Anhui Provincial Key Laboratory of Photonic Devices and Materials, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 51502292)
Received Date:28 July 2019
Accepted Date:27 September 2019
Available Online:17 December 2019
Published Online:05 January 2020
Abstract:The melting point of Ti:MgAl2O4 crystal is as high as 2130 °C, it is a challenge to obtain a large-sized and high-quality laser crystal. By optimizing the crystal growth process, Ti:MgAl2O4 crystal with a size of 30 mm× 70 mm is successfully grown by the Czochralski method under the condition of weak reducing atmosphere. The X-ray diffraction pattern is studied, and the x-ray rocking curve indicates that the grown crystal has a high crystalline quality in terms of the lower full width at half maximum(FWHM) intensity, which provides a material basis for the next laser output experiment. In a range of 100–1000 cm–1, there are four Raman vibration peaks located at 312, 410, 675 cm–1 and 771 cm–1 respectively. The grown crystal has an absorption cutoff range of 250–318 nm and two wide absorption bands of 395–495 nm and 550–1100 nm. Excited by 271 nm, the grown crystal shows a strong broadband emission ina range of 340–650 nm with a peak centered at 480 nm. After annealing in hydrogen atmosphere, shape of the transmittance spectrum and emission spectrum are both unchanged, but the fluorescent emission intensity is significantly reduced. After annealing in air atmosphere, the original two absorption bands disappear while none of the characteristics of fluorescence emission in a 340–650 nm range changes significantly. In addition, a new fluorescence emission peak near 725 nm is observed. Combining with the ESR spectrum, what we canconfirm is that the Ti:MgAl2O4 as-grown crystal contains Ti3+ and Ti4+ ions, and no ESR signal of Ti3+ is observed after annealing in air atmosphere. Moreover, excitationspectrum is also recorded. The fluorescence lifetime is 14 μs at room temperature, which is 4–5 times that of Ti:Al2O3 crystal and Ti:BeAl2O4 crystal. Furthermore, the emission cross section of the grown Ti:MgAl2O4 crystal is calculated from the Füchtbauer-Ladenburg (F-L) formula and its value is 2 × 10–20 cm2, large emission cross section which is beneficial for realizing laser oscillation. All the above results show that the Ti:MgAl2O4 crystal is a potential crystal material for realizing broadband tunable blue laser output. Keywords:Czochralski method/ crystal growth/ Ti:MgAl2O4 crystal/ blue luminescence
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2.1.晶体生长
生长Ti:MgAl2O4晶体原料的熔点接近铱坩埚的安全使用温度2200 ℃, 一旦温场条件不合适极易导致铱坩埚熔化损坏, 为此必须构建出温度梯度非常小的温场条件, 但对于掺杂晶体生长来说, 如果温场梯度过小, 熔体对流不顺畅, 极易导致晶体生长过程中的组分过冷, 出现气泡、云层等缺陷. 由于保温材料导热系数与其加工的密度密切相关, 并且关系到其在高温下的抗形变和抗硬化能力, 因此, 在温场搭建过程中需要综合考虑材质、密度、形状等因素. 采用积木式模块化的搭建方式, 并通过不同密度、不同材质保温材料的组合、构建出了既能保证坩埚安全, 又有一定的熔体自然对流的温度条件. 生长原料选择高纯的TiO2、MgO和Al2O3粉末, 压制成块后进行固相烧结, 然后将块料放入铱金坩埚中, 所用籽晶为 $ \langle 100\rangle $ 方向的MgAl2O4晶体, 生长气氛为90% N2和10% H2, 待锅内原料完全熔化后, 以12 r/min的转速和1.5 mm/h的拉速进行晶体生长, 首次成功生长出了等径部分为?30 mm × 70 mm的Ti:MgAl2O4晶体, 如图1所示, 晶体无散射、气泡和开裂等缺陷, 为下一步激光实验奠定了材料基础. 图 1 尺寸为?30 mm × 70 mm的Ti:MgAl2O4晶体 Figure1. As-grown Ti:MgAl2O4 crystal with the size of ?30 mm × 70 mm.
22.2.测 试 -->
2.2.测 试
粉末衍射测试在荷兰飞利浦公司生产的X′Pert PROX射线衍射仪进行, 测试角度范围为10o—70o. 沿垂直于晶体生长方向 $ \langle 100\rangle $ 切割并研磨出一定厚度的盘片, 将其中2片分别置于1200 ℃的空气气氛和氢气气氛下退火, 恒温时间为24 h. 单晶摇摆曲线所用设备为X'pert Pro MPD衍射仪, 拉曼光谱测试采用法国JY公司生产的LabRamHR拉曼光谱仪. 在室温条件下, 使用PE Lambda 950分光光度计测量其在250— 1200 nm波长范围的透射光谱; 在FLSP 920荧光光谱仪上进行了荧光光谱、发射光谱和荧光寿命的测量. 采用JES-FA 200型电子自旋共振谱仪测试了退火前后样品的电子自旋共振谱, 测试温度为130 K. -->
3.1.结构表征
图2是Ti:MgAl2O4晶体的粉末衍射图, 与MgAl2O4晶体的标准谱图(JCPDS, No. 77-0435)一致. 图3是Ti:MgAl2O4晶体(100)面的摇摆曲线, 其半峰宽(full width at half maximum intensity, FWHM)只有0.012°, 说明该晶体的结晶质量良好. 图 2 Ti:MgAl2O4晶体的粉末衍射图和MgAl2O4晶体的标准谱图(JCPDS, no. 77-0435) Figure2. X-ray diffraction patterns of the as-grown Ti:MgAl2O4 crystal and MgAl2O4 standard patterns (JCPDS, no. 77-0435).
图 3 Ti:MgAl2O4晶体(100)面的摇摆曲线 Figure3. X-ray rocking curve of (100) plane of the as-grown Ti:MgAl2O4 crystal.
表1几种不同的MgAl2O4的拉曼振动峰 Table1.Raman vibration peaks of several different MgAl2O4.
图 4 Ti:MgAl2O4晶体的拉曼谱图 Figure4. Raman spectra of the as-grown Ti:MgAl2O4 crystal.
23.2.光谱和发光性能 -->
3.2.光谱和发光性能
图5为室温下退火前后Ti:MgAl2O4晶体在250—1200 nm范围内的透过光谱. 图6是以271 nm的氙灯作为激发光, 采用相同的狭缝宽度, 在加340 nm滤波片的条件下, 测量了退火前后Ti:MgAl2O4晶体在340—900 nm范围内的荧光发射光谱. 图 5 退火前后Ti:MgAl2O4晶体在250?1200 nm范围内的透过光谱 Figure5. Transmittance spectra of the as-grown Ti:MgAl2O4 crystal before and after annealing in the range of 250?1200 nm.
图 6 退火前后Ti:MgAl2O4晶体在271 nm波长激发下的室温荧光发射光谱 Figure6. Emission spectra of the as-grown Ti:MgAl2O4 crystal before and after annealing excited by 271 nm at room temperature.