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Tm3+ grown by Bridgman method
Chemical Physics Letters 652 (2016) 68–72
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Fluorescence spectra of Na5Lu9F32 single crystals co-doped with Ho3+/Tm3+ grown by Bridgman method Zhigang Feng a, Haiping Xia a,⇑, Cheng Wang a, Zhixiong Zhang a, Dongsheng Jiang a, Jian Zhang a, Shinan He a, Qingyang Tang a, Qiguo sheng a, Xuemei Gu a, Yuepin Zhang a, Baojiu Chen b,⇑, Haochuan Jiang c a
Key laboratory of Photo-electronic Materials, Ningbo University, Ningbo, Zhejiang 315211, China Department of Physics, Dalian Maritime University, Dalian, Liaoning Province 116026, China c Ningbo Institute of Materials Technology and Engineering, The Chinese Academy of Sciences, Ningbo, Zhejiang 315211, China b
a r t i c l e
i n f o
Article history: Received 29 February 2016 Revised 2 April 2016 In final form 9 April 2016 Available online 12 April 2016 Keywords: Optical materials Crystal growth X-ray diffraction Optical properties Luminescence Na5Lu9F32 Energy transfer
a b s t r a c t This work presents the luminescent properties of Ho3+/Tm3+ co-doped Na5Lu9F32 single crystals that were grown by an improved Bridgman method for the first time. The J–O intense parameters of Ho3+ ions were calculated. An intense 2.0 lm emission was achieved with Tm3+ ions sensitizing Ho3+ ions by the processing of energy transfer from Tm3+ ions to Ho3+ ions under excitation of 800 nm LD. The maximum emission intensity at 2.0 lm is obtained, and the cross sections of Tm3+ ions and Ho3+ ions were calculated. The physical mechanism for energy transfer from Tm3+ to Ho3+ ions was analyzed by using Inokuti–Hirayama’s model. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Recent years, the development of 2 lm solid state lasers has attracted much attention due to their extensive applications including remote sensing, environment monitoring, military weapons, eye-safe laser radars et al. [1–3]. The rare earth doped single crystals have been practically applied in 2 lm solid state lasers because of their excellent physical–chemical properties and high luminous efficiency [4,5]. Among rare earth ions, the transition 5 I7 ? 5I8 of Ho3+ ion is well known for generating laser with 2.0 lm radiation, which has been reported in many single crystals [6,7]. Tm3+ ion has a strong absorption band at 800 nm that overlaps with the emission band of the commercially available AlGaAs laser diodes. As sensitizer for Ho3+ ion, Tm3+ ion can increase the pumping efficiency of 800 nm LD via energy transfer from Tm3+ ions to Ho3+ ions due to the very close energy levels between Tm3+:3F4 and Ho3+:5I7. Such performance between Ho3+ ions and Tm3+ ions has been demonstrated in previous studies [8].
⇑ Corresponding authors. E-mail addresses: [email protected] (H. Xia), [email protected] (B. Chen). http://dx.doi.org/10.1016/j.cplett.2016.04.025 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.
The host materials also play an important role in the optical properties. In order to gain highly efficient laser operation with low phonon energy and long lifetime of the excited electronic states, the fluoride hosts were investigated in previous researches [9,10]. The forms of these materials include nano-crystals, glasses, and ceramics. Generally, powders have strong scattering and low transmission, while glasses or ceramics have less chemical stability and lower luminescent efficiency. These disadvantages blocked the utilization of above materials in practical applications. In contrast, the high transparency and luminescence efficiency of single crystal make it more favorable as the host material for practical applications in optical advices. More recently, we have prepared aNaYF4, LiYF4 and LiLuF4 crystals in our previous studies [6,7,11]. Na5Lu9F32 is a kind of fluoride compounds with good physical– chemical properties and thermal stability [12]. As a host material, a high doping concentration for trivalent rare earth ions which take the place of the Lu3+ ions could be obtained in Na5Lu9F32 single crystal due to the similar size of rare earth ions. It is the lower phonon energy (440 cm1) and higher optical transparency in the range of infrared that make the Na5Lu9F32 single crystal more favorable as potential host material for practical applications in advices for the mid-IR solid state lasers. However, the study of
Z. Feng et al. / Chemical Physics Letters 652 (2016) 68–72
rare-earth ions doped Na5Lu9F32 matrix has been mainly focused on nano-crystallites [12,13]. There are scarcely any reports about growing bulk Na5Lu9F32 single crystals because of the difficulties in the crystal growth. In this work, Ho3+/Tm3+ co-doped Na5Lu9F32 single crystal was grown by an improved Bridgman method for the first time. The DTA/TG and refractive index curves were measured. And the density of the single crystal is 5.88 g/cm3 measured by Archimedes method. The optical spectra of Na5Lu9F32 single crystals with constant Tm3+ ions concentration of 1.3% and various Ho3+ ions concentrations from 0% to 2% were investigated. By analyzing the fluorescence properties, the mechanism of 2.0 lm emission in Ho3+/Tm3+ co-doped Na5Lu9F32 single crystal was revealed. 2. Experimental procedures The preparation of the material was from commercially available NaF and LuF3 powders of high purity (99.999%). The molar compositions of the raw materials were NaF:LuF3: TmF3 = 40:58.7:1.3, NaF:YF3:TmF3:HoF3 = 40:58.7 v:1.3:v (v = 0.5, 1, 1.5, 2) and NaF:LuF3:HoF3 = 40:59:1. The obtained samples were named, respectively, as SNT1, SNTH1, SNTH2, SNTH3, SNTH4, and SNH1. The initial compounds were mixed and ground sufficiently in a porcelain bowl, and put the mixtures into apparatus for fluoridation process about 6 h at 780 °C to remove the moisture and oxide from the mixture completely. The Na5Lu9F32 single crystals were grown in a resistively heated vertical Bridgman furnace. The temperature gradient cross solid–liquid interface was 60– 80 °C/cm. The seeding temperature is about 860–880 °C. The growth process was carried on by lowering the crucible at a rate of 0.06 mm/h. The detailed process is similar to the report of growth of NaYF4 single crystal [6]. The seed of pure a-NaYF4 single crystal with a-axis direction was placed at the bottom of the crucible. The grown crystal with about £10 mm 50 mm is shown in Fig. 1(a). The crystals were sliced into pieces and polished to be about 2.0 mm-thickness for the optical measurements. The crystal structure was performed by X-ray diffraction (XRD) pattern with XD-98X diffractometer (XD-3, Beijing). The concentrations of Ho3+ ions and Tm3+ ions in these samples were confirmed severally with an inductively coupled plasma (ICP) atomic emission spectroscopy (ICP-AES, PerkinElmer Inc., Optima 3000). The absorption spectra were measured with a Cary 5000 UV/VIS/NIR spectrophotometer in the wavelength region from 200 nm to 2500 nm. The emission spectra were recorded under the excitation of 800 nm LD by a Triax 320 spectrometer in the wavelength range of 1000–2200 nm. The fluorescence lifetimes were obtained with the FLSP920 fluorescence
Fig. 1. DTA/TG curves of Ho3+/Tm3+ doped Na5Lu-9F32 crystals.
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spectrophotometer. All the measurements were carried out at room temperature under the room temperature.
3. Results and discussion 3.1. DTA/TG and refractive index analysis Fig. 1 presents the DTA/TG curves of the powder ofNa5Lu9F32 single crystal. One can be noted from the TG curve in Fig. 1 that there is 1.3% weight loss in the temperature region of 25–916 °C, and then a huge weight loss, 21.7%, from 916 to 1017 °C. It can be seen from the DTA curve in Fig. 1 that as the temperature rises, the sample begins to absorb heat at about 35 °C and shows an endothermic peak about at 80 °C, which is related to the endotherm of H2O impurity mixed during the grind. As the temperature further rises, a strong endotherm at about 916.52 °C appears, and the weight shows a huge loss, which is caused by the heat absorbance due to the melting and vaporization of the crystal. And the refractive index of Na5Lu9F32 crystals in all the calculation is 1.44 as we measured. The maximum phonon energy of Na5Lu9F32 is also estimated to be 440 cm1 from the measured Raman spectrum, and there exists a high transmittance from 2500 nm to 7150 nm from measured infrared transmittance spectrum of Na5Lu9F32 single crystal. The excellent properties provide potential application for 2 lm mid-infrared laser material.
3.2. X-ray diffraction The measured XRD pattern for Ho3+/Tm3+ co-doped Na5Lu9F32 crystal (SNTH2) is presented in Fig. 2(b). The pattern appearing in JCPDS card (No. 27–0725) for Na5Lu9F32 is also shown in Fig. 2(c) for comparison. It is believed that all the diffraction peak positions of obtained samples doped with Ho3+/Tm3+ ions are matched perfectly with that in standard Na5Lu9F32, and there are no any other peaks belong to impurity phase from Fig. 2, which indicates that the obtained transparent crystals are cubic phase Na5Lu9F32.
a d ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 h þk þl
ð1Þ
According to formula (1) [14], the cell parameters of SNTH2 were calculated to be a = b = c = 0.5453 nm.
Fig. 2. (a) Photograph of Ho3+/Tm3+ co-doped Na5Lu9F32 single crystal. (b) XRD pattern of the Na5Lu9F32: Ho3+/Tm3+. (c) Standard line pattern of the Na5Lu9F32.
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Z. Feng et al. / Chemical Physics Letters 652 (2016) 68–72 Table 1 Molar fractions of Ho3+ and Tm3+ in feed material and concentration of Ho3+ ions and Tm3+ ions in all crystal samples. Samples
Molar fraction of Tm3+
1020 N (Tm3+)
Molar fraction of Ho3+
1020 N (Ho3+)
SNT1 SNTH1 SNTH2 SNTH3 SNTH4 SNH1
1.3% 1.3% 1.3% 1.3% 1.3% 0%
2.90 2.91 2.89 2.91 2.92 0
0% 0.5% 1% 1.5% 2% 1%
0 1.12 2.10 3.37 4.45 2.13
Table 2 Measured and calculated oscillator strength of Ho3+ ions in SNTH2 sample.
Fig. 3. Absorption spectra of Tm3+, Ho3+ singly doped and Ho3+/Tm3+ co-doped Na5Lu9F32 single crystals.
3.3. Absorption spectra and Judd–Ofelt analysis The measured concentrations of Ho3+ and Tm3+ are shown in Table 1. Fig. 3 shows the absorption spectra of Tm3+, Ho3+ singly doped Ho3+/Tm3+ co-doped Na5Lu9F32 samples. The characteristic absorption bands corresponding to the transition of Ho3+ ions and Tm3+ ions from the ground state to the excited ones are marked in spectra. Compared with the absorption spectra of Tm3+, Ho3+ singly doped Na5Lu9F32 crystals, the Fig. 3 shows that the position of all absorption peaks of Ho3+ ions and Tm3+ ions do not change any more with the doping of Ho3+ ions, which means that the absorption spectra of Ho3+/Tm3+ co-doped Na5Lu9F32 crystals is the superposition of Ho3+ ions and Tm3+ ions singly doped Na5Lu9F32 crystals. Fig. 4 shows the absorption spectra of different doping concentrations of Ho3+ ions in Ho3+/Tm3+ co-doped Na5Lu9F32 single crystals. As we can see from the Fig. 4, the corresponding absorption coefficient is proportional to the Ho3+ ions doping concentration. The intense absorption band and the widest full width at half maximum(FWHM) in all absorption bands centered at the approximately 1931 nm of the all samples indicates that the codoped Ho3+ ions can act as an efficient absorber of the energy from Tm3+:3F4 excited state to Tm3+:3H6 ground state. The Judd–Ofelt theory is extensively employed by determining the radiative properties of rare-earth ions within a matrix based on its absorption spectrum. According to the Judd–Ofelt theory
Absorption
Wavelength (nm)
I8 ? 5I7 5 I8 ? 5I6 5 I8 ? 5F5 5 I8 ? 5F4 5 I8 ? 5F3 5 I8 ? 5G6 5 I8 ? (5G, 3G)5 drms (107)
1927 1151 641 535 482 449 415 3.1
5
Oscillator strength (106) fexp
fcal
0.685 0.321 1.339 1.98 0.35 4.396 1.19
0.65 0.488 1.418 1.491 0.531 4.396 1.492
[15,16], through the least-squares fitting procedure, measured absorption oscillator strength fexp, calculated oscillator strength fcal and the Judd–Ofelt intensity parameters X2, X4 and X6 for Ho3+ ions in SNTH2 sample were calculated respectively from the absorption spectrum. The fexp and fcal of SNTH2 sample are shown in Table 2, and Xt (t = 2, 4, 6) parameters are compared with other Ho3+ doped glasses in Table 3. As is shown in Table 2, the rootmean-square error deviation of intensity parameters is 3.1 107, which indicates the validity of the Judd–Ofelt theory for predicting the spectral intensities of Tm3+ ions and the reliable calculations. The X4/X6 determines the spectroscopy quality of the host materials [17]. Among the glasses listed in Table 3, the present crystal has the largest X4/X6. Considering that largest X4/X6, the Tm3+ doped Na5Lu9F32 is an appropriate material for Ho3+ ions and 2 lm emission. 3.4. Fluorescence spectra and stimulated emission cross section The emission spectra of Tm3+/Ho3+ co-doped Na5Lu9F32 crystal samples from 1300 nm to 2200 nm under the excitation of 800 nm at room temperature are presented in Fig. 5. In these emission spectra, there are three emission bands at 1.47 lm (pretty weak), 1.8 lm and 2.0 lm, corresponding to Tm3+:3H4 ? 3F4 and Tm3+:3F4 ? 3H6 and Ho3+:5I7 ? 5I8 transitions, respectively. It is obviously noticeable that the fluorescence intensity at 2.0 lm continues to increase as the doping concentration of Ho3+ increases from 0.5% to 1.5%, corresponding to SNTH1 to SNTH3. When the current doping level arrives to the 2%, the clear luminescence quenching for Ho3+ ions appears. It can also be noticed from Fig. 5 that the characteristics emission at 1.8 lm of Tm3+:3F4 ? 3H6 Table 3 Judd–Ofelt intensity parameters Xt (1020 cm2) of Ho3+ in various host materials.
Fig. 4. Absorption spectra of Ho3+/Tm3+ co-doped Na5Lu9F32 single crystals.
Host materials
X2
X4
X6
X4/X6
drms (106)
Ref.
YAG LiYF4 a-NaYF4 Na5Lu9F32
0.04 1.14 1.65 0.67
2.67 0.79 0.84 1.51
1.89 1.33 1.29 0.86
1.41 0.59 0.52 1.75
– 0.43 – 0.31
[18] [7] [6] This work
Z. Feng et al. / Chemical Physics Letters 652 (2016) 68–72
Fig. 5. Emission spectra of Ho3+/Tm3+ co-doped Na5Lu9F32 single crystals upon 800 nm LD. The inset is the relationship between the values of emission intensity at 2.0 lm and doping concentration of Ho3+ ions.
for all three Tm3+/Ho3+ co-doped samples disappears while the Tm3+ singly doped one shows a strongly intensity. It indicates that the strong energy transfer from Tm3+ ions to Ho3+ ions occurs. The possible energy transfer mechanism for Ho3+/Tm3+ codoped Na5Lu9F32 samples can be explained by the simplified energy-level diagram in previous investigations [8]. The Tm3+ ions are initially excited from the 3H6 ground state to 3H4 level under the excitation of 800 nm, and then it transfers to the 3F4 manifold quickly by two ways. One is the excited Tm3+ ions at the state of 3 H4 radiatively transfer to 3F4 state to emit photons at 1.47 lm, and the other is the well-known two-for-one cross-relaxation (CR) process. Then, a fraction of the energy stored in 3F4 level is therefore transferred to the adjacent state 5I7 of Ho3+ ions, a strong interaction occurring between the two ions, yielding 2.0 lm emissions efficiently. According to the absorption spectra, the transition cross-section of Tm3+ ions from 3H6 level and Ho3+ ions from 5I8 can be calculated. The absorption cross-section (rabs) can be determined by [19]:
rabs ðkÞ ¼
2:303 logðI0 =IÞ a ¼ NL N
ð2Þ
where log(I0/I) is the optical density as a function of wavelength, N is the rare earth ions concentration of sample, and L is the thickness of the polished sample, a is the absorption coefficient. In general, the emission cross-section of rare earths ions can be calculated by the Futchbauer–Ladenburg theory. However, for the Yb3+:2F5/2 level, the spontaneous transition probability following the J–O intensity parameters cannot be obtained because Yb3+ presents only on transition, namely, Yb3+:2F5/2 ? 2F7/2. Therefore, the simulated emission cross-section based on absorption crosssection (rabs) is determined by using the McCumber formula as [20]:
rem ðkÞ ¼ rabs ðkÞ
Zl Ezl hck1 exp Zu kT
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Fig. 6. Absorption and emission cross-sections of SNTH2 sample.
emission cross-section of Tm3+ ions at 1.80 lm is calculated to be 0.705 1020 cm2. The large emission cross-section of Tm3+ ions is beneficial for energy transfer process from Tm3+:3F4 level to Ho3+:5I7 level. Ho3+ can be efficiently sensitized in Ho3+/Tm3+ codoped Na5Lu9F32 single crystal pumped by 800 nm LD because of the energy transfer of Tm3+. And the emission cross-section of Ho3+ ions at 2.0 lm is calculated to be 0.347 1020 cm2. From the above results, we can find that an appropriate co-doping concentration of Tm3+/Ho3+ is vital to get more suitable materials for 2.0 lm laser applications.
3.5. Fluorescence decay curves and energy transfer efficiency We measured the fluorescence decay curves for 2.0 lm emissions (Ho3+:5I7?5I8) of SNTH1, SNTH2, SNTH3, and SNTH4 samples in Fig. 7 and curves for 1.8 lm emissions (Tm3+:3F4 ? 3H6) of SNTH2 and SNT1 upon excitation at 800 nm in Fig. 8, in order to get insight of the energy transfer mechanism from Tm3+ ions to Ho3+ ions in Na5Lu9F32 -single crystals. It can be found from the Fig. 7 that the lifetimes of Ho3+:5I7 increase from 14.67 ms to 18.01 ms with the concentration of Ho3+ increasing from 0.5% to 1.5%. However, as the concentration of Ho3+ further increase to 2%, the lifetime of Ho3+:5I7 reduces to 17.02 ms. The concentration quenching of Ho3+ ions is in charge of the reduction of lifetime, which is well corresponding to the relationship between the
! ð3Þ
where Zl and Zu represent the partition functions between the lower and upper manifolds respectively, and Ezl is the zero line energy, which is defined as the energy separation between the lowest crystal field levels of the upper and lower manifolds. The calculated absorption cross-section is illustrated in Fig. 6. Fig. 6 gives the absorption and emission cross-sections from 1.55 lm to 2.15 lm. The absorption cross-section (rabs) of Tm3+:3H4 of SNTH2 lying at 1640 nm is 0.139 1020 cm2. The
Fig. 7. Fluorescence decay curves of 2.0 lm of SNTH1, SNTH2, SNTH3, and SNTH4 samples.
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Z. Feng et al. / Chemical Physics Letters 652 (2016) 68–72
emission cross section at 2.0 lm is 0.347 1020 cm2. All the parameters suggest that these materials have more advantages on 2.0 lm laser applications in the future. It is also confirmed that the energy transfer from Tm3+ ions to Ho3+ ions is charge of electric dipole–dipole interaction. All the parameters suggest that these materials have more advantages in the future 2.0 lm laser applications. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 51472125 and 51272109), K.C. Wong Magna Fund in Ningbo University, and Fundamental Research Funds for the Central Universities (Grant No. 3132014327). References Fig. 8. Fluorescent decays for 1800 nm emission (Tm3+:3F4 ? 3H6) of SNTH2 and SNT1. The solid red lines are the fitting curves. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
emission intensity at 2.0 lm and doping concentration of Ho3+ ions shown in Fig. 5. As seen from Fig. 7, the decay curve of 1.8 lm emission of SNT1 sample exhibits an almost single exponential decay. However, when Ho3+ is added into single crystal, the decays of 1.8 lm for SNTH2 sample turns to be non-exponential. The energy transfer process from Tm3+ ions to Ho3+ ions is responsible for the non-exponential decay, which follow the Inkuti–Hirayama’s model [21,22]. The fluorescence decay can be express as:
"
3s # 4 3 3 t R N IðtÞ ¼ Ið0Þ exp pC 1 s 0 s0 3 s0 t
ð4Þ
where s = 6, 8 and 10, respectively, denotes the electric dipole– dipole, dipole–quadrupole and quadrupole–quadrupole interaction between luminescent centers, N is the doping concentration, C(1–3/s) is a Gamma function, and R0 is critical transfer distance, s0 is the intrinsic radiative transition lifetime. By fitting the fluorescence decay for SNTH2 sample, the s value is confirmed to be 6.2, and which is close to the theoretical value 6 for the electric dipole–dipole interaction between Tm3+ and Ho3+. The I–H model fitting for SNTH2 sample and mono-exponential fitting for SNT1 are shown in Fig. 8. 4. Conclusions Our experiments demonstrated that Ho3+/Tm3+ co-doped Na5Lu9F32 bulk single crystals can were grown successfully by an improved Bridgman method. The Ho3+ concentration significantly affects the spectra properties and Tm3+ is an effective sensitizer for Ho3+ in present crystal to achieve an intense 2.0 lm emission because of the strong energy transfer from Tm3+ to Ho3+. The maximum emission intensity at 2.0 lm is obtained at about 1.5% doping concentration of Ho3+ ions when the concentration of Tm3+ ions is fixed constantly at 1.3% in the current research. The calculated
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Fluorescence spectra of Na5Lu9F32 single crystals co-doped with Ho3+/Tm3+ grown by Bridgman method Zhigang Feng a, Haiping Xia a,⇑, Cheng Wang a, Zhixiong Zhang a, Dongsheng Jiang a, Jian Zhang a, Shinan He a, Qingyang Tang a, Qiguo sheng a, Xuemei Gu a, Yuepin Zhang a, Baojiu Chen b,⇑, Haochuan Jiang c a
Key laboratory of Photo-electronic Materials, Ningbo University, Ningbo, Zhejiang 315211, China Department of Physics, Dalian Maritime University, Dalian, Liaoning Province 116026, China c Ningbo Institute of Materials Technology and Engineering, The Chinese Academy of Sciences, Ningbo, Zhejiang 315211, China b
a r t i c l e
i n f o
Article history: Received 29 February 2016 Revised 2 April 2016 In final form 9 April 2016 Available online 12 April 2016 Keywords: Optical materials Crystal growth X-ray diffraction Optical properties Luminescence Na5Lu9F32 Energy transfer
a b s t r a c t This work presents the luminescent properties of Ho3+/Tm3+ co-doped Na5Lu9F32 single crystals that were grown by an improved Bridgman method for the first time. The J–O intense parameters of Ho3+ ions were calculated. An intense 2.0 lm emission was achieved with Tm3+ ions sensitizing Ho3+ ions by the processing of energy transfer from Tm3+ ions to Ho3+ ions under excitation of 800 nm LD. The maximum emission intensity at 2.0 lm is obtained, and the cross sections of Tm3+ ions and Ho3+ ions were calculated. The physical mechanism for energy transfer from Tm3+ to Ho3+ ions was analyzed by using Inokuti–Hirayama’s model. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Recent years, the development of 2 lm solid state lasers has attracted much attention due to their extensive applications including remote sensing, environment monitoring, military weapons, eye-safe laser radars et al. [1–3]. The rare earth doped single crystals have been practically applied in 2 lm solid state lasers because of their excellent physical–chemical properties and high luminous efficiency [4,5]. Among rare earth ions, the transition 5 I7 ? 5I8 of Ho3+ ion is well known for generating laser with 2.0 lm radiation, which has been reported in many single crystals [6,7]. Tm3+ ion has a strong absorption band at 800 nm that overlaps with the emission band of the commercially available AlGaAs laser diodes. As sensitizer for Ho3+ ion, Tm3+ ion can increase the pumping efficiency of 800 nm LD via energy transfer from Tm3+ ions to Ho3+ ions due to the very close energy levels between Tm3+:3F4 and Ho3+:5I7. Such performance between Ho3+ ions and Tm3+ ions has been demonstrated in previous studies [8].
⇑ Corresponding authors. E-mail addresses: [email protected] (H. Xia), [email protected] (B. Chen). http://dx.doi.org/10.1016/j.cplett.2016.04.025 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.
The host materials also play an important role in the optical properties. In order to gain highly efficient laser operation with low phonon energy and long lifetime of the excited electronic states, the fluoride hosts were investigated in previous researches [9,10]. The forms of these materials include nano-crystals, glasses, and ceramics. Generally, powders have strong scattering and low transmission, while glasses or ceramics have less chemical stability and lower luminescent efficiency. These disadvantages blocked the utilization of above materials in practical applications. In contrast, the high transparency and luminescence efficiency of single crystal make it more favorable as the host material for practical applications in optical advices. More recently, we have prepared aNaYF4, LiYF4 and LiLuF4 crystals in our previous studies [6,7,11]. Na5Lu9F32 is a kind of fluoride compounds with good physical– chemical properties and thermal stability [12]. As a host material, a high doping concentration for trivalent rare earth ions which take the place of the Lu3+ ions could be obtained in Na5Lu9F32 single crystal due to the similar size of rare earth ions. It is the lower phonon energy (440 cm1) and higher optical transparency in the range of infrared that make the Na5Lu9F32 single crystal more favorable as potential host material for practical applications in advices for the mid-IR solid state lasers. However, the study of
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rare-earth ions doped Na5Lu9F32 matrix has been mainly focused on nano-crystallites [12,13]. There are scarcely any reports about growing bulk Na5Lu9F32 single crystals because of the difficulties in the crystal growth. In this work, Ho3+/Tm3+ co-doped Na5Lu9F32 single crystal was grown by an improved Bridgman method for the first time. The DTA/TG and refractive index curves were measured. And the density of the single crystal is 5.88 g/cm3 measured by Archimedes method. The optical spectra of Na5Lu9F32 single crystals with constant Tm3+ ions concentration of 1.3% and various Ho3+ ions concentrations from 0% to 2% were investigated. By analyzing the fluorescence properties, the mechanism of 2.0 lm emission in Ho3+/Tm3+ co-doped Na5Lu9F32 single crystal was revealed. 2. Experimental procedures The preparation of the material was from commercially available NaF and LuF3 powders of high purity (99.999%). The molar compositions of the raw materials were NaF:LuF3: TmF3 = 40:58.7:1.3, NaF:YF3:TmF3:HoF3 = 40:58.7 v:1.3:v (v = 0.5, 1, 1.5, 2) and NaF:LuF3:HoF3 = 40:59:1. The obtained samples were named, respectively, as SNT1, SNTH1, SNTH2, SNTH3, SNTH4, and SNH1. The initial compounds were mixed and ground sufficiently in a porcelain bowl, and put the mixtures into apparatus for fluoridation process about 6 h at 780 °C to remove the moisture and oxide from the mixture completely. The Na5Lu9F32 single crystals were grown in a resistively heated vertical Bridgman furnace. The temperature gradient cross solid–liquid interface was 60– 80 °C/cm. The seeding temperature is about 860–880 °C. The growth process was carried on by lowering the crucible at a rate of 0.06 mm/h. The detailed process is similar to the report of growth of NaYF4 single crystal [6]. The seed of pure a-NaYF4 single crystal with a-axis direction was placed at the bottom of the crucible. The grown crystal with about £10 mm 50 mm is shown in Fig. 1(a). The crystals were sliced into pieces and polished to be about 2.0 mm-thickness for the optical measurements. The crystal structure was performed by X-ray diffraction (XRD) pattern with XD-98X diffractometer (XD-3, Beijing). The concentrations of Ho3+ ions and Tm3+ ions in these samples were confirmed severally with an inductively coupled plasma (ICP) atomic emission spectroscopy (ICP-AES, PerkinElmer Inc., Optima 3000). The absorption spectra were measured with a Cary 5000 UV/VIS/NIR spectrophotometer in the wavelength region from 200 nm to 2500 nm. The emission spectra were recorded under the excitation of 800 nm LD by a Triax 320 spectrometer in the wavelength range of 1000–2200 nm. The fluorescence lifetimes were obtained with the FLSP920 fluorescence
Fig. 1. DTA/TG curves of Ho3+/Tm3+ doped Na5Lu-9F32 crystals.
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spectrophotometer. All the measurements were carried out at room temperature under the room temperature.
3. Results and discussion 3.1. DTA/TG and refractive index analysis Fig. 1 presents the DTA/TG curves of the powder ofNa5Lu9F32 single crystal. One can be noted from the TG curve in Fig. 1 that there is 1.3% weight loss in the temperature region of 25–916 °C, and then a huge weight loss, 21.7%, from 916 to 1017 °C. It can be seen from the DTA curve in Fig. 1 that as the temperature rises, the sample begins to absorb heat at about 35 °C and shows an endothermic peak about at 80 °C, which is related to the endotherm of H2O impurity mixed during the grind. As the temperature further rises, a strong endotherm at about 916.52 °C appears, and the weight shows a huge loss, which is caused by the heat absorbance due to the melting and vaporization of the crystal. And the refractive index of Na5Lu9F32 crystals in all the calculation is 1.44 as we measured. The maximum phonon energy of Na5Lu9F32 is also estimated to be 440 cm1 from the measured Raman spectrum, and there exists a high transmittance from 2500 nm to 7150 nm from measured infrared transmittance spectrum of Na5Lu9F32 single crystal. The excellent properties provide potential application for 2 lm mid-infrared laser material.
3.2. X-ray diffraction The measured XRD pattern for Ho3+/Tm3+ co-doped Na5Lu9F32 crystal (SNTH2) is presented in Fig. 2(b). The pattern appearing in JCPDS card (No. 27–0725) for Na5Lu9F32 is also shown in Fig. 2(c) for comparison. It is believed that all the diffraction peak positions of obtained samples doped with Ho3+/Tm3+ ions are matched perfectly with that in standard Na5Lu9F32, and there are no any other peaks belong to impurity phase from Fig. 2, which indicates that the obtained transparent crystals are cubic phase Na5Lu9F32.
a d ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 h þk þl
ð1Þ
According to formula (1) [14], the cell parameters of SNTH2 were calculated to be a = b = c = 0.5453 nm.
Fig. 2. (a) Photograph of Ho3+/Tm3+ co-doped Na5Lu9F32 single crystal. (b) XRD pattern of the Na5Lu9F32: Ho3+/Tm3+. (c) Standard line pattern of the Na5Lu9F32.
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Z. Feng et al. / Chemical Physics Letters 652 (2016) 68–72 Table 1 Molar fractions of Ho3+ and Tm3+ in feed material and concentration of Ho3+ ions and Tm3+ ions in all crystal samples. Samples
Molar fraction of Tm3+
1020 N (Tm3+)
Molar fraction of Ho3+
1020 N (Ho3+)
SNT1 SNTH1 SNTH2 SNTH3 SNTH4 SNH1
1.3% 1.3% 1.3% 1.3% 1.3% 0%
2.90 2.91 2.89 2.91 2.92 0
0% 0.5% 1% 1.5% 2% 1%
0 1.12 2.10 3.37 4.45 2.13
Table 2 Measured and calculated oscillator strength of Ho3+ ions in SNTH2 sample.
Fig. 3. Absorption spectra of Tm3+, Ho3+ singly doped and Ho3+/Tm3+ co-doped Na5Lu9F32 single crystals.
3.3. Absorption spectra and Judd–Ofelt analysis The measured concentrations of Ho3+ and Tm3+ are shown in Table 1. Fig. 3 shows the absorption spectra of Tm3+, Ho3+ singly doped Ho3+/Tm3+ co-doped Na5Lu9F32 samples. The characteristic absorption bands corresponding to the transition of Ho3+ ions and Tm3+ ions from the ground state to the excited ones are marked in spectra. Compared with the absorption spectra of Tm3+, Ho3+ singly doped Na5Lu9F32 crystals, the Fig. 3 shows that the position of all absorption peaks of Ho3+ ions and Tm3+ ions do not change any more with the doping of Ho3+ ions, which means that the absorption spectra of Ho3+/Tm3+ co-doped Na5Lu9F32 crystals is the superposition of Ho3+ ions and Tm3+ ions singly doped Na5Lu9F32 crystals. Fig. 4 shows the absorption spectra of different doping concentrations of Ho3+ ions in Ho3+/Tm3+ co-doped Na5Lu9F32 single crystals. As we can see from the Fig. 4, the corresponding absorption coefficient is proportional to the Ho3+ ions doping concentration. The intense absorption band and the widest full width at half maximum(FWHM) in all absorption bands centered at the approximately 1931 nm of the all samples indicates that the codoped Ho3+ ions can act as an efficient absorber of the energy from Tm3+:3F4 excited state to Tm3+:3H6 ground state. The Judd–Ofelt theory is extensively employed by determining the radiative properties of rare-earth ions within a matrix based on its absorption spectrum. According to the Judd–Ofelt theory
Absorption
Wavelength (nm)
I8 ? 5I7 5 I8 ? 5I6 5 I8 ? 5F5 5 I8 ? 5F4 5 I8 ? 5F3 5 I8 ? 5G6 5 I8 ? (5G, 3G)5 drms (107)
1927 1151 641 535 482 449 415 3.1
5
Oscillator strength (106) fexp
fcal
0.685 0.321 1.339 1.98 0.35 4.396 1.19
0.65 0.488 1.418 1.491 0.531 4.396 1.492
[15,16], through the least-squares fitting procedure, measured absorption oscillator strength fexp, calculated oscillator strength fcal and the Judd–Ofelt intensity parameters X2, X4 and X6 for Ho3+ ions in SNTH2 sample were calculated respectively from the absorption spectrum. The fexp and fcal of SNTH2 sample are shown in Table 2, and Xt (t = 2, 4, 6) parameters are compared with other Ho3+ doped glasses in Table 3. As is shown in Table 2, the rootmean-square error deviation of intensity parameters is 3.1 107, which indicates the validity of the Judd–Ofelt theory for predicting the spectral intensities of Tm3+ ions and the reliable calculations. The X4/X6 determines the spectroscopy quality of the host materials [17]. Among the glasses listed in Table 3, the present crystal has the largest X4/X6. Considering that largest X4/X6, the Tm3+ doped Na5Lu9F32 is an appropriate material for Ho3+ ions and 2 lm emission. 3.4. Fluorescence spectra and stimulated emission cross section The emission spectra of Tm3+/Ho3+ co-doped Na5Lu9F32 crystal samples from 1300 nm to 2200 nm under the excitation of 800 nm at room temperature are presented in Fig. 5. In these emission spectra, there are three emission bands at 1.47 lm (pretty weak), 1.8 lm and 2.0 lm, corresponding to Tm3+:3H4 ? 3F4 and Tm3+:3F4 ? 3H6 and Ho3+:5I7 ? 5I8 transitions, respectively. It is obviously noticeable that the fluorescence intensity at 2.0 lm continues to increase as the doping concentration of Ho3+ increases from 0.5% to 1.5%, corresponding to SNTH1 to SNTH3. When the current doping level arrives to the 2%, the clear luminescence quenching for Ho3+ ions appears. It can also be noticed from Fig. 5 that the characteristics emission at 1.8 lm of Tm3+:3F4 ? 3H6 Table 3 Judd–Ofelt intensity parameters Xt (1020 cm2) of Ho3+ in various host materials.
Fig. 4. Absorption spectra of Ho3+/Tm3+ co-doped Na5Lu9F32 single crystals.
Host materials
X2
X4
X6
X4/X6
drms (106)
Ref.
YAG LiYF4 a-NaYF4 Na5Lu9F32
0.04 1.14 1.65 0.67
2.67 0.79 0.84 1.51
1.89 1.33 1.29 0.86
1.41 0.59 0.52 1.75
– 0.43 – 0.31
[18] [7] [6] This work
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Fig. 5. Emission spectra of Ho3+/Tm3+ co-doped Na5Lu9F32 single crystals upon 800 nm LD. The inset is the relationship between the values of emission intensity at 2.0 lm and doping concentration of Ho3+ ions.
for all three Tm3+/Ho3+ co-doped samples disappears while the Tm3+ singly doped one shows a strongly intensity. It indicates that the strong energy transfer from Tm3+ ions to Ho3+ ions occurs. The possible energy transfer mechanism for Ho3+/Tm3+ codoped Na5Lu9F32 samples can be explained by the simplified energy-level diagram in previous investigations [8]. The Tm3+ ions are initially excited from the 3H6 ground state to 3H4 level under the excitation of 800 nm, and then it transfers to the 3F4 manifold quickly by two ways. One is the excited Tm3+ ions at the state of 3 H4 radiatively transfer to 3F4 state to emit photons at 1.47 lm, and the other is the well-known two-for-one cross-relaxation (CR) process. Then, a fraction of the energy stored in 3F4 level is therefore transferred to the adjacent state 5I7 of Ho3+ ions, a strong interaction occurring between the two ions, yielding 2.0 lm emissions efficiently. According to the absorption spectra, the transition cross-section of Tm3+ ions from 3H6 level and Ho3+ ions from 5I8 can be calculated. The absorption cross-section (rabs) can be determined by [19]:
rabs ðkÞ ¼
2:303 logðI0 =IÞ a ¼ NL N
ð2Þ
where log(I0/I) is the optical density as a function of wavelength, N is the rare earth ions concentration of sample, and L is the thickness of the polished sample, a is the absorption coefficient. In general, the emission cross-section of rare earths ions can be calculated by the Futchbauer–Ladenburg theory. However, for the Yb3+:2F5/2 level, the spontaneous transition probability following the J–O intensity parameters cannot be obtained because Yb3+ presents only on transition, namely, Yb3+:2F5/2 ? 2F7/2. Therefore, the simulated emission cross-section based on absorption crosssection (rabs) is determined by using the McCumber formula as [20]:
rem ðkÞ ¼ rabs ðkÞ
Zl Ezl hck1 exp Zu kT
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Fig. 6. Absorption and emission cross-sections of SNTH2 sample.
emission cross-section of Tm3+ ions at 1.80 lm is calculated to be 0.705 1020 cm2. The large emission cross-section of Tm3+ ions is beneficial for energy transfer process from Tm3+:3F4 level to Ho3+:5I7 level. Ho3+ can be efficiently sensitized in Ho3+/Tm3+ codoped Na5Lu9F32 single crystal pumped by 800 nm LD because of the energy transfer of Tm3+. And the emission cross-section of Ho3+ ions at 2.0 lm is calculated to be 0.347 1020 cm2. From the above results, we can find that an appropriate co-doping concentration of Tm3+/Ho3+ is vital to get more suitable materials for 2.0 lm laser applications.
3.5. Fluorescence decay curves and energy transfer efficiency We measured the fluorescence decay curves for 2.0 lm emissions (Ho3+:5I7?5I8) of SNTH1, SNTH2, SNTH3, and SNTH4 samples in Fig. 7 and curves for 1.8 lm emissions (Tm3+:3F4 ? 3H6) of SNTH2 and SNT1 upon excitation at 800 nm in Fig. 8, in order to get insight of the energy transfer mechanism from Tm3+ ions to Ho3+ ions in Na5Lu9F32 -single crystals. It can be found from the Fig. 7 that the lifetimes of Ho3+:5I7 increase from 14.67 ms to 18.01 ms with the concentration of Ho3+ increasing from 0.5% to 1.5%. However, as the concentration of Ho3+ further increase to 2%, the lifetime of Ho3+:5I7 reduces to 17.02 ms. The concentration quenching of Ho3+ ions is in charge of the reduction of lifetime, which is well corresponding to the relationship between the
! ð3Þ
where Zl and Zu represent the partition functions between the lower and upper manifolds respectively, and Ezl is the zero line energy, which is defined as the energy separation between the lowest crystal field levels of the upper and lower manifolds. The calculated absorption cross-section is illustrated in Fig. 6. Fig. 6 gives the absorption and emission cross-sections from 1.55 lm to 2.15 lm. The absorption cross-section (rabs) of Tm3+:3H4 of SNTH2 lying at 1640 nm is 0.139 1020 cm2. The
Fig. 7. Fluorescence decay curves of 2.0 lm of SNTH1, SNTH2, SNTH3, and SNTH4 samples.
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emission cross section at 2.0 lm is 0.347 1020 cm2. All the parameters suggest that these materials have more advantages on 2.0 lm laser applications in the future. It is also confirmed that the energy transfer from Tm3+ ions to Ho3+ ions is charge of electric dipole–dipole interaction. All the parameters suggest that these materials have more advantages in the future 2.0 lm laser applications. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 51472125 and 51272109), K.C. Wong Magna Fund in Ningbo University, and Fundamental Research Funds for the Central Universities (Grant No. 3132014327). References Fig. 8. Fluorescent decays for 1800 nm emission (Tm3+:3F4 ? 3H6) of SNTH2 and SNT1. The solid red lines are the fitting curves. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
emission intensity at 2.0 lm and doping concentration of Ho3+ ions shown in Fig. 5. As seen from Fig. 7, the decay curve of 1.8 lm emission of SNT1 sample exhibits an almost single exponential decay. However, when Ho3+ is added into single crystal, the decays of 1.8 lm for SNTH2 sample turns to be non-exponential. The energy transfer process from Tm3+ ions to Ho3+ ions is responsible for the non-exponential decay, which follow the Inkuti–Hirayama’s model [21,22]. The fluorescence decay can be express as:
"
3s # 4 3 3 t R N IðtÞ ¼ Ið0Þ exp pC 1 s 0 s0 3 s0 t
ð4Þ
where s = 6, 8 and 10, respectively, denotes the electric dipole– dipole, dipole–quadrupole and quadrupole–quadrupole interaction between luminescent centers, N is the doping concentration, C(1–3/s) is a Gamma function, and R0 is critical transfer distance, s0 is the intrinsic radiative transition lifetime. By fitting the fluorescence decay for SNTH2 sample, the s value is confirmed to be 6.2, and which is close to the theoretical value 6 for the electric dipole–dipole interaction between Tm3+ and Ho3+. The I–H model fitting for SNTH2 sample and mono-exponential fitting for SNT1 are shown in Fig. 8. 4. Conclusions Our experiments demonstrated that Ho3+/Tm3+ co-doped Na5Lu9F32 bulk single crystals can were grown successfully by an improved Bridgman method. The Ho3+ concentration significantly affects the spectra properties and Tm3+ is an effective sensitizer for Ho3+ in present crystal to achieve an intense 2.0 lm emission because of the strong energy transfer from Tm3+ to Ho3+. The maximum emission intensity at 2.0 lm is obtained at about 1.5% doping concentration of Ho3+ ions when the concentration of Tm3+ ions is fixed constantly at 1.3% in the current research. The calculated
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