispersion model and single electronic oscillator approximation. The parameters of room temperature monomial Sellmeier oscillator were calculated. And the refractive index dispersive parameter E₀/S₀ of KTa_0.5Nb_0.5O₃ thin films on Pt/Ti/SiO₂/Si substrates is (6.72 ± 0.04) × 10⁻¹⁴ eV m², which is consistent with those of KTN crystals and compounds with ABO₃ perovskite type structure.     

Photorefractive properties of potassium lithium niobate doped with copper

Potassium lithium niobate doped with copper (Cu:KLN) were grown by the Czochralski method for the first time. The structure of Cu:KLN was measured by the x-ray powder diffraction method, and its lattice constants were obtained. The position of copper ions in KLN crystal was determined. The exponential gain coefficient, response time and erasure time were measured. It was found that the exponential gain coefficient of Cu:KLN is 10.5 cm⁻¹, as two times high as that of KLN, and its response time of 1.53 s is one order of magnitude shorter than that of Cu:LiNbO₃. The type of light exciting carriers in Cu:KLN has been investigated. The result showed that the electron acts the main role in Cu:KLN.     

Electronic structure and optical properties of ordered compounds potassium tantalate and potassium niobate and their disordered alloys

The electronic energy band structure, site and angular momentum decomposed density of states (DOS) of cubic perovskite oxides KNbO₃ and KTaO₃ have been obtained from a first principles density functional based full potential linearized augmented plane wave (FLAPW) method within a generalized gradient approximation (GGA). The total DOS in valence region is compared with the experimental photo-emission spectra (PES). The calculated DOS is in good agreement with the experimental energy spectra and the features in the spectra are interpreted by comparison with the projected density of states (PDOS). The valence band PES is mainly composed of Nb-4d/Ta-5d and O 2p states in KNbO₃ and KTaO₃, respectively. Using the PDOS and the band structure we have analyzed the inter-band contribution to the optical properties of these materials. The real and imaginary parts of the dielectric function have been calculated and compared with experimental data. They are found to be in a reasonable agreement. The role of band structure on the optical properties have been discussed.     

Paramagnetic defects as probes for the study of ferroelastic phase transition in lithium niobate and lithium tantalate under high pressure

It was found by optical polarization microscopy and the EPR study that lithium niobate and tantalate crystals undergo irreversible lattice changes under anisotropic hydrostatic compression. Regions having different cell orientations were registered. The observed changes were explained in terms of "strain switching" of ferroelastic domains. Possible sequence of phase transitions in these crystals ( Pm3m\leftrightarrow R\ \bar{3}\ c\leftrightarrow R\ 3\ c ) and the symmetry of the condensed soft modes ( R 25 and o 15 , correspondingly) were obtained by the analysis of the Gibbs free energy under external pressure.     

Enhancement of upconversion luminescence in lithium niobate polycrystal via doped potassium ion

Lanthanide ions doped lithium niobate has many outstanding performances in upconversion (UC) emission. However, the low UC efficiency still constitutes the main limitation for practical applications. To improve the issue of low efficiency, whether visible green UC emission in holmium/ytterbium/lithium niobate can be improved through tridoping with 0, 2, 4, 6, 8, and 10?mol% potassium has been investigated. The results demonstrate that increasing the concentration of potassium ion enhances the fluorescence signal by 20 times of visible green UC emission and that the maximum fluorescence signal can be achieved at 8?mol%. Theoretical analysis, arising from the steady-state rate equations, proves that the enhancement of green UC fluorescence signal is due to the increased lifetime of the intermediate (⁵I₆) state of the holmium ion and excited (⁵F_5/2) state of the ytterbium ion.     

An application of the electron mirror in the time-of-flight spectrometer

We suggest applying of the spherical electron mirror in the time-of-flight electron spectrometer with a position sensitive detector in order to increase the effective acceptance solid angle of the spectrometer. The spherical electron mirror is placed near the specimen and it focuses electrons on a position sensitive detector as a converging electron flux. The electron mirror increases the acceptance angle of the spectrometer by a factor of 20. The electron mirror of the spectrometer consists of an inner spherical electrode of the radius R and an outer spherical electrode of the radius 1.1R. The central segment of the inner electrode inside the linear angle of 80° is made of a grid. The detector plate radius is about 0.23R. The acceptance solid angle of the spectrometer with this electron mirror is about 1.1sr, the range of the polar angle of emission is 20°–90° relative to the surface normal, and the range of the azimuth angle of emission at its maximum is ±36° relative to the basic plane of the spectrometer. The design of the spectrometer allows to recover the electron trajectory for each detected electron and to calculate the energy and the emission angle of the electron. The energy resolution of the spectrometer is about 0.2 eV/ns for the electron energy of 10 eV. The energy range is from E_min0.1eU_r up to eU_r, where U_r is the retarding potential of the electron mirror. The perturbing influence of the grid of the electron mirror limits mainly the angular resolution of the spectrometer and affects relatively slightly the energy resolution. The electron spectrometer with two detectors and two electron mirrors symmetric about the spectrometer axis allows to measure electron pairs in coincidence in a wide range of emission angles and energies with k-resolutions.     

Electro-optic coefficients of a non-congruent lithium niobate fabricated by vapour transport equilibration: Composition effect

Composition effect on electro-optic (EO) properties of a LiNbO₃ (LN) single-crystal has been investigated in a Li₂O-content range of 47.0–49.95 mol%. Some non-congruent LN crystals with different Li₂O-contents were prepared by performing Li-deficient or Li-rich vapour transport equilibration treatments on as-grown congruent LN crystals. Unclamped EO coefficients γ₁₃ and γ₃₃ of these samples were measured by a Mach–Zehnder interferometric method. The measurements show that in the Li-deficient regime both γ₁₃ and γ₃₃ increase by ～8% as Li₂O-content decreases from the congruent 48.6 mol% to the 47.0 mol% in the Li-deficient regime. The feature is desired for the EO application of the Li-deficient crystal. In the near-stoichiometric regime, both γ₁₃ and γ₃₃ reveal a non-monotonic dependence. As the Li₂O-content increases from the 48.6 mol%, the EO coefficient decreases. Around Li₂O-content 49.5 mol%, a minimum is reached. After that, the EO coefficient recovers slowly. At the stoichiometric composition, it recovers to a value close to that at the congruent point. Comparison shows that different crystal growth methods give rise to different defect structure features and hence different composition effects.     

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.