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Abstract
Pr3+-doped GdScO3 single crystal was successfully grown by the Czochralski (Cz) method. Polarized absorption spectra, polarized fluorescence spectra, and fluorescence decay curves were measured at room temperature. The polarized spectroscopic property of Pr:GdScO3 crystal for new-wavelength transition in the near infrared (NIR) region was discussed for the first time. The absorption cross-sections at 600 nm were calculated to be 0.39×10−20 cm2 (E∥a), 0.32×10−20 cm2 (E∥b) and 0.4×10−20 cm2 (E∥c). The emission cross sections at 1053 nm of 1D2 energy level were calculated to be 0.86×10−20 cm2 (E∥a), 0.79×10−20 cm2 (E∥b) and 0.78×10−20 cm2 (E∥c), with FWHM of 57.54 nm, 57.96 nm and 58.96 nm, respectively. The fluorescence lifetime and radiative lifetime of the 1D2 energy levels of Pr3+ ions were 45.46 µs and 393.94 µs, respectively. Those results show that the Pr:GdScO3 crystal should be the potential near infrared laser gain medium originated from the 1D2 transition.
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1. Introduction
Near infrared lasers have been widely used in biomedicine, data storage, optical diagnostic system and other fields [1–4]. The trivalent Pr3+ ion, which has the complex energy level structure and the possibility of laser emission from the blue to the infrared region, has attracted much attention [5–6]. The number of researches about 3P0 energy level laser operation of Pr3+ ion have been reported. Pr3+ -doped fluoride materials show obvious advantages due to the low phonon energy, which could effectively reduce the probability of non-radiative transitions and high 4f5d energy level position can weaken excited state absorption [7–14]. Especially, The Pr:LiYF4 crystal has achieved continuous watt-level laser output in seven different laser wavelengths of 523, 546, 604, 607, 640, 698, and 720 nm with GaN laser diode pumping [15].
Meaningly, Pr3+ ions doped laser materials has attracted extensive attention for new wavelength laser output in near infrared region, especially derived from 1D2 multiplet. In the past decade, a low threshold laser output of 1µm has only been achieved through the 1D2→3F3+3F4 transition in silica based fiber by 592 nm dye laser pumping [16]. Besides, spectral property of near infrared region only in Pr3+ -doped ZBLAN fiber and LiYF4 crystal has been reported [12,17]. Most of the spectral properties of 1D2 energy level have not been studied. compared with oxide materials, due to fluoride crystals have relative low thermal conductivity and poor physical-chemical properties, novel host crystals for new wavelength research of Pr3+ ions with low phonon energy and excellent physical-chemical properties are expected.
Gadolinium scandate (GdScO3) crystal have the structure of perovskite (type GdFe03) and belonging to orthorhombic system with the space group of Pnma (no.62) [18]. The phonon energy of GdScO3 is about 452 cm−1 [19]. The distance among Gd-O bond in GdScO3 crystal is 5.745 Å, which is much larger than the distance among Y-O bond in YAP crystal. Therefore, the 4f5d energy level splitting of Pr3+ ion in GdScO3 crystal is relative small, which is beneficial to reduce the excited state absorption. GdScO3 can be a good host material due to its perovskite structure, which has high tolerance to structural distortions, so all cationic sites can be substituted by various of ionic [20]. The crystal structure of GdScO3 is shown in Fig. 1. Compared with other ortho-aluminates, GdScO3 crystal has low-symmetry distortions structure in the local symmetry of activator center, which contributes to large inhomogeneous broadband spectra [21]. As a laser host crystal, In 2018, Fang et al. studied the yellow light emission of Dy:GdScO3 crystal [22]. The spectral property Nd3+ -doped GdScO3 single crystal was first reported in 1972 [23]. In 2019, Wang et al. reported the spectral characteristics and crystal field structure analysis of Cr3+ -doped GdScO3 crystals [24]. However, crystal growth and spectral properties of Pr3+ -doped GdScO3 crystal for new wavelength have not been investigated.
In this paper, the Pr:GdScO3 single crystal has been grown by Cz method. Polarized absorption spectra, polarized emission spectra for new-wavelength transition in the near infrared (NIR) region, J-O analysis and fluorescence decay curve have been discussed. Furthermore, the potential of Pr:GdScO3 crystal as a laser gain media with1D2 laser upper level is also evaluated.
2. Experimental
2.1 Crystal growth
0.6 at.% Pr3+ -doped GdScO3 crystal with high quality was grown by the Cz method. Pr6O11, Gd2O3 and Sc2O3 powders with the purity of 99.999% were weighed out according to the molecular formula Pr0.006Gd0.994ScO3 and mixed thoroughly. The mixtures were pressed into a block under 100 MPa pressure and then sintered in air at 1500 °C for 24 hours. The mixture was loaded into the Iridium crucible. High-purity nitrogen was used as protective atmosphere to prevent oxidation of the Ir. The crystal was grown along the crystalline c-axis with the pulling rate of 1 mm/h and rotation rate of 10-20 rpm. In order to avoid cracking, the crystal was slowly cooled to room temperature. The grown Pr:GdScO3 crystal was transparent and free from cracks. The size of the crystal was Φ25×68 mm3, which was shown in Fig. 2.
2.2 XRD and ICP-AES measurement
The top part of crystal sample was used for X-ray powder diffraction (XRD) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) measurements. The XRD diffraction pattern was shown in Fig. 3. The diffraction peaks are in consistent with the standard card JCPDS 27-0220 of GdScO3 crystal. Pr3+ -doped do not change lattice phase of GdScO3 crystal, which belongs to orthorhombic system. The cell parameters of Pr:GdScO3 crystal were calculated to be a = 5.429 Å, b = 5.707 Å and c = 7.899 Å, which were very close to the pure GdScO3 parameters (a = 5.487 Å, b = 5.756 Å and c = 7.926 Å) [25]. This is because Pr3+ and Gd3+ have similar ionic radii, and Pr3+ can easily replace Gd3+ into the GdScO3 lattice structure. The concentration of Pr3+ ions at the top of Pr:GdScO3 crystal was measured to be 1.32 at.%, the segregation coefficient is calculated to be 2.2.
3. Results and discussion
3.1 Absorption spectra
Pr:GdScO3 crystal with the size of 6 mm×5 mm×4 mm were polished for the polarized spectra measurements as shown in Fig. 4. The polarized absorption spectrum of Pr:GdScO3 crystal in the range from 400 to 2500 nm was shown in Fig. 5. The results show that the absorption spectra of different polarization directions are polarization-dependent and the absorption coefficients in each direction are different. The absorption spectra of Pr:GdScO3 are mainly composed with eight bands corresponding to 3H4→3P2, 3P1+1I6, 3P0, 1D2, 1G4, 3F3+3F4, 3F2 and 3H6 transitions. The different peaks of each absorption band are caused by the stark splitting of energy levels due to the function of crystal field. In three polarization directions, the absorption cross section of the 1D2 level at 600 nm are 0.39×10−20 cm2(E∥a), 0.32×10−20 cm2(E∥b) and 0.4×10−20 cm2 (E∥c), respectively. The large absorption cross-sections are conducive to the absorption of pump light, which can enhance the pumping efficiency.
3.2 Judd–Ofelt theory analysis
According to the absorption spectra, the calculated average wavelength, experimental line intensity Sexp (J→J’) and theoretical line intensity Scal (J→J’) of the transitions from 3H4 level to the excited state levels were calculated and shown in Table 1. Considering that small gap between the 4f2 fundamental and 4f15d1 excited configurations can be caused by the degeneration of disturbing 4f5d configuration in Pr:GdScO3 crystal. Therefore, it is unreasonable to make the assumption that the energy difference between the 4fN-15d configuration and the 4fN states is a constant [26,27]. Therefore, the 3H4→3P2 absorption band is ignored in the calculation process. While omitting the 3H4→3P2 absorption transition, there is a reasonable deviation between experimental and calculated line strengths.
Table 1. The calculated average wavelength λ, absorption line strength Sexp (J→J’) and calculated line strength Scal (J→J’) of Pr:GdScO3 crystal.
The J-O intensity parameters of a, b, and c polarization can be obtained by the least square fitting between Sexp (J→J’) and Scal (J→J’). The effective strength parameter of GdScO3 crystal can be calculated as: Ωt,eff = (Ωt a + Ωt,b + Ωt,c)/3 and the values of Ω2,eff, Ω4,eff, Ω6,eff are calculated to be 1.20×10−20 cm2, 1.09×10−20 cm2 and 2.25×10−20 cm2, respectively. The J-O intensity parameters of Pr3+ in GdScO3 and other crystals are listed in Table 2. The value of Ω2 represents the strength of covalency between dopant ion and the closed distance anion ligand, and reflects the symmetry of crystal field. The value Ω2 of Pr:GdScO3 is lower than that of Pr3+ -doped YAP [2] and CaYAlO4 [11] but higher than that of LaF3 [6], and YAG [29], which indicate that the covalency of Pr3+ ions in GdScO3 is lower than that in YAP [2] and CaYAlO4 [11], but higher than that of LaF3 [6], and YAG [29]. The Ω4/Ω6 is usually an important parameter to describe spectral quality, where Ω4 and Ω6 are related to the crystal structure, presenting the overall performance such as the rigidity of matrix. The value of Ω4/Ω6 of Pr:GdScO3 is 0.485, which is smaller than Pr3+ -doped YAP [2], CaYAlO4 [11], and Y3Al5O12 [29]. The results show that the rigidity of Pr:GdScO3 is smaller than that of Pr3+ -doped YAP [2], CaYAlO4 [11], and Y3Al5O12 [29].
Table 2. The J-O intensity parameters of Pr3+ in GdScO3 and other crystals.
The spontaneous transition rate, fluorescent branch ratio and radiation lifetime of the 1D2 energy level in Pr:GdScO3 crystal are listed in Table 3. The largest fluorescent branching ratio of 1D2→3F3+3F4 were calculated to be 36.02%, 35.99% and 35.23% for a, b and c polarizations, respectively. The radiation lifetime of the 1D2 energy level of Pr3+ was calculated to be 393.94µs, which is longer than that in Pr:CYA (117.09 µs) [11], Pr:SrWO4 (80.9 µs) [28], Pr:NaGd(WO4)2 (56.4 µs) [33] and Pr:KLu(WO4)2 (10.18 µs) [34] crystal. The long radiation lifetime of 1D2 energy level shows that Pr:GdScO3 crystal has high energy storage capacity.
Table 3. Radiative transition rates A (J→J’), branching ratios β (J→J’) and radiative lifetime τrad of Pr: GdScO3 crystal.
3.3 Fluorescence spectra
The polarized fluorescence spectra in the region of 610-1550 nm of the Pr:GdScO3 crystal under 600 nm excitation are shown in Fig. 6 and Fig. 7. The simplified energy level diagram of 1D2 in Pr:GdScO3 crystal are shown in Fig. 8. There are five distinguishable emission bands, which corresponds to 1D2→3H4 (around 630 nm), 1D2→3H5 (around 715 nm), 1D2→3H6+3F2 (around 896 nm), 1D2→3F3+3F4 (around 1053 nm) and 1D2→1G4 (around 1464 nm). Due to the anisotropy of GdScO3 host that the polarization dependence of the fluorescence spectrum is strong, where the position and intensity of the emission peak corresponding to each polarization are different. The stimulated radiation cross section is one of the important parameters that affect the performance of the laser, which can be calculated by the F-L formula [35]:
Fig. 6. Room temperature polarized fluorescence spectra of Pr:GdScO3 crystal in visible region under 600 nm excitation.
Fig. 7. Room temperature polarized fluorescence spectra of Pr:GdScO3 crystal in infrared region under 600 nm excitation.
Table 4. The emission peak wavelength λ, FWHM and emission cross section σem corresponding to each polarization of Pr:GdScO3 crystal.
3.4 Fluorescence lifetime
The room-temperature fluorescence decay curve of the 1D2 energy levels of Pr:GdScO3 crystal under 600 nm excitation is shown in Fig. 9. The measured attenuation curve exhibits a single exponential decay behavior, and the fluorescence lifetime is fitted to be 45.46 µs, which is longer than that of Pr:NaGd(WO4)2 (5.9 µs) [33] and Pr:KLu(WO4)2 (19.72 µs) [34] crystals. Furthermore, it can be compared with 2 at% Pr:LiYF4 (34 µs) [38], which has better laser performance. The long fluorescence lifetime is beneficial to laser operation. The fluorescence quantum efficiency of the 1D2 level is 11.54%, which is lower than that of Pr:CYA (43.3%) [11], but higher than Pr:NaGd(WO4)2 (10.4%) [33] crystal. The low quantum efficiency may be caused by the concentration quenching effect in the Pr3+ ion doped system. The σem·τ factor of 1D2→3F3+3F4 transition at 1053 nm was calculated to be 39.1×10−20 cm2·µs, which is close to that of Pr:CYA [11] and Pr:La2(WO4)3 [8] but higher than Pr:NaGd(WO4)2 [33] crystal. The above results demonstrate that Pr:GdScO3 crystal is a promising laser gain media for 1D2→3F3+3F4 transition.
Fig. 9. Room temperature fluorescence decay curve of the 1D2 energy levels of Pr:GdScO3 crystal under 600 nm excitation.
4. Conclusions
Pr:GdScO3 crystal was successfully grown by Cz method. Polarized absorption spectra, polarized emission spectra for new-wavelength transition in the near infrared (NIR) region, J-O analysis and fluorescence decay curve have been discussed for the first time. The absorption cross-sections at 600 nm were calculated to be 0.39×10−20 cm2, 0.32×10−20 cm2 and 0.4×10−20 cm2 for a, b and c polarization, respectively. The values of Ω2,eff, Ω4,eff and Ω6,eff are calculated to be 1.20×10−20 cm2, 1.09×10−20 cm2 and 2.25×10−20 cm2 respectively. In addition, the emission cross section at 1053 nm, corresponding to the1D2→3F3+3F4 transition, were calculated to be 0.86×10−20 cm2 for a polarization, 0.79×10−20 cm2 for b polarization and 0.78×10−20 cm2 for c polarization with FWHM of 57.54 nm, 57.96 nm and 58.96 nm, respectively. The largest fluorescent branching ratio of 1D2→3F3+3F4 were calculated to be 36.02%, 35.99% and 35.23 % for a, b and c polarizations, respectively. The fluorescence lifetime and radiative lifetime of the 1D2 energy levels of Pr:GdScO3 crystal are 45.46 µs and 393.94 µs, respectively. Those results show that the laser operation of 1053 nm is expected to be achieved in Pr:GdScO3 crystal.
Funding
Fund of Key Laboratory of Optoelectronic Materials Chemistry and Physics, Chinese Academy of Sciences (No. 2008DP173016); National Natural Science Foundation of China (No. 61621001, No. 61805177, No. 61861136007); Fundamental Research Funds for the Central Universities (No. 22120210432).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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