Cite this Article
Shi Jiaojiao, Liu Bin, Wang Qingguo, Tang Huili, Wu Feng, Li Dongzhen, Zhao Hengyu, Wang Zhanshan, Deng Wen, Xu Zian, Xu Jiayue, Xu Xiaodong, Xu Jun. Crystal growth and spectral properties of Tb:Lu2O3*

Project supported by the National Natural Science Foundation of China (Grant No. 61621001), the National Key Research and Development Program of China (Grant Nos. 2016YFB1102202 and 2016YFB0701002), and the Fundamental Research Funds for the Central Universities, China. We acknowledge the help of MOE Key Laboratory of Advanced Micro-Structured Materials and School of Physical Science and Technology, Guangxi University.

. Chinese Physics B, 2018, 27(9): 097801  
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Crystal growth and spectral properties of Tb:Lu2O3*

Project supported by the National Natural Science Foundation of China (Grant No. 61621001), the National Key Research and Development Program of China (Grant Nos. 2016YFB1102202 and 2016YFB0701002), and the Fundamental Research Funds for the Central Universities, China. We acknowledge the help of MOE Key Laboratory of Advanced Micro-Structured Materials and School of Physical Science and Technology, Guangxi University.

Shi Jiaojiao1, 2, Liu Bin1, 2, Wang Qingguo1, 2, Tang Huili1, 2, 4, Wu Feng1, 2, Li Dongzhen3, Zhao Hengyu1, 2, Wang Zhanshan1, 2, Deng Wen5, Xu Zian6, Xu Jiayue6, Xu Xiaodong3, †, Xu Jun1, 2, 7, ‡
School of Physics Science and Engineering, Institute for Advanced Study, Tongji University, Shanghai 200092, China
MOE Key Laboratory of Advanced Micro-Structure Materials, Shanghai 201899, China
Jiangsu Key Laboratory of Advanced Laser Materials and Devices, School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, China
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100,China
School of Physical Science and Technology, Guangxi University, Nanning 530004, China
Institute of Crystal Growth, School of Materials Science and Engineering, Shanghai Institute of Technology, Shanghai 201418, China
Shanghai Engineering Research Center for Sapphire Crystals, Shanghai 201899, China

 

† Corresponding author. E-mail: xdxu79@jsnu.edu.cn xujun@mail.shcnc.ac.cn

Project supported by the National Natural Science Foundation of China (Grant No. 61621001), the National Key Research and Development Program of China (Grant Nos. 2016YFB1102202 and 2016YFB0701002), and the Fundamental Research Funds for the Central Universities, China. We acknowledge the help of MOE Key Laboratory of Advanced Micro-Structured Materials and School of Physical Science and Technology, Guangxi University.

Abstract

The crystal growth, x-ray diffraction pattern, absorption spectrum, emission spectrum, and fluorescence lifetime of a Tb:Lu2O3 single crystal were studied. Excited at 483 nm, the peak absorption cross-section was calculated to be 3.5 × 10−22 cm2, and the full width at half maximum was found to be 2.85 nm. The Judd–Ofelt (J–O) intensity parameters Ω2, Ω4, and Ω6 were computed to be 3.79 × 10−20 cm2, 1.30 × 10−20 cm2, and 1.08 × 10−20 cm2, with a spectroscopic quality factor Ω4/Ω6 being 1.20. The emission cross-sections of green emission around 543 nm and yellow emission around 584 nm were calculated to be 9.43 × 10−22 cm2 and 1.32 × 10−22 cm2, respectively. The fluorescence lifetime τexp of 5D4 was fitted to be 1.13 ms. The data suggest that the Tb:Lu2O3 crystal could be a potential candidate for green and yellow laser operation.

PACS: 78.55.-m;42.70.Hj;78.47.da
Keyword:fluorescence spectra;laser materials;excited states
1. Introduction

Visible lasers are very attractive for their widespread applications, such as optical storage devices, display technology, medicine, materials processing, and more.[1,2] Recently, visible lasers have aroused much attention due to the invention of the InGaN laser diode,[3] because the emission wavelength of InGaN laser diode matches well with the absorption wavelength of most rare-earth ions, which is favorable for laser operation.

Pr3+ is a famous rare earth ion in the visible laser operation. Efficient laser operations in deep red, red, orange, green, and blue regions have been demonstrated for Pr3+-doped laser materials.[4] Unfortunately, for Pr3+, there is no yellow emission transition. The electronic configuration of trivalent terbium ions is [Xe]4f8, with a large energy gap (∼ 15000 cm−1) between the metastable 5D4 excited state and the lower 7FJ (J = 6, 5, …, 0) multiplets,[5], which determines the presence of multiple visible emissions in red, yellow, green, and blue spectral ranges, high luminescence quantum efficiency, and a long lifetime of the emission state (ranging from a few hundred microseconds to a few milliseconds).[6] Particularly, the yellow emission corresponding to the 5D4 → 7F4 transition makes Tb3+ ions a good candidate for yellow laser operation, which is not included by Pr3+ ions. Various materials doped with Tb3+ ions have been researched for fluorescence imaging and green phosphors.[7–9] In 1973, the stimulated emission at 544.5 nm was first demonstrated in Tb:LiYF4 crystals.[10] Recently, pumped by the InGaN laser diode, high-efficiency room temperature continuous wave lasers emitting green (5D4 → 7F5) and yellow (5D4 → 7F4) light from Tb3+-doped fluoride crystals have also been achieved, with the maximum slope efficiencies being 58% around 545 nm in the green region and 20% around 585 nm in the yellow region.[6,11]

The laser performance of rare-earth ions doped fluoride crystals strongly depends on the temperature. Thus it is necessary to find a new single crystal host, which not only owns the low phonon energy similar with fluoride crystals, but also has better thermal and mechanical properties than fluoride crystals. The sesquioxide Lu2O3 belonging to the bixbyite structure with the cubic space group Ia3[12] meets these requirements well. They possess higher thermal conductivity (∼ 12.5 W/m·K) than that of fluoride materials (e.g., CaF2 ∼ 7.0 W/m·K).[13,14] Compared with pure lutecia, the thermal conductivity of 3 at.% Yb3+-doped lutecia only decreases slightly to 11.0 W/m·K.[15] In addition, the phonon energy of Lu2O3 (∼ 430 cm−1) is lower than that of other oxides,[15] which means that the rare earth ions doped into Lu2O3 own lower non-radiative transition rates between the metastable electron levels, resulting in higher radiation probability and quantum efficiency. Up to now, the optical characteristics of Tm3+, Er3+, and Yb3+ doped Lu2O3 crystals[16,17] have been reported. However, very few researches were focused on the Lu2O3 crystals doped with Tb3+ ions.

In this work, we study the x-ray diffraction (XRD) pattern, absorption spectrum, Judd–Ofelt (J–O) theory, emission spectrum, and fluorescence lifetime of a Tb:Lu2O3 crystal grown by floating zone (Fz) method in detail.

2. Experiments
2.1. Crystal growth

The Lu2O3 doped with trivalent terbium ions was grown by the Fz method. The Lu2O3 (4N purity) and Tb4O7 (4N purity) powders were used as raw materials and weighed precisely according to the equation (TbxLu1−x)2O3 (x = 0.01). After mixed evenly in an agate mortar, the raw powders were pressed into two rods and then sintered in the air at 1780 °C for 24 h. The rotation rate and growth rate were selected to be 8–10 rpm and 1–2 mm/h, respectively. High-purity argon gas was aerated into the floating zone furnace as a protective atmosphere. The grown Tb:Lu2O3 was slowly cooled to room temperature. There was an internal stress in the crystal due to the large temperature gradient in the growth process of the Fz method. Therefore, the grown Tb:Lu2O3 crystal was annealed in air at 1700 °C for 24 h to eliminate the internal stress. Then the obtained Tb:Lu2O3 crystal was cut into 2.5 mm × 4 mm × 1 mm and polished for spectral measurements.

2.2. Spectral measurements

The following measurements were performed at room temperature. The sample of Tb:Lu2O3 was ground into powders for XRD measurement using an Ultima IV diffractometer from Japan. As shown in Fig. 1, the diffraction peaks of the Tb:Lu2O3 crystal are in very good accordance with the JCPDS card of pure Lu2O3 crystal. The lattice parameter of the Tb:Lu2O3 crystal was calculated to be 1.0424 nm, which is close to that of pure Lu2O3 (1.0391 nm), indicating that the doping of the Tb3+ ions into Lu2O3 did not affect the phase and crystal structure obviously. The concentration of terbium in Tb:Lu2O3 reached 1 at.% (i.e., 2.85 × 1020 cm−3), which was measured by an ICP-AES (Ultima2, Jobin–Yvon). The absorption spectrum of Tb:Lu2O3 from 300 nm to 2400 nm with the resolution of 1 nm was measured by a spectrometer (Lambda900). The emission spectrum together with the decay curve of Tb:Lu2O3 was obtained by a fluorospectrophotometer (FSP920).

Fig. 1. Powder XRD pattern of Tb:Lu2O3 crystal.
3. Results and discussion
3.1. Absorption spectrum

The absorption spectra of the Tb:Lu2O3 crystal in the ranges of 320–495 nm and 1600–2400 nm are presented in Fig. 2. The absorption peaks centered around 328 nm, 331 nm, 339 nm, 342 nm, 344 nm, 349 nm, 353 nm, 355 nm, and 483 nm, corresponding to the transitions from 7F6 to 5H6, 5H7+5D0, 5L9, 5G4, 5G5+5D2, 5L10, 5G6, 5D3, and 5D4, respectively, are assigned and marked in Fig. 2(a). Figure 2(b) shows four absorption bands at 1752 nm, 1843 nm, 1906 nm, and 2075 nm, corresponding to the transitions from 7F6 to 7F0, 7F1, 7F2, and 7F3, respectively. Some absorption bands overlap with each other due to the interaction of the crystal field. The absorption band around 483 nm corresponding to the 7F6 → 5D4 transition fits well with the emission wavelength of the InGaN laser diode.[18] At the peak wavelength of 483 nm, its absorption cross-section σabs and FWHM are 3.5 × 10−22 cm2 and 2.85 nm, respectively. For the Tb:Lu2O3 crystal, its σabs is larger than that of Tb:LiLuF4 crystal (2.0 × 10−22 cm2 at 488.8 nm for π-polarization)[11] and the FWHM is larger than that of Tb:KYbW (only 1.0 nm for E ∥ Nm).[19]

Fig. 2. Room temperature absorption spectrum of Tb:Lu2O3 crystal: (a) 320–495 nm, (b) 1600–2400 nm.
3.2. Judd–Ofelt analysis

The Judd–Ofelt (J–O) theory is a useful tool to calculate the 4fN radiative transition intensities of lanthanide ions in various host materials. The detailed calculation process is the same with the literature.[19–21] The refractive index formula of Lu2O3 and the transition matrix elements of Tb3+ ions required for the calculation are taken from Refs. [5], [19], and [22]. In our J–O theory analysis, six bands corresponding to the 7F6 → 5L10, 5G6+5D3, 5D4, 7F0+7F1, 7F2, and 7F3 transitions are chosen to confirm the J–O intensity parameters. The parameters of the average wavelength , the refractive index n, the calculated line strengths Scal, and the experimental line strengths Sexp are shown in Table 1. The small value of rms ΔS (0.011 × 10−20 cm2) indicates that the fitting results between Sexp(J,J′) and Scal(J,J′) could be considered as reasonable. The three J–O intensity parameters of Tb:Lu2O3 and Tb3+-doped other host materials are presented in Table 2. The parameter Ω2 depends on the asymmetry and covalence of the lanthanide ions coordination field. The high value of Ω2 means the increase of the covalent bonding and a higher asymmetry of the lanthanide ion site in host materials.[23] As shown in Table 2, Ω2 of Tb3+-doped Lu2O3 is 3.79, which is larger than that of NaPO3–BaF2–(GdF3/TbF3) and KYb(WO4)2, but smaller than that of CdF2, TbAl3(BO3)4, LiYF4, and GAGSe, indicating that the covalent bonding and asymmetry of Tb3+ in Lu2O3 are higher than those of NaPO3–BaF2–(GdF3/TbF3) and KYb(WO4)2, but lower than those of CdF2, TbAl3(BO3)4, LiYF4, and GAGSe. Furthermore, Ω2 is relatively sensitive because of the admixing of the configuration, while Ω4 and Ω6 are less sensitive to the environment of the crystal field.[23] As a spectroscopic quality factor, Ω4/Ω6 is generally used to predict the stimulated emission in laser hosts.[21,24] For Tb:Lu2O3, Ω4/Ω6 is calculated to be 1.20, which is larger than that of most Tb3+-doped host materials listed in Table 2. This suggests that Tb:Lu2O3 is a very promising medium for the visible laser operation.

Table 1.

The average wavelength ( ), refractive index (n), experimental line strength Sexp(J,J′), and calculated line strength Scal(J,J′) of Tb:Lu2O3 crystal.

.
Table 2.

Comparison of the J–O intensity parameters and the values of Ω4/Ω6 for Tb:Lu2O3 crystal and other Tb3+-doped materials.

.

For the Lu2O3 crystal, the radiative lifetime (τrad), fluorescence branching ratio (β), and spontaneous radiation transition probability (A) of 5D4 multiplet of Tb3+ ions are calculated and shown in Table 3. Here, we mainly pay attention to the 5D4 → 7F5 transition emitting green light and the 5D4 → 7F4 transition emitting yellow light. The fluorescence branching ratio of the 5D4 → 7F5 transition is 65.89%, which is the largest one among the listed transitions in Table 3, indicating the high possibility of a green emission around 543 nm. For the 5D4 → 7F4 transition at 584 nm, the fluorescence branching ratio is 4.50%. In addition, the radiative lifetime τrad is 3.02 ms, larger than that of Tb:KYb(WO4)2 (2.08 ms)[19] and Tb:CdF2 (2.77 ms),[21] suggesting the higher energy storage capability of the Tb:Lu2O3 crystal.

Table 3.

The spontaneous radiation transition probability (A), fluorescence branching ratio (β), and radiative lifetime (τrad) of Tb:Lu2O3 crystal.

.
3.3. Fluorescence spectrum

Excited by 483 nm, the room-temperature fluorescence spectrum of the Tb3+-doped Lu2O3 crystal is shown in Fig. 3, in which three observed emissions corresponding to the 5D4 → 7F5, 7F4, and 7F3 transitions are assigned and marked. The intensities of the three emission bands are in good agreement with the results of J–O analysis presented in Table 3. It can be seen that the 5D4 → 7F5 transition at 543 nm corresponding to green emission owns the largest emission cross-section (9.43 × 10−22 cm2) and the obtained FWHM is 4.15 nm. The emission cross-section is higher than that of Tb:KY3F10 (8.0 × 10−22 cm2 at 545 nm)[6] and Tb:LaF3 (7.5 × 10−22 cm2 at 543 nm).[6] Furthermore, the emission cross-section of the yellow emission band at 584 nm corresponding to the 5D4 → 7F4 transition is 1.32 × 10−22 cm2. The value is higher than that of Tb:PZABP (0.7 × 10−22 cm2 at 582 nm),[25] Tb:LBTAF (0.58 × 10−22 cm2 at 585 nm),[23] and Tb:TPP (1.0 × 10−22 cm2 at 587 nm).[26] The above data indicate that the Tb:Lu2O3 crystal is a promising candidate for a visible laser operation.

Fig. 3. Fluorescence spectrum of Tb:Lu2O3 crystal excited at 483 nm.

Excited at 483 nm, the fluorescence decay curve of 5D4 level measured at room temperature is shown in Fig. 4. Obviously, it is single-exponential. The fluorescence lifetime τexp of 5D4 level in the Tb:Lu2O3 crystal is fitted to be 1.13 ms. With the obtained radiative lifetime τrad (3.02 ms), the fluorescence quantum efficiency η (τexp/τred) of the Tb:Lu2O3 crystal is calculated to be 37.4%, which is comparable to that of Tb:KY3F10[27] (38%) and higher than that of Tb:KYb(WO4)2[19] (19%). Furthermore, with the concentration of Tb3+ ions decreasing from 2.0 mol% to 0.1 mol%, the quantum efficiency was reported to change from 46% to 93% in Tb:LBTAF.[23] Hence, the quantum efficiency of the Tb:Lu2O3 crystal could also be optimized by changing the doped concentration of the Tb3+ ions.

Fig. 4. (color online) Fluorescence decay curve of 5D4 level of Tb:Lu2O3 crystal.
4. Conclusion

Tb3+-doped Lu2O3 was successfully grown through the Fz method. The J–O theory was adopted to analyze the 4fN radiative transitions of Tb3+ ions doped in Lu2O3. Around 483 nm, the absorption cross-section was obtained to be 3.5 × 10−22 cm2 with an FWHM of 2.85 nm. As shown in the fluorescence spectrum, the 5D4 → 7F5 transition around 543 nm, corresponding to the green emission, has the largest emission cross-section of 9.43 × 10−22 cm2 with the FWHM of 4.15 nm and the emission cross-section of the 5D4 → 7F4 transition around 584 nm, corresponding to the yellow emission, is 1.32 × 10−22 cm2. With the fluorescence lifetime τexp of 5D4 level fitted to be 1.13 ms and the radiative lifetime τrad of 3.02 ms, the fluorescence quantum efficiency was calculated to be 37.4%. These results reveal that the Tb:Lu2O3 crystal would be promising for green and yellow laser operations.

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