11. Materials Genome Institute, Shanghai University, Shanghai 200444, China
22. 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
33. Jiangsu Key Laboratory of Advanced Laser Materials and Devices, School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, China
44. Nanjing Metalaser Photonics Company Ltd., Nanjing 210038, China
Calcium fluoride (CaF2) is an attractive laser material due to its broad transmission range (0.125-10 µm), good thermal conductivity (9.71 W/(m·K)) and low coefficient of nonlinear effect. In Pr:CaF2 crystal, the [Pr3+-Pr3+] cluster lead to fluorescence quenching at lower concentrations of Pr3+ ions. Therefore, co-doping of La ions in CaF2 to break the [Pr3+-Pr3+] cluster deserves further exploration. In this work, a series of Pr:CaF2 single crystals co-doped with different concertation of La3+ ions were successfully grown by temperature gradient technique (TGT). The X-ray powder diffraction, absorption spectra, fluorescence spectra and fluorescence decay lifetime of Pr,La:CaF2 crystals were measured. Obtained data demonstrated that the Pr:CaF2 crystals still had the cubic crystal structure after co-doped with La3+ ions. The largest stimulated emission cross-sections of 3P0→3H6 (604 nm) and 3P0→3F2 (640 nm) transitions were calculated to be 1.36×10-20 cm2 and 3.18×10-20 cm2 with FWHM of 17.0 nm and 3.8 nm, respectively. With the increase of La3+ ion concentration, the value of FWHM increased from 15.84 nm to 18.53 nm of 3P0→3H6. The largest fluorescent lifetime and spectral quality factor of 0.6%Pr, 10%La:CaF2 (atom fraction) is 45.82 μs and 1.458×10-18 cm2·μs, respectively. All above results show that the [Pr3+-Pr3+] ions quench clusters are broken by La3+ ions. The Pr, La:CaF2 crystal is a potential laser gain material for orange-red laser operation.
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 (3P0,1→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 U3+[13] and Sm2+[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.
1 Experimental
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
Figure 1.As-grown 0.6%Pr, x%La:CaF2 (x=3, 10, 18) single crystals
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).
2 Results and discussion
2.1 XRD and ICP-AES analysis
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].
Figure 2.XRD patterns of Pr,La:CaF2 crystals and standard pattern of CaF2 crystal
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.
2.2 Absorption spectra
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 σabs(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.
Figure 3.Absorption spectra of Pr,La:CaF2 crystals at room temperature Colorful figure is available on website
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 Scal(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, Ωt(t = 2, 4, 6) are the J-O intensity parameters.
The experiments line strength Sexp(J→J') of ED transitions can also be expressed as following formula:
π
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 Sexp(J→J') and calculation line strengths Scal(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 Sexp(J, Jʹ), calculated line strength Scal(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 Ωt (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 Ωt (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:
π
Where (t=2,4,6) is the emission squared reduced matrix elements of the tensorial operator, which is shown in Ref. [21]. Aed is the probability of electric-dipole transitions. is the probability of magnetic-dipole transitions. Smd is the magnetic dipole radiation intensity, which is an independent constant. Smd is ignored due to the forbidden of magnetic-dipole transitions for Pr3+ ions. Sed is the electric dipole radiation intensity which could be defined by following formula:
The fluorescence branching ratio (βJJ’) and radiation lifetime (τrad) 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.
2.4 Fluorescent spectra
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 ).
Figure 4.Fluorescent spectra of Pr,La:CaF2 crystals at room temperatureColorful figure is available on website
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 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 σem 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.
2.5 Fluorescent lifetime
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 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.
Figure 5.Fluorescence decay lifetime of 3P0 energy of 0.6%Pr, x%La:CaF2 crystals at room temperature(a) x=0; (b) x=3; (c) x=10; (d) x=18; Colorful figure is available on website
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(τf) and spectral quality factor (σem·τf) 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
Supporting materials related to this article can be found at
https://doi.org/10.15541/jim20220504.
Supporting Materials:
Luminescence Property and Judd-Ofelt Analysis of 0.6%Pr, x%La:CaF2Crystals
QIAN Xinyu1,2, 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)
Table 1.
Actual concentration of Pr3+ and La3+ ions in CaF2 crystals
Peak wavelength λ, FWHM and stimulated emission cross section σem
Concentration of La3+ ion
Transition
λem/nm
FWHM/nm
σem/(×10-20, cm2)
0.6%Pr:CaF2
3P0→3H4
482
5.20
0.77
3P0→3H5
535
26.68
0.00
3P0→3H6
605
15.84
1.14
3P0→3F2
640
5.13
0.96
3P0→3F3+3F4
723
4.51
0.59
0.6%Pr,3%La:CaF2
3P0→3H4
484
6.76
1.96
3P0→3H5
537
27.45
0.00
3P0→3H6
599
15.30
1.35
3P0→3F2
640
3.12
1.51
3P0→3F3+3F4
723
5.21
2.48
0.6%Pr,10%La:CaF2
3P0→3H4
484
8.36
2.52
3P0→3H5
535
26.76
0.00
3P0→3H6
599
16.98
1.36
3P0→3F2
640
3.80
3.18
3P0→3F3+3F4
723
5.49
3.03
0.6%Pr,18%La:CaF2
3P0→3H4
482
9.57
1.22
3P0→3H5
537
26.25
0.00
3P0→3H6
606
18.53
1.24
3P0→3F2
640
3.80
2.92
3P0→3F3+3F4
723
7.38
1.40
Table 6.
Wavelength λ, FWHM, stimulated emission cross section σem and spectral quality factor σem·τf for the transition of 3P0→3H6 and 3P0→3F2 of Pr,La:CaF2 and other crystals
Wavelength λ, FWHM, stimulated emission cross section σem and spectral quality factor σem·τf for the transition of 3P0→3H6 and 3P0→3F2 of Pr,La:CaF2 and other crystals
Crystal
Transition
FWHM/nm
σem/(×10-20, cm2)
τ/μs
σem·τf/(×10-20, cm2·μs)
Ref.
0.6%Pr:CaF2
3P0→3H6
15.84
1.14
45.30
51.65
This work
3P0→3F2
5.13
0.96
43.53
0.6%Pr,3%La:CaF2
3P0→3H6
15.3
1.35
45.33
61.20
3P0→3F2
3.12
1.51
68.65
0.6%Pr,10%La:CaF2
3P0→3H6
16.98
1.36
45.82
62.32
3P0→3F2
3.80
3.18
145.79
0.6%Pr,18%La:CaF2
3P0→3H6
18.53
1.24
39.75
49.29
3P0→3F2
3.80
2.92
116.10
LaF3
3P0→3H6
3
3.3
51.42
169.67
[22]
3P0→3F2
7.7
7.56
388.70
LuLiF4
3P0→3H6
1.2
12
37.90
454.80
[33]
3P0→3F2
0.7
21
795.90
YLiF4
3P0→3H6
1.4
14
35.70
499.80
[33]
3P0→3F2
0.7
22
785.40
GdLiF4
3P0→3H6
1.3
13
43.61
566.91
[33]
3P0→3F2
-
23
1003.00
BaY2F8
3P0→3H6
1.2
24.7
43.00
1062.10
[32]
3P0→3F2
0.6
12.1
520.30
[1] T H MAIMAN. Stimulated optical radiation in ruby. Nature, 4736:, 493(1960).
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[33] F CORNACCHIA, LIETO A DI, M TONELLI et al. Efficient visible laser emission of GaN laser diode pumped Pr-doped fluoride scheelite crystals. Optics Express, 15932(2008).
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Xinyu QIAN, Wudi WANG, Qingsong SONG, Yongjun DONG, Yanyan XUE, Chenbo ZHANG, Qingguo WANG, Xiaodong XU, Huili TANG, Guixin CAO, Jun XU. Luminescence Property and Judd-Ofelt Analysis of 0.6%Pr, x%La:CaF2 Crystals [J]. Journal of Inorganic Materials, 2023, 38(3): 357