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Abstract
High-quality Cr:LuScO3 crystals were grown by the FZ (floating zone) method. The absorption spectra from 400 nm to 750 nm were tested at room temperature. The absorption cross-sections (σabs) of 0.5at.% and 1at.% Cr3+:LuScO3 were calculated to be 0.70×10−20 cm2 and 0.64×10−20 cm2 at 483 nm corresponding to the 4A2→4T1 transition, and 0.30×10−20 cm2 and 0.27×10−20 cm2 at 661 nm corresponding to the 4A2→4T2 transition, respectively. The transition Cr3+:4T2→4A2 in 1at.% Cr3+:LuScO3 exhibited a broad emission band with the FWHM value reaching up to 297 nm and the emission cross-section (σem) was 2.35×10−20 cm2. From the spectra, the crystal field parameter (Dq) was calculated to be 1513 cm-1, Racah B and C were 544 cm-1 and 3331 cm-1, respectively. All the results indicated the great application potential of Cr3+:LuScO3 crystals in the field of ultrafast lasers.
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1. Introduction
Rare earth sesquioxides RE2O3 (RE = Lu, Sc, and Y) became a research hotspot because of their excellent properties, such as the high thermal conductivity (Sc2O3 ∼17 W/mK, Y2O3 ∼14 W/mK, and Lu2O3 ∼12.5 W/mK @300 K) higher than YAG (11 W/mK) [1,2], and the simple cubic structure (space group Ia$\bar{3}$) which was very suitable for high concentration doping of rare-earth ions [3–5]. In addition, the low phonon energy of sesquioxide crystals can also effectively avoid the non-radiative transition between energy levels and improve the fluorescence quantum efficiency of activated ions. As the excellent laser host crystals, rare-earth ions (Yb3+, Er3+, Ho3+, Dy3+, Nd3+, Tm3+, Tb3+, etc.) doped sesquioxide crystals had been studied [6–15]. But the high melting point of the sesquioxide crystals (Sc2O3 ∼2430°C, Y2O3 ∼2430°C, Lu2O3 ∼2450°C) limited their growth with large size and their further application [16].
Compared with the single-component sesquioxide crystals (Lu2O3, Sc2O3, and Y2O3), mixed sesquioxide crystals such as LuScO3 and YScO3, have the lower lattice symmetry and larger distortion, which can induce the non-homogeneous broadening of the emission spectra of laser-activated ions [17]. In another hand, the lower melting point of the mixed sesquioxide crystals (∼2200-2300°C) makes it easier to grow large single crystals. The FWHM value of the emission peak of Yb3+ ions in LuScO3 host reached up to 22 nm, while the values were 13 nm, 12 nm, 14 nm, and 9 nm in Lu2O3, Sc2O3, Y2O3 and YAG crystals, respectively [18,19]. A similar situation can also be seen in Er3+ and Tm3+ ions, the FWHM values of the emission band of Er3+ and Tm3 + doped LuScO3 were much higher than those in Lu2O3, Sc2O3, and Y2O3 [20]. 250 W-CW laser output of Yb3+ ions in LuScO3 crystal was obtained in a thin disk laser configuration, and the slope efficiency reached up to 81% [21]. The mode-locked pulse laser output of Yb:LuScO3 was generated by soft-aperture Kerr-lens, and the shortest pulse duration was 74 fs [22]. In Tm:LuScO3 crystal, tuned laser output from 1961 nm to 2165 nm with the output power of 0.71 W and a high slope efficiency of 55% under Ti:sapphire laser pumping was achieved [23], as well as a 148 fs-pulse laser output was created by passively mode-locking in this crystal [24]. Recently, laser output of Nd3+, Er3 + and Ho3+ ions doped LuScO3 were reported [25–27].
In this paper, Cr3+ doped LuScO3 crystals were grown successfully with the Floating Zone (FZ) method. The structure and the spectral properties of the grown crystals were measured and studied.
2. Experimental procedure
Lu2O3(5N), Sc2O3(5N), and Cr2O3(5N) powders were synthesized as raw materials for the Cr3+:LuScO3 crystal growth, and then homogeneously mixed according to the following proportions: Crx:Lu1Sc(1-x)O3 (X = 0.005, 0.01). The precursor bars for the crystal growth with the FZ method were prepared by secondary sintering. The mixtures were pressed into rods and then sintered at 1500°C for 48 hours in the air to make the Cr3+:LuScO3 precursor with a single phase. The sintered rods were grinded into powder, and then the powder was packed in the rubber balloon and pushed under 200 Mpa hydrostatic pressure to create a cylindrical rod, which was subsequently sintered at 1500°C for 48 hours in the air for the second time. The Cr3+:LuScO3 crystal was grown with the Floating Zone (FZ) method and in a four-ellipsoidal-mirror furnace (Crystal System Corporation, FZ-T-12000-X-I-S-SU). Focused radiation from four 3 kW Xenon lamps melted one end of the feed rod. The melting zone was gradually pushed upward to accomplish the crystallization process. Two Cr3+:LuScO3 single crystals with high quality were grown successfully in an argon environment with a growth rate of 4-8 mm/h and a rotation rate of 10-15 rpm. As shown in Fig. 1, the crystals are transparent without cracking, and the sizes of the grown crystals are Φ 20 mm×32 mm for 0.5at.% Cr3+:LuScO3 and Φ 20 mm × 27 mm for 1at.% Cr3+:LuScO3 crystal.
Fig. 1. The Cr3+:LuScO3 crystals were grown with the FZ (Floating Zone) method. (a) 0.5at.% Cr3+:LuScO3 single crystal; (b) 1at.% Cr3+:LuScO3 single crystal.
The powder samples for XRD tests were ground in the agate mortar, and the XRD patterns were recorded on a Bruker SMART APEX II 4 K CCD diffractometer with Cu Kα (50 kV, 40 mA) irradiation (λ=0.71073 Å). With a scanning rate of 0.015° and an exposure length of 0.9 s, the XRD data was recorded at the 2θ angle from 10° to 80°. The XRD curves were analyzed with Jade 6.0 software to determine the cell characteristics of the grown crystals.
Samples with the size of Φ 20 mm×0.8 mm were cut and polished for spectral measurements. The absorption spectra in the range of 400-750 nm were recorded at room temperature using a spectrophotometer (Lambda 900, Perkin-Elmer UV-VIS-NIR). The emission spectra and the corresponding fluorescence decay curves of the grown crystals under 483 nm excited were measured with Edinburgh Fluorescence Spectrophotometer (FLS980).
Micro-Raman scattering spectra were measured with a Raman spectrometer “Omni-λ 300i Zolix” (laser irradiation of 785 nm). The spectra were produced by averaging 20 runs over a period of 90 seconds (5 s for a run).
3. Results and discussion
3.1 Crystal structure
Compared with the standard cards PDF#05-0629 (Sc2O3) and PDF#43-1021 (Lu2O3), the XRD patterns of the grown crystals (as shown in Fig. 2) show that LuScO3 crystal has the bixbyite-type cubic structure and the same space group (Ia$\bar{3}$) as Sc2O3 and Lu2O3, there are two types of cationic sites, centrosymmetric C3i (Re1O6) and non-centrosymmetric C2 (Re2O6) in this structure, the ratio of the C2 to C3i sites is 3:1, in LuScO3 the two types of atom (Lu and Sc) are randomly dispersed to these cationic sites [17], and the radius of Lu3+ (0.86 Å) and Sc3+ (0.75 Å) ions are very different, so the LuScO3 has the lower lattice symmetry and larger distortion, and Cr3+ will substitute the lattice of the ScO6 octahedron. The lattice parameters of 0.5at.% Cr3+:LuScO3 and 1at.% Cr3+:LuScO3 was calculated to be 10.146 Å and 10.142 Å, while the values of Lu2O3 and Sc2O3 crystal were 10.39 Å and 9.845 Å, respectively. Due to the smaller radius of Sc3+ ions (0.75 Å) than Lu3+ ions (0.86 Å), the lattice parameter of LuScO3 lies between the Lu2O3 and Sc2O3. The lattice parameter of Cr:LuScO3 crystal decreases when increasing Cr3+ (0.615 Å) ions are substituted in the ScO6 octahedron lattice.
Fig. 2. XRD patterns of Cr3+:LuScO3 single-crystals compared with the standard PDF cards of Sc2O3 and Lu2O3.
3.2 Absorption spectra
The room temperature absorption spectra of the grown Cr3+:LuScO3 crystals from 400 nm to 750 nm are shown in Fig. 3. There are two broad absorption peaks from the transitions of 4A2→4T1 and 4A2→4T2, which are centered at 483 nm and 661 nm, respectively. The absorption cross-section σabs can be calculated with the following formula [28]:
where the σabs is the absorption cross-section, α is the absorption coefficient and Nc is the actual concentration of Cr3+ in Cr3+:LuScO3 crystal determined by ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry). The absorption cross-sections of 0.5at.% Cr3+:LuScO3 are calculated to be 0.70×10−20 cm2 and 0.30×10−20 cm2 corresponding to the transitions of 4A2→4T1 and 4A2→4T1, respectively. And the absorption cross-sections of 1at.% Cr3+:LuScO3 are 0.64×10−20 cm2 and 0.27×10−20 cm2. A weak absorption peak centered at 709 nm can be found, which comes from the 4A2→2E (R-line) transition.3.3 Crystal field parameters
In the Cr3+:LuScO3 crystal, Cr3+ ions replace the ScO6 distorted octahedral lattice with lower symmetry than that of the ortho-octahedron Oh, which can induce the splitting of the degenerate energy levels. However, this is not important when calculating the crystal field parameters, so it can be treated as an ortho-octahedral Oh crystal field when calculating the crystal field parameters. According to the Tanable-Sugano theory, the absorption spectra of Cr3+:LuScO3 crystals can be used to calculate the crystal field parameters Dq, Racah parameters B, and C by the following equations [29,30]:
Table 1. Comparison of the crystal parameters of Cr3+:LuScO3 and other Cr3+-doped crystals
3.4 Emission spectra and fluorescence lifetime
Figure 4 shows the emission spectra of the grown crystals under 489 nm Xe-lamp pumping at room temperature. Broadband emission peak centered at 950 nm from Cr3+ 4T2→4A2 transition from 700 nm to 1400 nm can be seen. The FWHM values of 0.5at.% and 1at.% Cr3+:LuScO3 crystals were 294 nm and 297 nm, respectively, which were substantially higher than that in Cr3+:Sc2O3 (61 nm) and Cr3+:GdScO3 (128 nm) crystals [8,28]. On the one hand, the LuScO3 has a more disordered structure than GdScO3 and Sc2O3 because Lu and Sc are randomly occupied the same cationic sites; on the other hand, the LuScO3 has a more strong electron-phonon coupling due to differences in the ionic radius of Lu3+ and Sc3+ create distorted octahedral ligands, which should be responsible for the luminescence broadening [17]. The higher Dq/B value (>2.3) indicates that the LuScO3 host crystal has the stronger lattice field and the Cr3+ 2E level below the Cr3+ 4T2 level, and the narrow R-line emission peak could be seen according to the Tanable-Sugano theory. Nevertheless, the 4T2→4A2 transition presents a broad emission band. In 1995, Marek Grinberg explained this phenomenon using Cr3+-doped gallogermanates. Distorted lattice can increase the effective electron-phonon coupling strength in the 4T2 state, reducing the minimum energy of the manifold. In the strong field, the 4T2 energy level can also be close to or even below the 2E energy level caused by strong electron-lattice interaction [42]. In LuScO3 the two types of atom (Lu and Sc) are randomly assigned to the same cationic sites, and the radius of Lu3+ (0.86 Å) and Sc3+ (0.75 Å) ions are very different, so the LuScO3 has the lower lattice symmetry and larger distortion, which indicates that there is strong electron-phonon coupling in Cr:LuScO3. In Cr:LuScO3, the 4T2 state is the lowest excited state because of the strong electron-lattice interaction even if the Dq/B of Cr:LuScO3 is larger than 2.3.
The emission cross-section (σem) can be calculated by the following formule [39]:
where λ is the central wavelength of emission peak, n is the refractive index at 882 nm, Δν is the frequency difference at FWHM and τƒ is the fluorescence lifetime of 4T2→4A2 emission transition. The fluorescence lifetimes of 0.5at% Cr3+:LuScO3 and 1at.% Cr3+:LuScO3 were measured to be 33.7 μs and 22.9 μs, respectively, as shown in Fig. 5. Due to the concentration quenching effect, the fluorescence lifetime of the 1at.% Cr3+:LuScO3 crystal is much shorter than that of 0.5at.% Cr3+:LuScO3 crystal. The emission cross-sections of 0.5at.% Cr3+:LuScO3 and 1at.% Cr3+:LuScO3 crystal were calculated to be 1.60×10−20 cm2 and 2.35×10−20 cm2, respectively, which is larger than Cr:GdScO3 (0.32×10−20 cm2) and Cr:BeAl2O4 (0.70×10−20 cm2) and the fluorescence lifetime of Cr:LuScO3 is smaller than Cr:GdScO3 (161 μs) and Cr:BeAl2O4 (260 μs) [28,43].3.5 Raman spectra
At room temperature, the Raman spectra of the Cr:LuScO3 crystal measured in the range of 100-800 cm-1 are shown in Fig. 6. The Raman spectral main peak is about 405 cm-1, and there are other weak peaks at 229 cm-1, 440 cm-1, 529 cm-1, and 672 cm-1, respectively. The peak at 529 cm-1 was attributed to the vibration mode of Lu2O3 and the peak at 672 cm-1 attributed to the vibration mode of Sc2O3 [44], they are non-dominant modes of vibration, the maximum phonon energy of Cr:LuScO3 crystal is 405 cm-1, which is lower than Sc2O3 (672 cm-1), Lu2O3 (618 cm-1), and Y2O3 (597 cm-1) [45], indicating that Cr:LuScO3 crystal can significantly reduce the possibility of radiation-free transition and reduce the heat generated at laser operation.
4. Conclusions
Cr3+ ions doped LuScO3 crystals with different concentrations (0.5at.% and 1at.%) were grown with the FZ (Floating Zone) method for the first time. The structure of the grown crystals was analyzed with XRD measurement. The crystal field parameter Dq, Racah B and C were calculated to be 1513 cm-1, 544 cm-1 and 3331 cm-1, respectively. The absorption spectra of Cr3+:LuScO3 crystals were recorded at room temperature. And the absorption cross-sections (σabs) of 0.5at.% Cr3+:LuScO3 were calculated to be 0.70×10−20 cm2 at 483 nm corresponding to the 4A2→4T1 transition, and 0.30×10−20 cm2 at 661 nm corresponding to the 4A2→4T2 transition, respectively. Under 489 nm Xe-lamp pumping, Cr3+:LuScO3 crystals present broadband emission centered at 950 nm with the FWHM of 297 nm. The fluorescence lifetime of 0.5at.% Cr3+:LuScO3 crystal is 33.7 µs and the emission cross-sections were calculated to be 1.60×10−20 cm2. The maximum phonon energy of Cr:LuScO3 crystal is 405 cm-1, according to its Raman spectrum. All the results show that Cr3+:LuScO3 crystal should be a potential broadband tunable laser and ultrafast laser material.
Funding
Fundamental Research Funds for the Central Universities (No.22120210432); National Natural Science Foundation of China (No.61861136007); National Natural Science Foundation of China (No.61621001).
Acknowledgment
This work is supported by the National Natural Science Foundation of China (No.61621001, No.61861136007) and the Fundamental Research Funds for the Central Universities (No.22120210432).
Disclosures
The authors declare no conflicts of interest.
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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