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Growth, structure, and spectroscopic properties of Yb,Ho,Pr:GYTO single crystal (Invited

He Yi Dou Renqin Zhang Haotian Liu Wenpeng Zhang Qingli Chen Yingying Gao Yuxi Luo Jianqiao

何異, 窦仁勤, 张昊天, 刘文鹏, 张庆礼, 陈迎迎, 高宇茜, 罗建乔. Yb,Ho,Pr:GYTO晶体生长、结构及光谱性能(特邀)[J]. 红外与激光工程, 2020, 49(12): 20201067. doi: 10.3788/IRLA20201067
引用本文: 何異, 窦仁勤, 张昊天, 刘文鹏, 张庆礼, 陈迎迎, 高宇茜, 罗建乔. Yb,Ho,Pr:GYTO晶体生长、结构及光谱性能(特邀)[J]. 红外与激光工程, 2020, 49(12): 20201067. doi: 10.3788/IRLA20201067
He Yi, Dou Renqin, Zhang Haotian, Liu Wenpeng, Zhang Qingli, Chen Yingying, Gao Yuxi, Luo Jianqiao. Growth, structure, and spectroscopic properties of Yb,Ho,Pr:GYTO single crystal (Invited)[J]. Infrared and Laser Engineering, 2020, 49(12): 20201067. doi: 10.3788/IRLA20201067
Citation: He Yi, Dou Renqin, Zhang Haotian, Liu Wenpeng, Zhang Qingli, Chen Yingying, Gao Yuxi, Luo Jianqiao. Growth, structure, and spectroscopic properties of Yb,Ho,Pr:GYTO single crystal (Invited)[J]. Infrared and Laser Engineering, 2020, 49(12): 20201067. doi: 10.3788/IRLA20201067

Yb,Ho,Pr:GYTO晶体生长、结构及光谱性能(特邀)

doi: 10.3788/IRLA20201067
详细信息
  • 中图分类号: O782

Growth, structure, and spectroscopic properties of Yb,Ho,Pr:GYTO single crystal (Invited

More Information
    Author Bio:

    何異(1993-),女,博士生,主要从事晶体的生长和数值模拟研究。Email:hy614@mail.ustc.edu.cn

    Corresponding author: 窦仁勤(1987-),女,博士,主要从事光学功能晶体的生长和表征。Email:drq0564@aiofm.ac.cn张庆礼(1973-),男,研究员,博士生导师,主要从事光功能晶体材料研究。Email:zql@aiofm.ac.cn
  • 摘要: 首次采用提拉法成功生长出了新型中红外激光晶体Yb,Ho,Pr:GYTO,采用X射线Rietveld精修方法得到了晶体的结构参数。测量了Yb,Ho,Pr:GYTO晶体(100)、(010)和(001)衍射面的X射线摇摆曲线,衍射峰的半峰宽分别为0.036°、0.013°和0.077°,表明生长出的晶体是单晶并且具有较高的结晶质量。采用激光剥蚀电感耦合等离子体质谱法测定了Yb,Ho,Pr:GYTO晶体中Yb3+、Ho3+、Pr3+和Y3+的浓度,Yb,Ho,Pr:GYTO晶体中Yb3+、Ho3+、Pr3+和Y3+的有效分凝系数分别为0.624、1.220、1.350和0.977。测量了Yb,Ho,Pr:GYTO晶体室温下的极化吸收谱,并指认了相应的能级吸收跃迁。940 nm半导体激光器激发的2.9 μm荧光光谱表明,最大发射波长为2908 nm。此外,还论证了GYTO中Yb-Ho-Pr的能量传递机制。与Ho:GYTO晶体相比,Yb,Ho,Pr:GYTO晶体的5I7能级寿命降低了87.13%,与上能级5I6的寿命相近,说明Yb,Ho,Pr:GYTO晶体更容易实现粒子数反转和激光输出。
  • Figure  1.  (a) Photograph of the as-grown Yb,Ho,Pr:GYTO crystal; (b) <100>, <010>, and <001>-oriented wafers of Yb,Ho,Pr:GYTO crystal

    Figure  2.  XRD patterns of Yb,Ho,Pr:GYTO single crystal

    Figure  3.  Rietveld refinement results from the XRD data of Yb,Ho,Pr:GYTO crystal

    Figure  4.  X-ray rocking curves of Yb,Ho,Pr:GYTO crystal

    Figure  5.  (a) Polarized absorption spectra of Yb,Ho,Pr:GYTO; (b) Comparization of Polarized absorption spectra of Yb,Ho,Pr:GYTO and Yb,Ho:GYTO in 850-1100 nm (a, b, c →Yb,Ho,Pr:GYTO; a’, b’, c’ →Yb,Ho:GYTO)

    Figure  6.  2.9 μm emission spectrum of Yb,Ho,Pr:GYTO crystal

    Figure  7.  Fluorescence decay curves. (a) 1204 nm (5I65I8); (b) 2068 nm(5I75I8)

    Figure  8.  Schematic of energy transfer processes among Yb3+, Ho3+, and Pr3+ ions

    Table  1.   Structural parameters obtained by Rietveld refinement

    AtomXYZWyckoff siteUiso
    Gd0.250.6210000.04a0.025
    Y0.250.6210000.04a0.025
    Yb0.250.6210000.04a0.025
    Ho0.250.6210000.04a0.025
    Pr0.250.6210000.04a0.025
    Ta0.250.1450000.04a0.025
    O10.0940000.4600000.2540008c0.025
    O2−0.007000.7170000.2930008c0.025
    Cell parameters: a=5.381 Å, b=11.023 Å, c=5.076 Å, β=95.59°; Cell volume: V=299.68 Å3; Space group: Monoclinic, I2/a (No.15); Density: ρ=8.630 g/cm3; Reliability factors(R-factor): Rp=9.72%, Rwp=7.21%
    下载: 导出CSV

    Table  2.   Effective segregation coefficients (keff) of Yb, Ho, Pr, and Y in Yb,Ho,Pr:GYTO crystal

    ElementStarting material (at %)Crystal (at %)keff (Cs/C0)
    Yb0.050.03120.624
    Ho0.010.01221.220
    Pr0.0020.00271.350
    Y0.20.19530.977
    下载: 导出CSV

    Table  3.   Comparison of the emission cross section for 2.9 μm in the different Ho3+ doped crystals

    CrystalsEmission cross section (10−20 cm2)
    Yb,Ho,Pr:GYTO (this work)14.4
    Ho:GYTO[25]12.6
    Yb,Ho:GYTO[27]18.9
    Ho:LaF3[29]0.63
    Ho:LuLF[30]1.7
    Ho:PbF2[31]1.44
    下载: 导出CSV

    Table  4.   Comparison of the lifetimes of 5I7 and 5I6 in different crystals

    CrystalHo (5I7)/msHo (5I6)/ μs
    Yb,Ho:YSGG[[32]]10.2585
    Tm,Ho:YAG[[33]]11.440
    Yb,Ho,Pr:YAP[[24]]1.258341
    Ho:GYTO[[25]]8.081311
    Tm,Ho:GYTO[[26]]4.09131
    Yb,Ho,Pr:GYTO (this work)0.939376
    下载: 导出CSV
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出版历程
  • 收稿日期:  2020-10-20
  • 修回日期:  2020-11-25
  • 网络出版日期:  2021-01-14
  • 刊出日期:  2020-12-25

Growth, structure, and spectroscopic properties of Yb,Ho,Pr:GYTO single crystal (Invited

doi: 10.3788/IRLA20201067
    作者简介:

    何異(1993-),女,博士生,主要从事晶体的生长和数值模拟研究。Email:hy614@mail.ustc.edu.cn

    通讯作者: 窦仁勤(1987-),女,博士,主要从事光学功能晶体的生长和表征。Email:drq0564@aiofm.ac.cn张庆礼(1973-),男,研究员,博士生导师,主要从事光功能晶体材料研究。Email:zql@aiofm.ac.cn
  • 中图分类号: O782

摘要: 首次采用提拉法成功生长出了新型中红外激光晶体Yb,Ho,Pr:GYTO,采用X射线Rietveld精修方法得到了晶体的结构参数。测量了Yb,Ho,Pr:GYTO晶体(100)、(010)和(001)衍射面的X射线摇摆曲线,衍射峰的半峰宽分别为0.036°、0.013°和0.077°,表明生长出的晶体是单晶并且具有较高的结晶质量。采用激光剥蚀电感耦合等离子体质谱法测定了Yb,Ho,Pr:GYTO晶体中Yb3+、Ho3+、Pr3+和Y3+的浓度,Yb,Ho,Pr:GYTO晶体中Yb3+、Ho3+、Pr3+和Y3+的有效分凝系数分别为0.624、1.220、1.350和0.977。测量了Yb,Ho,Pr:GYTO晶体室温下的极化吸收谱,并指认了相应的能级吸收跃迁。940 nm半导体激光器激发的2.9 μm荧光光谱表明,最大发射波长为2908 nm。此外,还论证了GYTO中Yb-Ho-Pr的能量传递机制。与Ho:GYTO晶体相比,Yb,Ho,Pr:GYTO晶体的5I7能级寿命降低了87.13%,与上能级5I6的寿命相近,说明Yb,Ho,Pr:GYTO晶体更容易实现粒子数反转和激光输出。

English Abstract

    • Previously, rare-earth orthotantalate (RETaO4) had been attracted as scintillator crystals and X-ray phosphors, due to its high chemical stability, high density, rich physical properties, and so on[1-2]. RETaO4 belongs to the fergusonite structure and exhibits excellent luminescent properties[3-6]. It usually exhibits two modifications, fergusonite M-type structure I2/a ($ {C}_{2h}^{6} $, #15, Z = 4), and fergusonite M’-type structure P2/a ($ {C}_{2h}^{4} $, #13, Z = 2)[7]. RE ions, with similar ion radius, can be substituted easily by other rare earth ions to realize characteristic emission[8]. Besides, RETaO4 belong to monoclinic system and the site of RE ions is C2 symmetry, which are advantageous to Stark levels splitting of active ions and realization of new emission and tunable wavelength. Therefore, RETaO4 can be used as new host matrices. Additionally, mixed crystal is an effective method to reduce lattice symmetry further and obtain the absorption and emission spectra with inhomogeneous broadening[9-10]. In the past decade, our group have finished a lot of research works on RETaO4, especially on GdTaO4 (GTO)[11]. On the base of the previous works, the GTO crystal field can be effectively regulated by mixing Y2O3. Moreover, the position of emission peaks can be regulated by the differentproportion of Y2O3 in GdYTaO4 (GYTO) crystal. Now, rare earth(Ho, Nd)-doped GTO and GYTO have been realized laser output in near infrared band[12-15].

      With the rapid development and application of laser technology, the search for new mid infrared laser materials has always been an important direction[16-20]. The 5I65I7 transition of Ho3+ is an effective approach to obtain 2.9 μm lasers[21-22]. However, the laser efficiency is poor. Because of the long lifetime of 5I7 level and the short lifetime of 5I6, it is hard to realize population inversion, that is self-terminating “bottleneck” effect. To overcome this “bottleneck” effect, Pr3+ ions are usually used as deactivators to reduce the lifetime of low laser level, which have been achieved good results in other crystals[23-24]. In our previous work, the detailed properties of Ho-doped GYTO, Yb,Ho-doped GYTO, and Tm,Ho-doped GYTO are studied[25-27]. Unfortunately, there is no laser output. Therefore, Pr3+ ions are doped into Yb,Ho:GYTO to reduce the lifetime of the laser low level 5I7.

      In this study, a Yb,Ho,Pr:GYTO crystal was grown successfully using Czochralski method for the first time. The crystal structure and quality are analyzed. The polarized absorption spectra are investigated. The optical properties, including fluorescence, lifetimes, and energy transfer mechanisms among the ions are measured and analyzed.

    • According to the chemical formula Yb0.05Ho0.01Pr0.002Gd0.738Y0.2TaO4, the high purity oxides were weighed, mixed, and calcined. The Yb,Ho,Pr:GYTO single crystal was grown by the Czochralski method. The temperature gradient, growth parameter, and growth process are the same as the previous work[26]. A transparent and crack free crystal with a size of Φ 23 mm × 40 mm was obtained, as shown in Fig.1(a). Under a 1 W 532 nm laser irradiation, no light-scattering points were observed in the as-grown Yb,Ho,Pr:GYTO crystal. The <100>, <010>, and <001>-oriented slice samples were cut with a thickness of 2 mm and polished on both sides for measurements (shown in Fig.1(b)).

      Figure 1.  (a) Photograph of the as-grown Yb,Ho,Pr:GYTO crystal; (b) <100>, <010>, and <001>-oriented wafers of Yb,Ho,Pr:GYTO crystal

    • The X-ray diffraction (XRD) patterns of the as-grown Yb,Ho,Pr:GYTO crystal were measured using a Philip X′pert PRO X-ray diffractometer equipped with Cu Kα radiation. The diffraction peaks were recorded in the 2θ range of 10°-90° with a scan step of 0.033°. A high resolution X′Pert Pro MPD diffractometer equipped with a hybrid Kα1 monochromator was employed to collect the X-ray rocking curve. The doping concentrations of Yb3+, Ho3+, Pr3+ and Y3+ ions in the Yb,Ho,Pr:GYTO crystal were measured by Laser Ablation Inductively-Coupled Plasma Mass Spectrometry (LA-ICP-MS). The analyses of the sample which was cut from the shoulder part of the as-grown crystal were carried out on an Agilent 7900 quadrupole ICP-MS coupled to a Photon Machines equipped with Analyte HE 193 nm ArF Excimer Laser Ablation system. The effective segregation coefficients (keff) of the doping ions were obtained by comparing the LA-ICP-MS results with the initial concentrations in the raw materials used for crystal growth. Polarized absorption spectra were recorded at room temperature by using a Perkin-Elmer UV-VIS-NIR spectrometer (Lambda-900). In addition, we used a fluorescence spectrometer (Edinburgh FLSP920) with an exciting source of 940 nm LD and Opolette (OPO) 355I lasers to record the fluorescence spectrum from 2850 to 3000 nm and the fluorescence decay curves.

    • The XRD patterns of the Yb,Ho,Pr:GYTO is shown in Fig.2. There are strong diffraction peaks,corresponding to (020), (110), (−121), (121), (040), (200), (002), (240), (042), (202), (−321), (−123), and (123) planes. The number and relative intensity of peaks are the same with the standard pattern of the GTO phase (ICSD-109186), which means that they belong to the same monoclinic space group of I2/a (No.15). Taking thestructure parameters of GTO as the initial values, the XRD data of Yb,Ho,Pr:GYTO crystal is fitted using the Rietveld refinement method to obtain the structural parameters. The refinement results of Yb,Ho,Pr:GYTO are shown in Fig.3 and Tab.1. The lattice parameters of Yb,Ho,Pr:GYTO are fitted to be a=5.381 Å(1Å=0.1 nm), b=11.023 Å, c=5.076 Å, β=95.59º, V=299.68 Å3, which are slightly smaller than the lattice parameters a=5.411, b=11.049, c=5.073, β=95.59º, V=302.56 Å3 of GTO. The reason for this is that the sites of Gd3+ in GTO are occupied by Yb3+, Ho3+, Pr3+ and Y3+, and their ionic radii are smaller than that of Gd3+.

      Table 1.  Structural parameters obtained by Rietveld refinement

      AtomXYZWyckoff siteUiso
      Gd0.250.6210000.04a0.025
      Y0.250.6210000.04a0.025
      Yb0.250.6210000.04a0.025
      Ho0.250.6210000.04a0.025
      Pr0.250.6210000.04a0.025
      Ta0.250.1450000.04a0.025
      O10.0940000.4600000.2540008c0.025
      O2−0.007000.7170000.2930008c0.025
      Cell parameters: a=5.381 Å, b=11.023 Å, c=5.076 Å, β=95.59°; Cell volume: V=299.68 Å3; Space group: Monoclinic, I2/a (No.15); Density: ρ=8.630 g/cm3; Reliability factors(R-factor): Rp=9.72%, Rwp=7.21%

      Figure 2.  XRD patterns of Yb,Ho,Pr:GYTO single crystal

      Figure 3.  Rietveld refinement results from the XRD data of Yb,Ho,Pr:GYTO crystal

      In recent years, inductively coupled plasma mass spectrometry (ICP-MS) is an effective detection for element concentrations measurement, especially trace element. However, the tested sample needs to be dissolved fully. Therefore, in this process, there are some shortcomings, such as insufficient dissolution, introduction of new impurities, which will lead to the incorrect results. The laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has become a preferred method for the measurement of major and trace element concentrations in mineral, gem, steel, ceramic, other synthetic and natural samples. It is a highly sensitive metal analytical technique and can realize microanalysis. Importantly, the tested sample does not need to be processed. In this study, a beam size of 15-40 µm and scan speeds of 15-40 µm/s (equal to beam size) were chosen. The repetition of 193 nm laser was 10 Hz with a constant energy output of 50 mJ, resulting in an energy density of 2-3 J/cm2 at the target. Meanwhile, multi-point measurements of samples were carried out. Then the average value was calculated and the concentrations of doping ions Yb3+, Ho3+, Pr3+ and Y3+ ions in the as-grown crystal are shown in Tab.2. The keff of elements Yb, Ho, Pr, and Y are calculated according to the equation keff= Cs/C0, where Cs and C0 are the ion concentrations in the crystal and melt, respectively. The keff of Yb, Ho, Pr, and Y in Yb,Ho,Pr:GYTO crystal is 0.624, 1.220, 1.350, and 0.977, respectively.

      Table 2.  Effective segregation coefficients (keff) of Yb, Ho, Pr, and Y in Yb,Ho,Pr:GYTO crystal

      ElementStarting material (at %)Crystal (at %)keff (Cs/C0)
      Yb0.050.03120.624
      Ho0.010.01221.220
      Pr0.0020.00271.350
      Y0.20.19530.977

      The X-ray rocking curves of the (100), (010), and (001) diffraction planes are shown in Fig.4. The three rocking curves are single diffraction peak with symmetric shape and without splitting, and the full widths at half maximum (FWHM) are 0.036°, 0.013°, and 0.077°, respectively. It indicates that the as-grown Yb, Ho,Pr:GYTO crystal is a single crystal with good crystalline quality.

      Figure 4.  X-ray rocking curves of Yb,Ho,Pr:GYTO crystal

    • The room-temperature polarized absorption spectra of Yb,Ho,Pr:GYTO in the wavelength between 350 nm and 2200 nm are shown in Fig.5(a). There are seven obvious absorption bands centered at around 360, 419, 450, 535, 645, 1175, and 1945 nm, which correspond to the transitions starting from the 5I8 ground state of Ho3+ to the excited states 5G5(1)+3H6, 5G5, 5F1+5G6, 5S2+5F4, 5F5, 5I6, and 5I7 of Ho3+, respectively. Importantly, the absorption bands from in the wavelength of 900-1050 nm corresponds to the transition of Yb3+ ions from the ground state 2F7/2 to the excited state 2F5/2, which matches well with the emission wavelength of commercially available high power InGaAs laser diodes (LD). For comparision, the 900-1050 nm absorption bands of Yb,Ho:GYTO crystal and Yb,Ho,Pr:GYTO crystal are shown in Fig.5(b) and expressed in a, b, c and a’, b’, c’ respectively. The absorption coefficient of E//c and E//c’ are larger than those along the other directions of themselves.Besides, the absorption coefficient of E//c is larger than that of E//c’, which indicated that the crystal absorption coefficient was not influenced by Pr3+ doped in Yb,Ho,Pr:GYTO crystal. The dopant of Yb3+ ions in Yb,Ho,Pr:GYTO crystal was calculated to be 4.16×1020 cm−3. Thus the absorption cross section of Yb,Ho,Pr:GYTO crystal can be calculated by the formula σabs=α(λ)/N. Where σabs is the absorption cross section, α(λ) is the absorption coefficient, and N is the unit volume concentration of Yb3+ ions. The strongest absorption peaks are located at 958 nm, 932 nm, and 1004 nm for E//c, corresponding to the absorption cross sections of 2.07×10−20 cm2, 1.63×10−20 cm2, and 1.03×10−20 cm2. These strong absorption peaks are beneficial to improve pumping efficiency and reduce the dependence on the temperature of pump source.

      Figure 5.  (a) Polarized absorption spectra of Yb,Ho,Pr:GYTO; (b) Comparization of Polarized absorption spectra of Yb,Ho,Pr:GYTO and Yb,Ho:GYTO in 850-1100 nm (a, b, c →Yb,Ho,Pr:GYTO; a’, b’, c’ →Yb,Ho:GYTO)

    • Figure 6 shows the emission spectrum of Yb,Ho,Pr:GYTO crystal in the wavelength range of 2850-3000 nm excited by 940 nm LD. In the 2.9 μm band, there is a strong emission peak, centered at 2908 nm. The FWHM of 2908 nm is about 15 nm. The wide emission peak is helpful to the tunability of laser wavelength. In addition, compared with that of Yb,Ho:GYTO crystal[27], the position of the strongest emission peak is shifted to the short wave direction by 2 nm, due to the little change of crystal field with the doped of Pr3+.

      Figure 6.  2.9 μm emission spectrum of Yb,Ho,Pr:GYTO crystal

      Furthermore, the stimulated emission cross section is calculated with the measured emission spectrum based on the Fchtbauer-Ladenburg equation:

      $$ {\sigma }_{{\rm{em}}}=\frac{\beta {\lambda }^{5}I\left(\lambda \right)}{8\pi {n}^{2}c{\tau }_{{\rm{rad}}}\displaystyle\int \lambda I\left(\lambda \right){\rm{d}}\lambda } $$ (1)

      where I(λ) is the emission intensity, λ is the emission wavelength, c is the speed of light, τ is the radiative lifetime of the upper energy level, and n is the refractive index, which is about 1.9[28]. The β factor is 16.324%, as reported in reference [25]. The maximum emission cross section at 2908 nm is 1.44 × 10−19 cm2. And the 2.9 μm emission cross section of Ho in GYTO and other hosts are presented in Tab.3. By comparison, the Yb,Ho,Pr:GYTO crystal possesses a larger emission cross section,which suggests it is easier to realize laser output. However, the emission cross section of Yb,Ho,Pr:GYTO crystal is smaller than that of Yb,Ho:GYTO crystal, because of the deactivation of Pr3+ on the 5I6 level. The details of the regulation of Pr3+ on energy level of Ho3+ are explained in the following part.

      Table 3.  Comparison of the emission cross section for 2.9 μm in the different Ho3+ doped crystals

      CrystalsEmission cross section (10−20 cm2)
      Yb,Ho,Pr:GYTO (this work)14.4
      Ho:GYTO[25]12.6
      Yb,Ho:GYTO[27]18.9
      Ho:LaF3[29]0.63
      Ho:LuLF[30]1.7
      Ho:PbF2[31]1.44

      The room temperature fluorescence decay curves of 1204 nm (5I65I8) and 2068 nm (5I75I8) emission of Yb,Ho,Pr:GYTO crystal excited by OPO pulse lasers are shown in Fig.7. Both of them are single exponential decay behavior. According to the fitted decay curves, the lifetimes of 5I6 and 5I7 level are 0.376 and 0.939 ms, respectively. Compared with the lifetimes of Yb,Ho:GYTO crystal as 0.419 and 7.298 ms, the Yb,Ho,Pr:GYTO crystal exhibits a remarkable attenuation of the 5I7 level lifetime and little influence on the 5I6 level. All these are attributed to the deactivation of Pr3+ through energy transfer between Ho-Pr in GYTO crystal. The energy transfer details are shown in Fig.8. The Yb3+ ions absorb pumping energy and transfer it to Ho3+ through cross-relaxation process. The emission from 5I65I7 of Ho3+ is located at 2.9 µm. Further doped with Pr3+, the energy transfer (ET) between Ho3+ and Pr3+ are through two processes: ET1, 5I63F4+3F3; ET2, 5I73F2+3H6. The efficiency of energy transfer ET1 and ET2 is directly related to the lifetime of 5I6 and 5I7 levels. The higher the efficiency is, the more the level lifetime is reduced. In addition, the efficiency of energy transfer from the Ho3+ to Pr3+ ions can be calculated based on the following equation:

      Figure 7.  Fluorescence decay curves. (a) 1204 nm (5I65I8); (b) 2068 nm(5I75I8)

      Figure 8.  Schematic of energy transfer processes among Yb3+, Ho3+, and Pr3+ ions

      $$ \eta =1-\frac{{\tau }_{{\rm{DA}}}}{{\tau }_{{\rm{D}}}} $$ (2)

      where τDA is the level lifetime of Yb,Ho,Pr:GYTO with deactivated ion, and τD is the level lifetime of Yb,Ho:GYTO without deactivated ion. According to equation (2) and the aforementioned level lifetimes of the Yb,Ho,Pr:GYTO and Yb,Ho:GYTO crystals, the energy transfer efficiencies of Ho3+ → Pr3+ in ET1 and ET2 processes are calculated to be about 10.26% and 87.13%, respectively. The energy transfer efficiency of ET2 is greater than that of ET1. Thus, the doping of Pr3+ ions can inhibit the self-termination phenomenon effectively. Population inversions between the 5I6 and 5I7 levels of the Ho3+ ions in Yb,Ho,Pr:GYTO crystal are likely to be realized at a lower pumping threshold.

      Moreover, the upper and lower laser level lifetimes of other hosts are presented in Tab.4. From the table, we can see that the Yb,Ho,Pr:GYTO crystal possesses a shorter lifetime of the lower level 5I7 and a similar lifetime of the upper level 5I6, which are easier to realizepopulation inversion and laser output.

      Table 4.  Comparison of the lifetimes of 5I7 and 5I6 in different crystals

      CrystalHo (5I7)/msHo (5I6)/ μs
      Yb,Ho:YSGG[[32]]10.2585
      Tm,Ho:YAG[[33]]11.440
      Yb,Ho,Pr:YAP[[24]]1.258341
      Ho:GYTO[[25]]8.081311
      Tm,Ho:GYTO[[26]]4.09131
      Yb,Ho,Pr:GYTO (this work)0.939376
    • High-quality Yb,Ho,Pr:GYTO single crystal was successfully grown using Czochralski method. It belongs to the monoclinic space group of I2/a (No.15) and the lattice parameters are fitted to be a=5.381 Å, b=11.023 Å, c=5.076 Å, β=95.59º, V=299.68 Å3. The keff of Yb, Ho, Pr, and Y in Yb,Ho,Pr:GYTO crystal are 0.624, 1.220, 1.350, and 0.977, respectively. The FWHM of X-ray rocking curves on the (100), (010), and (001) crystalline faces are 0.036˚, 0.013˚, and 0.077˚, respectively, suggesting a high crystalline quality. The polarized absorption spectra indicate that the coefficient of E//c is larger than that of the other direction. The strongest absorption peaks are located at 958 nm, 932 nm, and 1004 nm for E//c, corresponding to the absorption cross sections of 2.07×10−20 cm2, 1.63×10−20 cm2, and 1.03×10−20 cm2. The strongest emission peak is located at 2908 nm, and the FWHM is about 15 nm. Emission cross section at 2908 nm is as large as 1.44 × 10−19 cm2. Importantly, the lifetimes of 5I6 and 5I7 level are obtained to be 0.376 and 0.939 ms, 10.26% and 87.13% less than Yb,Ho:GYTO respectively. Therefore, the deactivator Pr3+ ions may be conducive to reducing the laser threshold and improving the conversion efficiency of the 2.9 µm laser in the Yb,Ho,Pr:GYTO crystal.

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