Growth and spectroscopic properties of Sm3+ doped Na3La9O3(BO3)8 crystal

Optical Materials xxx (2015) xxx–xxx

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Growth and spectroscopic properties of Sm3+ doped Na3La9O3(BO3)8 crystal Faxian Shan a,b, Guochun Zhang a,⇑, Xinyuan Zhang a,b, Tianxiang Xu c, Tao Xu a,b, Yicheng Wu a a Beijing Center for Crystal Research and Development, Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China b University of Chinese Academy of Sciences, Beijing 100190, China c State Key Laboratory of Crystal Materials and Institute of Crystal Materials, Shandong University, Jinan 250100, China

a r t i c l e

i n f o

Article history: Received 2 September 2014 Received in revised form 15 December 2014 Accepted 27 December 2014 Available online xxxx Keywords: Sm:Na3La9O3(BO3)8 Judd–Ofelt parameter Emission cross-section Orange-red laser

a b s t r a c t Sm3+ doped Na3La9O3(BO3)8 (Sm:NLBO) crystal has been grown by the top-seeded solution growth (TSSG) method. The absorption and emission spectra were measured at room temperature by using a (1 0 0) oriented crystal plate. According to the Judd–Ofelt theory, the spontaneous transition probabilities, fluorescence branch ratio, and radiation lifetime of 4G5/2 state were calculated. The unpolarized and polarized emission properties of 564, 601, and 651 nm under the 405 nm excitation were also evaluated. The decay curve of the 601 nm emission assigned to the 4G5/2–6H7/2 transition was also measured. The experimental results have shown that Sm:NLBO would be a promising orange-red laser material, which can be pumped directly by lighting 405 nm laser diodes (LDs). Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Orange-red lasers in the wavelength range of 580–620 nm have important applications in iatrology, astronomy, telecommunication, remote sensing, and color displays [1,2]. In the past there were few reports about the generation of orange-red lasers pumped directly by a light source because of the insufficient absorption properties for optical pumping with classical lamps [3]. This shortcoming can be overcome at near future with the rapid development of high power laser diodes (LDs) emitting 405 nm blue light. Therefore it is an urgent task to develop novel orange-red laser materials which can be pumped directly by 405 nm LDs [1,3]. In rare earth ion Sm3+ doped laser crystals, there is an emission of the 4G5/2–6H7/2 transition at about 600 nm in the orange-red range when pumped by 405 nm LDs [1,4]. Furthermore, the large energy between 4G5/2 and a next lower-lying state is up to 7000 cm1, which would result in a negligible nonradiative multiphonon relaxation and an efficient orange-red luminescence with a low laser threshold [1,3]. Since trivalent rare earth ions such as La3+, Y3+, and Gd3+ could be easily partially substituted by Sm3+ without charge imbalance and structure distortion, Sm3+ doped rare earth laser crystals have attracted lots of attention. Recently, the spectroscopic investigations of Sm3+ doped La2CaB10O19 (LCB) [5], YAlO3 (YAP) [6], YAl3(BO3)4 ⇑ Corresponding author. Tel./fax: +86 10 82543726. E-mail address: [email protected] (G. Zhang).

(YAB) [7], YCa4O(BO3)4 (YCOB) [8], Gd3Ga5O12 (GGG) [1,3], and GdVO4 [9] crystals have been reported. Na3La9O3(BO3)8 (NLBO) is a promising borate nonlinear optical crystal with excellent optical properties, such as a large nonlinear optical coefficient, a wide transparency range, and a moderate birefringence. In addition, the crystal exhibits high chemical stability and good mechanical properties [10]. To our knowledge, Sm3+ doped NLBO crystal are not available in previous literatures. In contrast to the above host crystals, NLBO possess a much larger density of rare earth ions (N = 1.51  1022 ions/cm3) [11]. Therefore its crystal structure can accommodate much larger amounts of Sm3+, which offers a very good opportunity to design a high-samarium-content luminescent system for orangered lasers. In this work, we report on the crystal growth of Sm:NLBO by the top-seeded solution growth (TSSG) method. The spectral properties of the crystal were also characterized by absorption and emission spectra, and the spectroscopic parameters were calculated on the basis of the Judd–Ofelt theory [12,13]. The fluorescence decay time of 4G5/2–6H7/2 transition has been reported. 2. Experimental 2.1. Crystal growth Sm:NLBO polycrystalline samples were synthesized by high temperature solid state reaction. According to the formula Na3Sm0.9La8.1B8O27, the 99.99% purity Sm2O3 and La2O3, analytical

http://dx.doi.org/10.1016/j.optmat.2014.12.031 0925-3467/Ó 2015 Elsevier B.V. All rights reserved.

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F. Shan et al. / Optical Materials xxx (2015) xxx–xxx

grade Na2CO3 and H3BO3 were weighted and mixed in an agate mortar thoroughly. The mixture was transferred into a platinum crucible, heated up to 950 °C and maintained for 72 h with several intermediate grindings, and finally followed by natural cooling to room temperature. Sm:NLBO single crystals were grown by the TSSG method from Na2O–B2O3–NaF flux system. The above-synthesized polycrystalline samples were mixed with Na2CO3, H3BO3, and NaF (analytical grade) in molar ratio of 1:7:6:9, and then the mixture were melted in a platinum crucible with a diameter of 70 mm and a height of 70 mm in several batches. The crucible was placed in a cylindrical resistance heated furnace controlled by a Shimaden FP23 controller with an accuracy of ±0.1 °C. The mixture was then heated to 1080 °C, stirred with a platinum stirrer for 48 h to ensure the solution melted completely and mixed homogeneously. The saturation temperature was determined to be 1012 °C by the testing seed crystal method. A seed with [1 0 1] orientation was slowly dipped into the solution at 1050 °C, and kept for half an hour to dissolve the outer surface of the seed. After that the temperature was lowered to 1012 °C at a rate of 30 °C/h, and then decreased at a rate of 0.2 °C/day until the end of growth. The growing crystal was rotated at 4 rpm. When the growth was finished after a month, the crystal was pulled out of the solution and cooled to room temperature at a rate of 10 °C/h. A rhombic-shaped crystal with size of 35  20  15 mm3 is shown in Fig. 1a. The Sm3+ concentration was determined by inductively coupled plasma optical emission spectrometry (Varian 710 ICP–OES). The content of Sm3+ in Sm:NLBO was 1.21  1021 ions/cm3, which is much higher than that in other Sm3+ doped crystals. The segregation coefficient is about 0.79 at doping concentration of 10%.

Fig. 1. As-grown Sm:NLBO crystal and schematic of (1 0 0) oriented rectangular crystal plate. (a) Sm:NLBO crystal. (b) Schematic of (1 0 0) oriented crystal plate.

where hðS; LÞJjjU ðtÞ jjðS0 ; L0 ÞJ0 i is the reduced matrix element of the tensorial operator corresponding to the transition from 6H5/2 to excited states, which is considered to be independent of host material and has been calculated in Ref. [15]. Xt (t = 2, 4, 6) are the three phenomenological intensity parameters of the host crystal field. The experimental line strengths of ED transition Sexp ed can be obtained from the absorption spectrum as following 0 Sexp ed ðJ; J Þ ¼

3hcð2J þ 1Þ 9n 2:3 8p3 e2 N ðn2 þ 2Þ2 kL

Z

ODðkÞdk

ð2Þ

2.2. Spectral measurements The as-grown crystal was cut into a (1 0 0) oriented rectangular plate with size of 4  4  2.5 mm3 and polished on both sides to 2 mm in thickness, as shown in Fig. 1b. The incident surface is the crystalline face (1 0 0). The absorption spectrum was measured by using a Lambda 900 UV–vis–NIR spectrophotometer at room temperature in the wavelength range of 200–2000 nm. The crystal plate was excited by a 450 W stable Xenon lamp at wavelength of 405 nm. Emission spectra in the wavelength range of 540–680 nm were measured at room temperature by a FLS920 spectrofluorimeter (Edinburg Instrument) with a scan step width of 0.25 nm and a fixed counting time of 0.2 s. The signal was recorded by an InGaAs solid-state detector with a resolution of <0.09 nm. The fluorescence decay time for 4G5/2 to 6H7/2 transition monitored by 601 nm and excited by 405 nm was also measured with the above spectrofluorimeter. 3. Results and discussion 3.1. Absorption spectrum and Judd–Ofelt theory analysis The absorption spectra of Sm:NLBO crystal in the wavelength range of 200–800 and 800–2000 nm are shown in Fig. 2. The dominant absorption bands correspond to the 4f electronic transitions of Sm3+ from the ground state 6H5/2 to excited states. Since Judd– Ofelt theory is uncertain to be applied to the transitions to high energy levels [6,14], we choose the low energy region bands in our calculation. The absorption peaks in 800–2000 nm are labeled by the usual SLJ designation [15], as shown in Table 1. According to the Judd–Ofelt theory, the calculated line strengths of the electric-dipole (ED) transition from the initial state |J > to the final state |J0 > are given by 0 Scal ed ðJ; J Þ ¼

X t¼2;4;6

Xt jhðS; LÞJkU ðtÞ kðS0 ; L0 ÞJ0 ij2

ð1Þ

Fig. 2. Absorption spectra of Sm:NLBO crystal at room temperature. (a) Visible region. (b) Near infrared region.

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F. Shan et al. / Optical Materials xxx (2015) xxx–xxx Table 1 Experimental and calculated line strengths (ED transitions) as well as Judd–Ofelt parameters of Sm:NLBO. R  Transitions 6H5/2 ? OD(k) dk (nm) k (nm) sexp (1020 cm2) ed 6

F11/2 F9/2 6 F7/2 6 F5/2 6 F3/2 + 6H15/2 + 6F1/2

931 1075 1229 1377 1481

6

9.478 66.992 112.245 66.924 59.514

0.364 2.230 3.275 1.745 1.444

20 cm2) scal ed (10

DS

DS2

0.307 2.128 3.363 1.707 1.447

0.057 0.102 0.088 0.038 0.003

0.003 0.010 0.008 0.001 0.000

X2 = 1.868  1020 cm2; X4 = 5.791  1020 cm2; X6 = 5.886  1020 cm2

k is the mean wavelength of the transition, L is the crystal where  thickness, OD(k) is the measured optical density, N is the Sm3+ concentration, n is the refractive index of NLBO crystal from Ref. [10], c is the velocity of light, e is the charge of electron, and h is the Planck constant. The three intensity parameters Xt were fitted by a leastroot-means-square fitting based on Eqs. (1) and (2). The fitted parameters are X2 = 1.868  1020 cm2, X4 = 5.791  1020 cm2, and X6 = 5.886  1020 cm2. The root-mean-squares deviation (rms–DS) is 1.071  1021 cm2, which indicates that the calculated results are in good agreements with the experimental ones. Table 1 represents the experimental and calculated line strengths of ED transition from the ground state 6H5/2 to excited states. The spontaneous transition probability A from the excited state 4 G5/2 to other lower-lying state could be calculated by the following equations:

Table 2 Calculated spontaneous transition probability (A), fluorescence branch ratio (bc), and the radiation lifetime (srad) of 4G5/2 state in Sm:NLBO.

A ¼ Aed þ Amd

transitions obey the selection rule DJ = ±1, 0. The 4G5/2–6H5/2 transition is mainly dominated by MD and partly by ED, the 4G5/2–6H7/2 transition is mainly dominated by ED and partly by MD, and the 4 G5/2–6H9/2 transition is purely ED transition. The emission band occurs at 601 nm has an extremely high intensity, which indicates that Sm:NLBO could be used in the generation of orange-red light. It is clear from Fig. 3 that the emission intensity for E\(0 0 1) plane is much higher than that for E\(1 1 0) plane, for example, the emission intensity at 601 nm for E\(0 0 1) plane is 2.5 times that for E\(1 1 0) plane. The emission cross-sections of the above three emission peaks in the unpolarized emission spectrum could be calculated with the following equation:

ð3Þ 2

Aed ðJ; J 0 Þ ¼

64p4 e2 nðn2 þ 2Þ 0 cal S 3hk3 9ð2J þ 1Þ ed

ð4Þ

Amd ðJ; J 0 Þ ¼

64p4 e2 n3 S0 cal 3hk3 2J þ 1 md

ð5Þ

cal S0md ðJ; J 0 Þ ¼



h 4pmc

2

jhðS; LÞJjjL þ 2SjjðS0 ; L0 ÞJ 0 ij2

ð6Þ

where Aed is the contribution from ED to the transition probability, Amd is the contribution from magnetic-dipole (MD), s0edcal was calculated based on Xt and the new reduced matrix element [16] related cal to the transition from 4G5/2 to other lower-lying state, s0md is the calculated line strength of the MD transition, and m is the mass of eleccal tron. It is clear from Eq. (6) that s0md does not change with the host material. Therefore we adopted the values reported in Ref. [16]. The fluorescence branch ratio bc and the radiation lifetime srad of the excited state 4G5/2 could be calculated with the following equations. The calculated luminescence parameters are summarized in Table 2.

AðJ; J 0 Þ bc ¼ P 0 J 0 AðJ; J Þ 1 AðJ; J0 Þ J

srad ¼ P

4

G5/2

6

k (nm)

F11/2 1460 6 F9/2 1198 6 F7/2 1038 6 F5/2 953 6 F3/2 908 6 H15/2 902 6 F1/2 893 6 H13/2 795 6 H11/2 713 6 H9/2 649 6 H7/2 601 6 H5/2 564 srad = 1.485 ms

s0edcal (1022 cm2)

cal s0md (1022 cm2)

A (s1)

bc

0.352 0.628 1.102 2.447 0.263 0.118 0.187 1.175 4.305 7.149 10.237 0.404

0 0 0.267 0.662 0.846 0 0 0 0 0 0.581 0.573

0.710 2.303 7.864 23.179 9.938 1.022 1.673 15.000 76.674 170.797 327.252 37.196

0.001 0.003 0.012 0.034 0.015 0.002 0.002 0.022 0.114 0.254 0.486 0.055

ð7Þ

ð8Þ

0

3.2. Emission spectra Fig. 3 represents the unpolarized and polarized emission spectra of Sm:NLBO under the 405 nm excitation at room temperature. E represents the electric field of the incident light. There are three emission bands at about 564 nm, 601 nm, and 651 nm, which assigned to the transition from the excited state 4G5/2 to 6H5/2, 6 H7/2, and 6H9/2, respectively. As is known, the MD allowed

Fig. 3. Unpolarized and polarized emission spectra of the (1 0 0) oriented plate after excited at 405 nm at room temperature. (The blue curve is the emission spectrum with unpolarized electric field of the incident light. The black and red ones are the spectra when the electric field is perpendicular to the (1 1 0) plane and (0 0 1) plane, respectively.) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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F. Shan et al. / Optical Materials xxx (2015) xxx–xxx

Table 3 Comparison of spectral parameters in Sm:NLBO with those in other Sm3+ doped rare-earth crystals. Crystal

Transition

k (nm)

A (s1)

bc

rem (1022 cm2)

Sm:NLBO Sm:LCB Sm:YAP Sm:YAB Sm:GGG Sm:GdVO4

4

G5/2– H5/2

564 563 567 564 566 567

37.196 23.921 36.249 1.877 20.38 97

0.055 0.076 0.086 0.001 0.054 0.052

12.25 5.7 14.9 – 11.2 59.2

Sm:NLBO Sm:LCB Sm:YAP Sm:YAB Sm:GGG Sm:GdVO4

4

G5/2–6H7/2

601 599 607 600 613 604

327.252 170.47 163.95 719.40 178.52 305.29

0.486 0.538 0.387 0.374 0.470 0.164

72.48 15.7 17.2 – 31.79 76.2

Sm:NLBO Sm:LCB Sm:YAP Sm:YAB Sm:GGG Sm:GdVO4

4

G5/2–6H9/2

651 648 648 647 662 646

170.797 57.388 135.324 829.72 73.94 844.12

0.254 0.181 0.320 0.432 0.195 0.454

101.79 5.2 26.2 – 5.12 58.8

rem ðkÞ ¼

6

bk5 IðkÞ R 8pcn2 srad kIðkÞdk

rem  sm (1022 cm2 ms)

21.019 30.772 – – – 41.148

ð9Þ

where k is the wavelength and I(k) is the emission intensity. The spectral parameters of 4G5/2 to 6H5/2, 6H7/2, and 6H9/2 transitions in Sm:NLBO were presented in Table 3. For comparison, the spectral parameters of other Sm3+ doped rare-earth crystals [1–9] are also summarized in Table 3. Obviously, Sm:NLBO has a large emission cross-section (7.248  1021 cm2) at 601 nm, which is much larger than that of other Sm3+ doped crystals except for Sm:GdVO4 [9] whose dominant emission is not in the region of 580–620 nm. The large cross-section is an important parameter influencing the potentials on low threshold and high gain laser applications. In addition, the large bc value at 601 nm of Sm:NLBO is comparable to that of Sm:LCB [5] and Sm:GGG [1]. The fluorescence decay curve for the 4G5/2–6H7/2 transition in Sm:NLBO crystal has been recorded and analyzed. The (1 0 0) crystal plate was excited by 405 nm, and the orange-red emission at 601 nm was observed. The obtained curve (Fig. 4) was found to be not a single exponential function when the intensity was presented in logarithmic scale. It is a common phenomenon for the materials with a high doped concentration of rare-earth ions due to the strong interaction between doped ions [17,18], which imply that Sm3+ in Sm:NLBO crystal might locate in different sites with a rather short distance between each other. To our knowledge, there are some studies on the crystallographic sites in which rare-earth ions could be located in NLBO crystal. For example, Balda verified the existence of at least two different sites (mainly La1 and La2) for Nd3+ ions in Nd:NLBO crystal (initial doping content is 4.6%) at 4.2 K by using steady state and time-resolved laser spectroscopy [19]. Cascales studied the site-resolved luminescence of Eu3+:NLBO crystal (initial doping content is 2.3%) at 10 K, which shows that doped ions may occupy the crystallographic sites for La1, La2, Na, and B3, respectively [20]. However, the 4G5/2–6HJ/2 (J = 5, 7, 9, 11) emissions of Sm3+ in Sm:NLBO crystal result in a complex spectrum for lacking of non-degenerate levels. In addition, the extremely high Sm3+ concentration in Sm:NLBO crystal (initial doping content is 10.0%) caused a strong level overlapping of emission peaks from different sites. Therefore it is difficult to identify the crystallographic sites of Sm3+ ions in Sm:NLBO crystal at room temperature. The mean lifetime [3,18] was evaluated to be R R smean ¼ IðtÞt dt= IðtÞ dt ¼ 0:29 ms, which is shorter than that of Sm:GdVO4 (0.54 ms) [9]. The luminescence quantum efficiency was calculated to be g = smean/srad = 19.5%. In addition, the product of the lifetime and the emission cross-section at 601 nm is

Fig. 4. Fluorescence decay curve for the 4G5/2–6H7/2 transition in Sm:NLBO crystal.

2.1019  1021 cm2 ms, which is comparable to that of other Sm3+ doped rare-earth crystals, as shown in Table 3. The above results indicate that the 4G5/2–6H7/2 transition in Sm:NLBO crystal is suitable for the generation of orange-red light at about 600 nm. 4. Conclusions In conclusion, Sm:NLBO crystals have been grown by the TSSG method from Na2O–B2O3–NaF flux system. The spectroscopic properties of Sm:NLBO crystal have been investigated at room temperature. Based on the Judd–Ofelt theory, the intensity parameters obtained by the least-root-means-square fitting method are X2 = 1.868  1020 cm2, X4 = 5.791  1020 cm2, and X6 = 5.886  1020 cm2. The spontaneous transition probability A, fluorescence branch ratio bc, radiation lifetime srad, and emission cross-section rem have also been calculated. The emission cross-section, fluorescence branch ratio, and florescence lifetime at 601 nm related to the transition 4G5/2–6H7/2 are 7.248  1021 cm2, 48.6%, and 0.29 ms, respectively. In comparison with other Sm3+ doped rare earth crystals, Sm:NLBO has a larger emission cross-section and branch ratio at 601 nm, indicating that the 4G5/2–6H7/2 transition in Sm:NLBO is well suitable for the generation of orange-red light.

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