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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+.
Atom X Y Z Wyckoff site Uiso Gd 0.25 0.621000 0.0 4a 0.025 Y 0.25 0.621000 0.0 4a 0.025 Yb 0.25 0.621000 0.0 4a 0.025 Ho 0.25 0.621000 0.0 4a 0.025 Pr 0.25 0.621000 0.0 4a 0.025 Ta 0.25 0.145000 0.0 4a 0.025 O1 0.094000 0.460000 0.254000 8c 0.025 O2 −0.00700 0.717000 0.293000 8c 0.025 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% Table 1. Structural parameters obtained by Rietveld refinement
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.
Element Starting material (at %) Crystal (at %) keff (Cs/C0) Yb 0.05 0.0312 0.624 Ho 0.01 0.0122 1.220 Pr 0.002 0.0027 1.350 Y 0.2 0.1953 0.977 Table 2. Effective segregation coefficients (keff) of Yb, Ho, Pr, and Y in Yb,Ho,Pr:GYTO crystal
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.
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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.
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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+.
Furthermore, the stimulated emission cross section is calculated with the measured emission spectrum based on the Fchtbauer-Ladenburg equation:
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
The room temperature fluorescence decay curves of 1204 nm (5I6→5I8) and 2068 nm (5I7→5I8) 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 5I6→5I7 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, 5I6→3F4+3F3; ET2, 5I7→3F2+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:
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.
Growth, structure, and spectroscopic properties of Yb,Ho,Pr:GYTO single crystal (Invited)
doi: 10.3788/IRLA20201067
- Received Date: 2020-10-20
- Rev Recd Date: 2020-11-25
- Available Online: 2021-01-14
- Publish Date: 2020-12-24
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Key words:
- Yb,Ho,Pr:GYTO /
- crystal growth /
- spectra /
- lifetime
Abstract: A new mid infrared laser material Yb,Ho,Pr:GYTO crystal was grown successfully using Czochralski method for the first time. The structural parameters were obtained by the X-ray Rietveld refinement method. The X-ray rocking curves of the (100), (010), and (001) diffraction face of Yb,Ho,Pr:GYTO crystal were measured. The full widths at half maximum of those diffraction peaks are 0.036°, 0.013°, and 0.077°, respectively, which indicates a high crystalline quality of the as-grown crystal. Laser Ablation Inductively-Coupled Plasma Mass Spectrometry was used to measure the concentrations of Yb3+, Ho3+, Pr3+, and Y3+ ions in the Yb,Ho,Pr:GdYTaO4 crystal. The effective segregation coefficients of Yb3+, Ho3+, Pr3+, and Y3+ in Yb,Ho,Pr:GYTO crystal are 0.624, 1.220, 1.350, and 0.977, respectively. The room-temperature polarhosized absorption spectra of Yb,Ho,Pr:GdYTaO4 was measured and the corresponding absorption transitions were assigned. The 2.9 μm fluorescence spectrum excited by 940 nm LD presents that the strongest emission is located at 2908 nm. In addition, the Yb-Ho-Pr energy transfer mechanism in GYTO was also demonstrated. Compared with Ho:GYTO crystal, the lifetime of 5I7 level of Yb,Ho,Pr:GYTO crystal is reduced by 87.13%, which is close to that of the upper level 5I6, indicating that Yb,Ho,Pr:GYTO crystal is easier to realize population inversion and laser output.