*Article* **Mid-IR Optical Property of Dy:CaF2-SrF2 Crystal Fabricated by Multicrucible Temperature Gradient Technology**

**Lihe Zheng 1, Jianbin Zhao 1, Yangxiao Wang 2, Weichao Chen 3,\*, Fangfang Ruan 4,\*, Hui Lin 5, Yanyan Xue 6, Jian Liu 6, Yang Liu 1, Ruiqin Yang 1, Haifeng Lu 1, Xiaodong Xu <sup>7</sup> and Liangbi Su <sup>2</sup>**


**Abstract:** Dy3+-doped CaF2-SrF2 crystals with various Dy3+ dopant concentrations were synthesized by multicrucible temperature gradient technology (MC-TGT). Dy:CaF2-SrF2 crystals were fluorite structured and crystallized in cubic *Fm*3*m* space group, as characterized by X-ray diffraction. The crystallographic site concentration was calculated from the measured density by Archimedes' hydrostatic weighing principle. The optical transmission reached over 90% with a sample thickness of 1.0 mm. The Sellmeier dispersion formula was obtained following the measured refractive index in a mid-IR range of 1.7–11 μm. Absorption coefficients of 6.06 cm−<sup>1</sup> and 12.71 cm−<sup>1</sup> were obtained at 804 nm and 1094 nm in 15% Dy:CaF2-SrF2 crystal. The fluorescence spectra of 15 at.% Dy:CaF2-SrF2 showed the strongest wavelength peak at 2919 nm with a full width at half maximum (FWHM) of 267 nm under an excitation wavelength of 808 nm. The fluorescence lifetimes were illustrated for different Dy3+ dopant levels of 5%, 10% and 15%. The results indicate that the Dy:CaF2-SrF2 crystal is a promising candidate for compact mid-IR lasers.

**Keywords:** Dy:CaF2-SrF2; crystal growth; temperature gradient technology; midinfrared crystal; Sellmeier dispersion formula

#### **1. Introduction**

Midinfrared (mid-IR) lasers are considered of great importance due to their wide applications in fundamental and practical fields such as directional infrared countermeasures, atmospheric monitoring, biomedicine, medical laser, optical communication and highenergy physics [1]. Directly pumped mid-IR solid-state lasers have attracted significant attention due to their advantages such as simple system composition, compact size, high efficiency and high output power. Active ions and host materials are both important in obtaining diode laser pumped solid-state lasers.

Rare-earth ions (Tm3+, Er3+, Ho3+ and Dy3+) are the preferred active ions for laser emission in the mid-IR spectral range. Tm3+, Er3+, Ho3+ ions have been reported with 2–3 μm lasing in crystalline or glass hosts such as Tm:YAG [2], Tm:YLF [3], Tm:ZBLAN [4],

**Citation:** Zheng, L.; Zhao, J.; Wang, Y.; Chen, W.; Ruan, F.; Lin, H.; Xue, Y.; Liu, J.; Liu, Y.; Yang, R.; et al. Mid-IR Optical Property of Dy:CaF2-SrF2 Crystal Fabricated by Multicrucible Temperature Gradient Technology. *Crystals* **2021**, *11*, 907. https:// doi.org/10.3390/cryst11080907

Academic Editors: Ludmila Isaenko, Xiaoming Duan, Renqin Dou, Linjun Li and Xiaotao Yang

Received: 1 July 2021 Accepted: 27 July 2021 Published: 31 July 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Tm:SrF2 [5], Ho:CaF2 [6], Ho:YAG [7], Er:SrF2 [8] and Er:YSGG [9]. With energy transitions corresponding to 2.3–3.4 μm and 4.0–6.2 μm, Dy3+ is recognized as an active ion with high potential applications for mid-IR lasers. However, compared with other rare-earth ions, such as Tm3+, Er3+ and Ho3+, laser emissions from Dy3+ ion-based solid-state systems are relatively rare. In addition, the intrinsic emission wavelengths of Dy3+ include emission band peaks around 2.4, 3.4, 4.3 and 5.4 μm. Owing to the even electron number of 4*f* electron shells in Dy3+ and Stark effects in different crystal fields, a smooth broadband emission spectrum around 2.3–3.4 μm could lead to tunable or ultrafast laser output.

Recently, Dy3+ laser has been investigated in fluorozirconate ZBLAN glass fiber with compositions of ZrF4, BaF2, LaF3, AlF3 and NaF. In 2003, Jackson reported a 2.9 μm CW laser from Dy:ZBLAN with maximum output power of 275 mW and slope efficiency of 4.5% [10]. In 2011, Tsang and El-Taher demonstrated an efficient Dy:ZBLAN fiber lasing around 3 μm with slope efficiency of 23%. However, the output power was limited to 100 mW [11]. In 2016, Majewski et al. reported a Dy:ZBLAN laser at 3.04 μm, with a slope efficiency of 51% and maximum output of 80 mW [12]. In 2018, Woodward et al. successfully obtained watt-level Dy:ZBLAN fiber laser at 3.15 μm with slope efficiency of 73% [13]. In contrast, Dy3+-doped crystals are yet to be explored for mid-IR lasers. Currently, the Dy3+-doped crystalline host is limited in fluoride and thiogallate crystals such as Dy:BaY2F8 [14], Dy:LYF [15], Dy:PbGa2S4 [16,17] and Dy:CaGa2S4 [18]. Due to superior thermal, mechanical and moisture-proof properties, the Dy3+-doped crystalline host is expected to have good performance in the mid-IR spectral range compared with those in ZBLAN fiber.

In this work, a family of CaF2-SrF2 crystals doped with Dy(III) was prepared to generate efficient mid-IR emission properties for potential applications in mid-IR lasers. CaF2-SrF2 crystal was selected as host material with a molar ratio of 1:1 for Ca/Sr, as inspired by predominant optical properties when doped with rare-earth ions such as Yb3+ [19,20]. Meanwhile, CaF2-SrF2 possesses low phonon energy, which is of benefit to weaken nonradiative decay from intermediate states to lower ground states in rare-earth ions [21,22]. Moreover, CaF2-SrF2 is an azeotrope system, and even the composition ratio of Ca/Sr is different [23,24]. Karimov et al. reported the growth of mixed crystals at the azeotrope point [25].

This research work focuses on crystal growth of Dy:CaF2-SrF2 with different Dy3+ dopant concentrations. The refractive index and related Sellmeier dispersion formula were obtained in the range of 1.7–11 μm. Optical characterization was conducted in the mid-IR spectral range, followed by the discussions of the energy transfer path in Dy:CaF2-SrF2 crystal.

#### **2. Materials and Methods**

#### *2.1. Crystal Growth*

Dy:CaF2-SrF2 crystal boules, with Dy3+ concentrations of 5 at.%, 10 at.% and 15 at.%, were fabricated by multicrucible temperature gradient technology (MC-TGT). TGT is a directional solidification technique adapted for the growth of high-temperature crystals [26]. The traditional TGT method allows for one-crucible crystal growth. To enhance growth efficiency, we developed an MC-TGT that allows obtaining multiple crystal boules in the one-growth process [22]. Six crucibles were fixed in the furnace. Benefiting from the azeotrope properties of CaF2-SrF2, no crystal seed was used. A stable temperature gradient was then built around the crucible to conduct latent heat and to promote crystallization. The starting materials were CaF2, SrF2 and DyF3 powders with purity higher than 99.995%. The raw materials were mixed according to Ca/Sr molar ratio of 1:1 before placing it in a graphite crucible. Then, 1 wt.% PbF2 was added to remove oxygen. The melting points of DyF3, CaF2 and SrF2 are 1360 ◦C, 1420 ◦C and 1477 ◦C, respectively. The furnace was insulated for 3 h at 1530 ◦C while evacuating to 10−<sup>3</sup> Pa. The cooling rate for crystal growth was set at 1.5 ◦C/h. After growth, the crucible was cooled to room temperature

at a cooling rate of 20 ◦C/h. The Dy:CaF2-SrF2 crystals were then cut and processed for subsequent tests.

#### *2.2. Characterizations*

The segregation coefficient of Dy3+ ion in the CaF2-SrF2 host was detected by inductively coupled plasma atomic emission spectrometry (ICP-AES). The sample was cut from the initial part of the crystal and then ground into fine powder in an agate mortar. The solvent was a mixture of phosphoric acid and boric acid. After obtaining the weight percentage of Dy3+, Ca2+ and Sr2+ in the solvent, the dopant level of Dy3+ was calculated. The segregation coefficient could thus be calculated by dividing the measured dopant value of Dy3+ by the theoretical value. The segregation coefficient was measured for the sample adjacent to the initial part of the as-grown crystal boule.

The structure of Dy:CaF2-SrF2 crystal was measured by a powder-X-ray diffractometer (P-XRD, RIGAKU TTRIII-18KW, Tokyo, Japan ) using a Cu target at room temperature. The scan rate was 3◦/min. The raw data from the P-XRD pattern were analyzed to fix the diffraction peak by comparing with the standard Power Diffraction File (PDF) data from Jade software. The diffractograms were gathered by Origin software.

The refractive index in the mid-IR spectral range was characterized using an infrared ellipsometer (J.A. Woollam IR-Vase II, Lincoln, NE, USA). The single crystal along the growth axis was cut with a diameter of 20 mm and a thickness of 1 mm. One surface was polished, while the other surface was kept rough for measurement. The measured data were then analyzed by nonlinear fitting by Origin software to obtain dispersion formula.

The optical quality of as-grown crystal boules was characterized by mid-IR transmission spectra with a sampling step of 2.8 nm (Bruker TENSOR27, Karlsruhe, Germany). Absorption spectra were measured with a UV/vis/NIR spectrophotometer (Varian Cary 5000, Palo Alto, CA, USA) using Xe light as a pump source. Fluorescence spectra were measured with an 808 nm pump source (Edinburg Instruments FLS1000, Livingston, UK) and an InSb detector. As a comparison, a 1320 nm pump source from a pulse generator (Thurlby Thandar Instruments TGP 110, Huntingdon, UK) was used for fluorescence spectra, emitting at 1200–3500 nm, recorded by a digital phosphor oscilloscope with sampling rate of 1.25 GS/s and frequency of 100 MHz (Tektronix TDS 3012C, Beaverton, OR, USA). The fluorescence lifetime measurement was carried out by a computer-controlled transient digitizer decay curve of emission under a pump wavelength of 808 nm. Single crystals with a thickness of 1 mm along the growth axis were polished and then used for the above-mentioned spectroscopic measurements. All measurements were performed at room temperature.

#### **3. Results and Discussion**

#### *3.1. Crystal Structure and Optical Quality*

The traditional TGT method allows for one-crucible crystal growth. To enhance growth efficiency, we developed an MC-TGT that allows obtaining multiple crystal boules in the one-growth process. Figure 1 shows the obtained Dy:CaF2-SrF2 crystal boules up to 20 mm in diameter and 68 mm in length. The as-grown yellow boules are homogeneous without bubbles. The dopants of Dy3+ ions are 5 at.%, 10 at.% and 15 at.%. The segregation coefficient of Dy3+ ion in 5 at.% Dy:CaF2-SrF2 and 15 at.% Dy:CaF2-SrF2 is 1.0, while that in 10 at.% Dy:CaF2-SrF2 is 0.96. This indicates the high solubility of Dy3+ ion in the CaF2-SrF2 host lattice.

**Figure 1.** As-grown Dy:CaF2-SrF2 crystal boules using MC-TGT. (**a**) 5at.% Dy; (**b**) 10at.% Dy; (**c**) 15at.% Dy.

The method of Archimedes' hydrostatic weighing principle aids in the determination of density following Equation (1).

$$
\rho = \mathbf{m}\_1 \cdot \rho\_1 / (\mathbf{m}\_1 - \mathbf{m}\_2) \tag{1}
$$

Here, m1 and m2 are sample weights measured in air and in water, respectively. ρ<sup>1</sup> is the density of water marked as 1 g·cm−3. <sup>ρ</sup> is the sample density in air to be determined. The test results are listed in Table 1.

**Table 1.** Density determination following Archimedes' hydrostatic weighing principle.


Figure 2 gives the measured XRD pattern of Dy:CaF2-SrF2 crystal compared with that of pure CaF2 referring to PDF Number 75-0363 and that of pure SrF2 referring to PDF Number 06-0262. The main three strongest peaks in CaF2 are located at 2θ of 28.272◦ for (111), 47.008◦ for (200) and 55.764◦ for (311), while those in SrF2 are located at 2θ of 26.57◦ for (111), 44.123◦ for (200) and 52.273◦ for (311). In the case of Dy:CaF2-SrF2 crystal, the three strongest peaks are located at 2θ of 27.46◦, 45.52◦ and 53.96◦. The diffraction angles 2θ of Dy:CaF2-SrF2 situates in-between those of CaF2 and SrF2. This indicates that Dy:CaF2-SrF2 crystal is fluorite structured and crystallizes in the cubic *Fm*3*m* space group.

**Figure 2.** XRD pattern of Dy:CaF2-SrF2 crystal.

The optical quality of polished Dy:CaF2-SrF2 crystals was characterized by transmission spectra. Figure 3 gives the transmission spectra of Dy:CaF2-SrF2 at 2–11 μm. The transmission curves in Figure 3 are based on measured raw data subtracting the background without considering the reflections on both surfaces. It shows that the transmission of all three polished samples is above 90% from 4 to 9 μm. The infrared transmittance cut-off wavelength is around 11 μm. It could be concluded that the crystallinity and optical quality of Dy:CaF2-SrF2 crystal boules are good.

**Figure 3.** Transmission spectra of Dy:CaF2-SrF2 crystals at 2–11 μm.

#### *3.2. Refractive Index in the Mid-IR Spectral Range*

Figure 4 gives the refractive index of Dy:CaF2-SrF2 in the mid-IR spectral range of 1.7–11 μm. As seen from Figure 4, the refractive index of Dy:CaF2-SrF2 increases along with a higher dopant level of Dy3+. The Sellmeier dispersion formula is used for nonlinear fitting. The dispersion formula for Dy:CaF2-SrF2 with Dy3+ dopant levels of 5%, 10% and 15% is thus achieved and described in Equations (2)–(4), respectively. The reduced Chi-square is 2.70 × <sup>10</sup><sup>−</sup>5. The adjusted R2 is 0.9986.

$$m^2 - 1 = 0.20292 + \frac{0.69272\lambda^2}{\lambda^2 - 0.94855^2} + \frac{0.87102\lambda^2}{\lambda^2 - 60.10^2} + \frac{5.64914\lambda^2}{\lambda^2 - 45.04^2} \tag{2}$$

$$m^2 - 1 = 0.21687 + \frac{0.70758\lambda^2}{\lambda^2 - 0.97111^2} + \frac{1.84394\lambda^2}{\lambda^2 - 70.44^2} + \frac{6.11963\lambda^2}{\lambda^2 - 48.55^2} \tag{3}$$

$$m^2 - 1 = 0.20292 + \frac{0.73684\lambda^2}{\lambda^2 - 0.97075^2} + \frac{2.30876\lambda^2}{\lambda^2 - 69.93^2} + \frac{6.15892\lambda^2}{\lambda^2 - 50.30^2} \tag{4}$$

**Figure 4.** Refractive index of Dy:CaF2-SrF2 in the mid-IR spectral range of 1.7–11 μm.

#### *3.3. Absorption and Emission Spectra*

Figure 5 shows the room-temperature absorption spectra in the spectral range of 650–1900 nm and the assignment of energy level in Dy:CaF2-SrF2 crystals. The assignment of energy level could refer to that in Dy:BaY2F8 [27]. In the mid-IR region, Dy3+ ions show broad absorption bands in 5% Dy:CaF2-SrF2 crystal peaks at 804 nm, 907 nm, 1092 nm, 1276 nm and 1714 nm corresponding to absorption coefficients of 2.12 cm−1, 2.69 cm−1, 4.04 cm<sup>−</sup>1, 2.54 cm−<sup>1</sup> and 1.18 cm<sup>−</sup>1, respectively. In the case of 10% Dy:CaF2-SrF2 crystal, the absorption coefficients are 3.56 cm−1, 4.46 cm−1, 7.08 cm−1, 4.47 cm−<sup>1</sup> and 2.02 cm−1, corresponding to absorption band peaks at 804 nm, 908 nm, 1093 nm, 1273 nm and 1716 nm. For 15% Dy:CaF2-SrF2 crystal, it gives the strongest absorption coefficients of 6.06 cm−1, 7.56 cm<sup>−</sup>1, 12.71 cm−1, 8.25 cm−<sup>1</sup> and 3.63 cm<sup>−</sup>1, corresponding to absorption band peaks at 804 nm, 907 nm, 1094 nm, 1277 nm and 1716 nm. The absorption band peaks at 1308 nm and 1720 nm correspond to energy level transitions of 6H15/2→6H9/2, 6F11/2 and 6H15/2→6H11/2 in Dy3+ ion. With the increase in Dy3+ ion dopant levels from 5% to 15%, the absorption coefficient becomes 2.8–3.3 times stronger. Broad absorption bands are profitable in increasing the diode-pumping efficiency, as laser diodes typically emit in a narrow spectral range and present a thermal shift in the peak wavelength.

**Figure 5.** Absorption coefficient of Dy:CaF2-SrF2 crystal at 650–1900 nm.

Table 2 gives the calculated absorption cross-section of Dy3+ according to the expression *σabs* = *α*/*N*. Here, α is the absorption coefficient of Dy3+. *N* is the concentration of Dy3+ ions with 1.21 × <sup>10</sup><sup>21</sup> ion·cm<sup>−</sup>3, 2.42 × <sup>10</sup><sup>21</sup> ion·cm−<sup>3</sup> and 4.03 × <sup>10</sup><sup>21</sup> ion·cm−<sup>3</sup> in 5% Dy:CaF2-SrF2, 10% Dy:CaF2-SrF2 and 15% Dy:CaF2-SrF2, respectively.


Figure 6 shows the emission spectra and peak assignment of Dy:CaF2-SrF2 crystal in the mid-IR range under excitation wavelengths of 808 nm and 1320 nm. In the case of using a pump wavelength of 1320 nm, as shown in Figure 6a, the fluorescence band peak at 1970 nm, corresponding to the energy level transition from 6H9/2 + 6F11/2 to 6H15/2 [16], was detected with an intensity variation in the order of the Dy3+ doping concentration. The strongest fluorescence intensity appears in 5% Dy:CaF2-SrF2, while the lowest fluorescence intensity appears in 15% Dy:CaF2-SrF2. It is interesting to note that the intensity of the fluorescence band peak at 2882 nm is not affected by the Dy3+ doping concentration.

**Figure 6.** Emission spectra of Dy:CaF2-SrF2 crystal under different pump source. (**a**) 1320 nm pump; (**b**) 808 nm pump.

Figure 6b shows the emission spectra by using an excitation wavelength of 808 nm. The emission intensity of 10% Dy:CaF2-SrF2 is the lowest, while that of 15% Dy:CaF2-SrF2 is the highest. In order to illustrate the mid-IR fluorescence bands, nonlinear fittings for multiple peaks are used in the form of the Lorentz function. Taking 15% Dy:CaF2-SrF2 for example, the full width at half maximum (FWHM) is 267 nm for the emission band peak at 2919 nm, corresponding to the energy level transition from 6H13/2 to 6H15/2 of Dy3+ ion. In the case of Dy:CaF2-SrF2 with Dy3+ dopant levels of 10% and 5%, the values of FWHM are both 237 nm. All emission bands peak at 2913 nm.

Figure 7 gives the measured fluorescence lifetime curves for the energy transfer channel 6H13/2→6H15/2 of Dy3+ ions in Dy:CaF2-SrF2 crystals. The obtained fluorescence lifetime curves are processed with nonlinear fitting according to the second-order exponential formula (ExpDec2), as described in Equation (5).

$$y = A\_0 + A\_1 e^{-\frac{\varphi}{l\_1}} + A\_2 e^{-\frac{\varphi}{l\_2}} \tag{5}$$

**Figure 7.** Measured and nonlinear fitted fluorescence lifetime curves in Dy:CaF2-SrF2. (**a**) 5% Dy; (**b**) 10% Dy; (**c**) 15% Dy.

Here, *A*0, *A*1, *A*2, *t*<sup>1</sup> and *t*<sup>2</sup> are constant values that could be obtained from the fitting results. Accordingly, the second-order exponential formula for Dy:CaF2-SrF2 with various Dy3+ dopant levels is thus written in Equations (6)–(8). The average fluorescence lifetime could be obtained from Equation (9). The fluorescence lifetimes are calculated as 743.6 μs, 339.2 μs and 194.7 μs for Dy:CaF2-SrF2 with Dy3+ dopant levels of 5%, 10% and 15%, respectively. The reduction of fluorescence lifetime along with a higher Dy3+ concentration could be attributed to the increase in interstitial fluoride ions. The mechanism could be described in the defect reaction equation as shown in Equation (10). It would be interesting to investigate the mechanism of enhancing fluorescence intensity while maintaining lifetime in future work.

$$y = 1046.495 + 331.646e^{-\frac{r}{1723767}} + 4722.538e^{-\frac{r}{572767}}\tag{6}$$

$$y = 793.798 + 5605.837e^{-\text{\footnotesize grow\text{\footnotesize}}} + 60.970e^{-\text{\footnotesize work}} \tag{7}$$

$$y = 1050.882 + 2892.258e^{-\frac{\pi}{10777}} + 2892.258e^{-\frac{\pi}{10777}}\tag{8}$$

$$\tau = \frac{A\_1 t\_1^2 + A\_2 t\_2^2}{A\_1 t\_1 + A\_2 t\_2} \tag{9}$$

$$D y F\_3 \stackrel{CaF\_2 - SrF\_2}{\rightarrow} D y\_{Ca/Sr} + 2F\_F + F\_i' \tag{10}$$

#### **4. Summary**

This paper concerns the important issue of novel emissive materials for laser applications in the significant mid-IR range. Dy3+-doped CaF2-SrF2 crystals with fluorite structure and cubic *Fm*3*m* space group were synthesized by MC-TGT. The crystallographic site concentrations were up to 4.034 × 1021 ions·cm−<sup>3</sup> in 15 at.% Dy:CaF2-SrF2. The optical transmission of Dy:CaF2-SrF2 crystal reached over 90% with a sample thickness of 1.0 mm. The Sellmeier dispersion formula for 1.7–11 μm was obtained from the refractive index. The strongest absorption coefficients of 6.06 cm−1, 7.56 cm−1, 12.71 cm−1, 8.25 cm−<sup>1</sup> and 3.63 cm−<sup>1</sup> were obtained in 15% Dy:CaF2-SrF2 crystal corresponding to absorption band peaks at 804 nm, 907 nm, 1094 nm, 1277 nm and 1716 nm. The value of FWHM was 267 nm for the emission band peak at 2919 nm in 15% Dy:CaF2-SrF2 under an excitation wavelength of 808 nm. The fluorescence lifetime in 5% Dy:CaF2-SrF2 was 3.8 times longer than that in 15% Dy:CaF2-SrF2. Further research work will focus on the enhancement of lifetime while maintaining strong emission in the mid-IR spectral range.

**Author Contributions:** Methodology—W.C.; formal analysis—J.Z., H.L. (Hui Lin), Y.X., Y.L. and R.Y.; investigation—H.L. (Haifeng Lu), Y.W., H.L. (Hui Lin), R.Y., J.L. and Y.X.; validation—F.R. and X.X.; funding acquisition—L.S.; supervision—L.Z.; writing—original draft preparation—W.C. and J.Z.; writing—review and editing—L.Z., F.R. and L.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by National Natural Science Foundation of China, Grant Number U1830104; Opening Project of the State Key Laboratory of Transparent Opto-functional Inorganic Materials, Chinese Academy of Science, Grant Number KLTOIM202001; Yunnan Fundamental Research Projects, Grant Number 202101AT070162; Yunnan University First-class University Construction Project, Grant Number C176220100155; Innovative entrepreneurial training for Yunnan College Students, Grant Number 202010673075.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors acknowledge the experimental support from Shaohua Wu and Huajin Wang.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

