Near-infrared photorefractivity in Cr-doped potassium sodium strontium barium niobate single crystal

We report on the photorefractive properties of a Cr-doped (K_1–x Na _x )_2A–2 (Sr _y Ba_1–y )_2–A Nb₁₀O₃₀ (x=0.586, y=0.659, A=1.12) single crystal in the near-infrared spectrum. The sample exhibits photorefractivity for wavelengths up to at least 840 nm where the steady-state two-beam coupling gain is found to be larger than 2 cm^–1. Photorefractive gain and decay rate are measured as a function of wavelength, grating spacing and intensity. The wavelength dependence of gain fluctuations in two-beam coupling are also measured.     

铁电钾钠铌酸锶钡薄膜电光性能的研究

采用溶胶-凝胶法在氧化镁单晶衬底上制备了符合化学计量比的完全填充型铁电钾钠铌酸锶钡(KNSBN)薄膜，通过X射线衍射，摇摆曲线，X射线Φ扫描，扫描电子显微镜等方法研究了薄膜的微结构，采用Adachi法研究了薄膜的电光特性. 实验发现，KNSBN薄膜在氧化镁(001)单晶衬底上沿c轴外延生长，K⁺，Na⁺的引入有效地提高了薄膜的横向电光系数r₅₁. 成分为K_0.2Na_0.2Sr_0.24Ba_0.56Nb₂O₆, K_0.2Na_0.2Sr_0.6Ba_0.2Nb₂O₆, K_0.2Na_0.2Sr_0.72Ba_0.08Nb₂O₆的三种KNSBN薄膜的r₅₁值分别为108.52pm/V, 119.98pm/V, 126.96pm/V，r₅₁的数值随Sr²⁺含量增加而增大. 关键词： 横向电光系数 钾钠铌酸锶钡 外延生长     

Planar optical waveguide in Cu-doped potassium sodium strontium barium niobate crystal formed by mega-electron-volt He-ion implantation

The first planar optical waveguide to the authors' knowledge has been formed in Cu-doped potassium sodium strontium barium niobate crystal by mega-electron-volt He(+) implantation. Both TE and TM modes are observed. The profiles of the ordinary and the extraordinary refractive indices are deduced from dark-line mode spectroscopy. The results show that the mega-electron-volt He implantation results in a decrease in refractive index in barriers for both n(o) and n(e), but for n(e) there is an obvious increase in the waveguide region. From an experiment in photorefractive two-wave mixing, it is found that the erasure time for two-wave mixing is prolonged by ion implantation.     

Ferroelectric properties of strontium barium niobate crystals doped with rare-earth metals

The influence of doping of the Sr_xBa_1?x Nb₂O₆ (x=0.61) crystals (SBN-0.61) by rare-earth element (RE) dopants has been investigated. Taking into account the data available in the literature, it is demonstrated that the introduction of all rare-earth metals is accompanied by a substantial decrease in the temperature of the phase transition T _p and an increase in its smearing. The shift in the T _p temperature for certain rare-earth metals ranges up to 20 K per atomic percent of dopant in the crystal. It is experimentally established that, in the SBN-0.61: Yb, SBN-0.61: Ce, SBN-0.61: Tm, SBN-0.61: La, and SBN-0.61: (Ce + La) crystals, a decrease in T _p brings about a considerable increase in the permittivity and the piezoelectric and electrooptical coefficients. The conclusion is drawn that the doping by rare-earth metals provides a means of optimizing the properties of strontium barium niobates. The pulse switching of the SBN-0.61 and SBN-0.61: RE crystals is studied for the first time. It is found that the switching is characterized by a number of features, the most important of which is a decrease in the switched charge after the application of external fields. This effect associated with the specific features of the switching in the relaxor ferroelectric is assumed to be responsible for the instability of the parameters for strontium barium niobate material.     

Parametric holographic scattering processes in photorefractive strontium–barium niobate

Light-induced holographic scattering is investigated in photorefractive Sr_0.61Ba_0.39Nb₂O₆ (SBN) crystals, in which diffusion charge transport dominates. A crystal is illuminated with two coherent pump beams and different scattering patterns are observed on a screen behind the crystal. We attribute the patterns to certain phase-matching conditions, which allows the identification of the parametric processes that are involved . PACS 42.65.Hw; 42.70.Nq     

Nanocrystals formation on Ho doped strontium barium niobate glass

The study of two different methods to obtain strontium barium niobate nanocrystals immersed in a glass matrix has been carried out. Ho₂O₃-doped SrO-BaO-Nb₂O₅-B₂O₃ glasses were fabricated using the melt quenching method. Glass ceramic samples were obtained from the precursor glass by thermal treatment in a furnace and by laser irradiation. These glass ceramic samples are formed by a glassy phase and a crystalline phase of strontium barium niobate nanocrystals. This structure was confirmed by X-ray diffraction and Atomic Force Microscope images. The incorporation of Ho³⁺ ions in the strontium barium niobate nanocrystals were corroborated by optical measurements, which produced an increment in the luminescence intensity compared to the precursor glass.     

Thermally induced jump-like pulse processes in a strontium barium niobate crystal doped with metal impurities

The specific features of jump-like pulse processes of spontaneous polarization switching in relaxor ferroelectric crystals of strontium barium niobate are investigated. Based on the fixed parameters of the jump-like switching pulses, three possible mechanisms of their formation are revealed.     

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.     

Incommensurate phases in lead potassium niobate and lead sodium niobate,homologues of barium sodium niobate.

Electron diffraction and high resolution electron microscopy reveal the presence of incommensurate structures in lead potassium niobate, lead sodium niobate and in mixed crystals. The incommensurability can be described either as being due to the presence of quasi-periodic “diffuse” anti-phase boundaries or as being due to the presence of deformation waves, the deformation mode being shearing of the NbO₆ octahedra. The incommensurability depends on composition as well as on temperature.     

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.     

Photorefractive properties of ion-implanted waveguides in strontium barium niobate crystals

0.61 Ba_0.39Nb₂O₆, SBN61), either by proton or helium ion implantation. Proton-implanted samples show a large increase of dark conductivity that reduces or even prevents the recording of refractive index gratings. For waveguides formed by helium implantation this effect is absent, and they can be used for efficient holographic recording. Photorefractive properties of the waveguides are investigated by two-beam coupling. After implantation with 2.0 MeV He⁺ and doses of (0.5-5)×10¹⁵ cm^-2, the samples have to be polarized again, because heating or charge effects at the crystals surface during the implantation process decreases or even reverses the effective electrooptic coefficients in the waveguiding layer. For repoled samples, we find logarithmic gain coefficients of up to 45 cm^-1 with time constants for the build-up of the purely π/2-shifted refractive index grating of the order of 1 ms for the blue lines of an Ar⁺ laser. Photoconductivity depends nonlinearly on light intensity with an exponent x≈0.55. With increasing implanted helium dose, both electronic and nuclear damage of the waveguiding layer grows, and the photorefractive properties of the waveguides are considerably degraded. Received: 20 February 1997/Revised version: 1 May 1997     

Observation and thermal fixing of holographic gratings in lead barium niobate crystal

We report what we believe to be the first observation of photorefractive gratings in lead barium niobate (Pb(1-x)Ba (x)Nb(2)O>( 6) ; PBN). A diffraction efficiency of 9% and a beam-coupling coefficient of 9 cm(-1) are obtained. After thermal fixing a grating of 0.5% is revealed at room temperature. We can improve this diffraction efficiency by optimizing fixing temperature, timing, and material parameters.     

Conoscopic interferometry for probing electro-optic coefficients of strontium calcium barium niobate crystal

The relation between the linear electro-optic effect and conoscopic interference has been investigated in a biased electro-optic crystal strontium calcium barium niobate (CSBN). Based on the change of interference patterns with applied field, an interferometric method for probing electro-optic coefficients of electro-optic crystals has been proposed. In our case, the linear electro-optic coefficients of CSBN:50 was first probed at γ₃₃=(141.0±0.5)×10⁻¹² m/V and γ₁₃=(85.0±0.5)×10⁻¹² m/V.     

Transverse modulational instability of periodic light patterns in photorefractive strontium barium niobate crystal

Interference fringes produced by the intersection of two plane waves in photorefractive strontium barium niobate crystal break in the transverse direction if the external electric field is high enough. For a certain relation between fringe spacing and nonlinearity strength, the tendency to regular pattern formation has been observed. We present a simple theory that qualitatively explains this effect.     

Noncollinear optical frequency doubling in strontium barium niobate

The observation of a novel noncollinear optical second-harmonic generation mechanism is reported. In strontium barium niobate crystals, a circular cone of second-harmonic light is generated when a fundamental beam of intensive laser light is directed along the crystallographic c axis. It can be shown that the effect is caused by the nonlinear polarization of antiparallel ordered ferroelectric microdomains.     

Second harmonic generation in a strontium barium niobate crystal with a random domain structure

The phenomenon of the second-harmonic (SH) generation in a strontium barium niobate (SBN) crystal in directions not obeying the conditions of phase matching for spatially homogeneous media has been studied. The appearance of the SH signal is interpreted as resulting from the quasi-phase-matching nonlinear wave interaction due to the random domain structure of the SBN crystal containing acicular ferroelectric microdomains. A theoretical model describing the SH generation in such two-dimensional, spatially inhomogeneous nonlinear optical structures is proposed. An analysis of the second-harmonic emission directivity patterns shows that the main contribution to the noncollinear SH generation in SBN is due to near-surface domains extending to a depth of up to several dozen microns.     

Ionoluminescence of trivalent rare-earth-doped strontium barium niobate

Ionoluminescence spectra for different rare-earth ion (Pr³⁺ and Eu³⁺)-activated Sr_xBa_1?xNb₂O₆ strontium barium niobate crystals (x=0.33 and 0.60) have been induced with a 3 MeV proton beam for a variety of beam current intensities (45, 40 and 20 nA). The proton-beam induced luminescent spectra have shown features associated with the presence of the rare-earth ion and some spectral features mostly related to the host crystal, which appear only for high beam current intensities. We have compared the ionoluminescence results to those obtained under UV light excitation (photoluminescence technique) where a direct excitation of the band gap would occur.     

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.     

Glassy properties of the relaxor ferroelectric strontium barium niobate at low temperatures

Relaxor ferroelectrics are known to show a low-temperature behaviour that quantitatively resembles the features of canonical glasses. In order to get a comprehensive characterisation of the responsible low-energy excitations, single crystals of strontium barium niobate were investigated below 10 K. The density of localised states was obtained from heat capacity, and their coupling to phonons from sound velocity and thermal conductivity. The presence of slowly relaxing systems could be estimated from heat release data, respectively.

In contrast with structural glasses, however, SBN and further relaxor ferroelectrics show an energy dependent density of states and an upper limit of the relaxation time spectrum. Considering these properties, which appear to be typical also for other glassy crystals, a modification of the standard tunneling model is proposed.     

Scanning probe microscopy investigation of ferroelectric properties of barium strontium niobate crystals

The ferroelectric properties of relaxor ferroelectric materials Sr _x Ba_{1 − x} Nb₂O₆ of different compositions have been investigated using scanning probe microscopy and macroscopic polarization methods. An analysis of the results obtained from microscopic and macroscopic measurements of the hysteresis loops has demonstrated that a frequency dependence of the coercive field E _c is observed at frequencies f in the range from 0.03 to 250 Hz, so that the coercive field E _c regularly decreases with decreasing frequency. The observation of the kinetics of domains under a direct-current voltage applied to the probe of the atomic force microscope has revealed a lateral motion of domain walls in response to electric fields E ≪ E _c . This result qualitatively explains a slow polarization relaxation occurring for a giant time in Sr _x Ba_{1 − x} Nb₂O₆.