Absolute frequency control of a 1560 nm (192 THz) DFB laser lockedto a rubidium absorption line using a second-harmonic-generated signal

We demonstrate second-harmonic generation from a DFB laser at 1560 nm in a type I critically phase-matched KNbO₃ crystal. We obtain 2.2 nW at 780 mm with 11.3 mW at 1560 nm incident on the crystal. The conversion efficiency is 17.2 pW/(mW)². The 780 nm beam is used to interrogate a resonance of the ⁸⁷Rb-D₂ line at 780 nm and lock the laser frequency. To characterize the absolute frequency stability, two 1560 nm DFB lasers are respectively stabilized on a Doppler resonance of the ⁸⁷Rb-D₂ line (780.246 nm) and of the ⁸⁵Rb-D₂ line (780.244 nm). The square root of the Allan variance measured from the beat note is around 1.5×10^-9 for averaging times between 3 and 100 s. To improve the precision of the frequency locking, we realize a setup to observe a saturated absorption profile. We use a 780 nm stabilized laser as a pump and the SHG signal as a probe. A saturated absorption profile is observed over the Doppler envelope. Work is under progress to use this saturated resonance for an improved frequency control     

稀土氧化物对SPS烧结AlN陶瓷电性能的影响

以AlN粉末为原料, 添加稀土氧化物(Sm₂O₃、Y₂O₃), 在氮气气氛下, 采用SPS烧结方法制备AlN陶瓷, 研究稀土氧化物的掺杂对AlN烧结试样相组成、微观结构和电性能的影响。实验表明: Sm₂O₃、Y₂O₃与Al₂O₃反应生成的液相稀土金属铝酸盐会提高AlN陶瓷致密度, 且在晶界处形成导电通路降低了AlN陶瓷电阻率。随着Sm₂O₃掺杂量的增加, 晶界相逐渐由Sm₄Al₂O₉过渡到SmAlO₃, 且Sm₄Al₂O₉对电阻率贡献最大。其中, 3wt% Sm₂O₃掺杂AlN陶瓷电阻率最低, 为      

Enhanced piezoelectric properties of potassium niobate single crystals with fine engineered domain configurations

Potassium niobate (KNbO₃) single crystals were grown by a top seed solution growth (TSSG) method. At first, the electric field was applied along [0 0 1]_c (cubic notification system) direction of KNbO₃ crystals to induce the engineered domain configurations into KNbO₃ crystals. Prior to domain engineering, the piezoelectric properties of [0 0 1]_c oriented KNbO₃ single-domain crystals were measured. These measured values were completely consistent with the calculated apparent d₃₁ and d₃₂. Finally, the engineered domain configurations were induced into KNbO₃ crystals. As a result, piezoelectric properties increased with decreasing domain sizes of the engineered domain configuration.     

Effects of Gallium on the Structure and Electrical Properties of 0.65 Bi0.94La0.06) (GaxFe1?x)O3-0.35PbTiO3 Ceramics

Crystalline solutions of 0.65 (Bi_0.94La_0.06) (Ga_xFe_1-x)O₃-0.35PbTiO₃ ceramics (BLGF-PT) for x = 0 and 0.05 have been fabricated by the solid-state reaction method. X-ray diffraction (XRD) was utilized to characterize the crystal structure and examine any possible impurities existing in the ceramics. The effects of Ga substitution on dielectric properties of the samples were studied at frequencies from 10² to 10⁶ Hz over a temperature range from 20 to 620degC. The results indicate that Ga modification can reduce the room temperature dielectric loss. The conduction mechanism of the material was investigated using ac conductivity. It is concluded that electrons originating from Fe²⁺ and oxygen ion vacancies are the main charge carriers, and Ga doping could decrease the electronic conduction effectively. The frequency dependence of ac conductivity was found to follow Jonscher's universal power law.     

氧化气氛对CaCu3Ti4O12陶瓷显微结构及介电性能的影响

采用固相法合成了CaCu₃Ti₄O₁₂陶瓷, 研究了氧化气氛热处理对其显微结构和介电性能的影响。结果表明, 氧化气氛热处理后, CaCu₃Ti₄O₁₂陶瓷的介电常数略微下降, 介电损耗角正切值得到有效抑制, 降至0.03~0.04。在183~273 K温度范围内, CaCu₃Ti₄O₁₂陶瓷总的介电损耗可以分解成低频电导损耗和高频的两个弛豫损耗峰, 氧化气氛热处理后低频电导损耗的贡献下降较为明显。对CaCu₃Ti₄O₁₂陶瓷的晶粒、晶界进行阻抗分析, 发现在393 K时晶界电阻值从1.8×10⁴ Ω增大到8.6×10⁴ Ω。伏安特性(J-E)结果表明氧化气氛热处理使CaCu₃Ti₄O₁₂陶瓷的击穿场强和非线性系数分别从226 V/mm、3.52上升到397 V/mm、4.51, 晶界势垒高度从0.56 eV增大到0.63 eV。     

Infrared optical properties of Bi2Ti2O7 thin films by spectroscopic ellipsometry

Bi₂Ti₂O₇ thin films have been grown directly on n-type GaAs (1 0 0) by the chemical solution decomposition technique. X-ray diffraction analysis shows that the Bi₂Ti₂O₇ thin films are polycrystalline. The optical properties of the thin films are investigated using infrared spectroscopic ellipsometry (3.0–12.5 μm). By fitting the measured ellipsometric parameter (Ψ and Δ) data with a three-phase model (air/Bi₂Ti₂O₇/GaAs), and Lorentz–Drude dispersion relation, the optical constants and thickness of the thin films have been obtained simultaneously. The refractive index and extinction coefficient increase with increasing wavelength. The fitted plasma frequency ω_p is 1.64×10¹⁴ Hz, and the electron collision frequency γ is 1.05×10¹⁴ Hz, and it states that the electron average scattering time is 0.95×10⁻¹⁴ s. The absorption coefficient variation with respect to increasing wavelength has been obtained.     

Ab initio supported model simulations of ferroelectric perovskites

While ferroelectric KNbO₃ and BaTiO₃ present the cubic-tetragonal-orthorhombic-rhombohedral phase sequence with decreasing T, their related compounds KTaO₃ and SrTiO₃ are incipient ferroelectrics which exhibit the ferroelectric soft mode but remain paraelectric up to 0 K. Despite the large amount of research attracted by the above phenomenology since long ago, quite a few realistic model simulations are available. Many lattice dynamical calculations have been performed for each isolated material, but the only unified view of their ferroelectric behavior has been provided by the nonlinear oxygen polarizability model. Although initially based on phonon data for the cubic phase, recent ab initio calculations confirm that the model is basically correct with regards to the energetics involved in the various ferroelectric distortions in KNbO₃. Our molecular dynamics simulations show the crossover from a soft mode to an order-disorder dynamics in the cubic phase of KNbO₃, the appearance of its various ferroelectric phases, and the soft-mode behavior of KTaO₃.     

Toward a unified description of nonlinearity and frequency dispersion of piezoelectric and dielectric responses in Pb(Zr,Ti)O3

A phenomenological approach is proposed describing both nonlinearity and frequency dispersion in dielectric and piezoelectric properties of lead zirconate titanate, Pb(Zr,Ti)O₃ (PZT), thin films and ceramics. The approach couples the frequency dependent response in form of the power law, 1/ω^β, with the rate-independent nonlinear response described by the Rayleigh law. The main experimental trends are well described by the model.     

Computer modelling of point defects in ABO3 perovskites and MgO

We present results for basic intrinsic defects: F-type electron centers (O vacancy which trapped one or two electrons) and hole polarons bound to Mg or K vacancy in ionic MgO and partly covalent KNbO₃ perovskite, respectively. We demonstrate that a considerable covalency of the perovskite chemical bonding makes the F-type centers therein much more similar to defects in partly-covalent quartz-type oxides rather than the conventional F centers in alkali halides and ionic MgO. Both one-site (atomic) and two-site (molecular) polarons are expected to coexist in KNbO₃ characterized by close absorption energies. Our calculations confirm existence of the self-trapped electron polarons in KNbO₃, KTaO₃, BaTiO₃, and PbTiO₃ crystals. The self-trapped electron is mostly localized on B-type ion due to a combination of breathing and Jahn–Teller modes of nearest six oxygen ion displacements. The relevant lattice relaxation energies are typically 0.2–0.3 eV, whereas the optical absorption energies 0.7–0.8 eV, respectively. According to our calculations, the absorption energy of a bound electron polaron in KNbO₃ by 0.1 eV exceeds that for the self-trapped electron polaron and equals 0.88 eV.     

BaTiO3陶瓷的低温冷烧结制备及性能研究

近年来, 冷烧结低温制备陶瓷引起了很大关注, 并在BaTiO₃陶瓷的制备上取得了一定进展。为了提高冷烧结BaTiO₃陶瓷性能, 本研究采用水热法制备了分散性好、粒径为100 nm的四方相(晶格参数c/a为1.0085) BaTiO₃粉末。采用0.1 mol/L的乙酸在100 ℃/1 h的条件下对粉末进行水热活化处理。以质量分数10% Ba(OH)₂·8H₂O为熔剂, 在350 MPa、400 ℃/1 h的条件下对粉体进行冷烧结, 最后经600 ℃/0.5 h退火获得了相对密度为96.62%、晶粒尺寸为180 nm, 常温介电(ε_r)为2836, 介电损耗(tanδ)低至0.03的BaTiO₃陶瓷。乙酸处理后高活性粉末表面形成的非晶钛层有效促进了陶瓷的致密化, 抑制了杂相的生成和晶粒长大, 提高了介电性能, 大幅改善了冷烧结BaTiO₃陶瓷出现的介电弥散现象, 从而实现了BaTiO₃陶瓷的低温冷烧结制备。     

Spectral Diagnostics of Helium-Carbon Arc Plasma During Carbon Nanostructure Formation

The quantitative diagnostics of carbon arc during the process of carbon nanostructure formation is presented. Temperature within 3500-5500 K and concentration within 2*10¹⁵ - 2*10¹⁶ cm^-2 of C₂(a ³II_u, v=0) were determined upon input power and plasma coordinates at arc gap 0.5 ± 0.2 mm favoring the process yield. The total carbon vapor pressure in the discharge zone was estimated as equal to 30 kPa.     

Fe高温浸渗法制备防静电3Y-TZP陶瓷及其性能研究

以3Y-TZP陶瓷为基体, 采用还原Fe粉包埋、高温浸渗的表面改性方法, 制备防静电性能与力学性能兼备的ZrO₂陶瓷。研究了浸渗时间、浸渗温度对ZrO₂陶瓷表面电阻率及硬度的影响, 结果表明：随着浸渗时间延长, 浸渗温度升高, 表面电阻率降低, 硬度下降。在1000℃浸渗4 h, ZrO₂防静电陶瓷表面电阻率由10¹⁴ ?/□以上降低至8.3×10⁷ ?/□, 硬度由12.7 GPa降低至11.23 GPa。采用XRD、SEM、XPS等方法对防静电ZrO₂陶瓷的显微结构、化学组成及防静电机理进行了分析。结果表明, 浸渗过程中, 发生由t-ZrO₂转变为m-ZrO₂相变; Fe元素以Fe₃O₄、FeO、单质Fe的形式存在晶界处, 从而使ZrO₂陶瓷具备了防静电性能。     

Eu掺杂KNN陶瓷的制备及可调性发光研究

稀土离子掺杂铁电陶瓷是一类新型光致变色材料, 在光开关、光信息存储等领域具有潜在应用价值。本研究采用水热法制备了(K_0.5 Na_0.5)_1-xEu_xNbO₃(KNN:xEu)前驱体粉体, 随后利用高温烧结得到对应陶瓷样品。在465 nm激发下, 观察到615 nm处有强的红色发光, 对应于Eu ³⁺的 ⁵D₀→ ⁷F₂跃迁。通过紫外光照射, KNN:Eu陶瓷从乳白色变为深灰色。随后经过200 ℃加热10 min, 着色陶瓷又变回到初始颜色, 显示出良好的光致变色行为。紫外照射和反复加热循环可以有效调控该陶瓷的发光强度。且经过多次循环之后, 发光强度没有明显衰减。在紫外光照射下, KNN:0.06Eu陶瓷发光强度的可调比(ΔR_t)高达83.9%, 说明发光具有良好的可调性。进而结合发光中心和色心之间的能量转移, 对KNN:Eu陶瓷的光致变色和发光机理进行了解释。     

Frequency dependent electrical conductivity and dielectric relaxation behavior in amorphous (90V2O5–10Bi2O3) oxide semiconductors doped with SrTiO3

We report on the experimental results of frequency dependent a.c. conductivity and dielectric constant of SrTiO₃ doped 90V₂O₅–10Bi₂O₃ semiconducting oxide glasses for wide ranges of frequency (500–10⁴ Hz) and temperature (80–400 K). These glasses show very large dielectric constants (10²–10⁴) compared with that of the pure base glass (≈10²) without SrTiO₃ and exhibit Debye-type dielectric relaxation behavior. The increase in dielectric constant is considered to be due to the formation of microcrystals of SrTiO₃ and TiO₂ in the glass matrix. These glasses are n-type semiconductors as observed from the measurements of the thermoelectric power. Unlike many vanadate glasses, Long's overlapping large polaron tunnelling (OLPT) model is found to be most appropriate for fitting the experimental conductivity data, while for the undoped V₂O₅–Bi₂O₃ glasses, correlated barrier hopping conduction mechanism is valid. This is due to the change of glass network structure caused by doping base glass with SrTiO₃. The power law behavior (σ_ac=A(ω^s) with s<1) is, however, followed by both the doped and undoped glassy systems. The model parameters calculated are reasonable and consistent with the change of concentrations (x).     

Structure, nucleation and Eu luminescence in alkaline earth rare earth borate glass

In La₂O₃-MO-B₂O₃ ternary system, various glasses/ glass ceramics with M=Ca, Sr and Ba have been prepared. In this ternary system, homogeneous nucleation occuring in the B₂O₃ melt appears to be the cause for the formation of nanocrystallites, hence glass ceramics. The nucleation process is very much dependent on the alkaline earth used viz., with smaller alkaline earths like Ca, boron prefers (BO₄)⁵⁻ tetrahedral coordination with oxygen, while in the case of bigger alkaline earths like Ba and Sr, (BO₃)³⁻ triangular coordination seems to be predominant. Eu³⁺ in this glass system yields intense ⁵D₀→⁷F_j emission. A cursory view on the dependence of the various Judd-Ofelt parameters (Ω_k) indicates that Ω₂ parameter is very much dependent on the immediate vicinity of the luminescent ion (Eu³⁺) while Ω₄ is not. Various results based on these are discussed.     

Semiconducting barium titanate ceramics prepared by using yttrium hexaboride as sintering aid

Nb-doped BaTiO₃ semiconducting ceramics have been prepared at 1150 °C by employing YB₆ as the sintering aid. The low-temperature sintered samples exhibit a positive temperature coefficient of resistivity (PTCR) effect. The room-temperature resistivity and the magnitude of PTCR effect depend on the amount of YB₆ added, the Nb₂O₅ content and the sintering conditions. At 1 mol% of YB₆, the sample has a low room-temperature resistivity and a resistivity jump of about 10⁴. Transmission electron microscope studies revealed that the intergranular region between the crystalline BaTiO₃ grains has an amorphous structure. Y, Ba and Ti are present in the intergranular region but no Y is detectable in the crystalline grains. A liquid phase is believed to be present during sintering and it helps to facilitate the densification process and to enhance the PTCR effect.     

尖晶石型CuAl2O4掺杂对CaCu3Ti4O12陶瓷介电性能的影响

研究了尖晶石型CuAl₂O₄掺杂对CaCu₃Ti₄O₁₂陶瓷显微结构、介电性能以及松弛特征和缺陷结构的影响。在频率为10^-1~10⁷ Hz、温度为153~453 K的条件下测量了样品的介电性能。研究表明, 适量添加CuAl₂O₄, 使样品晶粒尺寸减小并趋于均匀, 击穿场强从CaCu₃Ti₄O₁₂陶瓷样品的3.0 kV/cm提高到13.0 kV/cm, 低频介电损耗减小。介电松弛中的高频松弛过程起源于晶粒本征缺陷的电子松弛过程, 其活化能~0.10 eV基本不变; 随着CuAl₂O₄含量增加, 与界面相关的松弛活化能从0.50 eV减小到0.22 eV, 可能与CuAl₂O₄在样品中引入杂质及更复杂的界面有关; 电导活化能从0.66 eV增至0.86 eV, 归因于CuAl₂O₄第二相抑制了晶界处的载流子跳跃, 提高了Schottky势垒高度。CuAl₂O₄掺杂量大于100mol%, 过量CuAl₂O₄会导致样品晶界势垒崩塌, 样品失去非欧姆特性和巨介电性能。     

Structural, electrical, and optical studies on rapid thermally processed ferroelectric BaTiO3 thin films prepared by metallo-organic solution deposition technique

Polycrystalline BaTiO₃ thin films having the perovskite structure were successfully produced on platinum coated silicon, bare silicon, and fused quartz substrate by the combination of the metallo-organic solution deposition technique and post-deposition rapid thermal annealing treatment. The films exhibited good structural, electrical, and optical properties. The electrical measurements were conducted on metal-ferroelectric-metal (MFM) and metal-ferroelectric-semiconductor (MFS) capacitors. The typical measured small signal dielectric constant and dissipation factor at a frequency of 100 kHz were 255 and 0.025, respectively, and the remanent polarization and coercive field were 2.2 μC cm⁻² and 25 kV cm⁻¹, respectively. The resistivity was found to be in the range 10¹⁰–10¹² Ω·cm, up to an applied electric field of 100 kV cm⁻¹, for films annealed in the temperature range 550–700 °C. The films deposited on bare silicon substrates exhibited good film/substrate interface characteristics. The films deposited on fused quartz were highly transparent. An optical band gap of 3.5 eV and a refractive index of 2.05 (measured at 550 nm) was obtained for polycrystalline BaTiO₃ thin film on fused quartz substrate. The optical dispersion behavior of BaTiO₃ thin films was found to fit the Sellmeir dispersion formula well.     

Nd3+:Lu2O3放电等离子烧结及光学性能

Lu₂O₃是具有高热导率而成为极具潜力的高功率激光介质材料。实验以商用氧化物粉体为原料, LiF为烧结助剂, 采用放电等离子烧结法制备了不同Nd³⁺掺杂浓度(C_Nd=0, 1at%, 3at%和5at%) Lu₂O₃透明陶瓷, 并研究了Nd³⁺掺杂浓度对Lu₂O₃陶瓷的物相、烧结性能、微观结构及光学性能的影响。结果表明：在高Nd³⁺浓度(5at%)掺杂后烧结样品仍为纯Lu₂O₃相;Nd³⁺掺杂对Lu₂O₃陶瓷烧结性能及微观形貌的影响有限;所有样品最终均表现出高致密性(99.5%以上)和优异的透光性能, 其中3at% Nd³⁺:Lu₂O₃的透过率最高, 在1064和2000 nm处的透过率分别为82.7和83.2%。Nd³⁺:Lu₂O₃透明陶瓷的最强发射峰位于1076和1080 nm;且随着Nd³⁺掺杂浓度的增加, 荧光强度降低, 寿命变短, 发生浓度淬灭。     

共沉淀纳米粉体制备Yb:CaF2激光陶瓷及其性能研究

利用共沉淀法合成的粉体, 通过真空烧结结合热压烧结后处理制备了掺镱的氟化钙透明陶瓷(Yb:CaF₂)。在600 ℃预烧1 h, 700 ℃热压烧结2 h制备的5at%Yb:CaF₂透明陶瓷在1200 nm处的直线透射率达到92.0%。对陶瓷的显微结构、光谱特性和激光性能进行了测试和讨论。研究结果表明, 陶瓷样品的显微结构均匀, 平均晶粒尺寸为360 nm。此外, 计算得到Yb:CaF₂陶瓷在977 nm处的吸收截面和1030 nm处的发射截面分别为0.39×10 ^-20和0.26×10 ^-20cm ²。最后, 对Yb:CaF₂陶瓷激光性能进行了表征, 得到最大输出功率为0.9 W, 最大斜率效率为23.6%。