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.     

Photocarrier transport in iron-doped potassium lithium tantalate niobate studied by time-of-flight measurement

The photocarrier mobility of Fe 0.03 wt%-doped potassium lithium tantalate niobate (K_0.95Li_0.05Ta_0.61Nb_0.39O₃) was investigated by time-of-flight (TOF) measurement. The longitudinal photocarrier response due to pulsed excitation leads to values of the drift mobility of μ_h = 1.45 × 10⁻² cm²/V s for holes, μ_e = 0.325 × 10⁻² cm²/V s for electrons, and a value for the range of holes (μτ)_h = 4.38 × 10⁻⁵ cm²/V at room temperature and at low field 3 KV/cm. The response time of holes and electrons (or the relaxation time) is determined to be 3.02 × 10⁻³ s and 3.74 × 10⁻³ s, respectively. The mobility of holes strongly depends on the field strength, and is observed to decrease with increasing bias field.     

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.     

Electric field controlled higher-order diffraction images of paraelectric potassium lithium tantalate niobate

We report some electric field controlled photorefractive higher-order diffraction phenomena of a paraelectric phase potassium lithium tantalate niobate crystal doped with iron. In experiments, a p-polarized semiconductor laser (532 nm) was used to record grating at a small incident angle. Higher-order diffraction images were observed when the signal beam was focused behind and in front of the crystal. Then the higher-order diffraction images were reconstructed by a p-polarized He–Ne laser (632.8 nm) in the direction perpendicular to the surface. The higher-order diffraction images could be controlled by the external electric field. A theory about the higher-order diffraction images of the K and 2K grating is developed. The results show that the even order diffraction images of the K grating and the odd order diffraction of the 2K grating overlap each other. The odd order diffraction images of the K grating are diffracted in unattached direction. The electric field controlled higher-order diffraction image provides a useful method for optical information processing.     

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.     

The two photorefractive centres in iron doped nearly stoichiometric lithium niobate crystals

This paper investigates the photorefractive properties of iron doped lithium niobate with different [Li]/[Nb] ratios The experimental results show two photorefractive centres for iron doped near-stoichiometric lithium niobate crystal Besides Fe^2＋ and Fe^3＋ ions, small polarons and bipolarons are considered as another photoactive centre.     

Photorefractive properties of iron-doped lithium tantalate crystals

Iron-doped lithium tantalate crystals are grown by the Czochralski method and their photorefractive properties are examined with holographic methods. Dynamic range, holographic sensitivity, photoconductivity, and dark storage time are measured in dependence on the iron concentration and light intensity. The largest refractive-index change for ordinarily polarized light is 3.5×10^-4, in comparison with 6.2×10^-4 for iron-doped lithium niobate. Due to a small mobility of protons the dark storage time of holograms in lithium tantalate is larger than that in lithium niobate. PACS 42.40.Pa; 42.70.Ln     

Photorefractive properties of ruthenium-doped potassium niobate

Photoconductivities and photovoltaic currents of ruthenium-doped KNbO₃ are orders of magnitude larger than that of undoped and ion-doped crystals. KNbO₃:Ru is very sensitive for holographic recording with red light and the photovoltaic current increases sublinearly with light intensity.     

Photorefractive properties of iron-doped stoichiometric lithium niobate

The photorefractive properties of stoichiometric LiNbO(3) crystals with a small number of defect densities grown by the double-crucible Czochralski method are investigated and compared with the defect densities of commercially available congruent Fe-doped LiNbO(3) crystals. Two-wave-mixing experiments show that novel stoichiometric crystals exhibit larger photorefractive gain and considerably faster response times than congruent ones. The results indicate that the nonstoichiometry defect control of photorefractive crystals is of key importance for the improvement of their properties.     

Optical properties of lithium niobate and lithium tantalate crystals with impurities and defects

The photoinduced and Raman scattering in lithium niobate and lithium tantalate crystals with impurities and defects have been studied. An exciting laser beam propagated either along the ferroelectric Z axis or perpendicular to it. The conditions for exciting transverse and longitudinal polar optical modes in Raman spectra are established. The regularities of the excitation of Raman spectra in several polarization geometries (X(ZZ)Y, Z(XX, Y Y)Z, Z(XX, Y Y)Z, X(ZX)Y, X(ZX)X and X(ZX)X) have been investigated. Additional (extra) spectral lines are interpreted as a manifestation of a biphonon enhanced by the Fermi resonance and the result of violation of selection rules for pseudoscalar modes of the A ₂ type due to the reduction of the point symmetry group caused by the presence of impurities and defects in real crystals. The conditions for exciting coherent longitudinal and transverse modes in lithium niobate and lithium tantalate single crystals upon stimulated Raman scattering are analyzed. The temperature evolution of the spectra recorded in the X(ZZ)Y geometry near the ferroelectric phase transition point is explained based on the concept of effective soft mode and analysis of the isofrequency opalescence effect. Strong photoluminescence is found in copper-doped lithium niobate crystals.     

Dependence of gain and phase-shift on crystal parameters and pump intensity in unidirectional photorefractive ring resonators

The steady-state amplification of light beam during two-wave mixing in photorefractive materials has been analysed in the strong nonlinear regime. The oscillation conditions for unidirectional ring resonator have been studied. The signal beam can be amplified in the presence of material absorption, provided the gain due to the beam coupling is large enough to overcome the cavity losses. Such amplification is responsible for the oscillations. The gain bandwidth is only a few Hz. In spite of such an extremely narrow bandwidth, unidirectional oscillation can be observed easily at any cavity length in ring resonators by using photorefractive crystals as the medium and this can be explained in terms of the photorefractive phase-shift. The presence of such a phase-shift allows the possibility of the non-reciprocal steady-state transfer of energy between the two light beams. Dependence of gain bandwidth on coupling constant, absorption coefficient of the material’s cavity length (crystal length) and modulation ratio have also been studied.      

Superior real-time holographic storage properties in doped potassium sodium strontium barium niobate crystal

We demonstrate superior holographic storage performance in a cobalt doped potassium sodium strontium barium niobate (Co:KNSBN) crystal that possesses a fast response time of 1.4 ms, a large photorefractive sensitivity of 13 x 10(-3) cm(3) J(-1) under a total writing intensity of 1 W/cm(2) , and high spatial resolution of 45 line pairs/mm. Reconstructed images with high fidelity have been obtained in real-time holographic storage. The dynamic properties of the index grating, the dependence of response time on writing intensity, and the dark decay of diffraction signal with increased writing intensity indicate that two species and shallow traps exist in Co:KNSBN crystal.     

Ca2+掺杂铌酸锶钡晶体的光折射变化特性研究

基于Ca2+掺杂铌酸锶钡晶体的透射特性,探讨了Ca2+在晶体中的电子行为机制,分析了Ca2+掺杂而引起的光致折射率变化特性.采用Michelson干涉装置测得了样品折射率随时间变化的特性曲线.实验结果分析表明,适当的ca2+掺杂可以有效改善铌酸锶钡晶体的光折射特性.     

掺杂铌酸锂He—Ne相位共轭激光器的稳态输出特性

实验研究了以Ce:Fe:LiNbO_3晶体为相位共轭镜的He-Ne外泵浦相位共轭激光器的稳态输出特性.并建立了在强泵浦条件下的理论公式.理论与实验曲线在趋势上相一致.     

Photorefractive properties of highly-doped lithium niobate crystals in the visible and near-infrared

Light-induced refractive-index changes, bulk-photovoltaic current densities, and photoconductivities of photorefractive iron-doped lithium niobate crystals (iron concentrations between 0.02 and 0.17 wt. %) are investigated in detail using visible and near-infrared light. It turns out that the one-center model predicts the material performance correctly for small iron concentrations (c_Fe<0.06 wt. % Fe₂O₃), only. A strong increase of the photoconductivity for higher doping levels limits the space-charge fields. Refractive-index changes up to 7×10^-4 for green and 2.8×10^-4 for near-infrared ordinarily polarized light are obtained. The corresponding hologram multiplexing numbers are 11 for green and 5 for near-infrared light. Received: 18 November 1998 / Revised version: 8 January 1999 / Published online: 7 April 1999     

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.     

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.     

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.