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Crystal growth and spectroscopic properties of Sm3+:Sr3Gd(BO3)3 crystal
Journal of Luminescence 149 (2014) 7–11
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Crystal growth and spectroscopic properties of Sm3 þ :Sr3Gd(BO3)3 crystal Houping Xia a,b, Jianghe Feng a, Yuexia Ji c, Jinlong Xu a, Zhaojie Zhu a, Yan Wang a, Zhenyu You a, Jianfu Li a, Hongyan Wang d, Chaoyang Tu a,n a Key Laboratory of Optoelectronic Materials Chemistry and Physics of CAS, Fujian Institute of Research on the Structure of Matter, CAS, Fuzhou, Fujian 350002, PR China b University of Chinese Academy of Sciences, Beijing 100039, PR China c School of Mathematics and Physics, Anhui University of Technology, Maanshan, Anhui 243002, PR China d Crystech Inc., Qingdao 266107, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 23 October 2013 Received in revised form 18 December 2013 Accepted 20 December 2013 Available online 31 December 2013
The Sm3 þ :Sr3Gd(BO3)3 single crystal with dimensions of Φ20 25 mm3 was grown by the Czochralski method in nitrogen atmosphere. The effective segregation coefficient of Sm3 þ was measured as 0.975. The polarized absorption spectra, emission spectra and fluorescence decay curves of the crystal were measured. Based on the Judd–Ofelt theory, spectroscopic parameters of the crystal, including the oscillator intensity parameters Ωt (t ¼ 2, 4, 6), spontaneous emission probabilities, fluorescence branching ratios, and radiative lifetimes were calculated and analyzed. The crystal exhibited a broad absorption band from 390 nm to 418 nm, a broad emission band from 585 nm to 625 nm, as well as a fluorescence lifetime up to 2.04 ms. These results indicate Sm3 þ :Sr3Gd(BO3)3 crystal is promising for lasing in visible region. & 2013 Elsevier B.V. All rights reserved.
Keywords: Sm3 þ :Sr3Gd(BO3)3 crystal Czochralski growth Spectroscopic properties Judd–Ofelt parameters
1. Introduction Recently, with the increasing demand of visible lasers, investigations on praseodymium and dysprosium ions doped laser materials are becoming more significant [1–4]. In theory, trivalent samarium ions (Sm3 þ ) doped laser crystal also has prominent advantages as a candidate of visible solid-state laser materials: a large energy gap between the excitation state 4G5/2 and the next lower energy state 6F11/2, which makes the multiphonon relaxation tiny and a highly efficient visible emission of Sm3 þ in inorganic matrices; transitions from 4G5/2 to 6H5/2, 6H7/2, 6H9/2 and 6 H11/2 are in the visible range. However, the studies about Sm3 þ doped laser materials were very few in the past, it is mainly attributed to the lack of ultraviolet and blue laser diode (LD). Recently, the present development of blue LD is expected to change this situation [5,6]. Therefore, searching for new laser materials that can be used to get visible lasers directly through pumped by 405 nm blue LD is an urgent task. A few Sm3 þ doped glasses and crystals have been grown and shown good luminescence properties [7–16]. However, there has been no research on Sm3 þ doped Sr3Gd(BO3)3 (SGB) single crystal. SGB is a member of double borate compounds with formula M3R
n
Corresponding author. Tel.: þ 86 59183711368; fax: þ86 59183714946. E-mail address: [email protected] (C. Tu).
0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.12.048
(BO3)3 (M ¼Sr,Ba,Ca; R¼ rare earth). This crystal belongs to trigonal system with the space group R3 and the unit cell parameters are a¼ b¼12.58 Å, c ¼9.28 Å and Z ¼ 6 [17–19]. Like other borate crystals, the SGB crystal also has excellent physicochemical properities, wide range of optical transmittance and high damage threshold. In addition, the ionic radius of Sm3 þ is similar to that of Gd3 þ , which allows high doping concentration of Sm3 þ , and can improve the absorption efficiency for pumping laser. So the SGB crystal was chosen as the candidate material for Sm3 þ in this work. The Sm3 þ :SGB crystal was grown by the Czochralski method. The polarized absorption spectra, emission spectra and the lifetime decay curve were measured at room temperature. The effective J-O intensity parameters (Ωt,eff), spontaneous transition probabilities (A), fluorescent branching ratios (β), radiative lifetimes (τr) and emission cross sections (se) were calculated. The fluorescence lifetime (τf) was also obtained. 2. Experimental details 2.1. Crystal growth Sm3 þ :SGB crystal melts congruently at 1353.8 1C, it can be grown by Czochralski method. The constituent raw materials of Sm3 þ :SGB crystal were synthesized by the solid-state reaction method. The chemicals used were Gd2O3 and Sm2O3 with a purity
8
H. Xia et al. / Journal of Luminescence 149 (2014) 7–11
of 99.99%, SrCO3 and H3BO3 with a purity of 99.5%. The stoichiometric amounts of raw materials were weighed according to the following chemical reaction: (1 x)/2Gd2O3 þ x/2Sm2O3 þ 3SrCO3 þ 3H3BO3 Sr3Gd1 xSmx(BO3)3 þ 3CO2 þ 9/2H2O where x is the concentration of Sm3 þ and the value is 0.02. In order to compensate the evaporation losses of the H3BO3 in the growing process, excess 1.0 wt.% H3BO3 was added to the raw materials. Before the solid-state reaction the chemical powders were mixed, ground thoroughly and pressed into pellets. Then the pellets were placed in the platinum crucible and sintered at 1100 1C for 48 h. The process was repeated and the pellets were held at 1200 1C for 48 h to assure complete reaction. Sm3 þ :SGB crystal was grown by 2 kHz intermediate-frequency furnace in a nitrogen atmosphere. The synthesized polycrystalline materials formed by solid-state reaction were placed and melted in the iridium crucible. A rectangular [001] orientated seed crystal was introduced at a proper temperature higher than the melting point of the melt. The pulling rate varied from 1 to 1.6 mm/h and the crystal rotation speed was kept 10–20 rpm. After growth was completed, the crystal was cooled to room temperature at a rate of 10–25 K/h. 2.2. Spectra measurements The crystal plate for spectra experiments was cut along the [100] direction, with dimensions of 10 8 1.5 mm3 (b c a) and with the two 10 8 mm2 faces polished. It is shown in Fig. 1 (b). The polarization absorption spectra were measured using a Perkin-Elmer UV–VIS–NIR Spectrometer (Lambda-900). The polarized emission spectra and fluorescence lifetime were measured using an Edinburgh Instruments FLS920 spectrophotometer with a continuous Xe-flash lamp (excited with 404 nm radiation). The π and s polarizations are defined in terms of the electric field direction of the incident light being perpendicular and parallel to the c-axis of the Sm3 þ :SGB crystal, respectively. All the spectral experiments were carried out at room temperature.
The concentrations of elemental Sm, Sr, Gd, and B in the crystal were measured using an inductively coupled plasma atomic emission spectrometry (ICP-AES, Ultima2, Jobin-Yvon). The concentration of Sm3 þ ions was measured to be 0.925 1020 cm 3 (1.95 at.%) in this crystal. Taking the 2.0 at.% Sm3 þ concentration in the raw material into account, the effective segregation coefficient of Sm3 þ ions is 0.975 in SGB crystal. The hardness of Sm3 þ :SGB crystal was measured by a microVikers hardness tester (401 MVA™) at room temperature. Two slices of (100) and (001) faces were used for the test. Ten points were collected for each direction. The average values of the hardness are 453.6 N/mm2 for (100) face and 473.4 N/mm2 for (001) face, respectively. The results show that the anisotropy of the hardness of Sm3 þ :SGB crystal is inconspicuous, and the crystal is fit for machining. 3.2. Absorption spectra The luminescence properties of Sm3 þ are closely connected with the transitions between numerous and complicated 4 f5 multiplets. Fig. 2 is a simplified energy lever scheme of the Sm3 þ in the SGB crystal. It can help to analyze the absorption and emission spectra below. The room-temperature polarized absorption spectra for the 2.0 at% Sm3 þ : SGB crystal is shown in Fig. 3 and the line groups are labeled by the usual SLJ designation. The intense absorption bands distribute in the near infrared region corresponding to transitions from the ground state to the 6H and 6F multiplets. In the region between 500 nm and the UV absorption edge of the host crystal, another group of intense absorption bands can be observed. Among them, a prominent absorption band centered at 404 nm offers a possibility of optical pumping with blue LD. It also can be seen in Fig. 3 that the anisotropy of the Sm3 þ absorption transition intensities is small. The absorption band peak at 404 nm has a full width at half maximum (FWHM) of 11 nm for π polarization, and 9.5 nm for s
3. Results and discussion 3.1. Crystal growth The as-grown Sm3 þ :SGB crystal with dimensions of about Φ20 25 mm3 is shown in Fig. 1(a). This crystal has no cracks and the color is pale yellow.
Fig. 1. Photograph of Sm3 þ :SGB crysta (a) and the polished crystal plate (b).
Fig. 2. Simplified energy level scheme of Sm3 þ . Arrows indicate observed absorption and emission transitions.
H. Xia et al. / Journal of Luminescence 149 (2014) 7–11
polarization. Such a broad absorption band makes the crystal suitable for diode laser pumping. The polarized absorption cross-sections could be obtained by:
sðλÞ ¼
2:303 ODðλÞ N0 U l
where λ is the wavelength, L is the thickness of the crystal, OD is the optical density and N0 is the Sm3 þ concentration in the crystal. The absorption cross-sections at 404 nm are about 1.3 10 20 cm2 for π polarization and 1.1 10 20 cm2 for s polarization, which are larger than that of Sm3þ :GGG crystal (about 1.0 10 20 cm2) [13]. 3.3. Spectral parameters The Judd–Ofelt theory [20,21] is a popular method for estimating the spectroscopic parameters of rare-earth ions in crystals and glasses. The J–O intensity parameters Ωt (t¼ 2, 4, 6) can be calculated from the measured absorption spectra. In this work, all the seven absorption bands shown in Fig. 3 were used to fit the J–O parameters for both π and s polarizations. It is worth noting that among the absorption transitions of Sm3 þ ions, 6H5/2-4F5/2, 6F3/2, 6F5/2 contains the contribution of both the electric-dipole (ED) and magnetic dipole (MD) transitions; the absorption line strength of the MD transition should be excluded before the J–O calculation. For this reason, the experimental line strength Sexp(J,J0 ) of an ED transition from the initial state J to the final state J0 can be expressed as follows: Z 3hcð2J þ 1Þ 9n ln 10 ODðλÞ dλ Smd ðJ; J 0 Þ ð1Þ Sexp ðJ; J 0 Þ ¼ 3 2 2 8π e ðn2 þ 2Þ U λ N0 l Smd ðJ; J 0 Þ ¼
3hð2J þ 1Þλ 0 n U f md ðJ; J 0 Þ 8π 2 mc
ð2Þ
9
where λ is the mean wavelength of the transition, n is the refractive index (here we use the average value 1.73 in the entire wavelength region), h is the Planck constant, m is the mass of the electron, c is the velocity of the light, and e is the charge of the electron. In Eq. (2), f0 md(J,J0 ) is the vacuum oscillator strength which can be considered to be constant, here we use the value given by Ref. [22]. The calculated line strength Scal(J,J0 ) of ED transition depends on three Ωt parameters (t ¼2,4,6) as follows: Scal ðJ; J 0 Þ ¼
∑ Ωt j〈φJjjU ðtÞ jjφ0 J 0 〉j2
ð3Þ
t ¼ 2;4;6
where j〈φJjjU ðtÞ jjφ0 J 0 〉j2 is the square of the matrix elements of the tensorial operator, which is considered to be independent of host matrix and has been calculated in Ref. [23]. The root-mean-square deviation between experiment and calculation line strengths is defined by the following equation: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ∑J 0 ðSexp Scal Þ2 RMS ΔS ¼ ð4Þ ðN 3Þ where N is the number of transitions used in our analysis. The experimental and calculated line strengths Sexp and Scal, respectively, and the energies ν (cm 1) of the various absorption bands for π and s polarizations are listed in Table 1. The rootmean-square deviations (RMS ΔS) between Sexp and Scal and the relative errors are also listed in this Table 1. The value of RMSΔS indicates the fitting results are in good agreement with the experiments. By a least-square fitting between Eqs. (1) and (3), the three intensity parameters could be obtained and listed in Table 2. For the polarized absorption, the effective intensity parameters were defined by Ωt,eff ¼ (2Ωt,s þΩt,π)/3. Generally, Ω2 is sensitive to the local environment of the RE3 þ ions. Large value of Ω2 indicates high degree of covalency and lower symmetry of the coordination structure in the vicinity of RE3 þ ions. On the other hand, Ω4 and Ω6 depend on the rigidity of the host matrix. The spectroscopic quality factor X¼ Ω4/Ω6 for the Sm3 þ :SGB crystal is determined to be 1.18, which is compared with the values of Sm3 þ ions in different hosts presented in Table 4. The value of Sm3 þ :SGB crystal is similar to that of other crystals and higher than that of Sm3 þ :YAP and Sm3 þ :GdVO4 crystals, implying it is a promising material for efficient laser action. The probabilities for spontaneous transition from an excited manifold J to a lower manifold J0 can be calculated by the following: AðJ; J 0 Þ ¼ Aed ðJ; J 0 Þ þ Amd ðJ; J 0 Þ " # 64π 4 e2 nðn2 þ 2Þ2 0 0 3 S ðJ; J Þ þ n S ðJ; J Þ ¼ ed md 3 9 3hð2J þ 1Þλ
Fig. 3. Polarized absorption spectra of the Sm3 þ : SGB crystal.
ð5Þ
Table 1 The measured and calculated line strength of Sm3 þ :SGB crystal. Transition 6H5/2-
ν (cm-1)
Line strength (10 20 cm2) π
4
6
4
4
D1/2, P7/2, L17/2, K13/2, L13/2, 4F7/2, 6P3/2, 4K11/2, 4M21/2, 4 I11/2, 4I13/2,4F5/2 6 F11/2 6 F9/2 6 F7/2 6 F1/2, 6H15/2, 6F3/2, 6F5/2 4
26596 24814 21231 10638 9328 8189 6835
σ
Sexp
Scal
Sexp
Scal
0.287 0.699 0.133 0.184 1.314 2.101 2.351 RMS ΔS¼ 0.471 10 22
0.286 0.702 0.135 0.19 1.319 2.097 2.351
0.14 0.456 0.073 0.095 0.662 1.139 1.542 RMS ΔS¼ 0.582 10 22
0.144 0.46 0.069 0.095 0.669 1.133 1.541
10
H. Xia et al. / Journal of Luminescence 149 (2014) 7–11
Table 2 J–O intensity parameters of Sm3 þ :SGB crystal. Direction
Ω2 ( 10 20 cm2)
Ω4 ( 10 20 cm2)
Ω6 ( 10 20 cm2)
π
2.12 1.35 1.61
3.68 2.45 2.86
3.64 1.81 2.42
s
Ωt,eff
Table 3 The calculated luminescence parameters of Sm3 þ :SGB single crystal. Transition 6G5/2-
Aed
Amd
A (s 1)
β (%)
6
0.319 1.471 2.719 10.944 1.576 0.382 1.318 5.673 32.291 88.657 123.52 7.078
0.000 0.000 0.945 2.988 4.358 0.000 0.000 0.000 0.000 0.000 17.908 21.160
0.319 1.471 3.664 13.932 5.934 0.382 1.318 5.673 32.291 88.657 141.428 28.238 ΣA ¼323.307
0.10 0.45 1.13 4.31 1.84 0.12 0.41 1.75 9.99 27.42 43.75 8.73
F11/2 F9/2 F7/2 6 F5/2 6 F3/2 6 H15/2 6 F1/2 6 H13/2 6 H11/2 6 H9/2 6 H7/2 6 H5/2 6 6
τr (ms)
Fig. 4. Polarized emission spectra of the Sm3 þ : SGB crystal under 404 nm excitation.
3.09
where Aed and Amd are the ED and MD contributions, respectively. The ED line strength Sed is expressed as the following and the MD line strength Smd is given by the standard formula (7): Z 3hcð2J þ1Þ 9n ln 10 ODðλÞ dλ ð6Þ Sed ðJ; J 0 Þ ¼ 8π 3 e2 ðn2 þ 2Þ2 U λ N 0 l Smd ðJ; J 0 Þ ¼
2 h 〈φJ L þ 2J φ0 J 0 〉j2 2 2 4m c
ð7Þ
It is obvious that the Smd is a value does not change with the host environment. Here we use the value given by Ref. [24]. The fluorescence branching ratio of each of the transitions and the radiative lifetime of 4G5/2 can be determined from the spontaneous transition probability according to the equations: βðJ; J 0 Þ ¼
τr ¼
AðJ; J 0 Þ ∑J 0 AðJ; J 0 Þ
1 ∑J 0 AðJ; J 0 Þ
ð8Þ
ð9Þ
The spontaneous transition probabilities of electric-dipole transition Aed, the magnetic-dipole transition Amd, the total spontaneous transition probabilities Atotal, the fluorescence branching ratios β as well as the radiative lifetimes τr are summarized in Table 3. It should be noticed that the 4G5/2-6H5/2 transition is mainly dominated by MD and partly by ED, the 4G5/2-6H7/2 transition is MD allowed but ED dominated, the result fulfils the selection rules for magnetic dipole transitions ΔJ ¼0, 1. From Table 3 it can be seen that the radiative transition probability of the 4G5/2-6H7/2 transition is the highest, which comes to 43.75%. It indicates that the emission spectrum of Sm3 þ : SGB crystal should be dominated by this transition.
Fig. 5. Fluorescence decay curve of Sm3 þ : SGB crystal (λex ¼ 404 nm, λem ¼ 607 nm).
equate the overall intensity to 100%. The experimental branching ratios obtained are 17%, 56%, 23% and 4% for transitions to the terminal 6H5/2, 6H7/2, 6H9/2 and 6H11/2 levels, respectively. Thus, the agreement between the calculated and experimental branching ratios is moderately good [15]. The stimulated emission crosssections can be calculated via the following expression:
se ðλÞ ¼
β 8π 2 cn2 τ
R r
λ5 IðλÞ λIðλÞ dλ
ð10Þ
where I(λ) is the emission intensity. The stimulated emission cross-sections at 600 nm are 6.4 10 22 cm2 with FWHM of 12 nm for π polarization and 4.8 10 22 cm2 with FWHM of 24 nm for s polarization, respectively. Fluorescence decay curve of the 4G5/2-6H7/2 transition in Sm3 þ :SGB is as shown in Fig. 5, which is monitored by 607 nm and excited by 404 nm. It can be seen that the fluorescence lifetime is 2.04 ms, which is much longer than that of Sm3 þ doped other crystals listed in Table 4. It is well known that a long upper laser level lifetime is beneficial in achieving population inversion and increasing energy storage, the Sm3 þ :SGB crystal having a long lifetime will make it desirable for laser operation.
3.4. Emission spectra and fluorescence lifetime The emission spectra is shown in Fig. 4. The transitions from G5/2 level to the 6HJ (J ¼5/2, 7/2, 9/2, 11/2) levels are observed. The reliability of the calculated branching ratio values can be now confirmed roughly by integrating the emission bands in Fig. 4, evaluating relative percentage for each band on condition that
4. Conclusions
4
A Sm3 þ :SGB crystal was grown by the Czochralski method. The Vickers hardness of (100) and (001) faces are 453.6 and 473.4 N/mm2, respectively. The room temperature polarized
H. Xia et al. / Journal of Luminescence 149 (2014) 7–11
11
Table 4 A comparison of J–O parameters, spectroscopic quality factor, radiative lifetime and fluorescence lifetime of Sm3 þ :SGB with other crystals. Host crystal
Ω2 ( 10 20 cm2)
Ω4 ( 10 20 cm2)
Ω6 ( 10 20 cm2)
Ω4/Ω6
τr (ms)
YAP GdVO4 GGG CaNb2O6 Gd2SiO5 Sr3Gd(BO3)3
2.28 2.75 3.90 6.33 1.12 1.61
1.62 3.22 2.48 6.49 5.57 2.86
2.21 4.97 1.83 3.72 2.78 2.42
0.73 0.65 1.36 1.74 2.00 1.18
2.4 0.54 2.54 0.75 1.78 3.09
absorption spectra, emission spectra, and fluorescence decay curve of the grown crystal were measured. The absorption cross-sections at 404 nm are 1.3 10 20 cm2 with FWHM of 11 nm for π polarization and 1.1 10 20 cm2 with FWHM of 9.5 nm for s polarization. The Judd–Ofelt theory was applied to calculate the spectral parameters. The emission spectra indicates that visible fluorescence of Sm3 þ consists of green light (563 nm), orange light (600 nm) and red light (647 nm) under 404 nm excitation in SGB crystal. The emission cross-sections (se) at 600 nm were calculated to be 6.4 10 22 cm2 for π polarization and 4.8 10 22 cm2 for s polarization. The fluorescence lifetime was measured to be 2.04 ms. Above results indicate that the Sm3 þ :SGB crystal is a potential visible laser material matched with pumping of blue LD.
Acknowledgments This work was supported by Science and Technology Plan Major Projects of Fujian Province (2010I0015, 2012H0048), National Nature Science Foundation of China (50902129, 61078076, 91122033, 11304313), Knowledge Innovation Program of Chinese Academy of Sciences (KJCX2-EW-H03). References [1] M. Fibrich, H. Jelinkova, J. Sulc, K. Nejezchleb, V. Skoda, Appl. Phys. B: Lasers Opt. 97 (2009) 363.
τf (ms)
Ref.
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Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Crystal growth and spectroscopic properties of Sm3 þ :Sr3Gd(BO3)3 crystal Houping Xia a,b, Jianghe Feng a, Yuexia Ji c, Jinlong Xu a, Zhaojie Zhu a, Yan Wang a, Zhenyu You a, Jianfu Li a, Hongyan Wang d, Chaoyang Tu a,n a Key Laboratory of Optoelectronic Materials Chemistry and Physics of CAS, Fujian Institute of Research on the Structure of Matter, CAS, Fuzhou, Fujian 350002, PR China b University of Chinese Academy of Sciences, Beijing 100039, PR China c School of Mathematics and Physics, Anhui University of Technology, Maanshan, Anhui 243002, PR China d Crystech Inc., Qingdao 266107, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 23 October 2013 Received in revised form 18 December 2013 Accepted 20 December 2013 Available online 31 December 2013
The Sm3 þ :Sr3Gd(BO3)3 single crystal with dimensions of Φ20 25 mm3 was grown by the Czochralski method in nitrogen atmosphere. The effective segregation coefficient of Sm3 þ was measured as 0.975. The polarized absorption spectra, emission spectra and fluorescence decay curves of the crystal were measured. Based on the Judd–Ofelt theory, spectroscopic parameters of the crystal, including the oscillator intensity parameters Ωt (t ¼ 2, 4, 6), spontaneous emission probabilities, fluorescence branching ratios, and radiative lifetimes were calculated and analyzed. The crystal exhibited a broad absorption band from 390 nm to 418 nm, a broad emission band from 585 nm to 625 nm, as well as a fluorescence lifetime up to 2.04 ms. These results indicate Sm3 þ :Sr3Gd(BO3)3 crystal is promising for lasing in visible region. & 2013 Elsevier B.V. All rights reserved.
Keywords: Sm3 þ :Sr3Gd(BO3)3 crystal Czochralski growth Spectroscopic properties Judd–Ofelt parameters
1. Introduction Recently, with the increasing demand of visible lasers, investigations on praseodymium and dysprosium ions doped laser materials are becoming more significant [1–4]. In theory, trivalent samarium ions (Sm3 þ ) doped laser crystal also has prominent advantages as a candidate of visible solid-state laser materials: a large energy gap between the excitation state 4G5/2 and the next lower energy state 6F11/2, which makes the multiphonon relaxation tiny and a highly efficient visible emission of Sm3 þ in inorganic matrices; transitions from 4G5/2 to 6H5/2, 6H7/2, 6H9/2 and 6 H11/2 are in the visible range. However, the studies about Sm3 þ doped laser materials were very few in the past, it is mainly attributed to the lack of ultraviolet and blue laser diode (LD). Recently, the present development of blue LD is expected to change this situation [5,6]. Therefore, searching for new laser materials that can be used to get visible lasers directly through pumped by 405 nm blue LD is an urgent task. A few Sm3 þ doped glasses and crystals have been grown and shown good luminescence properties [7–16]. However, there has been no research on Sm3 þ doped Sr3Gd(BO3)3 (SGB) single crystal. SGB is a member of double borate compounds with formula M3R
n
Corresponding author. Tel.: þ 86 59183711368; fax: þ86 59183714946. E-mail address: [email protected] (C. Tu).
0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.12.048
(BO3)3 (M ¼Sr,Ba,Ca; R¼ rare earth). This crystal belongs to trigonal system with the space group R3 and the unit cell parameters are a¼ b¼12.58 Å, c ¼9.28 Å and Z ¼ 6 [17–19]. Like other borate crystals, the SGB crystal also has excellent physicochemical properities, wide range of optical transmittance and high damage threshold. In addition, the ionic radius of Sm3 þ is similar to that of Gd3 þ , which allows high doping concentration of Sm3 þ , and can improve the absorption efficiency for pumping laser. So the SGB crystal was chosen as the candidate material for Sm3 þ in this work. The Sm3 þ :SGB crystal was grown by the Czochralski method. The polarized absorption spectra, emission spectra and the lifetime decay curve were measured at room temperature. The effective J-O intensity parameters (Ωt,eff), spontaneous transition probabilities (A), fluorescent branching ratios (β), radiative lifetimes (τr) and emission cross sections (se) were calculated. The fluorescence lifetime (τf) was also obtained. 2. Experimental details 2.1. Crystal growth Sm3 þ :SGB crystal melts congruently at 1353.8 1C, it can be grown by Czochralski method. The constituent raw materials of Sm3 þ :SGB crystal were synthesized by the solid-state reaction method. The chemicals used were Gd2O3 and Sm2O3 with a purity
8
H. Xia et al. / Journal of Luminescence 149 (2014) 7–11
of 99.99%, SrCO3 and H3BO3 with a purity of 99.5%. The stoichiometric amounts of raw materials were weighed according to the following chemical reaction: (1 x)/2Gd2O3 þ x/2Sm2O3 þ 3SrCO3 þ 3H3BO3 Sr3Gd1 xSmx(BO3)3 þ 3CO2 þ 9/2H2O where x is the concentration of Sm3 þ and the value is 0.02. In order to compensate the evaporation losses of the H3BO3 in the growing process, excess 1.0 wt.% H3BO3 was added to the raw materials. Before the solid-state reaction the chemical powders were mixed, ground thoroughly and pressed into pellets. Then the pellets were placed in the platinum crucible and sintered at 1100 1C for 48 h. The process was repeated and the pellets were held at 1200 1C for 48 h to assure complete reaction. Sm3 þ :SGB crystal was grown by 2 kHz intermediate-frequency furnace in a nitrogen atmosphere. The synthesized polycrystalline materials formed by solid-state reaction were placed and melted in the iridium crucible. A rectangular [001] orientated seed crystal was introduced at a proper temperature higher than the melting point of the melt. The pulling rate varied from 1 to 1.6 mm/h and the crystal rotation speed was kept 10–20 rpm. After growth was completed, the crystal was cooled to room temperature at a rate of 10–25 K/h. 2.2. Spectra measurements The crystal plate for spectra experiments was cut along the [100] direction, with dimensions of 10 8 1.5 mm3 (b c a) and with the two 10 8 mm2 faces polished. It is shown in Fig. 1 (b). The polarization absorption spectra were measured using a Perkin-Elmer UV–VIS–NIR Spectrometer (Lambda-900). The polarized emission spectra and fluorescence lifetime were measured using an Edinburgh Instruments FLS920 spectrophotometer with a continuous Xe-flash lamp (excited with 404 nm radiation). The π and s polarizations are defined in terms of the electric field direction of the incident light being perpendicular and parallel to the c-axis of the Sm3 þ :SGB crystal, respectively. All the spectral experiments were carried out at room temperature.
The concentrations of elemental Sm, Sr, Gd, and B in the crystal were measured using an inductively coupled plasma atomic emission spectrometry (ICP-AES, Ultima2, Jobin-Yvon). The concentration of Sm3 þ ions was measured to be 0.925 1020 cm 3 (1.95 at.%) in this crystal. Taking the 2.0 at.% Sm3 þ concentration in the raw material into account, the effective segregation coefficient of Sm3 þ ions is 0.975 in SGB crystal. The hardness of Sm3 þ :SGB crystal was measured by a microVikers hardness tester (401 MVA™) at room temperature. Two slices of (100) and (001) faces were used for the test. Ten points were collected for each direction. The average values of the hardness are 453.6 N/mm2 for (100) face and 473.4 N/mm2 for (001) face, respectively. The results show that the anisotropy of the hardness of Sm3 þ :SGB crystal is inconspicuous, and the crystal is fit for machining. 3.2. Absorption spectra The luminescence properties of Sm3 þ are closely connected with the transitions between numerous and complicated 4 f5 multiplets. Fig. 2 is a simplified energy lever scheme of the Sm3 þ in the SGB crystal. It can help to analyze the absorption and emission spectra below. The room-temperature polarized absorption spectra for the 2.0 at% Sm3 þ : SGB crystal is shown in Fig. 3 and the line groups are labeled by the usual SLJ designation. The intense absorption bands distribute in the near infrared region corresponding to transitions from the ground state to the 6H and 6F multiplets. In the region between 500 nm and the UV absorption edge of the host crystal, another group of intense absorption bands can be observed. Among them, a prominent absorption band centered at 404 nm offers a possibility of optical pumping with blue LD. It also can be seen in Fig. 3 that the anisotropy of the Sm3 þ absorption transition intensities is small. The absorption band peak at 404 nm has a full width at half maximum (FWHM) of 11 nm for π polarization, and 9.5 nm for s
3. Results and discussion 3.1. Crystal growth The as-grown Sm3 þ :SGB crystal with dimensions of about Φ20 25 mm3 is shown in Fig. 1(a). This crystal has no cracks and the color is pale yellow.
Fig. 1. Photograph of Sm3 þ :SGB crysta (a) and the polished crystal plate (b).
Fig. 2. Simplified energy level scheme of Sm3 þ . Arrows indicate observed absorption and emission transitions.
H. Xia et al. / Journal of Luminescence 149 (2014) 7–11
polarization. Such a broad absorption band makes the crystal suitable for diode laser pumping. The polarized absorption cross-sections could be obtained by:
sðλÞ ¼
2:303 ODðλÞ N0 U l
where λ is the wavelength, L is the thickness of the crystal, OD is the optical density and N0 is the Sm3 þ concentration in the crystal. The absorption cross-sections at 404 nm are about 1.3 10 20 cm2 for π polarization and 1.1 10 20 cm2 for s polarization, which are larger than that of Sm3þ :GGG crystal (about 1.0 10 20 cm2) [13]. 3.3. Spectral parameters The Judd–Ofelt theory [20,21] is a popular method for estimating the spectroscopic parameters of rare-earth ions in crystals and glasses. The J–O intensity parameters Ωt (t¼ 2, 4, 6) can be calculated from the measured absorption spectra. In this work, all the seven absorption bands shown in Fig. 3 were used to fit the J–O parameters for both π and s polarizations. It is worth noting that among the absorption transitions of Sm3 þ ions, 6H5/2-4F5/2, 6F3/2, 6F5/2 contains the contribution of both the electric-dipole (ED) and magnetic dipole (MD) transitions; the absorption line strength of the MD transition should be excluded before the J–O calculation. For this reason, the experimental line strength Sexp(J,J0 ) of an ED transition from the initial state J to the final state J0 can be expressed as follows: Z 3hcð2J þ 1Þ 9n ln 10 ODðλÞ dλ Smd ðJ; J 0 Þ ð1Þ Sexp ðJ; J 0 Þ ¼ 3 2 2 8π e ðn2 þ 2Þ U λ N0 l Smd ðJ; J 0 Þ ¼
3hð2J þ 1Þλ 0 n U f md ðJ; J 0 Þ 8π 2 mc
ð2Þ
9
where λ is the mean wavelength of the transition, n is the refractive index (here we use the average value 1.73 in the entire wavelength region), h is the Planck constant, m is the mass of the electron, c is the velocity of the light, and e is the charge of the electron. In Eq. (2), f0 md(J,J0 ) is the vacuum oscillator strength which can be considered to be constant, here we use the value given by Ref. [22]. The calculated line strength Scal(J,J0 ) of ED transition depends on three Ωt parameters (t ¼2,4,6) as follows: Scal ðJ; J 0 Þ ¼
∑ Ωt j〈φJjjU ðtÞ jjφ0 J 0 〉j2
ð3Þ
t ¼ 2;4;6
where j〈φJjjU ðtÞ jjφ0 J 0 〉j2 is the square of the matrix elements of the tensorial operator, which is considered to be independent of host matrix and has been calculated in Ref. [23]. The root-mean-square deviation between experiment and calculation line strengths is defined by the following equation: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ∑J 0 ðSexp Scal Þ2 RMS ΔS ¼ ð4Þ ðN 3Þ where N is the number of transitions used in our analysis. The experimental and calculated line strengths Sexp and Scal, respectively, and the energies ν (cm 1) of the various absorption bands for π and s polarizations are listed in Table 1. The rootmean-square deviations (RMS ΔS) between Sexp and Scal and the relative errors are also listed in this Table 1. The value of RMSΔS indicates the fitting results are in good agreement with the experiments. By a least-square fitting between Eqs. (1) and (3), the three intensity parameters could be obtained and listed in Table 2. For the polarized absorption, the effective intensity parameters were defined by Ωt,eff ¼ (2Ωt,s þΩt,π)/3. Generally, Ω2 is sensitive to the local environment of the RE3 þ ions. Large value of Ω2 indicates high degree of covalency and lower symmetry of the coordination structure in the vicinity of RE3 þ ions. On the other hand, Ω4 and Ω6 depend on the rigidity of the host matrix. The spectroscopic quality factor X¼ Ω4/Ω6 for the Sm3 þ :SGB crystal is determined to be 1.18, which is compared with the values of Sm3 þ ions in different hosts presented in Table 4. The value of Sm3 þ :SGB crystal is similar to that of other crystals and higher than that of Sm3 þ :YAP and Sm3 þ :GdVO4 crystals, implying it is a promising material for efficient laser action. The probabilities for spontaneous transition from an excited manifold J to a lower manifold J0 can be calculated by the following: AðJ; J 0 Þ ¼ Aed ðJ; J 0 Þ þ Amd ðJ; J 0 Þ " # 64π 4 e2 nðn2 þ 2Þ2 0 0 3 S ðJ; J Þ þ n S ðJ; J Þ ¼ ed md 3 9 3hð2J þ 1Þλ
Fig. 3. Polarized absorption spectra of the Sm3 þ : SGB crystal.
ð5Þ
Table 1 The measured and calculated line strength of Sm3 þ :SGB crystal. Transition 6H5/2-
ν (cm-1)
Line strength (10 20 cm2) π
4
6
4
4
D1/2, P7/2, L17/2, K13/2, L13/2, 4F7/2, 6P3/2, 4K11/2, 4M21/2, 4 I11/2, 4I13/2,4F5/2 6 F11/2 6 F9/2 6 F7/2 6 F1/2, 6H15/2, 6F3/2, 6F5/2 4
26596 24814 21231 10638 9328 8189 6835
σ
Sexp
Scal
Sexp
Scal
0.287 0.699 0.133 0.184 1.314 2.101 2.351 RMS ΔS¼ 0.471 10 22
0.286 0.702 0.135 0.19 1.319 2.097 2.351
0.14 0.456 0.073 0.095 0.662 1.139 1.542 RMS ΔS¼ 0.582 10 22
0.144 0.46 0.069 0.095 0.669 1.133 1.541
10
H. Xia et al. / Journal of Luminescence 149 (2014) 7–11
Table 2 J–O intensity parameters of Sm3 þ :SGB crystal. Direction
Ω2 ( 10 20 cm2)
Ω4 ( 10 20 cm2)
Ω6 ( 10 20 cm2)
π
2.12 1.35 1.61
3.68 2.45 2.86
3.64 1.81 2.42
s
Ωt,eff
Table 3 The calculated luminescence parameters of Sm3 þ :SGB single crystal. Transition 6G5/2-
Aed
Amd
A (s 1)
β (%)
6
0.319 1.471 2.719 10.944 1.576 0.382 1.318 5.673 32.291 88.657 123.52 7.078
0.000 0.000 0.945 2.988 4.358 0.000 0.000 0.000 0.000 0.000 17.908 21.160
0.319 1.471 3.664 13.932 5.934 0.382 1.318 5.673 32.291 88.657 141.428 28.238 ΣA ¼323.307
0.10 0.45 1.13 4.31 1.84 0.12 0.41 1.75 9.99 27.42 43.75 8.73
F11/2 F9/2 F7/2 6 F5/2 6 F3/2 6 H15/2 6 F1/2 6 H13/2 6 H11/2 6 H9/2 6 H7/2 6 H5/2 6 6
τr (ms)
Fig. 4. Polarized emission spectra of the Sm3 þ : SGB crystal under 404 nm excitation.
3.09
where Aed and Amd are the ED and MD contributions, respectively. The ED line strength Sed is expressed as the following and the MD line strength Smd is given by the standard formula (7): Z 3hcð2J þ1Þ 9n ln 10 ODðλÞ dλ ð6Þ Sed ðJ; J 0 Þ ¼ 8π 3 e2 ðn2 þ 2Þ2 U λ N 0 l Smd ðJ; J 0 Þ ¼
2 h 〈φJ L þ 2J φ0 J 0 〉j2 2 2 4m c
ð7Þ
It is obvious that the Smd is a value does not change with the host environment. Here we use the value given by Ref. [24]. The fluorescence branching ratio of each of the transitions and the radiative lifetime of 4G5/2 can be determined from the spontaneous transition probability according to the equations: βðJ; J 0 Þ ¼
τr ¼
AðJ; J 0 Þ ∑J 0 AðJ; J 0 Þ
1 ∑J 0 AðJ; J 0 Þ
ð8Þ
ð9Þ
The spontaneous transition probabilities of electric-dipole transition Aed, the magnetic-dipole transition Amd, the total spontaneous transition probabilities Atotal, the fluorescence branching ratios β as well as the radiative lifetimes τr are summarized in Table 3. It should be noticed that the 4G5/2-6H5/2 transition is mainly dominated by MD and partly by ED, the 4G5/2-6H7/2 transition is MD allowed but ED dominated, the result fulfils the selection rules for magnetic dipole transitions ΔJ ¼0, 1. From Table 3 it can be seen that the radiative transition probability of the 4G5/2-6H7/2 transition is the highest, which comes to 43.75%. It indicates that the emission spectrum of Sm3 þ : SGB crystal should be dominated by this transition.
Fig. 5. Fluorescence decay curve of Sm3 þ : SGB crystal (λex ¼ 404 nm, λem ¼ 607 nm).
equate the overall intensity to 100%. The experimental branching ratios obtained are 17%, 56%, 23% and 4% for transitions to the terminal 6H5/2, 6H7/2, 6H9/2 and 6H11/2 levels, respectively. Thus, the agreement between the calculated and experimental branching ratios is moderately good [15]. The stimulated emission crosssections can be calculated via the following expression:
se ðλÞ ¼
β 8π 2 cn2 τ
R r
λ5 IðλÞ λIðλÞ dλ
ð10Þ
where I(λ) is the emission intensity. The stimulated emission cross-sections at 600 nm are 6.4 10 22 cm2 with FWHM of 12 nm for π polarization and 4.8 10 22 cm2 with FWHM of 24 nm for s polarization, respectively. Fluorescence decay curve of the 4G5/2-6H7/2 transition in Sm3 þ :SGB is as shown in Fig. 5, which is monitored by 607 nm and excited by 404 nm. It can be seen that the fluorescence lifetime is 2.04 ms, which is much longer than that of Sm3 þ doped other crystals listed in Table 4. It is well known that a long upper laser level lifetime is beneficial in achieving population inversion and increasing energy storage, the Sm3 þ :SGB crystal having a long lifetime will make it desirable for laser operation.
3.4. Emission spectra and fluorescence lifetime The emission spectra is shown in Fig. 4. The transitions from G5/2 level to the 6HJ (J ¼5/2, 7/2, 9/2, 11/2) levels are observed. The reliability of the calculated branching ratio values can be now confirmed roughly by integrating the emission bands in Fig. 4, evaluating relative percentage for each band on condition that
4. Conclusions
4
A Sm3 þ :SGB crystal was grown by the Czochralski method. The Vickers hardness of (100) and (001) faces are 453.6 and 473.4 N/mm2, respectively. The room temperature polarized
H. Xia et al. / Journal of Luminescence 149 (2014) 7–11
11
Table 4 A comparison of J–O parameters, spectroscopic quality factor, radiative lifetime and fluorescence lifetime of Sm3 þ :SGB with other crystals. Host crystal
Ω2 ( 10 20 cm2)
Ω4 ( 10 20 cm2)
Ω6 ( 10 20 cm2)
Ω4/Ω6
τr (ms)
YAP GdVO4 GGG CaNb2O6 Gd2SiO5 Sr3Gd(BO3)3
2.28 2.75 3.90 6.33 1.12 1.61
1.62 3.22 2.48 6.49 5.57 2.86
2.21 4.97 1.83 3.72 2.78 2.42
0.73 0.65 1.36 1.74 2.00 1.18
2.4 0.54 2.54 0.75 1.78 3.09
absorption spectra, emission spectra, and fluorescence decay curve of the grown crystal were measured. The absorption cross-sections at 404 nm are 1.3 10 20 cm2 with FWHM of 11 nm for π polarization and 1.1 10 20 cm2 with FWHM of 9.5 nm for s polarization. The Judd–Ofelt theory was applied to calculate the spectral parameters. The emission spectra indicates that visible fluorescence of Sm3 þ consists of green light (563 nm), orange light (600 nm) and red light (647 nm) under 404 nm excitation in SGB crystal. The emission cross-sections (se) at 600 nm were calculated to be 6.4 10 22 cm2 for π polarization and 4.8 10 22 cm2 for s polarization. The fluorescence lifetime was measured to be 2.04 ms. Above results indicate that the Sm3 þ :SGB crystal is a potential visible laser material matched with pumping of blue LD.
Acknowledgments This work was supported by Science and Technology Plan Major Projects of Fujian Province (2010I0015, 2012H0048), National Nature Science Foundation of China (50902129, 61078076, 91122033, 11304313), Knowledge Innovation Program of Chinese Academy of Sciences (KJCX2-EW-H03). References [1] M. Fibrich, H. Jelinkova, J. Sulc, K. Nejezchleb, V. Skoda, Appl. Phys. B: Lasers Opt. 97 (2009) 363.
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Ref.
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