Since the first solid-state ruby laser (Cr:Al₂O₃) was manufactured, the laser has played an important role in our life^[1]. In the past few decades, with the development of GaN/InGaN laser diode (LD), the solid-state laser source at visible light range has very important prospects in various applications, such as biomedical, data storage, remote sensing and quantum optics^{[2⇓⇓⇓-6]}. Trivalent praseodymium ions (Pr³⁺) can achieve transition emission from deep red (³P₀→³F₄), red (³P₀→³F₂), orange (³P₀→³H₆), green (³P_0,1→³H₅) to blue (³P₀→³H₄) region due to its plentiful energy level structures^[7⇓⇓-10]. Therefore, Pr³⁺- doped laser materials have attracted much attention in the development process of all visible solid-state lasers^[11-12].

Compared with oxide crystals, fluoride crystals have lower phonon energy and excellent mechanical properties. Among them, calcium fluoride (CaF₂) crystals have been widely concerned and studied due to their excellent thermal conductivity (9.8 W/(m·K)), wide transmission range (0.157-10 μm), low refractive index and small non-linear effects, which is the excellent laser gain medium and high transmittance window materials. In addition, calcium fluoride belongs to the cubic crystal system which is easy to obtain large-sized and high- quality single crystals. In the 1960s and 1970s, Sorokin and Stevenson, et al reported that the first laser output was obtained by U^3+[13] and Sm^2+[14] ion-doped calcium fluoride (CaF₂) crystals.

In trivalent praseodymium ions (Pr³⁺) doped CaF₂ crystals, Pr³⁺ ions replace the site of divalent Ca²⁺ ions, and interstitial F^- ions are introduced into the lattice vacancies to maintain the charge balance of the lattice. Meanwhile, [Pr³⁺-Pr³⁺] clusters can form even at lower Pr³⁺ ions doping concentration due to the dipole interaction between Pr³⁺ and F^- ions, which leads to the fluorescence quench. Therefore, a new mixed fluoride crystals system by co-doping R³⁺ (R= La³⁺, Y³⁺, Lu³⁺ and Gd³⁺) is designed to break clusters and release more luminescent centers. Compared with Pr³⁺ ions single doped, Pr³⁺:RF₃-CaF₂ (R= La³⁺, Y³⁺, Lu³⁺ and Gd³⁺) crystals have better spectral property. As for Pr:YF₃-CaF₂ crystals, Beck, et al^[15] reported that the emission intensity has been greatly increased and crystals show inhomogeneous and broad emission spectrum characteristics of glasses. The incorporation of La³⁺ ions could reduce fluorescence quenching in fluorites^[16]. In CaF₂ crystals, the luminescence intensity of Pr³⁺ ions is also enhanced by co-doping Lu³⁺ ions^[17]. The red laser output power at 642 nm reaches 22.2 mW, and the slope efficiency is 7.5% in CaF₂ crystal by co-doping Gd³⁺ ions^[4].

In present work, a series of Pr:CaF₂ crystals co-doping different concertation of La³⁺ ions were successfully grown by the temperature gradient technique (TGT). The modulation effects of different concentrations of La³⁺ ions on the spectral property and Judd-Ofelt (J-O) theory analysis crystals were studied and analyzed.

A series of Pr:CaF₂ crystals by co-doping different concertation La³⁺ ions were successfully grown by TGT, as shown in Fig. 1. In previous experiments, we found that concentration quenching effect of Pr³⁺ ions occurred at concentrations above 0.6% in CaF₂ crystal. Therefore, high purity of PrF₃ (99.99%), LaF₃ (99.99%) and CaF₂ (99.99%) were used as raw materials and weighted according to the formula: 0.6%Pr, x%La:CaF₂ (x=3, 10, 18). These raw powders were grounded about 30 min in an agate mortar. Then the raw materials were put into a porous graphite crucible in the cylindrical part. After the chamber was vacuumed to 15 Pa, the entire chamber was filled with high-argon gas to 110 kPa. The temperature was raised to 1430 ℃ for 6 h until the raw powders melted completely. The crystal grown procedure was controlled at the rate of 1 ℃/h. After crystals grown, slices were cut from the middle part of crystal and double-side polished to 1 mm thickness

The structure of Pr,La:CaF₂ crystals were studied and analyzed by X-ray powder diffraction (XRD) using Ultima IV diffractometer with a step size of 0.02°. The concentrations of Pr³⁺ and La³⁺ ions were analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES). At room temperature, the absorption spectra of 400-2500 nm were tested by using a UV-VIS-NIR spectrophotometer (Lambda 900, Perkin-Elmer). The room temperature fluorescence spectra of 450-800 nm and fluorescence lifetime at 483 nm were measured by a FLS-1000 fluorescence spectrometer (Edinburgh Company, English).

The samples of XRD were cut from as-grown crystals and grounded into powder. Fig. 2 shows XRD patterns of Pr,La:CaF₂ crystals. The diffraction peak position and intensity of 0.6%Pr, x%La:CaF₂ (x=0, 3, 10, 18) were consistent with that of pure CaF₂ crystal. This means that no other impurity phase is generated. From XRD pattern, the lattice parameter a of 0.6%Pr, x%La:CaF₂ (x=0, 3, 10, 18) were calculated to be 0.54727, 0.54851, 0.55192, 0.55606 nm. The result show that the lattice parameters of Pr, La:CaF₂ increased with increasing of doping concentration of La³⁺ ions, The lattice parameters are increased when Pr³⁺ and La³⁺ ions entry into lattice replace Ca²⁺ (0.100 nm) sites. The interstitial F^- ions (0.133 nm) are introduced to keep the balance of the system^[17].

As shown in Table S1, the concentration of Pr³⁺ and La³⁺ ions were tested by ICP-AES. The result shows that actual concentration of Pr³⁺ and La³⁺ are closed to initial concentration. The segregation coefficient of Pr,La:CaF₂ are closed to 1, which shows that the dopant ions are uniformly distributed in the host material and Pr,La:CaF₂ crystal have good quality.

The room temperature absorption spectra of Pr,La:CaF₂ crystals from 400 to 2500 nm show in Fig. 3. There are eight absorption bands of Pr³⁺ ions, corresponding to transitions from ground state of ³H₄ to excited states of ³P₂, ³P₁, ³P₀, ¹D₂, ³F₄, ³F₃, ³F₂ and ³H₆, respectively. In the blue visible region of 440-480 nm, absorption peak positions are 442 or 443 nm, corresponding to absorption transition of ³H₄→³P₂. The absorption peak matches the emission wavelength of the high-efficiency blue laser diode. The absorption cross-section σ_abs(λ) can be calculated by the following formula:

Where OD(λ) is the optical density, N₀ is actual concentration of Pr³⁺ ion, λ is wavelength and L is the thickness of these samples. Among all samples, 0.6%Pr,10% La:CaF₂ has the largest absorption cross-section reaching 1.743×10^-20 cm². The large absorption cross-section is very favorable for the absorption of pump wavelength.

In 1962, in order to explain the energy level transition of rare earth ions, the Judd-Ofelt (J-O) theory was proposed^[18-19], which is now widely used to analyze the spectral properties of rare earth ions in host materials.

For Pr³⁺ ions, magnetic-dipole (MD) transitions is forbidden because transition rule is not complied, therefore electric-dipole (ED) transitions is only considered^[20-21]. The selection rules of MD transition in the lanthanides are: ΔS=ΔL=0, ΔJ=0, ±1. According to these rules, Pr³⁺ ions does not satisfy the selection rule of MD transition. So the magnetic-dipole transitions would not be taken into account in the calculations of spectroscopic parameters of the Pr,La:CaF₂ crystal. According to J-O theory, the calculated line strength S_cal(J→J′) can be expressed as following formula:

Where J and J' are the angular momentum quantum numbers of the initial and final levels, ||U^(t)|| is the squared reduced matrix elements of the tensorial operator, Ω_t(t = 2, 4, 6) are the J-O intensity parameters.

The experiments line strength S_exp(J→J') of ED transitions can also be expressed as following formula:

Where h is the Planck constant, c is the speed of light in vacuum, e is the electron charge, N is the concentration of Pr³⁺ ion doping, n represents the refractive index of the Pr,La:CaF₂ crystal, λ is the average wavelength of the absorption band, L is the thickness of the samples, and OD(λ) represents optical density.

The root-mean-square (RMS) deviation between the experiment line strengths S_exp(J→J') and calculation line strengths S_cal(J→J') can be defined as the following formula:

Where P is the number of absorption bands in the process of calculation. For 0.6% Pr, x% La:CaF₂ (x=3, 10, 18) crystals, the average wavelength λ, absorption line strength S_exp(J, Jʹ), calculated line strength S_cal(J, Jʹ) and RMSΔS are calculated by the formula (2-4), as shown in Table S2. The value of RMSΔS is 0.744×10^-20, 0.290×10^-20, and 0.793×10^-20 cm², respectively.

The J-O intensity parameters Ω_t (t=2, 4, 6) of Pr,La:CaF₂ crystals and other host crystals are compared and shown in Table S3. For J-O intensity parameters Ω_t (t=2, 4, 6), Ω₂ is represents the symmetry and covalent bond strength of the Pr³⁺ ions coordination structure. The value of Ω₂(1.68×10^-20 cm²) of Pr,La:CaF₂ crystals is higher than those of LaF₃^[22], BaY₂F₈^[23], Y₃Al₅O₁₂^[24] and SrAl₁₁O₁₉^[25], but smaller than those of LYF₄^[22], KYF₄^[23], and CaYAlO₄^[26]. Ω₄ and Ω₆ represent the whole performance of crystals such as rigidity and viscosity of matrix. Ω₄/Ω₆ is spectroscopic quality factor that can indicate the stimulated emission efficiency in the laser gain medium^[27]. It can be found that the value of Ω₄/Ω₆(0.61) of CaF₂ crystal is higher than those of LYF₄^[22], KYF₄^[23] and SrAl₁₁O₁₉^[25] crystal from Table S3.

The radiative transition rates A(J→J’) from the excited state ³P₀ to other lower state is defined by following formula:

Where ||Ut||=2,4,6 (t=2,4,6) is the emission squared reduced matrix elements of the tensorial operator, which is shown in Ref. [21]. A^ed is the probability of electric-dipole transitions. Amdis the probability of magnetic-dipole transitions. S^md is the magnetic dipole radiation intensity, which is an independent constant. S^md is ignored due to the forbidden of magnetic-dipole transitions for Pr³⁺ ions. S^ed is the electric dipole radiation intensity which could be defined by following formula:

The fluorescence branching ratio (β_JJ’) and radiation lifetime (τ_rad) can be calculated by the following formula:

The spontaneous transition rate, branch ratio and radiation lifetime of the ³P₀ energy level in Pr,La:CaF₂ crystals are shown in Table S4. For ³P₀ energy level, the radiation lifetime of 0.6% Pr, x% La:CaF₂ (x=0, 3, 10 and 18) crystals are 137.3, 201.1, 136.3, and 168.9 μs, respectively. The radiation lifetime is longer than those of Pr³⁺:SrWO₄ (9.5 µs)^[28], Pr:KLu(WO₄)₂ (10.18 µs)^[29], Pr:YAlO₃ (19.99 µs)^[30], Pr³⁺:CaYAlO₄ (10.19 μs)^[26], Pr³⁺:LiYF₄ (37 μs), Pr³⁺:KYF₄ (54 μs) and Pr³⁺:BaY₂F₈ (54 μs)^[23]. The results represent that Pr,La:CaF₂ has high energy storage capacity, which is relatively easy to realize laser operation of ³P₀ energy level.

The visible fluorescent spectra of 0.6%Pr, x% La:CaF₂ (x=0, 3, 10, 18) crystals under excitation of 443 nm is shown in Fig. 4. It can be seen that there are six emission spectra bands, which correspond to transition of ³P₀→³ H₄(484 nm), ³P₀→³H₅ (537 nm), ³P₀→³H₆ (599 nm ), ³P₀→ ³F₂ (640 nm), ³P₀→³F₃ (700 nm), and ³P₀→³F₄ (723 nm ).

Stimulated emission cross section is one of the important parameters, which is usually used to evaluate laser performance of crystal. The stimulated emission cross section can be calculated by the Füchtbauere Ladenburg (F-L) formula combining the emission spectrum^[31]:

Where A(J→J′) is the radiative transition rates, I(λ) is the experimental fluorescent intensity at the wavelength λ, c presents the light velocity, n stands for the refractive index. The peak wavelength λ, FWHM and stimulated emission cross section σ_em are shown in Table S5. For 0.6%Pr,10%La:CaF₂ crystal, the largest stimulated emission cross section at 640 nm reaches 3.18×10^-20 cm² with FWHM of 6.8 nm, corresponding to the ³P₀→³F₂ transition. Furthermore, the stimulated emission cross section and FWHM of ³P₀→³H₆ transition are 1.36×10^-20 cm² and 16.98 nm, respectively. The FWHMs of ³P₀→³F₂ and ³P₀→³H₆ transition are larger than those of Pr:YLiF₄, Pr:LuLiF₄, Pr:BaY₂F₈, Pr:GdLiF₄ and Pr:LaF₃^{[22,31⇓-33]}. With La³⁺ ions concentration increasing, FWHM increases from 15.84 to 18.53 nm for ³P₀→³H₆ transition. FWHM of emission band is mainly broadened by the Stark splitting of different energy level. After co-doping La³⁺, La³⁺ ion interacts with the inherent electric dipole moment of the matrix, which enhances the electron- phonon interaction, resulting in the increasing of energy level splitting of La³⁺, and the closed energy levels of Stark splitting overlap each other, which lead to the broadening of corresponding emission peaks. This result indicate that co-doping with La³⁺ ions facilitate the ultrafast orange laser output of Pr:CaF₂ crystals.

From Fig. 4 and Table S5, the fluorescence intensity can be greatly enhanced by co-doping with La³⁺ ions, which is due to the fact that the [Pr³⁺-Pr³⁺] cluster is broken, more fluorescence centers are released and thus decreasing the probability of energy transfer among luminescence center ions. It is worth noting that co-doping with La³⁺ ions can significantly improve the emission coefficients. The results indicates that the Pr,La:CaF₂ crystals display great potential to realize laser operation at orange and red light.

The fluorescent lifetime decay of ³P₀→³F₂ transition at room temperature were measured by 443 nm Xenon lamp (Fig. 5). The fluorescence lifetime decay conforms to the characteristics of single exponential decay. The fitted formula is y=y0+A1e−x/t1.After co-doping with La³⁺ ions, the fluorescence lifetime Pr³⁺ ions are not affected only because the local coordination structure is changed. The fluorescence lifetimes of 0.6%Pr, x% La:CaF₂ (x=0, 3, 10, 18) crystals are fitted to be 43.75, 45.82, and 39.75 μs, respectively. The longest fluorescence lifetime of 0.6%Pr,10% La:CaF₂ reaches 45.82 μs, which is longer than those of Pr:LuLiF₄, Pr:YLiF₄, Pr:GdLiF₄ and Pr:BaY₂F₈ crystals.

A series of Pr:CaF₂ crystals by co-doping different concertation La³⁺ ions are successfully grown by temperature gradient (TGT) method. The X-ray powder diffraction, absorption spectra, fluorescent spectra and fluorescent decay lifetime are measured and analyzed at room temperature. The largest absorption cross-sections at 442/443 nm are calculated to be 1.743×10^-20 cm² of 0.6%Pr,10% La:CaF₂. The value of J-O intensity parameters Ω₂, Ω₄ and Ω₆ are fitted to 2.30×10^-20, 2.93×10^-20, and 5.84×10^-20 cm², respectively. The largest stimulated emission cross-sections of ³P₀→³H₆ and ³P₀→³F₂ transitions at 604 and 640 nm are calculated to be 1.36×10^-20 and 3.18×10^-20 cm² in 0.6%Pr,10%La:CaF₂ crystal, with FWHM of 15.3 and 3.8 nm, respectively. The largest fluorescent lifetime(τ_f) and spectral quality factor (σ_em·τ_f) of 0.6%Pr,10%La:CaF₂ of ³P₀→³F₂ transition are 45.82 μs and 145.8×10⁻²⁰ cm²·μs. These results show that 0.6%Pr,10%La:CaF₂ crystal is potential laser gain medium for 604 and 640 nm laser operation.

Supporting materials related to this article can be found at https://doi.org/10.15541/jim20220504.

Luminescence Property and Judd-Ofelt Analysis of 0.6%Pr, x%La:CaF₂Crystals

QIAN Xinyu^1,2, WANG Wudi², SONG Qingsong², DONG Yongjun⁴, XUE Yanyan², ZHANG Chenbo², WANG Qingguo², XU Xiaodong³, TANG Huili², CAO Guixin¹, XU Jun²

(1. Materials Genome Institute, Shanghai University, Shanghai 200444, China; 2. Key Laboratory of Advanced Micro-Structured Materials of Ministry of Education, School of Physics Science and Engineering, Institute for Advanced Study, Tongji University, Shanghai 200092, China; 3. Jiangsu Key Laboratory of Advanced Laser Materials and Devices, School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, China; 4. Nanjing Metalaser Photonics Company Ltd., Nanjing 210038, China)