1. Introduction

3 μm mid-infrared (MIR) laser sources with short duration, high-beam quality and relatively high repetition rates are used in medical treatments, including dental tissue, vitreoretinal surgery, and cardiovascular surgery, due to the strong absorption of water at this region. Additionally, these lasers can be used for free-space communication, light detection [1] and as pump sources for far-infrared optical parametric oscillators (OPOs) [2]. Therefore, 3μm pulsed lasers play a critical role and attract much attention in wide application fields [3].

Ho³⁺ ion is one of the most important active ions applied to luminescence lasers because of favorable energy level structure. It has been demonstrated that infrared laser emission of Ho³⁺ ion acts in the range of 1.2∼4.9 μm including mid-infrared emission with transition ⁵I₇→⁵I₈ (2.0 μm) and ⁵I₆→⁵I₇ (2.9 μm) which attracted much attention in recent years [4–5]. Up to now, the 2μm and 3μm efficient continuous-wave (CW) laser operation could be achieved at some matrix such as YLF, LLF, CaF₂, CYA, YAG, CNGG, YAP [6–25] and so on. However, the lifetime of laser lower level (Ho^{3+ 5}I₇) is much longer than that of upper level (Ho^{3+ 5}I₆), which is not conducive to the laser operation of 2.9 μm. So, the deactivation of the lower Ho^{3+ 5}I₇ level is the key problem for the 2.9 μm laser output at present, which is not beneficial to the laser operation of 2μm. Pr³⁺ ion has been proved to be the effective deactivation ion for Ho^{3+ 5}I₇ level from the resonance energy transfer process: Ho^{3+ 5}I₇ → Pr^{3+ 3}F₂ [22].

Compared with fluoride crystal, many oxide host materials exhibit superior thermo-mechanical properties, including excellent hardness and good thermal conductivity, and can be easily grown by using the Czochralski method. Sc₂O₃ crystal has the high thermal conductivities of 15.5Wm^-1K^-1, which is higher than that of YAG (10.7Wm^-1K^-1) [26–27] and relatively low phonon energy (656cm^-1). Besides, the cubic structure of Sc₂O₃ crystal minimizes the thermal lensing effect and cracking from uneven thermal expansion [28]. All the results show that Sc₂O₃ crystal will be an excellent laser host crystal. But there were few reports about the rare earth ions doped Sc₂O₃ laser crystal due to its super high melting point ∼2430°C [29]. It is very difficult to grow Sc₂O₃ single crystals with high optical quality and large sizes.

In this work, Ho³⁺, Pr³⁺: Sc₂O₃ single crystal has been successfully grown by temperature gradient technique for the first time. The structure, spectral properties and J-O (Judd-Ofelt) theory analysis of Ho³⁺, Pr³⁺: Sc₂O₃ crystal were investigated.

2. Experiment

Ho³⁺, Pr³⁺ co-doped Sc₂O₃ crystal was grown by TGT (Temperature gradient technique) method in an induction heated furnace. Sc₂O₃, Ho₂O₃, Pr₆O₁₁ powders with the purity of 99.99% have been used as the raw materials with the following molar compositions: 1 at.% Ho₂O₃, 0.1 at.% Pr₆O₁₁, 98.9 at.% Sc₂O₃, respectively. The mixed powders were pressed into bulks and put into the tungsten crucible. When the air pressure in the furnace reduced down to 8 Pa, the argon gas was filled into the furnace as the protective atmosphere. Then the crucible was heated up to 2500℃, and kept for 2 hours. The crystal growth was controlled by decreasing the temperature at the rate of 1℃/h from 2500℃ to 2400℃. Figure 1 shows the grown Ho³⁺, Pr³⁺ co-doped Sc₂O₃ crystal.

 

Fig. 1. Ho³⁺/Pr³⁺ co-doped Sc₂O₃ crystal sample with the size of 20mm*8mm*3mm

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The Ho³⁺ and Pr³⁺ concentrations in crystal were measured by an inductively coupled plasma (ICP) atomic emission spectrometry analysis. The Ho³⁺ and Pr³⁺ molar concentrations are 0.40 at.% and 0.03 at.% in the crystal. Structures of Ho³⁺/Pr³⁺ co-doped Sc₂O₃ crystal was investigated by X-ray diffraction (XRD) by using XD-98X diffractometer (XD-3, Beijing) with CuKa radiation at 0.15403 nm, and the scanning 2θ was from10 to 90 with 0.02 increments. The absorption spectra in the range of 300-3000 nm of samples were carried out by using Spectrometer (Cary 5000). The emission spectra and decay time were tested with a FLS 1000 (Edinburgh Instruments, England) in the range of 1700-2300 nm, and 2700-3100 nm excited by 640 nm LD pump. All measurements were carried out at room temperature.

3. Results and discussion

The XRD pattern of the Ho³⁺, Pr³⁺: Sc₂O₃ crystal was shown in Fig. 2, which was in good agreement with the standard card: PDF#43-1028 of Sc₂O₃ single crystal. It can be confirmed that the crystal structure of Ho³⁺, Pr³⁺: Sc₂O₃ does not change significantly after co-doping with Ho³⁺ and Pr³⁺. The cell parameters of Ho³⁺, Pr³⁺: Sc₂O₃ crystal were calculated as 0.98352 nm by Jade software, which was close to the cell parameters of pure Sc₂O₃ crystal (0.9845 nm).

 

Fig. 2. (a) XRD pattern of the Sc₂O₃:Ho³⁺/Pr³⁺; (b) standard line pattern of the orthorhombic phase Sc₂O₃ (PDF#43-1028).

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The sample with thickness of 3 mm was cut and polished for spectroscopic measurements. The absorption spectra in the range of 400-2200 nm was recorded and shown in Fig. 3. There are 6 absorption peaks centered at 415 nm, 449 nm, 536 nm, 636 nm, 1144 nm and 1920nm with FWHM values of 9.51 nm, 3.52 nm, 7.56 nm, 16.35 nm, 21.31 nm and 8.13 nm respectively, which are corresponding to the transitions from the ground state Ho^{3+ 5}I₈ level to the excited states: ³G₅, ⁵F₁+⁵G₆, ⁵S₂ +⁵F₄, ⁵F₅, ⁵I₆, ⁵I₇ level. The absorption coefficients were calculated to be 0.67 cm^-1, 8.52 cm^-1, 1.17 cm^-1, 0.80 cm^-1, 0.39 cm^-1 and 0.52 cm^-1, respectively. And the corresponding absorption cross sections were 0.37×10⁻²⁰ cm², 4.77×10⁻²⁰ cm², 0.66×10⁻²⁰ cm², 0.45×10⁻²⁰ cm², 0.22×10⁻²⁰ cm² and 0.29×10⁻²⁰ cm², respectively. The high absorption cross section with broad FWHM at 636 nm were conducive to be pumped by the 640 nm LD.

 

Fig. 3. Absorption spectra of Ho³⁺/Pr³⁺ co-doped Sc₂O₃ crystal.

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According to the framework of J-O theory [30–32], absorption line strength S_ed(J,J′) and calculated line strength S_cal(J,J′) of Ho³⁺ in Ho³⁺/Pr³⁺ co-doped Sc₂O₃ crystal were calculated and listed in Table 1.

Table 1. The calculated average wavelength $\bar{{\boldsymbol{\lambda} }}$, absorption line strength S_ed(J,J′) and calculated line strength S_cal(J,J′) of Ho³⁺ in Ho³⁺/Pr³⁺ co-doped Sc₂O₃ crystal.

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About Ho³⁺/Pr³⁺ co-doped Sc₂O₃ crystal, the values of Ω_2,4,6 are calculated to be 4.90×10^–20 cm², 1.06×10^–20 cm², 0.18×10^–20 cm², respectively. The J-O intensity parameters of Ho³⁺/Pr³⁺ co-doped different crystals are listed in Table 2. As we know, the Ω₄/Ω₆ is usually used as a parameter for spectroscopic quality [33]. The value of Ω₄/Ω₆ of Ho³⁺/Pr³⁺ co-doped Sc₂O₃ is 3.21, which is higher than that of YLF [4], LLY [34], PbF₂ and YAG [35]. The results indicate that the rigidity of Ho³⁺/Pr³⁺ co-doped Sc₂O₃ crystal is higher than Ho³⁺/Pr³⁺ co-doped YLF, LLY, PbF₂, and YAG.

Table 2. The J-O intensity parameters of Ho³⁺/Pr³⁺ co-doped in different crystals.

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The radiative rate A(J,J′), The fluorescent branching ratio β_JJ′ and the radiative lifetime $\tau _{rad}$ were listed in Table 3. The radiative τ_rad was closely related to laser power of potential transitions. The radiative lifetime τ_rad of ⁵I₆ state of Ho³⁺ was calculated to be 12 ms in Ho³⁺/Pr³⁺ co-doped Sc₂O₃ crystal. The fluorescence branching ratio of the ⁵I₆→⁵I₇ transition was calculated to be 36.2% of the Ho³⁺/Pr³⁺ co-doped Sc₂O₃ crystal. The results make ⁵I₆ energy level of Ho³⁺/Pr³⁺ co-doped Sc₂O₃ crystal to be a promising energy level for laser operation.

Table 3. Radiative transition rates A(J-J′), branching ratios β_JJ′ and radiative lifetime τ_rad of Ho³⁺/Pr³⁺ co-doped crystal

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The emission spectra of Ho³⁺/Pr³⁺ co-doped Sc₂O₃ crystal excited at 640 nm at room temperature were shown in Fig. 4. The emission bands at ∼2 μm and ∼3 μm regions corresponding toHo^{3+ 5}I₇ → ⁵I₈ and ⁵I₆ → ⁵I₇ transitions, respectively.

 

Fig. 4. Emission spectra of Ho³⁺/Pr³⁺ co-doped Sc₂O₃ crystal.

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The emission cross section ${\sigma _{em}}$ can be calculated by F-L formula [36]:

Table 4. The emission peak wavelength λ, FWHM and emission cross section σ_em corresponding to Ho³⁺/Pr³⁺ co-doped Sc₂O₃ crystal.

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The decay time of Ho^{3+ 5}I₇ and ⁵I₆ levels excited at 640 nm were tested at room temperature and shown in Fig. 5. With single exponential fitting, the fluorescence lifetime of ⁵I₇ and ⁵I₆ level were fitted to be 5.74 ms and 1.64 ms, respectively. Compared with 0.42% Ho:Sc₂O₃, the lifetime of ⁵I₇ level was 8.2 ms. This is because the ³F₂ energy level of Pr³⁺ ion is close to and the ⁵I₇ energy level of Ho³⁺ ion. The two energy levels have energy transfer, shown in Fig. 6, ET process represents that the ⁵I₇ of Ho³⁺ ion transfers energy to the ³F₂ of Pr³⁺ ion. The above results indicate that Pr³⁺ ion can be used as an effective deactivation ion of Ho³⁺ ion, which reduces the lifetime of the ⁵I₇ of Ho³⁺ ion, and good for the number of population inversion between the upper and lower energy levels.

 

Fig. 5. Fluorescence decay time of the ⁵I₇ and ⁵I₆ of Ho³⁺/Pr³⁺ co-doped Sc₂O₃ crystal.

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Fig. 6. Schematic diagram of energy transfer between Ho³⁺ and Pr³⁺

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4. Conclusion

1at%Ho,0.1at%Pr: Sc₂O₃ crystal was successfully grown by TGT method. The spectroscopic properties of the crystal were analyzed, and the absorption cross section of Ho,Pr: Sc₂O₃ at 636 nm is 1.58×10^–20 cm². J-O theory was applied to analyze spectroscopic properties of Ho³⁺ at 2.9 μm. The intensity parameters Ω_2,4,6, radiative transition rates, branching ratios and radiative lifetime were calculated. Ho,Pr: Sc₂O₃ exhibits strong emission at 2110 nm and 2864 nm with stimulated emission cross section of 3.5×10^–21 cm² and 5.4×10^–21 cm², respectively. The fluorescence lifetimes of the ⁵I₆ and ⁵I₇ levels were measured to be 1.64 ms and 5.74 ms. All the results show that the Ho,Pr: Sc₂O₃ crystal is a potential laser material.