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Abstract
High quality 0.3 at.% Pr, 2.5 at.% Yb:BaF2 and 0.6 at.% Pr, 3 at.% Yb:BaF2 single crystals were grown by the temperature gradient technique (TGT). The absorption spectra, fluorescence spectra, fluorescence decay curves and energy transfer process are measured and analyzed at room temperature. By co-doping Yb3+ ions as a sensitizer of Pr3+ ions, the broad near-infrared band emission from 1G4 energy level at 1.3 µm is observed in the Pr3+/Yb3+: BaF2 crystal with low maximum phonon energy for the first time. In addition, the visible fluorescence emission can be detected and the luminescence components are mainly blue-green transition. The emission cross sections of 1G4→3H5 and 3P0→3H4 transitions are calculated to be 0.21 × 10−20 cm2 and 1.12 × 10−20 cm2 with the larger FWHM of 103.8 nm and 4.2 nm in 0.6 at.% Pr, 3 at.% Yb:BaF2 crystal, respectively. The luminescence lifetime of 1G4 and 3P0 energy levels are fitted to be 259.6 µs and 126.1 µs in 0.3 at.% Pr, 2.5 at.% Yb:BaF2 crystal, which is much increased by Yb3+ ions co-doping. The energy transfer efficiency in BaF2 crystal from Yb3+ to Pr3+ ions is calculated to be 60.3% and 70.5%, respectively. All results show that the Pr3+/Yb3+:BaF2 crystal is a promising candidate of laser gain medium for near-IR laser at 1.3 µm and blue laser operation.
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1. Introduction
The trivalent Pr3+ ion, due to the abundant energy level structure and the potential laser emission from ultraviolet (UV) to mid-infrared (MIR), has been extensively studied [1,2]. Especially in the visible region, Pr3+ ion-doped fluoride materials due to low phonon energy and weak lattice field have achieved high-efficiency watt-scale laser output from blue (3P0→3H4), green (3P0,1→3H5), orange (3P0→3H6), red (3P0→3F2) deep red (3P0→3F3,4) [3–5]. Furthermore, Pr3+ ions doped LiYF4 (LYF) crystal have achieved a quasi-continuous 8.1W laser output with a slope efficiency of 51.5% in the 640 nm band, which is the highest output power among all materials [6]. Besides, the maximum output power reaches up to 2.4 W at 915 nm, corresponding to 3P0→1G4 transition [7]. As we concerned, other than 3P0 energy level, such as1D2 and 1G4 energy levels are usually used as the broad band near-infrared transitions. Particularly the 1G4 energy level is able to meet demands of optical fiber communication at 1.3 μm, which also has significant prospects for application [8].
The development of the 1D2 energy level of Pr3+ ions has been limited due to the lack of efficient pump sources. In the past, only a low threshold laser output at 1 µm by 592 nm dye laser pumping has been achieved in silica based fiber due to the 1D2→3F3 + 3F4 transition [9]. In 2012, X. Liu et al. reported ultra-broadband near-infrared emission of 1D2→1G4 in Pr3+ ion doped germanium tellurite glasses for optical fiber amplifier operating [10]. In 2019, the spectral properties of tellurite glasses doped with Pr3+, Nd3+ and Yb3+ ions in the near-infrared region are compared [11]. Besides, spectral property of near infrared region only in Pr3+-doped LYF and GdScO3 crystal has been reported [12–14]. Most of the near infrared region spectral properties of Pr3+ ions doped crystal are not discussed.
More interestingly, the1G4 level also acts as a laser upper level, which has broadband transition emission from the near-infrared (NIR) region to the mid-infrared (MIR) region. It is noted that the specific transition of 1G4→3H5 at 1.3 µm is one of the two central wavelengths used in conventional optical telecommunication system. In 1991, laser output at 1294 nm in a praseodymium-doped ZBLAN fiber has been demonstrated by Y. Durteste et al. [ 15], and spectral performance of Pr3+ doped doped fluoride fiber at 1310 nm has also been reported by Yasutake Ohishi et al. [16]. In 1994, M. A. Newhouse et al. found that the fluorescence lifetime of 1G4 level in Pr3+-doped Mixed-Halide Glasses is three times larger than fluorozirconate, which is especially helpful for 1300 nm fiber amplifier [17]. In 1996, the lifetime of 1G4 level, optical and thermal characteristics of Pr3+-doped GeS2-chalcohalide glasses have been analyzed by Diego Marchese et al. [18]. In 1997, L.B. Shaw reported infrared spectroscopy of the 1G4 transitions for laser and amplifier in Pr3+ doped heavy metal selenide glasses (BaInGaGeSe) [19]. In addition, H.W. Song also reported Ni2+ and Pr3+ Co-doped CsPbCl3 perovskite quantum dots with efficient infrared emission at 1300 nm in 2021 [20].
The reason why the 1G4 energy level of Pr3+ ions cannot be used for efficient signal amplification is that the absorption of the 1G4 energy level is very weak, only one-tenth absorption of the3P0 energy level. Spectral properties and laser output of 1G4 energy level in Pr3+ ion-doped crystals have not been reported. The problem of weak absorption of 1G4 level in Pr3+ ions doped crystal can be solved by sensitization of Yb3+ ions due to matched energy levels between Pr3+ and Yb3+ ions. BaF2 crystal is a good laser gain material, due to low maximum phonon energy (∼242 cm-1) [21], wide transmission range (from 250 nm to 15 µm) [22], relatively high thermal conductivity [23] and stable mechanical properties, which has been widely concerned and studied.
In this work, 0.3 at.% Pr, 2.5 at.% Yb:BaF2 and 0.6 at.% Pr, 3 at.% Yb:BaF2 crystals were grown by temperature gradient technique (TGT). The broadband emissions of 1G4 energy level and up-conversion emission ware observed under 976 nm InGaAs Laser diode (LD) excitation. In addition, spectral characteristics and energy transfer of Pr3+/Yb3+:BaF2 crystals are particularly investigated and analyzed by Judd–Ofelt theory.
2. Experiment
High quality 0.3 at.% Pr, 2.5 at.% Yb:BaF2 and 0.6 at.% Pr, 3 at.% Yb:BaF2 crystals were successfully grown by temperature gradient technique (TGT) with an inductive heated growth furnace. The PrF3, YbF3 and BaF2 powders with purity of 99.99% are precisely prepared and weighted according to the stoichiometric formulation PrxYbyBa1-x-yF2 (x = 0.3,0.6; y = 2.5,3). The raw ingredients are placed in cylindrical porous carbon crucible after well grounded and mixed in an agate mortar. After entire chamber is evacuated to be less than 8 Pa, the high purity argon is used as the protective atmosphere at 110 KPa pressure. The melt of raw powder, equal diameter growth and cooling down to the room temperature are three processes of crystal growth. The raw materials were melted at 1400 °C for 12 hours. The equal diameter growth is a critical stage that is controlled by slowly decreasing temperature at the rate of 2°C/hour. After the equal diameter growth, the crystals are cooled to room temperature in about 1000 minutes, and then the crystals are taken out. The Pr3+/Yb3+:BaF2 crystal samples for various spectral measurements are cut and double-side polished in the size of Φ15 × 1.5 m, without inclusion and graphite. The polished thin slices are shown in Fig. 1.
The crystal structure of grown sample is studied by an X-ray powder diffraction ((XRD, Bruker-D2, Germany) in 2θ range from 20° to 80° at room temperature. The actual concentrations of Pr3+ and Yb3+ ions in BaF2 crystals are tested by the inductively coupled plasmas atomic emission spectroscopy (ICP-AES) method.The absorption spectra are measured by a spectrometer (Cary 5000, UV-VIS-IR) from 400 to 2500 nm at the step of 1 nm. The fluorescence spectra as well as decay lifetime curves are attained by a FLS-1000 fluorescence spectrometer (Edinburgh Company, English) by 980 nm InGaAs laser diode (LD) excitation. All the measurements are carried out at room temperature.
3. Results and discussions
3.1 Crystal structure and ICP-AES measurement
The XRD pattern of Pr3+/Yb3+:BaF2 crystals and the standard pattern of BaF2 phase are shown in Fig. 2. The diffraction peaks of PrxYbyBa1-x-yF2 (x = 0.3,0.6; y = 2.5, 3) crystals are identical to pure BaF2 crystal (PDF#04-0452), which shows that the as-grown crystals belong to the space group Fm-3 m (225) and crystalline phase is not changed by Pr3+ and Yb3+ ions doping. According to the obtained pattern, the lattice parameters of 0.3 at.% Pr, 2.5 at.% Yb:BaF2 and 0.6 at.% Pr, 3 at.% Yb:BaF2 crystals are calculated to be a = b = c = 6.183 Å and 6.179 Å, which are quite close to pure BaF2 crystal (a = b = c = 6.200 Å). The small lattice cell is due to the fact that the ionic radius of Pr3+ ions (0.99 Å) and Yb3+ ions (0.868 Å) are smaller than Ba2+ ions (1.35 Å). The actual concentration and ions number of Pr3+ and Yb3+ in BaF2 crystals are listed in Table 1, which is close to the design concentration.
Table 1. Actual doping concentration and ions number of Pr3+ and Yb3+ in BaF2 crystals
3.2 Absorption spectra
The transmittance spectra of Pr3+/Yb3+:BaF2 crystals with 1.5 mm thick are tested in left of Fig. 3. A transmittance of up to 90% indicates that the crystal is a high-quality crystalline. Besides, the absorption spectra of different concentration with the range from 420 nm to 2500 nm are calculated and shown in right of Fig. 3. As we can see, there are six main absorption bands of Pr3+ ion from the visible to infrared spectral region, which corresponds to the transition from the 3H4 ground state to 3P2, 3P1, 3P0, 1D2, 3F4 + 3F3 and 3F2 + 3H6 excited states. In addition, there is an obvious absorption band of Yb3+ ion at ∼976 nm, corresponding to the 2F7/2 →2F5/2 transition and matching the emission wavelength of the high-efficiency and commercial InGaAs laser diode (LD). For different doping concentration PrxYbyBa1-x-yF2 (x = 0.3,0.6; y = 2.5,3) crystals, the absorption coefficient at 976 nm are calculated to be 3.09 cm-1 and 3.56 cm-1. Furthermore, the absorption cross-section can be calculated by the following equation [24]:
Fig. 3. Room temperature transmittance spectra (left) and absorption spectra (right) of Pr3+/Yb3+:BaF2 crystals.
3.3 Judd-Ofelt analysis
Judd–Ofelt (J–O) theory is a widely adapted method to analyze spectroscopic property of rare earth doped crystals, glass and powder samples [25–27]. For Pr3+ ion, only the electric dipole transitions are taken into account in the J-O theory since the magnetic dipole transitions are not satisfied with transition selection rules. As shown in Fig. 3, absorption bands such as 3P1 + 1I6, 3F3 + 3F4 and 3H4 + 3F2 are overlapped, which are often regarded as a band. The reduced matrix element of unit tensor operator is almost independent of the host crystal, which is referred to Ref. [28]. The sum of corresponding matrix element is applied for overlapping absorption transitions. According to the J-O theory, the average wavelength, the experimental line strength Sexp(J-J′) and theoretical line strength Scal(J-J’) of electric dipole transitions from 3H4 level to excited stated level is calculated and listed in Table 2. In addition, three J-O intensity parameters Ω2,4,6 are obtained and shown by the least square fitting between the measured line strength and reduced matrix elements. Especially, the Ω2 is related with covalency and symmetry of coordination structure around the Pr3+ ions, Ω4 and Ω6 represent the rigidity of matrix, not directly related to the ligand symmetry of rare earth ions. The value of the root mean square(RMΔS) deviation between experimental and calculated line strengths are 0.352 × 10−20 cm2 and 0.606 × 10−20 cm2. The relatively small values indicate a reasonable result.
Table 2. Average wavelength, the experimental line strength Sexp(J-J′) and the theoretical line strength Scal(J-J′), intensity parameters Ω2,4,6 and root mean square(RMΔS) deviation of Pr3+/Yb3+:BaF2
3.4 Fluorescent spectra and up-conversion luminescence
The near-infrared fluorescent spectra of Pr3+/Yb3+:BaF2 crystal in the range of 1200-1500 nm are measured under the excitation of 976 nm, which is shown in Fig. 4. It is clear that the broad band emission at 1210-1500 nm is observed in Pr3+/Yb3+:BaF2 crystals, which derives from1G4→3H5 transition. The peak wavelength of emission spectra is 1316 nm. The broad emission band of 1G4→3H5 has important application prospect, which can meet the demand the application of 1.3 µm fiber amplifier to a large extent.
Besides, the visible fluorescent spectra from 450 nm to 800 nm of Pr3+/Yb3+:BaF2 crystals are also measured by 976 nm laser diode (LD) excitation, which is shown in Fig. 5. As we can see, there are four main emission bands from 3P0 energy level centered at blue 480 nm, green 543 nm, orange 606 nm and deep red 750 nm, corresponding to 3P0→3H4, 3P0,1→3H5, 3P0→3H6, and 3P0→3F4 transition, respectively. In order to better understand the up-conversion (UC) luminescence mechanism of 3P0 energy level, the dependence of up-conversion emission intensity on pump power at 976 nm has been measured. The up-conversion emission intensity (IUC) is directly proportional to pump power (Ppump), which is shown by the relationship of IUC ∝ (Ppump)n [29]. The slope of up-conversion emission intensity is a function of pump power in logarithmic form, which is used to calculate the involved photon number. In Fig. 6, the fitting linear slope values of 480 nm (3P0→3H4), 544 nm (3P0,1→3H5), and 750 nm (3P0→3F4) emission bands are 1.95, 1.48, and 1.38, respectively, which indicates that the two- photon pumping process is required for the up-conversion emission of Pr3+:3P0 energy level.
According to characterized fluorescent spectra, the schematic diagram of energy transfer in Pr3+/Yb3+:BaF2 crystal is shown in Fig. 7. Under the mature 976 nm LD excitation, Yb3+ ions absorb a photon from 2F7/2 to 2F5/2 energy level and transfers energy to 1G4 energy level of Pr3+ ions by cross relaxation process [Yb3+ (2F5/2) + Pr3+ (3H4) → Yb3+ (2F7/2) + Pr3+ (1G4)], which increases the particle population of 1G4 energy level. After that, the abundant particles of 1G4 energy level transition to3H5 energy level for broad near infrared transition emission. In addition, some particles of 1G4 energy level also directly absorb the pumping photon to reach the 3P0 energy level of Pr3+ ions (so-called 1G4 energy level excited state absorption), realizing the blue-green light emission. There is no literature report that Yb3+ ions are used as a sensitizer of Pr3+ ions to achieve efficient broadband fluorescence emission of 1G4 energy level.
The Füchtbauere Ladenburg (F-L) formula can be used to calculate the stimulated emission cross section [32], which is an important indicator to evaluate potential laser performance [30].
Table 3. The emission peak wavelength λ, FWHM and emission cross section σem of Pr3+/Yb3+ -doped BaF2 crystal
3.5 Fluorescence lifetime
The fluorescence decay curves of 1G4 and 3P0 energy level in Pr3+/Yb3+:BaF2 crystals are shown in Fig. 8. The luminescence lifetime of 1G4 energy level is fitted to be 259.6 µs and 195.8 µs for 0.3 at.% Pr, 2.5 at.% Yb:BaF2 and 0.6 at.% Pr, 3 at.% Yb:BaF2, respectively. Besides, the decay lifetime of 3P0 energy level are fitted to be 126.1µs and 80.8 µs by single exponent form, which is much higher than that of LiYF4 (35.7 µs) [31], Sr0.5La0.5Mg0.5Al11.5O19 (39.6 µs) [32] and YAlO3 (19.2 µs) [33] crystals. The long fluorescence lifetime is related to long decay of Yb3+ ions 2F5/2 energy level and energy transfer from Yb3+ ions to Pr3+ ions, compared with single doped crystal. The long luminescence lifetime of laser upper energy level is beneficial to population inversion and energy storage, which shows that Pr3+/Yb3+:BaF2 crystal may be a potential laser material. Besides, the energy transfer efficiency between Yb3+ and Pr3+ ions is calculated by the following formula [34]:
where ${\tau _{Yb,Pr}}$ is the lifetime of the donor in the presence of acceptor, and ${\tau _{Yb\; }}$ is lifetime of the donor in the absence of acceptor. In Pr3+/Yb3+ co-doped and Yb3+ single doped systems, the luminescence lifetime of Yb3+ ions are 833.9 µs and 2.1 ms in 0.3 at.% Pr, 2.5 at.% Yb:BaF2 crystal, 488.1 µs and 1.6 ms in 0.6 at.% Pr, 3 at.% Yb:BaF2 crystal. The energy transfer efficiency from Yb3+ to Pr3+ ions is calculated to be 60.3% and 70.5%, respectively. The high efficiency energy transfer is very helpful for ∼1.3 µm emission and up-conversion emission of 3P0 energy level.4. Conclusion
The high quality 0.3 at.% Pr, 2.5 at.% Yb:BaF2 and 0.6 at.% Pr, 3 at.% Yb:BaF2 single crystals were grown by temperature gradient technique (TGT). The absorption spectra, fluorescence spectra, fluorescence decay curves and energy transfer process were measured and characterized at room temperature. By co-doping Yb3+ ions as sensitizer of Pr3+ ions, the broad near-infrared band emission from 1G4 energy level at 1.3 µm can be observed in the Pr3+/Yb3+:BaF2 crystal with low maximum phonon energy. In addition, the visible fluorescence emission can be detected and the luminescence component are mainly blue-green transition. The emission cross sections of 1G4→3H5 and 3P0→3H4 transitions are calculated to be 0.21 × 10−20 cm2 and 1.12 × 10−20 cm2 with the larger FWHM of 103.8 nm and 4.2 nm in 0.6 at.% Pr, 3 at.% Yb:BaF2 crystal, respectively. The luminescence lifetime of 1G4 energy level is fitted to be 259.6 µs and 195.8 µs in 0.3 at.% Pr, 2.5 at.% Yb:BaF2 and 0.6 at.% Pr, 3 at.% Yb:BaF2 crystals. Besides, the up-conversion luminescence lifetime of 3P0 energy level are fitted to 121.6 µs and 80.8 µs, which is much increased by Yb3+ ions co-doping. The energy transfer efficiency from Yb3+ to Pr3+ ions is calculated to be 60.3% and 70.5% in 0.3 at.% Pr, 2.5 at.% Yb:BaF2 and 0.6 at.% Pr, 3 at.% Yb:BaF2 crystals. All of favorable properties show that the Pr3+/Yb3+:BaF2 crystal is a potential laser gain medium for near-IR laser at 1.3 µm and blue laser operation.
Funding
National Key Research and Development Program of China (2022YFB3605701); National Natural Science Foundation of China (No.61621001, No.61805177, No.62075166); Fundamental Research Funds for the Central Universities (No.22120210432, No.22120220252).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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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