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Growth and characterization of Ti:MgAl2O4 laser crystal by Czochralski method

Sun Gui-Hua Zhang Qing-Li Luo Jian-Qiao Sun Dun-Lu Gu Chang-Jiang Zheng Li-Li Han Song Li Wei-Min

Growth and characterization of Ti:MgAl2O4 laser crystal by Czochralski method

Sun Gui-Hua, Zhang Qing-Li, Luo Jian-Qiao, Sun Dun-Lu, Gu Chang-Jiang, Zheng Li-Li, Han Song, Li Wei-Min
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  • 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.
      Corresponding author: Sun Gui-Hua, ghsun2011@163.com
    [1]

    章佶, 孙真荣, 王祖赓, 司继良, 王静雅, 杭寅, 徐军 2005 人工晶体学报 34 657

    Zhang J, Sun Z R, Wang Z G, Si J L, Wang J Y, Hang Y, Xu J 2005 J. Synth. Cryst. 34 657

    [2]

    Gourier D, Colle L, Lejus A M, Vivein D, Moncorge R 1988 J. Appl. Phys. 63 1144

    [3]

    夏海平, 徐铁峰, 张新民, 王金浩, 章践立 2009 光学技术 35 307

    Xia H P, Xu T F, Zhang X M, Wang J H, Zhang J L 2009 Opt. Tech. 35 307

    [4]

    Basun S A, Danger T, Kaplyanskii A A, McClure D S, Petermann K, Wong W C 1996 Phys. Rev. B 54 6141

    [5]

    Bausa l E, Vergara I, Garcia-Sole J, Strek W, Deren P J 1990 J. Appl. Phys. 68 736

    [6]

    Sato T, Shirai M, Tanaka K, Kawabe Y, Hanamura E 2005 J. Lumin. 114 155

    [7]

    Jouini A, Sato H, Yoshikawa A, Fukuda T 2006 J. Mater. Res. 21 2337

    [8]

    Kuleshov N V, Shcherbitsky V G, Mikhailov V P, Kiick S, Koetke J, Petermann K, Huber G 1997 J. Lumin. 71 265

    [9]

    Tomita A, Sato T, Tanaka K, Kawabe Y, Shirai M, Tanaka K, Hanamura E 2004 J. Lumin. 109 19

    [10]

    Jouini A, Yoshikawa A, Fukuda T, Boulon G 2006 J. Cryst. Growth 293 517

    [11]

    Lombard P, Boizot B, Ollier N, Jouini A, Yoshikawa A 2009 J. Cryst. Growth 311 899

    [12]

    Wood D L, Imbusch G F, Macfarlane R M, Kisliuk P, Larkin D M 1968 J. Chem. Phys. 48 5255

    [13]

    王成思, 沈锡田, 刘云贵, 张倩 2019 光谱学与光谱分析 39 109

    Wang C S, Shen X T, Liu Y G, Zhang Q 2019 Spectrosc. Spect. Anal. 39 109

    [14]

    O'Horo M P, Frisillo A L, White W B 1973 J. Phys. Chem. Solids 34 23

    [15]

    Takahashi S, Kan A, Ogawa H 2017 J. Eur. Ceram. Soc. 37 1001

    [16]

    Frass L W, Moore J E, Salzberg J B 1973 J. Chem. Phys. 58 3585

    [17]

    Simeone D, Dodane-Thiriet C, Gosset D, Daniel P, Beauvy M 2002 J. Nucl. Mater. 300 151

    [18]

    Dash S, Sahoo R K, Das A, Bajpai S, Debasish D, Singh S K 2017 J. Alloy. Compd. 726 1186

    [19]

    Watterich A, Hofstaetter A, Wuerz’ R and Scharmann A 1996 Solid State Commun. 100 513

    [20]

    Jiang Y Q, Halliburton L E, Roth M, Tseitlin M, Angert N, 2007 Physica B 400 190

    [21]

    Dong S Y, Wang X Y, Shen L F, Li H S, Wang J, Nie P, Wang J J, Zhang X G 2015 J. Electroanal. Chem. 757 1

  • 图 1  尺寸为Ø30 mm × 70 mm的Ti:MgAl2O4晶体

    Figure 1.  As-grown Ti:MgAl2O4 crystal with the size of Ø30 mm × 70 mm.

    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)

    Figure 2.  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

    Figure 3.  X-ray rocking curve of (100) plane of the as-grown Ti:MgAl2O4 crystal.

    图 4  Ti:MgAl2O4晶体的拉曼谱图

    Figure 4.  Raman spectra of the as-grown Ti:MgAl2O4 crystal.

    图 5  退火前后Ti:MgAl2O4晶体在250−1200 nm范围内的透过光谱

    Figure 5.  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

    Figure 6.  Emission spectra of the as-grown Ti:MgAl2O4 crystal before and after annealing excited by 271 nm at room temperature.

    图 7  Ti:MgAl2O4晶体在130 K时的ESR谱

    Figure 7.  ESR spectrum of the as-grown Ti:MgAl2O4 crystal before and after annealing at 130 K.

    图 8  退火前后Ti:MgAl2O4晶体480 nm发射的激发光谱

    Figure 8.  Excitation spectra of the as-grown Ti:MgAl2O4 crystal before and after annealing with 480 nm as monitoring.

    图 9  室温下Ti:MgAl2O4晶体的荧光衰减曲线

    Figure 9.  Emission decay curve of the as-grown Ti:MgAl2O4 crystal at room temperature.

    表 1  几种不同的MgAl2O4的拉曼振动峰

    Table 1.  Raman vibration peaks of several different MgAl2O4.

    不同的MgAl2O4振动模式/cm–1
    F2g(1)EgF2g(2)A1g
    Natural Cr:MgAl2O4 crystal[13]312407667769
    MgAl2O4 crystal[14]311410671772
    MgAl2O4 ceramic[15]312407666767
    Natural Cr, V:MgAl2O4 crystal[16]305405663770
    Non-stoichiometric MgAl2O4 powder[17]306406670766
    MgAl2O4 polycrystalline [18]311407667767
    DownLoad: CSV
  • [1]

    章佶, 孙真荣, 王祖赓, 司继良, 王静雅, 杭寅, 徐军 2005 人工晶体学报 34 657

    Zhang J, Sun Z R, Wang Z G, Si J L, Wang J Y, Hang Y, Xu J 2005 J. Synth. Cryst. 34 657

    [2]

    Gourier D, Colle L, Lejus A M, Vivein D, Moncorge R 1988 J. Appl. Phys. 63 1144

    [3]

    夏海平, 徐铁峰, 张新民, 王金浩, 章践立 2009 光学技术 35 307

    Xia H P, Xu T F, Zhang X M, Wang J H, Zhang J L 2009 Opt. Tech. 35 307

    [4]

    Basun S A, Danger T, Kaplyanskii A A, McClure D S, Petermann K, Wong W C 1996 Phys. Rev. B 54 6141

    [5]

    Bausa l E, Vergara I, Garcia-Sole J, Strek W, Deren P J 1990 J. Appl. Phys. 68 736

    [6]

    Sato T, Shirai M, Tanaka K, Kawabe Y, Hanamura E 2005 J. Lumin. 114 155

    [7]

    Jouini A, Sato H, Yoshikawa A, Fukuda T 2006 J. Mater. Res. 21 2337

    [8]

    Kuleshov N V, Shcherbitsky V G, Mikhailov V P, Kiick S, Koetke J, Petermann K, Huber G 1997 J. Lumin. 71 265

    [9]

    Tomita A, Sato T, Tanaka K, Kawabe Y, Shirai M, Tanaka K, Hanamura E 2004 J. Lumin. 109 19

    [10]

    Jouini A, Yoshikawa A, Fukuda T, Boulon G 2006 J. Cryst. Growth 293 517

    [11]

    Lombard P, Boizot B, Ollier N, Jouini A, Yoshikawa A 2009 J. Cryst. Growth 311 899

    [12]

    Wood D L, Imbusch G F, Macfarlane R M, Kisliuk P, Larkin D M 1968 J. Chem. Phys. 48 5255

    [13]

    王成思, 沈锡田, 刘云贵, 张倩 2019 光谱学与光谱分析 39 109

    Wang C S, Shen X T, Liu Y G, Zhang Q 2019 Spectrosc. Spect. Anal. 39 109

    [14]

    O'Horo M P, Frisillo A L, White W B 1973 J. Phys. Chem. Solids 34 23

    [15]

    Takahashi S, Kan A, Ogawa H 2017 J. Eur. Ceram. Soc. 37 1001

    [16]

    Frass L W, Moore J E, Salzberg J B 1973 J. Chem. Phys. 58 3585

    [17]

    Simeone D, Dodane-Thiriet C, Gosset D, Daniel P, Beauvy M 2002 J. Nucl. Mater. 300 151

    [18]

    Dash S, Sahoo R K, Das A, Bajpai S, Debasish D, Singh S K 2017 J. Alloy. Compd. 726 1186

    [19]

    Watterich A, Hofstaetter A, Wuerz’ R and Scharmann A 1996 Solid State Commun. 100 513

    [20]

    Jiang Y Q, Halliburton L E, Roth M, Tseitlin M, Angert N, 2007 Physica B 400 190

    [21]

    Dong S Y, Wang X Y, Shen L F, Li H S, Wang J, Nie P, Wang J J, Zhang X G 2015 J. Electroanal. Chem. 757 1

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  • Received Date:  28 July 2019
  • Accepted Date:  27 September 2019
  • Available Online:  17 December 2019
  • Published Online:  01 January 2020

Growth and characterization of Ti:MgAl2O4 laser crystal by Czochralski method

    Corresponding author: Sun Gui-Hua, ghsun2011@163.com
  • Anhui Provincial Key Laboratory of Photonic Devices and Materials, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China

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.

    • 当前微细加工、激光医疗、激光化学、激光印刷、军事应用、水下通信和轴同位素分离等领域迫切要求固体激光器向短波长发展. 在众多晶体中, Ti掺杂的晶体中如Ti:Al2O3[1]、Ti:LaMgAl11O19[2]、Ti:BeAl2O4[3]、Ti:YAlO3[4]等有蓝光波段的荧光发射, 但其吸收波段覆盖了蓝光波段, 如Ti:Al2O3晶体虽然出现了有中心波长为420 nm的荧光发射, 荧光范围为340—480 nm, 但是其在400—600 nm处有宽峰吸收, 峰值在490 nm处, 并且其荧光寿命很短. MgAl2O4是一种典型的尖晶石结构, 属于立方晶系, 面心立方点阵, 空间群属于 Fd3 m, 晶格常数为 0.8085 nm, 具有良好的导热特性, 室温下热导率为25 W/(m·K). 在一个MgAl2O4晶胞中, 32个O2–作立方密堆时形成64个四面体空隙和32个八面体空隙, 8个Mg2+填充1/8四面体空隙, 16个Al3+填充1/2八面体空隙, 结构中存在较多空位; 如果16个Al3+中有8个Al3+占据8个四面体空隙, 另8个Al3+与8个Mg2+占据16个八面体空隙, 形成的结构称反尖晶石结构. 高温处理会引起Mg2+和Al3+的位置互换. 最近一些报道认为Ti:MgAl2O4晶体有可能成为蓝光激光并且是宽带可调谐的激光材料, 但这些报道结果也不完全一致, 其光谱特性依赖于其生长的环境和条件, 如Bausa等[5]研究表明Ti:MgAl2O4晶体有两个吸收带395—520 nm和520—1200 nm, Sato等[6]和Jouini等[7]研究结果则在250—900 nm没有发现明显的吸收带.

      另一方面, 由于MgAl2O4晶体的熔点高达2130 ℃, 并且高温条件下熔体表面的MgO和Al2O3存在非比例挥发, 严重影响晶体质量, 因此一直以来MgAl2O4及其掺杂晶体的生长方法主要有火焰法[5,8]、浮区法[6,9]、微下拉法[7,10,11]或熔盐法[12], 但是这些方法生长的晶体内部往往有包裹体、内部核芯等缺陷, 或者晶体外形为长条状、直径小, 难以满足实用化的需要. 提拉法可以通过温场的调节及其晶体旋转的搅拌效应的充分运用, 获得合适的自然对流和强迫对流, 从而可以进行有效的杂质输运, 这是目前所报道的其他晶体生长方法所不具备的优点. 另外, 由于提拉法生长时坩埚不与晶体接触, 从而可以减少寄生成核和生长应力, 有利于提高晶体质量.

      为了获得优质的大尺寸Ti:MgAl2O4晶体, 并进一步探索其在蓝光激光方面应用的可能性, 通过构建合适的温场, 优化生长工艺, 首次生长出了优质大尺寸Ti:MgAl2O4晶体. 在此基础上对晶体的结构、结晶质量、拉曼光谱、透过和吸收光谱、荧光发射及寿命进行了测试分析.

    2.   实 验
    • 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Ø30 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.

      Figure 1.  As-grown Ti:MgAl2O4 crystal with the size of Ø30 mm × 70 mm.

    • 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.   实验结果与讨论
    • 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.

      Figure 2.  X-ray diffraction patterns of the as-grown Ti:MgAl2O4 crystal and MgAl2O4 standard patterns (JCPDS, no. 77-0435).

      Figure 3.  X-ray rocking curve of (100) plane of the as-grown Ti:MgAl2O4 crystal.

      群理论计算认为MgAl2O4晶体具有以下的声子振动模式:

      式中A1gEgF2g为拉曼振动模式. 图4为以532 nm激光作为激发光源得到的Ti:MgAl2O4晶体的拉曼光谱, 可以看到有四个振动峰, 分别位于312, 410, 675 cm–1和771 cm–1, 振动模式分别对应于F2g (1)、EgF2g (2)和A1 g. 表1是文献报道的不同形态、不同掺杂离子的MgAl2O4的拉曼振动峰, 可以看出本文所述结果与文献[14]中MgAl2O4 晶体拉曼振动峰的位置最为接近.

      不同的MgAl2O4振动模式/cm–1
      F2g(1)EgF2g(2)A1g
      Natural Cr:MgAl2O4 crystal[13]312407667769
      MgAl2O4 crystal[14]311410671772
      MgAl2O4 ceramic[15]312407666767
      Natural Cr, V:MgAl2O4 crystal[16]305405663770
      Non-stoichiometric MgAl2O4 powder[17]306406670766
      MgAl2O4 polycrystalline [18]311407667767

      Table 1.  Raman vibration peaks of several different MgAl2O4.

      Figure 4.  Raman spectra of the as-grown Ti:MgAl2O4 crystal.

    • 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.

      Figure 5.  Transmittance spectra of the as-grown Ti:MgAl2O4 crystal before and after annealing in the range of 250−1200 nm.

      Figure 6.  Emission spectra of the as-grown Ti:MgAl2O4 crystal before and after annealing excited by 271 nm at room temperature.

      对于我们生长的Ti:MgAl2O4晶体来说, 其生长时的气氛为弱还原性, 掺杂浓度为0.05 at%, 退火前Ti:MgAl2O4晶体的吸收截止边的范围为250—318 nm, 有两个吸收带分别位于395—495 nm和550—1100 nm; 在271 nm波长的激发下, 在340—650 nm范围内都有一个强的宽带发射带, 中心波长位于480 nm. Bausa等[5]采用火焰法制备的Ti:MgAl2O4晶体也有两个吸收带, 位于395—520 nm和520—1100 nm, 他们认为前者是由Ti3+T2 gEg的跃迁引起的; 后者在Ti:Al2O3[1]和Ti:LaMgAl11O19[2]晶体中都有此吸收带出现, 与Fe2+—Ti4+有关, 还原气氛退火后这两种晶体在此处的吸收带的强度会明显减弱甚至消失; 而Bausa等[5]研究表明其所生长的Ti:MgAl2O4晶体在还原气氛退火后中晶体中的Ti4+没有转变为Ti3+. 我们生长的晶体在氢气退火前后的透过光谱和荧光发射光谱的外形基本一致, 但氢气退火后的发射光谱强度明显降低, 说明其荧光发射的有效成分减弱了. 当Ti:MgAl2O4晶体在1200 ℃空气气氛下退火后, 在250—1200 nm范围内未见到明显的吸收峰; 但在340—650 nm范围内仍有一个强的荧光发射带, 并且光谱的强度变化不大; 此外在725 nm附近还出现了一个弱的荧光发射峰, 这与晶体样品在此处吸收带的消失相对应. Sato等[6]和Jouini等[7]采用微下拉法在氧化气氛下生长的Ti:MgAl2O4晶体也具有与此类似的光谱特征.

      从退火前后的透过光谱和荧光发射光谱的变化可以看出, Ti:MgAl2O4晶体中Ti离子具有明显的变价特征. 为了进一步表征所生长的Ti:MgAl2O4晶体中Ti离子的价态, 对退火前后的晶体样品进行了电子自旋共振(electron spin resonance, ESR)谱的测定. ESR是处于恒定磁场中的电子自旋磁矩在射频电磁场作用下发生的一种磁能级间的共振跃迁现象, 其研究对象是具有未偶电子的物质, 测试结果如图7所示. 可以看出, 退火前和氢气气氛退火后的Ti:MgAl2O4晶体的谱图中都有Ti3+离子的ESR信号[11, 19-21]; 结合透过光谱的特征可以确定退火前的Ti:MgAl2O4晶体中含有Ti3+和Ti4+离子, 氢气气氛退火后仍然有Ti4+离子存在; 后者的ESR信号强度更大则可能与所用测试样品较大或部分Ti4+可能已被还原成Ti3+有关. 空气气氛退火后的ESR谱图没有明显的Ti3+离子信号, 可以认为空气退火后晶体中的Ti3+全部被氧化为Ti4+. 空气退火后的晶体没有明显的吸收带, 而退火前和氢气退火后的样品中都有两个吸收带, 由此可见这两个吸收带都与Ti3+有关. 此外, 由于退火前后的晶体都有很强的宽带蓝光荧光, 因此可以推断该荧光的产生是来自于晶体中Ti4+的贡献.

      Figure 7.  ESR spectrum of the as-grown Ti:MgAl2O4 crystal before and after annealing at 130 K.

      以480 nm作为监测波长, 测量了退火前Ti:MgAl2O4晶体在250—340 nm范围内的激发光谱, 如图8所示. 可以看到Ti:MgAl2O4晶体的激发波长范围为250—320 nm, 中心激发波长为271 nm. 图9是退火前Ti:MgAl2O4晶体的荧光衰减曲线, 拟合得到的荧光寿命为14 μs, 寿命是Ti:Al2O3、Ti:BeAl2O4晶体的4—5倍, 大的荧光寿命有利于储能. Bausa等[5]研究表明其制备的Ti:MgAl2O4晶体在77 K时的荧光寿命为25 μs, 随着温度的升高, 寿命逐渐缩短, 在300 K时荧光寿命小于10 μs; Sato等[6]制备的Ti:MgAl2O4晶体的荧光寿命为6.6 μs; 荧光寿命的差别可能来自于不同的晶体生长条件. 利用Füchtbauer-Ladenburg(F-L)公式:

      Figure 8.  Excitation spectra of the as-grown Ti:MgAl2O4 crystal before and after annealing with 480 nm as monitoring.

      Figure 9.  Emission decay curve of the as-grown Ti:MgAl2O4 crystal at room temperature.

      式中c 为光速, ι为拟合得到荧光寿命, β为荧光分支比, n 为折射率, 计算了Ti:MgAl2O4晶体的发射截面, 其发射截面具有较大值, 为2 × 10–20 cm2, 较大的发射截面利于实现激光输出.

    4.   结 论
    • 针对Ti:MgAl2O4晶体的熔点高达2130 ℃的特点, 设计并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场结构, 通过优化晶体生长工艺, 在弱还原气氛的条件下, 首次生长出了Ø30 mm × 70 mm的Ti:MgAl2O4晶体, X射线摇摆曲线测试结果表明其FWHM只有0.012°, 晶体的结晶质量良好, 为下一步激光实验输出奠定了材料基础. 在100—1000 cm–1范围内, 有4个拉曼振动峰, 分别位于312, 410, 675 cm–1和771 cm–1. Ti:MgAl2O4晶体的吸收截止边的范围为250—318 nm, 有两个吸收带分别位于395—495 nm和550—1100 nm; 在271 nm氙灯激发下, 有一个340—650 nm强的荧光发射带, 中心波长位于480 nm. 氢气退火前后的透过光谱和荧光发射光谱的外形基本一致, 但氢气退火后的发射光谱强度明显降低; 空气气氛下退火后, 原有的两个吸收带消失, 但在340—650 nm范围内仍有一个强的荧光发射, 并且光谱的强度变化不大, 同时在725 nm附近还出现了一个弱的荧光发射峰. 结合ESR谱图可以确定退火前的Ti:MgAl2O4晶体中含有Ti3+和Ti4+离子, 晶体的两个吸收带与Ti3+有关, 而蓝光荧光是来自于Ti4+的贡献. 室温下测得退火前Ti:MgAl2O4晶体的荧光寿命为14 μs, 为Ti:Al2O3、Ti:BeAl2O4晶体的5—6倍; 同时利用F-L公式计算了Ti:MgAl2O4晶体的发射截面, 其发射截面具有较大值, 为2 × 10–20 cm2. 以上结果表明, Ti:MgAl2O4晶体是潜在的能够实现宽带可调谐蓝光激光输出的晶体材料.

Reference (21)

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