Rare-earth ion-doped solid-state materials emitting visible laser light, especially in the yellow region, are currently applied in many fields, such as medical treatment, telecommunications, microscopy, and biomedicine.^[1,2] Some strategies have been developed to acquire yellow laser light, such as Raman-induced lasing and frequency-doubling lasing in semiconductors. Nevertheless, these methods have some shortcomings, namely high cost, system complexity, and indirect yellow emission. Among rare-earth ions, trivalent dysprosium is suitable for studying luminescent behavior because its ⁴F_9/2→⁶H_13/2 transition emits yellow light, and the ion is a potential candidate for achieving yellow emission directly and efficiently. In addition, the emergence of the InGaN laser diode (LD) has advanced the study of Dy³⁺-doped laser materials because it is convenient to pump the ⁴I_15/2 level in dysprosium (∼ 446 nm) with an InGaN LD. Up to now, a series of spectral results regarding Dy³⁺-doped materials have been reported,^[3–6] and yellow laser emission from Dy:YAG^[7] and Dy,Tb:LiLuF₄^[8] have been demonstrated.

Lu₂O₃^[11] has attracted much attention because of its low phonon energy (430 cm⁻¹), robust thermal stability, and high thermal conductivity.^[9,10] However, the high melting point (∼ 2450 °C)^[12] of Lu₂O₃ presents a great challenge to its growth. Some methods have been attempted to grow Lu₂O₃ crystals, for example, the heat exchanger method,^[13] Czochralski technique,^[14] laser-heated pedestal growth,^[15] Bridgman method,^[16] and micro-pulling technique.^[17] However, few Lu₂O₃ crystals with good quality have been obtained for applications. So far, the optical characteristics of Tm³⁺-,^[18] Er³⁺-,^[19] and Yb³⁺-doped^[19] Lu₂O₃ crystals have been investigated. Unfortunately, very few studies were focused on Dy³⁺-doped Lu₂O₃ crystals.

In this work, crystal growth, spectroscopic properties, and JO theory analysis of Dy:Lu₂O₃ are discussed in detail. Furthermore, we compare the optical characteristics of Dy³⁺-doped Lu₂O₃ crystals with other materials.

Lu₂O₃ (99.99%), Dy₂O₃ (99.99%) powders were used as raw materials and were weighed accurately according to (Dy_xLu_1−x)₂O₃ (x = 0.03). The weighed material was mixed in a beaker and then shaped into two rods. After pressed at 220 MPa for 3 min, the two rods were sintered in air at 1700 °C for 24 h. One was used as the feed rod and the other was used as the seed rod. The two rods should be placed on a same vertical axis during the growth process. High-purity argon gas was used as a protective atmosphere. The rotation rate and growth rate were set to be 8–12 rpm and 1–2 mm/h for both rods, respectively. After growth, the as-grown Dy:Lu₂O₃ was cooled to room temperature slowly and then annealed in air at 1700 °C for 24 h. A 3 mm×4 mm×1 mm sample was cut and polished for spectral measurements. A photo of the Dy:Lu₂O₃ sample is shown in Fig. 1.

Fig. 1.

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	Fig. 1. (color online) The photo of a Dy:Lu2O3 sample.

The following measurements were performed at room temperature. XRD analysis of Dy:Lu₂O₃ was performed with an Ultima IV diffractometer (Japan). The absorption spectrum was measured with a Lambda 900 spectrophotometer. The concentration of dysprosium in Dy:Lu₂O₃ was determined to be 2.8 at.% (i.e., 7.99 × 10²⁰ cm⁻³) using inductively coupled plasma-atomic emission spectrometry (ICP-AES) measurements. The emission spectrum together with the decay curve of Dy:Lu₂O₃ were gathered with a fluorospectrophotometer (FSP920).

The XRD pattern of the Dy:Lu₂O₃ crystal is shown in Fig. 2. The diffraction peaks of Dy:Lu₂O₃ are in accordance with JCPDS 86-2475 of Lu₂O₃, and no impurity peaks can be seen in the spectra. The lattice parameter of Dy:Lu₂O₃ was calculated to be 1.0411 nm, which is similar with that of a pure Lu₂O₃ crystal (1.0391 nm).^[30] The results indicate that the obtained Dy:Lu₂O₃ crystal has bixbyite structure with Ia3 cubic space group, and the adulteration of Dy³⁺ into Lu₂O₃ does not obviously affect the crystal structure.

Fig. 2.

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	Fig. 2. The XRD pattern of Dy:Lu2O3 crystal.

The absorption spectrum of Dy³⁺-doped Lu₂O₃ is presented in Fig. 3, where the corresponding transitions of Dy³⁺ were assigned and marked. As shown in Fig. 3, some absorption bands were overlapped with each other due to the interaction between crystal fields. The absorption cross-section σ_abs around 446 nm corresponding to the ⁶H_15/2 → ⁴I_15/2 transition was determined to be 0.71 × 10⁻²¹ cm² with 4.0 nm full width at half maximum (FWHM). These qualities make the Dy:Lu₂O₃ crystal highly desirable for pumping with an InGaN laser diode.^[20]

Fig. 3.

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	Fig. 3. Absorption spectrum of Dy:Lu2O3 crystal.

JO theory^[21–23] was used to evaluate the optical characteristics of lanthanide ions doped in host materials in this work. The detailed calculation process used here is the same as that presented in Ref. [2]. The matrix elements for chosen transitions in Dy³⁺ used for calculating the JO parameters were taken from Refs. [32] and [38]. The calculated average wavelength , refractive index n,^[24] calculated and experimental line strengths (S_cal and S_exp), and the root-mean-square deviation (rms Δs) are presented in Table 1. The results show that rms Δs = 0.027 × 10⁻²⁰ cm², which informs the accuracy of the fitting results. The relatively small value demonstrates that the fitting results between S_exp(J,J′) and S_cal(J,J′) are reasonable.

Table 1.

Table 1.

Table 1. The average wavelength ( ), refractive index (n), experimental line strength Sexp(J,J′), and calculated line strength Scal(J,J′) of Dy:Lu2O3 crystal. . Transition (from 6H15/2) /nm n Sexp/10−20 cm2 Scal/10−20 cm2 6P7/2+4I11/2 350 2.009 1.003 1.090 6P5/2+4D3/2+4M19/2 364 2.000 0.377 0.417 4F7/2+4I13/2+4M21/2+4K17/2 386 1.987 0.626 0.660 4I15/2 446 1.963 0.130 0.151 6F5/2 795 1.919 0.531 0.608 6H7/2+6F9/2 1076 1.910 2.472 2.499 6H9/2+6F11/2 1254 1.906 6.932 6.982 6H11/2 1674 1.900 1.567 1.651 rms Δs/10−20 cm2 0.027

Table 1. The average wavelength ( ), refractive index (n), experimental line strength Sexp(J,J′), and calculated line strength Scal(J,J′) of Dy:Lu2O3 crystal. .

The JO intensity parameters Ω_t (t = 2, 4, 6) were obtained through least squares fitting between the calculated and experimental line strength. The JO intensity parameters Ω_t (t = 2, 4, 6) of Dy³⁺-doped Lu₂O₃ and other hosts are presented in Table 2. The calculated Ω₂, Ω₄, and Ω₆ values of Dy:Lu₂O₃ are 4.86 × 10⁻²⁰ cm², 2.02 × 10⁻²⁰ cm², and 1.76 × 10⁻²⁰ cm², respectively. Normally, Ω₂ is related to the asymmetry of the crystal field and matrix environment around lanthanide ions. The value of Ω₂ is proportional to the asymmetry and is inversely proportional to the contravalency. Ω₄/Ω₆ is used as a spectroscopic quality factor and is a significant characteristic parameter for estimating the stimulated emission efficiency.^[27] As shown in Table 2, the value of Ω₄/Ω₆ for Dy:Lu₂O₃ is larger than that for most other crystals, revealing that Dy:Lu₂O₃ is an excellent candidate for laser emission. The radiative lifetime τ_rad, fluorescence branching ratio β and spontaneous radiation transition probability A for a Dy:Lu₂O₃ crystal were calculated, and the results are shown in Table 3. In a Dy:Lu₂O₃ crystal, the ⁴F_9/2 → ⁶H_13/2 transition shows the largest fluorescence branching ratio of 61.19%, which indicates a high possibility of yellow emission around 574 nm. Moreover, the radiative lifetime τ_rad of Dy:Lu₂O₃ (756 μs) is comparable to Dy:YAP (757 μs)^[2] and is larger than that of Dy:CaGdAlO₄ (501 μs),^[27] Dy:Li₃Ba₂La₃(MoO₄)₈ (215 μs),^[28] and Dy:NaGd(WO₄)₂ (281 μs),^[35] indicating Dy:Lu₂O₃ has excellent energy storage capability.

Table 2.

Table 2.

Table 2. The J–O intensity parameters with Ω4/Ω6 values for different Dy3+-doped crystals. . Hosts Ω2/10−20 cm2 Ω4/10−20 cm2 Ω6/10−20 cm2 Ω4Ω6 Ref. CaGdAlO4 1.80 1.00 0.50 2.00 [27] YSGG 0.13 0.73 1.06 0.68 [25] K2GdF5 2.51 0.94 2.12 0.44 [26] LiNbO3 5.42 1.14 2.51 0.45 [29] ZnWO4 7.76 0.57 0.31 1.84 [31] LiYF4 2.01 1.34 2.39 0.89 [32] YAG 0.20 1.11 1.46 0.76 [36] YAP 3.93 1.64 3.79 0.43 [2] LiLuF4 2.04 0.91 1.09 0.83 [37] Lu2O3 4.86 2.02 1.76 1.15 This work

Table 2. The J–O intensity parameters with Ω4/Ω6 values for different Dy3+-doped crystals. .

Table 3.

Table 3.

Table 3. Spontaneous emission transition probability A, fluorescence branching ratio β, and radiative lifetime τrad of Dy:Lu2O3 crystal. . Transitions A/s−1 β/% 4F9/2→ 6F5/2 17.005 1.29 6F7/2 9.904 0.75 6H5/2 6.062 0.46 6H7/2, 6F9/2 42.341 3.20 6H9/2, 6F11/2 51.371 3.88 6H11/2 107.858 8.15 6H13/2 809.418 61.19 6H15/2 278.911 21.08 Radiative lifetime/μs τrad = 756

Table 3. Spontaneous emission transition probability A, fluorescence branching ratio β, and radiative lifetime τrad of Dy:Lu2O3 crystal. .

The emission spectrum of Dy:Lu₂O₃ under 446 nm excitation is shown in Fig. 4. The ⁴F_9/2 → ⁶H_9/2+⁶F_11/2, ⁶H_11/2, ⁶H_13/2, and ⁶H_15/2 transitions are centered at 487 nm, 574 nm, 673 nm, and 761 nm, respectively. Among these transitions, the ⁴F_9/2 → ⁶H_13/2 transition at 574 nm exhibits the largest intensity, which agrees well with the JO theory results shown in Table 3. Near 574 nm, the emission cross-section σ_em and FWHM are 0.53 × 10⁻²⁰ cm² and 2.69 nm, respectively. The corresponding σ_em of Dy:Lu₂O₃ is larger than that of Dy:YAP^[2] and is comparable to those of Dy:GdVO₄^[33] and Dy:ZnWO₄^[31] with a magnitude of 10⁻²⁰ cm². For the ⁴F_9/2 → ⁶H_13/2 and ⁴F_9/2 → ⁶H_15/2 transitions in Dy³⁺, the intraconfigurational transitions split into maximum number energy levels,^[39] which have a more important influence on the luminescence than other transitions. Thus, we compared the ⁴F_9/2 → ⁶H_15/2 transition (blue emission) with the ⁴F_9/2 → ⁶H_13/2 transition (yellow emission). The yellow to blue intensity ratio (Y/B) is an important parameter of reflecting the possibility of yellow laser emission. A large Y/B value indicates a material is more likely to emit yellow laser light.^[27,28] To the best of our knowledge, it has been reported that the emission from Dy:LTT was yellowish-white, which is primarily due to its large Y/B value.^[40] For the Dy:Lu₂O₃ crystal, the yellow (⁴F_9/2 → ⁶H_13/2 transition around 574 nm) to blue (⁴F_9/2 → ⁶H_15/2 transition around 487 nm) intensity ratio is 12.9, which is relatively larger than that in Dy:Li₃Ba₂La₃(MoO₄)₈ (4.52),^[28] Dy:YAl₃(BO₃)₄ (3.37),^[29] and Dy:CaGdAlO₄ (3.68).^[27]

Fig. 4.

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	Fig. 4. Emission spectrum of Dy:Lu2O3 crystal.

The fluorescence decay curve of the ⁴F_9/2 multiplet under 446 nm excitation is presented in Fig. 5. The fluorescence lifetime τ was measured to be 112.1 μs. Thus, the fluorescence quantum efficiency η (η = τ/τ_rad) is 14.8%. This result is principally caused by the high concentration (7.99 × 10²⁰ cm⁻³) of Dy³⁺ in Lu₂O₃. The increased concentration will arouse cross-relaxations of Dy³⁺ via ⁴F_9/2+⁶H_15/2 to (⁶F_3/2, ⁶F_1/2)+(⁶H_9/2, ⁶F_11/2).^[34] It has been reported that the fluorescence quantum efficiency changed from 94.3% to 5.5% as the concentration of Dy³⁺ increases from 0.4 at.% to 15.0 at.% in a Dy:LiLa(MoO₄)₂ crystal.^[34] Therefore, the quantum efficiency of Dy:Lu₂O₃ would be optimized by altering the doped concentration of Dy³⁺.

Fig. 5.

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	Fig. 5. (color online) Fluorescence decay curve of the 4F9/2 level in a Dy:Lu2O3 crystal.

A comparison of the optical parameters in different Dy³⁺-doped hosts is shown in Table 4. Among them, yellow laser emission has been observed in Dy:YAG and Dy, Tb:LiLuF₄. As for Dy:LiNbO₃ crystals, gain of the yellow transition has also been realized. As shown in Table 4, the absorption cross-section σ_abs corresponding to ⁶H_15/2 → ⁴I_15/2 transition of Dy:Lu₂O₃ is smaller than that of Dy:YAG and Dy:LiLuF₄, but is larger than that of Dy:LiNbO₃. The emission cross-section σ_em corresponding to ⁴F_9/2 → ⁶H_13/2 transition of Dy:Lu₂O₃ is larger than that of Dy:YAG, Dy:LiLuF₄, and Dy:LiNbO₃ near 574 nm, indicating that Dy:Lu₂O₃ is beneficial for high gain and low threshold laser emission. The fluorescence branching ratio β of Dy:Lu₂O₃ is larger than that of YAG (50.96%) and is comparable to that of LiLuF₄ (65.40%) and LiNbO₃ (65.50%). The above data reveal that Dy:Lu₂O₃ is quite promising for yellow laser emission.

Table 4.

Table 4.

Table 4. A comparison of absorption wavelength and absorption cross-section σabs for the 6H15/2 → 4I15/2 transition, emission wavelength, fluorescence branching ratio β, and emission cross-section σem for the 4F9/2 → 6H13/2 transition in different Dy3+-doped hosts. . Hosts Absorption Wavelength/nm σabs/10−21 cm2 Emission Wavelength/nm σem/10−20 cm2 β/% Ref. YAG 447 1.60 582 0.30 50.96 [7], [36] LiLuF4 447 0.98 574 0.45 65.40 [8], [37] LiNbO3 – 0.60 574 0.32 65.50 [31] Lu2O3 446 0.71 574 0.53 61.19 This work

Table 4. A comparison of absorption wavelength and absorption cross-section σabs for the 6H15/2 → 4I15/2 transition, emission wavelength, fluorescence branching ratio β, and emission cross-section σem for the 4F9/2 → 6H13/2 transition in different Dy3+-doped hosts. .

In conclusion, a Dy:Lu₂O₃ single crystal was successfully grown via the Fz method. The optical properties and JO analysis were studied in detail. The XRD diffraction peaks of Dy:Lu₂O₃ are in accordance with JCPDS 86-2475. The absorption cross-section and FWHM near 446 nm were found to be 0.71 × 10⁻²¹ cm² and 4.0 nm, respectively. Based on the absorption spectrum, the JO intensity parameters Ω_t (t = 2, 4, 6) were calculated to be Ω₂ = 4.86 × 10⁻²⁰ cm², Ω₄ = 2.02 × 10⁻²⁰ cm², and Ω₆ = 1.76 × 10⁻²⁰ cm², respectively. The Y/B value ranges up to 12.9, reflecting the possibility of yellow laser emission. The emission cross-section and FWHM near 574 nm were found to be 0.53 × 10⁻²⁰ cm² and 2.69 nm, respectively. The fluorescence lifetime corresponding to the ⁴F_9/2 → ⁶H_13/2 emission line at 574 nm was fitted to be 112.1 μs. Through the above analysis, Dy:Lu₂O₃ crystal could be a promising material for yellow laser emission.