信息时代的铌酸锂晶体:进展与展望

铌酸锂晶体具有非线性效应、电光效应、声光效应、光折变效应、压电效应与热释电效应等多种物理特性,在表面声波器件、光电器件、声光器件等方面获得广泛的应用。经历了六十多年的发展,铌酸锂晶体历久弥新,随着材料特性的不断开发,新功能、新器件、新应用层出不穷,尤其是铌酸锂单晶薄膜在薄膜滤波器、集成光电器件等领域的性能具有明显优势,被称为新一代信息和通信技术的关键材料。应用器件的发展正迫切要求基质晶体材料的发展,本文通过简述近年来铌酸锂的新发现、新应用,相应地探讨了铌酸锂晶体未来发展方向。     

铜铁镁三掺铌酸锂晶体的第一性原理研究

利用基于密度泛函理论的第一性原理,研究了Cu:Fe:Mg:LiNbO3晶体及对比组的电子结构和光学特性.研究显示,单掺铜或铁铌酸锂晶体的杂质能级分别由Cu 3d轨道或Fe 3d轨道贡献,禁带宽度分别为3.45和3.42 eV;铜、铁共掺铌酸锂晶体杂质能级由Cu和Fe的3d轨道共同贡献,禁带宽度为3.24 eV,吸收峰分别在3.01,2.53和1.36 eV处;Cu:Fe:Mg:LiNbO3晶体中Mg^2+浓度低于阈值或高于阈值(阈值约为6.0 mol%)的禁带宽度分别为2.89 eV或3.30 eV,吸收峰分别位于2.45 eV,1.89 eV或2.89 eV,2.59 eV,2.24 eV.Mg^2+浓度高于阈值,会使吸收边较低于阈值情况红移;并使得部分Fe^3+占Nb位,引起晶体场改变,从而改变吸收峰位置和强度.双光存储应用中可选取2.9 eV作为擦除光,2.5 eV作为读取和写入光,选取Mg^2+浓度达到阈值的三掺晶体在增加动态范围和灵敏度等参量以及优化再现图像的质量等方面更具优势.     

Ultraviolet photorefractive effect in Mg-doped near-stoichiometric LiNbO3

The ultraviolet photorefractive effect of Mg-doped near-stoichiometric LiNbO₃ crystals prepared by vapor transport equilibration (VTE) technique was studied at 351 nm. It was found in the near-stoichiometric LiNbO₃ crystals that the ultraviolet photorefractive effect could be enhanced greatly with the increase of Mg concentration. Based on the activation energy of dark decay of the photorefractive grating, possible centers responsible for the ultraviolet photorefractive effect were also discussed.     

All-optical amplification via the interaction among guided and leaky propagation modes in lithium niobate waveguides exploiting the cascaded second-order nonlinearity

The aim of this paper consists of performing a theoretical investigation of an all-optical amplifier and discussing the problems for a realistic implementation. An original periodically poled lithium niobate channel waveguide, having the optical c-axis opportunely oriented on the xz plane was considered. The waveguiding structure, designed to operate in the second communication window of optical fibers, shows versatile properties as was theoretically demonstrated. In fact, it enables both the selection and the coupling strength of four different propagation modes, two guided and two leaky, interacting via the cascaded second-order nonlinearity. The coupling among guided and leaky modes is obtained by means of two different techniques: (i) birefringence phase-matching (BPM) via a suitable orientation of the optical c-axis crystal on the xz plane and (ii) quasi-phase-matching by periodical inversion of the lithium niobate ferroelectric domains. A strong diffusion of magnesium in the region external to the periodically poled waveguiding channel is needed in order to achieve the suitable BPM and to maximize the bounding of the electromagnetic field and, thus, the overlapping integrals. Finally, a powerful refinement of device operation via the use of the seeded second harmonic waves is proposed. Gains of the order of 20 dB are achieved with reduced bias powers. The order of magnitude of the bias power decreases, with respect to the unseeded second harmonic operation, from 145 to 32 W.     

Heat capacity, enthalpy and entropy of strontium bismuth niobate and strontium bismuth tantalate

The heat capacities and enthalpy increments of strontium bismuth niobate SrBi₂Nb₂O₉ (SBN) and strontium bismuth tantalate SrBi₂Ta₂O₉ (SBT) were measured by the relaxation method (2–150 K), Calvet-type heat-conduction calorimetry (305–570 K) and drop calorimetry (773–1373 K). The temperature dependences of non-transition heat capacities in the form C_pm = 324.47 + 0.06371T − 5.0755 × 10⁶/T² J K⁻¹ mol⁻¹ (298–1400 K) and C_pm = 320.22 + 0.06451T − 4.7001 × 10⁶/T² J K⁻¹ mol⁻¹ (298–1400 K) were derived for SBN and SBT, respectively, by the least-squares method from the experimental data. Furthermore, the standard molar entropies at 298.15 K S_m°(SBN)=327.15±0.80 and S_m°(SBT)=339.23±0.72 J K⁻¹ mol⁻¹ were evaluated from the low-temperature heat capacity measurements.     

类钙钛矿新铌酸盐Ba5NdTi2Nb3O18的合成、结构与介电特性

为满足现代通信技术的小型化、集成化与高可靠性的迫切要求,探索具有高介电常数、低介电损耗与低温度系数的微波介电材料引起了材料科学、化学、物理、电子等领域科学工作者的广泛关注,并已开发出复合钙钛矿结构Ba(Zn_(1/3)Ta_(2/3))O_3、钨青铜结     

Optical waveguide with homogeneous refractive index profile in LiNbO3 by double-low-energy Oxygen ion implantation

We report on the formation of the planar waveguide by 550 keV O ion followed by 250 keV O ion implantation in lithium niobate (LiNbO₃), at fluences of 6 × 10¹⁴ ions/cm² and 3 × 10¹⁴ ions/cm², respectively. The Rutherford backscattering/channeling spectra have shown the atomic displacements in the damage region before and after annealing. A broad and nearly homogeneous damage layer has been formed by double-energy ion implantation after annealing. Both the dark mode spectra and the data of refractive index profile verified that the extraordinary refractive index was enhanced in the ion implanted region of LiNbO₃. A homogeneous near-field intensity profile was obtained by double-low-energy ion implantation. There is a reasonable agreement between the simulated modal intensity profile and the experimental data. The estimated propagation loss is about 0.5 dB/cm.     

Comparative Solution Synthesis of Mn Doped (Na,K)NbO3 Thin Films

(K_0.5Na_0.5)NbO₃ (KNN) is a promising lead-free alternative for ferroelectric thin films such as Pb(Zr,Ti)O₃. One main drawback is its high leakage current density at high electric fields, which has been previously linked to alkali non-stoichiometry. This paper compares three acetate-based chemical solution synthesis and deposition methods for 0.5 mol % Mn-doped KNN film fabrication, using lower crystallization temperature processes in comparison to the sintering temperatures necessary for fabrication of KNN ceramics. This paper shows the crucial role of the A site homogenization step during solution synthesis in preserving alkali chemical homogeneity of Mn doped KNN films. Chemically homogeneous films show a uniform grain size of 80 nm and a leakage current density under 2.8×10⁻⁸ A cm⁻² up to electric fields as high as 600 kV cm⁻¹, which is the highest breakdown strength reported for KNN thin films. Solution synthesis involving two-step pyrolysis resulted in films with dense, columnar microstructures, which are interesting for orientation control and enhancement of piezoelectric properties. This study reports detailed solution synthesis and deposition processes with good dielectric, ferroelectric and breakdown field properties. An optimized fabrication method that should couple low leakage current density with dense and oriented microstructures is proposed.     

周期性极化铌酸锂晶体的电光复合逻辑门

根据准相位匹配线性电光效应耦合波理论,基于单个周期性极化铌酸锂晶体,提出了外电场可控的电光复合与门、或门的逻辑计算方法.采用级联周期性极化铌酸锂晶体,探讨了外电场可控的电光复合逻辑异或门、半加法器、半减法器的逻辑计算方法.在外加电场分别为0.184KV/mm和0.368KV/mm条件下,分析了光波的波长、方位角和极角、晶体的温度对逻辑输出的影响.研究结果表明,基本逻辑门的逻辑计算能够成功实现,但其逻辑计算对光波波长、晶体温度、方位角和极化角非常敏感,这些参量轻微的变化会导致逻辑输出的改变.该方案可推广到更为复杂的基本逻辑门以及三个以上逻辑输入与输出的基本逻辑门中.