Wavelength dependence of light-induced domain nucleation in MgO-doped congruent LiNbO3 crystal

Within the wavelength range from 351 to 799 nm, the different reductions of nucleation field induced by the focused continuous laser irradiation are achieved in the 5 mol % MgO-doped congruent LiNbO3 crystals. The reduction proportion increases exponentially with decreasing irradiation wavelength and decreases exponentially with increasing irradiation wavelength. At one given wavelength, the reduction proportion increases exponentially with increasing irradiation intensity. An assumption is proposed that the reduction of nucleation field is directly related to the defect structure of crystal lattice generated by the complex coaction of incident irradiation field and external electric field. (c) 2007 American Institute of Physics.

高浓度掺Er^3＋铌酸锂晶体的光谱参数计算

用提拉法成功地生长了6mol％的高浓度掺铒铌酸锂晶体。测量了晶体的两个非偏振方向（X和Z）以及两个偏振方向（π和δ）的吸收光谱。高浓度掺铒铌酸锂晶体的吸收系数高，有利于提高泵浦效率。根据所测的吸收光谱用Judd-Ofelt理论拟合出了Er^3＋离子的强度参数Ωλ。所得的均方差结果显示偏振拟合的误差要小于非偏振拟合。利用偏振吸收数据计算了各能级跃迁的自发辐射跃迁几率（AJJ’）、辐射寿命（τ）、荧光分支比（β）和积分发射截面（σp）等参数，对计算结果进行了讨论并与其他文献的报道结果进行了比较。

Electrical properties of LiNbO3 (electrolyte)/Cu (anode) bi-layers

In this work specific film structures of Li-Nb-O/Li/Li-Nb-O are investigated by AC Impedance Spectroscopy measurements at different temperatures. This gives the opportunity to investigate properties of the material itself and, at the same time, to consider the influence of the grain boundaries on the ionic behavior of the polycrystalline Lithium Niobate. On the other hand, LiNbO3/Li/Cu multi-layers are studied as electrolyte/anode bi-layers and potential parts of "Li-free" microbatteries. The Li deficiency in the as deposited Li-Nb-O films is cured by forming a "sandwich" of Li-Nb-O/Li/Li-Nb-O, which after annealing becomes ionic conductor. The electrical behavior of an annealed film depends on two sources. The first is due to properties of the material itself and the second is based on the network of the grain boundaries. The average size of the grains is strongly influenced by the structure of the ohmic-contact/substrate. The electrical behavior of the electrolyte/anode interface of the "Li-free" structure LiNbO3/Li/Cu/Au is very similar to the impedance measurements of the single LiNbO3 single films. The whole multilayer structure, though, presents a third relaxation time which is consistent of a small resistance. This resistance is independent of temperature and it seems that is due to the metallic interface Li/Cu/Au. © 2010 Elsevier B.V. All rights reserved.

The effect of stoichiometry on nonlinear optical properties of LiNbO3

From the chemical bond viewpoint, second-order nonlinear optical (NLO) tensor coefficients of LiNbO3 have been investigated. The single-bond contributions to the second-order NLO susceptibility and the linear susceptibility were determined. The tensor values thus calculated are in good agreement with experimental data. Based on theoretical results of LiNbO3 with Li/Nb = 1, we also have calculated linear and nonlinear optical properties of nonstoichiometric samples with Li/Nb < 1. In the calculation, we find that the Li-O bond is an important type of chemical bond in these LiNbO3 samples, which have large NLO contributions to the total nonlinearities. The refractive indices and second-order NLO tensor coefficients have been determined as a function of the stoichiometry.

Wall thickness dependence of the scaling law for ferroic stripe domains

The periodicity of 180 degrees. stripe domains as a function of crystal thickness scales with the width of the domain walls, both for ferroelectric and for ferromagnetic materials. Here we derive an analytical expression for the generalized ferroic scaling factor and use this to calculate the domain wall thickness and gradient coefficients ( exchange constants) in some ferroelectric and ferromagnetic materials. We then use these to discuss some of the wider implications for the physics of ferroelectric nanodevices and periodically poled photonic crystals.

Assessment of biocompatibility of selected ferroelectric ceramics

Desde há muitas décadas que é sabido que os organismos vivos, em especial os tecidos, reagem fisicamente a estímulos eléctricos, podendo esses efeitos reproduzirem-se numa libertação de químicos endógenos, ou deformar a sua estrutura física. O tecido ósseo por si só é considerado um material/tecido piezoeléctrico, deformando-se mecanicamente quando lhe é induzido um estímulo eléctrico e vice-versa, ou seja, produz um potencial eléctrico quando sofre uma tracção ou compressão mecânica. A hipótese de que um material ferroeléctrico possa vir a produzir efeitos no desempenho deste tipo de tecidos é então proposta, como por exemplo, para uma melhor, mais rápida e eficaz regeneração óssea. Estes mesmos materiais ferroeléctricos podem porventura alterar as cargas de superfície dos tecidos vivos de modo a atrair, atrasar ou até impedir o fluxo iónico de elementos químicos específicos responsáveis pelo processo de regeneração. São escolhidos então o niobato de lítio e o tantalato de lítio como cerâmicos ferroeléctricos e foi estudada pela primeira vez a sua bioactividade in vitro, esperando-se encontrar pistas relativas à sua bioactividade in vivo. Estes cerâmicos ferroeléctricos foram seleccionados devido às suas importantes propriedades piezoeléctricas e ferroeléctricas. Estas propriedades podem abrir um novo e importante leque de aplicações biomédicas caso estes cerâmicos sejam bioactivos. Este trabalho foi dividido em 3 fases: (i) sintetização dos pós de niobato de lítio e tantalato de lítio, (ii) caracterização dos pós e (iii) preparação das amostras e (iv) estudo da bioactividade destes cerâmicos ferroeléctricos. Os pós foram produzidos através de um processo simples de mistura/moagem seguido de calcinação. Foram estudadas as fases cristalinas presentes através de Difracção de raios-X (DRX) e avaliadas as características morfológicas destes pós, nomeadamente o diâmetro de partículas e área superficial específica. De modo a simular o ambiente do plasma humano, foi produzido sinteticamente um “Simulated Body Fluid” (SBF). Seguidamente as amostras foram imersas nesse ambiente líquido por 1, 3, 7, 15 e 21 dias. Após remoção dos pós foram realizadas uma série de análises de modo a estudar a sua bioactividade. De entre estes testes destacam-se a microscopia electrónica de varrimento (SEM/EDS), DRX e espectroscopia de Infravermelho por transformada de Fourier com reflectância total atenuada (FTIR-ATR). Embora não tenham sido detectadas alterações no DRX realizado aos pós, verificou-se a formação de aglomerados de fosfato de cálcio na superfície dos pós através do SEM, resultados estes, reforçados pelo EDS e FTIR-ATR. Estes precipitados de fosfato de cálcio indiciam a capacidade destes pós cerâmicos ferroeléctricos se comportarem como bioactivos em contacto com tecidos ósseos in vivo.

Radio over fiber broadband access networks architectures based on wavelength division multiplexing techniques

The recent remarkable growth in bandwidth of both wired optical and wireless access networks supports a burst of new high bandwidth Internet applications such as: peer-topeer file sharing, cloud storage, on-line gaming, video streaming, etc. Within this scenario, the convergence of fixed and wireless access networks offers significant opportunities for network operators to satisfy user demands, and simultaneously reduce the cost of implementing and running separated wireless and wired networks. The integration of wired and wireless network can be accomplished within several scenarios and at several levels. In this thesis we will focus on converged radio over fiber architectures, particularly on two application scenarios: converged optical 60 GHz wireless networks and wireless overlay backhauling over bidirectional colorless wavelength division multiplexing passive optical networks (WDM-PONs). In the first application scenario, optical 60 GHz signal generation using external modulation of an optical carrier by means of lithium niobate (LiNbO3) Mach- Zehnder modulators (MZM) is considered. The performance of different optical modulation techniques, robust against fiber dispersion is assessed and dispersion mitigation strategies are identified. The study is extended to 60 GHz carriers digitally modulated with data and to systems employing subcarrier multiplexed (SCM) mm-wave channels. In the second application scenario, the performance of WDM-PONs employing reflective semiconductor optical amplifiers (RSOAs), transmitting an overlay orthogonal frequency-division multiplexing (OFDM) wireless signal is assessed analytically and experimentally, with the relevant system impairments being identified. It is demonstrated that the intermodulation due to the beating of the baseband signal and wireless signal at the receiver can seriously impair the wireless channel. Performance degradation of the wireless channel caused by the RSOA gain modulation owing to the downstream baseband data is also assessed, and system design guidelines are provided.

Heterodyne Interferometry Applied to the Characterization of Acousto-Electro-Optic Light Modulators

When the electro-optic and acousto-optic effects are combined into a single device, the resulting acousto-electro-optic (AEO) modulator shows improved flexibility to overcome some limitations of the individual modulators or their cascade combinations. By using optical interferometry, it is possible to investigate the AEO modulator behavior as a function of this applied voltage. By this way, a lithium niobate AEO modulator is positioned in one of the arms of a Mach-Zehnder interferometer and operates at 62 MHz frequency, which constitutes the intermediate frequency of the heterodyne interferometer. Operating the AEO modulator in the acousto-optic small diffraction efficiency regime, the photodetected signal amplitude and phase are analyzed, and the induced phase shift, transmission curve and linearity response are obtained. The experimental results show good agreement with that expected from the coupled-mode theory. The possibility of linear control of the optical phase shift by the external voltage, from 0 to 2 p radians, is demonstrated.

Study on the K3Li2Nb5O15 formation during the production of (Na0.5K0.5)((1-x))LixNbO3 lead-free piezoceramics at the morphotropic phase boundary

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

LiNbO3 thin films prepared through polymeric precursor method

Thin films of lithium niobate were deposited on (100) silicon by the polymeric precursor method (Pechini method). Annealing in static air was performed at 500degreesC for 3 h. The films obtained were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Electrical characterization of the films pointed to ferroelectricity via hysteresis loop. The dielectric constant, dissipation factor and resistance were measured in the frequency region from 10 Hz to 10 MHz. At 1 MHz, the dielectric constant was 46 and the dissipation factor was 0.043. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Influence of thickness on crystallization and properties of LiNbO3 thin films

Thin films of lithium niobate were deposited on Pt/Ti/SiO2 (111) substrates by spin coating from the polymeric precursor method (Pechini process). Annealing in static air was performed at 500 degreesC for 3 h. The obtained films were characterized by X-ray diffraction and atomic force microscopy. The dielectric constant, dissipation factor and resistance were measured in frequency region from 10 Hz to 10 MHz and the hysteresis loop was obtained. The influence of number of layers on crystallization, morphology and properties of LiNbO3 thin films is discussed. (C) 2003 Elsevier B.V. All rights reserved.

Influence of oxygen atmosphere on crystallization and properties of LiNbO3 thin films

Thin films of lithium niobate were deposited on the Pt/Ti/SiO2 (111) substrates by spin coating from the polymeric precursor method (Pechini process). Annealing in static air and oxygen atmosphere was performed at 500 degreesC for 3 h. The films obtained were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The dielectric constant and dissipation factor were measured in frequency region from 10 Hz to 10 MHz. Electrical characterization of the films pointed to ferroelectricity via hysteresis loop. The influence of oxygen atmosphere on crystallization, morphology and properties of LiNbO3 thin films is discussed. (C) 2003 Elsevier Ltd. All rights reserved.

Preparação de LiNbO3 e LiNbO3:Eu3+ pelo método dos precursores poliméricos

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Epitaxially grown LiNbO3 thin films by polymeric precursor method

LiNbO3 thin films were grown on (0001) sapphire substrates by a chemical route, using the polymeric precursor method. The overall process consists of preparing a coating solution from the Pechini process, based on metallic citrate polymerization, the precursor films, deposited by dip coating, are then heat treated to eliminate the organic material and to synthesize the phase. In this work, we studied the influence of the heat treatment on the structural and optical properties of single-layered films. Two routes were also investigated to increase the film thickness: increasing the viscosity of the coating solution and/or increasing the number of successively deposited layers. The x-ray diffraction theta -2 theta scans revealed the c-axis orientation of the single- and multilayered films and showed that efficient crystallization can be obtained at temperatures as low as 400 degreesC, the phi-scan diffraction evidenced the epitaxial growth with two in-plane variants, A microstructural study revealed that the films were crack free, homogeneous, and relatively dense. Finally, the investigation of the optical properties (optical transmittance and refractive index) confirmed the good quality of the films. These results indicate that the polymeric precursor method is a promising process to develop lithium niobate waveguides.

Dielectric properties of Na1-xLixNbO3 ceramics from powders obtained by chemical synthesis

Ultra-fine powders of Na1-xLixNbO3 (x=0; 0.06; 0.09; 0.12) were synthesized by the Polymeric Precursors Method. Such powders had their orthorhombic structures determined by X-ray diffraction and their surface area determined by BET isotherms (less than 10 m(2) g(-1)). Densification was followed by dilatometric study. The powders, calcined at 700 degrees C for 5 h, were sintered at 1290 degrees C during 2 h under ambient atmosphere with no application of extra pressure. The samples with relative densities higher than 95% were analyzed by impedance spectroscopy at room temperature, under a signal amplitude of 1 V-rms. Dielectric constants of about 180 and dielectric loss factor of about 0.03 were measured showing small dependence with frequency. The electrical properties were similar to those obtained for samples sintered by hot pressing. (C) 1999 Elsevier B.V. Limited and Techna S.r.l. All rights reserved.