Growth and spectroscopic properties of Sm3+-doped La2CaB10O19 crystal

Journal of Crystal Growth 399 (2014) 39–42

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Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro

Growth and spectroscopic properties of Sm3 þ -doped La2CaB10O19 crystal Xinyuan Zhang a,b, Yang Wu a,b, Faxian Shan a,b, Yin Li a,n, Jianxiu Zhang a, 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, PR China b University of Chinese Academy of Sciences, Beijing 100049, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 6 January 2014 Received in revised form 17 April 2014 Accepted 17 April 2014 Communicated by M. Roth Available online 24 April 2014

Sm3 þ -doped La2CaB10O19(Sm:LCB) crystal with size of 40  25  15 mm3 has been grown by the topseeded solution growth (TSSG) method. The absorption and fluorescence spectra of Sm:LCB were measured at room temperature, and the fundamental spectral parameters, including transition probability A, fluorescence branch ratio βc, radioactive lifetime τr, emission cross-sections sem and intensity parameters Ωt(2,4,6), have been calculated by applying the Judd–Ofelt theory. In Sm:LCB crystal, a strong 599 nm orange fluorescence emitting peak was observed under 410 nm excitation, which may be applied in the medicine and biological detection. Sm:LCB crystal may be a promising orange laser material. & 2014 Published by Elsevier B.V.

Keywords: A1. Doping A2. Single crystal growth B1. Borate B3. Self-frequency doubling

1. Introduction La2CaB10O19 (LCB) possesses a relatively large effective nonlinear optical coefficient (1.05 pm/V), a wide optical transparency range (185–3000 nm), a moderate birefringence (Δn ¼ 0.053 at 1064 nm), a high laser damage threshold (11.5 GW/cm2 for 8 ns pulses at 1064 nm), and stable chemical and mechanical properties (non-hygroscopic, Moh's hardness 6.5) [1–4]. All these advantages lead to the development of LCB crystal as an UV nonlinear optical material. Presently, a 31.6 W of 355 nm laser radiation with the optical conversion efficiency of 28.9% was obtained by using the LCB crystal [5]. Since La3 þ ions in the LCB lattice can be partially substituted by other rare earth ions, LCB crystals doped with rare earth active ions, including Pr3 þ [6], Nd3 þ [7,31], Dy3 þ [8], Ho3 þ [9], Er3 þ [10] and Yb3 þ [11], have been extensively investigated. These doped LCB crystals exhibit excellent spectroscopic and laser properties, which indicate that the doped LCB crystals will be promising selffrequency doubling materials. For example, in Nd3 þ :LCB crystal, the output power of the three green visible lasers (525 nm, 529 nm, and 533.6 nm) generated by multi-self-frequency-conversion of the fundamental laser is up to 26.64 mW, and the light–light conversion efficiency is up to 4.85% [12]. Yb3 þ lasers in Yb:LCB crystal could be easily achieved at the wavelength having the highest emission n

Corresponding author. E-mail address: [email protected] (Y. Li).

http://dx.doi.org/10.1016/j.jcrysgro.2014.04.020 0022-0248/& 2014 Published by Elsevier B.V.

intensity or at a specified wavelength, or emitting two frequencies with a specified frequency difference [13]. Up to now the spectroscopic and laser properties have not been studied in the Sm3 þ -doped La2CaB10O19 (Sm:LCB) crystal. The Sm3 þ ion (4f5) exhibits intense fluorescence in the visible region, where the emission wavelengths are located at 550–570 nm (4G5/2–6H5/2), 580–610 nm (4G5/2–6H7/2), 630–650 nm (4G5/2–6H9/2), and 700–710 nm (4G5/2–6H11/2) yellow–red band [14]. Among them, the laser emissions at 560–580 nm (4G5/2–6H5/2) and 580– 610 nm (4G5/2–6H7/2) may be applied in telecommunication and color display technology. Nowadays, the research of Sm3 þ ions concentrates upon the measurement and analysis of luminescent properties of Sm3 þ -doped tellurite, fluoroborate, phosphate, fluorophosphates glasses, and crystals [14–22]. In this work, Sm:LCB crystals have been grown by the topseeded solution growth (TSSG) method. Sm3 þ ion oscillator strength in Sm:LCB crystal has been analyzed by the Judd–Ofelt theory on the basis of absorption spectrum. The emission spectrum and fluorescence lifetime of Sm3 þ ion are also reported.

2. Experimental 2.1. Crystal growth Sm:LCB crystals were grown by the top-seeded solution growth (TSSG) method in Li2O–CaO–B2O3 flux system. The starting

40

X. Zhang et al. / Journal of Crystal Growth 399 (2014) 39–42

Fig.1. The Sm3 þ -doped LCB crystal.

materials were 99.99% purity La2O3 and Sm2O3, analytical grade Li2CO3, CaCO3, and H3BO3. The mixture of the starting charges taken in appropriate proportion was thoroughly homogenized in an agate mortar, and then melted in a platinum crucible with a diameter of 80 mm and a height of 100 mm in several batches. The crucible was placed in a cylindrical resistance heated furnace, which is controlled by a Shimaden FP23 controller with an accuracy of 70.1 1C. The mixture was heated to about 30 1C above the expected saturation temperature, stirring with a platinum stirrer for 48 h in order to ensure that the solution melted completely and mixed homogeneously. A tentative seed was used for determining the saturation temperature. A seed with [110] orientation was slowly dipped into the solution at a temperature of 10 1C above the saturation temperature and kept for 10 min to dissolve the rough surfaces. After that, the temperature was lowered to saturation temperature within 30 min, and then decreased at a rate of 1 1C/day during the first 10 days. The cooling rate was changed to 2–4 1C/day subsequently. The growing crystal was rotated at 4.5 rpm. After about 2 months, the crystal was pulled out of the solution, and cooled down to room temperature at the rate of 20–30 1C/h. A rhombic-shaped crystal with size of 40  25  15 mm3 is shown in Fig. 1. Sm3 þ concentration was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). The result showed that content of Sm3 þ ions in Sm:LCB was 1.34  1020 ions/cm3, and the segregation coefficient of Sm3 þ ions in LCB crystal was about 0.45 at doping concentration 2.25%.

Fig.2. Absorption spectra of Sm:LCB crystal. (a) Visible region. (b) Near infrared region.

Table 1 The calculated parameters of absorption spectra for Sm3 þ in Sm:LCB crystal. Transitions H5/2-

λ(nm)

R OD(λ)dλ (nm)

Smeas (10  20 cm2)

Scal ΔS (10  20 cm2)

ΔS2

6

935 1070 1242 1356 1547

0.228 2.156 3.892 1.570 0.887

0.171 1.418 2.211 0.818 0.406

0.220 1.502 2.226 0.887 0.467

0.002 0.007 0.000 0.005 0.004

6

2.2. Spectra measurements The as-grown Sm:LCB crystal was cut into slices with size of 6  6  1 mm3 and polished along the (010) plane. The absorption spectra were measured using a Lambda 900 UV–vis–NIR spectrophotometer at room temperature in a range from 250 to 1800 nm. The Sm:LCB sample was excited with an Edinburgh Instruments FLS920 spectrophotometer xenon lamp laser that emits 410 nm radiations. The measurement range is from 530 to 700 nm.

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

0.049 0.084 0.015 0.069 0.061

rms-ΔS is 0.0949  10–20. Ω2 ¼ 7.49  10  22 cm2; Ω4 ¼ 3.10  10  20 cm2; Ω6 ¼ 4.22  10  20 cm2.

the terminal J0 level are given by 3. Results and discussion

Sed ðJ-J 0 Þ ¼

3.1. Absorption spectra and Judd–Ofelt theory analysis The absorption spectra of Sm:LCB crystal are shown in Fig. 2. The dominant peak positions correspond to the characteristic absorption of 4f electronic transitions in Sm3 þ . According to the energy level diagram of Sm3 þ [23], specific transition energies have been identified. The results are listed in Table 1. According to the Judd–Ofelt theory [24,25], the electric and magnetic dipole line strengths for a transition from initial J level to

∑ Ωt j〈ðS; LÞJ‖U ðtÞ ‖ðS0 ; L0 ÞJ 0 〉j2

t ¼ 2;4;6

ð1Þ

and
Smd ¼

h 4πmc

2

 〈ðS; LÞJ‖L þ2S‖ðS0 ; L0 ÞJ 0 〉j2

ð2Þ

where 〈ðS; LÞJ‖U ðtÞ ‖ðS0 ; L0 ÞJ 0 〉is the reduced matrix elements that depends only on the RE ion. Here the calculated values of the squares of the reduced matrix elements are cited from Carnall's

X. Zhang et al. / Journal of Crystal Growth 399 (2014) 39–42

data. Ωt (t ¼2, 4, 6) are the three phenomenological intensity parameters arising from the static crystal field. The measured line strengths Smeans ðJ-J 0 Þ from the absorption ed spectrum can be given by Z 9n 3hc 2J þ 1 2:3 ODðλÞdλ ð3Þ Smeans ðJ-J 0 Þ ¼ ed 2 8π 3 e2 2 λ Nc L ðn þ 2Þ where λ is the mean wavelength of the absorption band, d is the crystal thickness, Nc is the Sm3 þ ions concentration, e is the electron charge, n is the refractive index, and ODðλÞpresents the measured optical density. From the measured line strengths Smeans ðJ-J 0 Þ, the three ed intensity parameters Ωt ( t¼2,4, 6) fitted by the least-square method are 7.49  10  22 cm2, 3.10  10–20 cm2, and 4.22  10  20 cm2, respectively. The root-mean-square deviation (rms)ΔS is 0.0949  10–20, which indicates that the fitting results are in good agreement with the experiments. Table 1 presents measured and calculated line strengths and intensity parameters of Sm3 þ in Sm:LCB crystal. On the basis of the Judd–Ofelt theory, the luminescence parameters can be calculated from the following equations:   64π 4 e2 nðn2 þ 2Þ2 Aed J-J 0 ¼ Sed 3 3hλ 9ð2J þ 1Þ Amd ðJ-J 0 Þ ¼

64π 4 e2 3hλ

3

ð4Þ

n3 S ð2J þ1Þ md

41

The emission cross sections were calculated using the following formula [27]:

sp ¼

λ4p AðJ-J 0 Þ 2 8π cn2 Δλef f

ð9Þ

In this expression, λp is the peak fluorescence wavelength, n is the refractive index at the peak fluorescence wavelength, and Δλef f is its effective line width derived by dividing the area of the emission band by its average height. The emission cross-section values correspond to peaks at 563 nm, 599 nm and 648 nm, which are presented in Table 3. For comparison, the spectral parameters of other Sm3 þ -doped laser crystals are also listed in Table 3. The dominant peak emission cross section amounts to 15.7  10  22 cm2, which is at 599 nm and correspond to 4G5/2-6H7/2 transition. The product of the lifetime and the emission crosssection at 599 nm is 3.08  10  21 cm2 ms, which is larger than that of Sm3 þ in LiNbO3 crystal (1.088  10  21 cm2 ms at 603 nm) and Sm:(Mg, Ca, Zr) GGG crystal (1.06  10  21 cm2 ms at 601 nm) [29,30]. The large emission cross section of 4G5/2-6H7/2 transition is a parameter influencing the potential on low threshold and high gain laser applications, which indicates that Sm:LCB crystal is a potential orange laser crystal.

599

100

ð5Þ

Excit410nm

A ¼ Aed þ Amd 1 τrad

ð6Þ

¼ ∑AðJ-J 0 Þ J

ð7Þ

0

0

AðJ-J Þ βc ¼ ∑J 0 AðJ-J 0 Þ

Emission (a.u.)

80

F11/2 F9/2 6 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 τrad ¼ 3.159 ms

0 550

600

650

700

Wavelength (nm)

Table 3 Comparison of spectral parameters in Sm:LCB with those in Sm:YAB, Sm:LiNbO3, and Sm:(Mg, Ca, Zr)GGG. Crystals

Table 2 The spectral parameters for Sm3 þ ions in Sm:LCB crystal.

6

648

Fig.3. Emission of Sm3 þ in LCB corresponding to the 4G5/2-6HJ transitions ( J¼ 5/2–9/2).

Fig.3 shows the fluorescence spectrum of Sm:LCB, which includes three transition bands at about 563 nm, 599 nm and 648 nm, corresponding to the 4G5/2–6H5/2, 4G5/2–6H7/2, and 4G5/2–6H9/2 transitions, respectively [26].

6

40

20

3.2. Emission spectrum

G5/2

60

ð8Þ

where βc is the fluorescence branch ratio, τr is the radioactive lifetime of a given upper level, and AðJ-J 0 Þ is the transition probability of spontaneous emission. The calculated results are listed in Table 2.

4

563

λ (nm)

Sed (10  22 cm2)

Smd (10  22 cm2)

A (s  1)

1460 1198 1038 953 908 902 893 795 713 648 601 564

0.242 0.191 0.611 0.581 0.039 0.084 0.007 0.821 2.528 3.004 6.421 0.188

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

0.392 0.565 4.136 7.746 6.629 0.592 0.054 8.45 36.245 57.388 170.47 23.921

βc (%) 0.12 0.178 1.31 2.44 2.09 0.187 0.017 2.67 11.45 18.12 53.83 7.55

Sm:LCB Sm:YABa Sm:LiNbO3b Sm:(Mg, Ca, Zr)GGGc Sm:LCB Sm:YABa Sm:LiNbO3b Sm:(Mg, Ca, Zr)GGGc Sm:LCB Sm:YABa Sm:LiNbO3b Sm:(Mg, Ca, Zr)GGGc a b c

Ref. [28]. Ref. [29]. Ref. [30].

Transitions

λ (nm) A (s  1)

563 G5/2-6H5/2 564.4 561.8 566 599 4 G5/2-6H7/2 600 603 613 648 4 6 G5/2- H9/2 646.8 652 649 4

23.921 1.877 27.96 20.92 170.47 719.400 356.45 186.94 57.388 829.720 311.34 86.3

βc

sem

0.0755 0.00097 0.0314 0.0517 0.5383 0.37400 0.3998 0.4615 0.1812 0.43200 0.3494 0.213

5.7 – – 0.19 15.7 – 9.7 4.28 5.2 – 25 0.51

(  10  22 cm2)

42

X. Zhang et al. / Journal of Crystal Growth 399 (2014) 39–42

¼7.49  10–22 cm2, Ω4 ¼3.10  10–20 cm2, and Ω6 ¼4.22  10  20 cm2. The radiative probabilities Ar, fluorescence lifetime τrad, fluorescence branching ratios βc and emission cross-section sem have been calculated. The emission cross-section and the fluorescence lifetime at 599 nm are 15.7  10  22 cm2 and 1.976 ms, respectively. In comparison with other Sm3 þ -doped laser crystals, the measured results and calculated parameters show that Sm:LCB crystal satisfies the fundamental spectral condition for laser emission.

Acknowledgment This work was financially supported by the National Natural Science Foundation of China (No. 51132008).

References Fig.4. Decay curve of the 4G5/2-6H7/2 luminescence of the Sm:LCB crystal under 410 nm excitation.

The fluorescence lifetime decay curve at 401 nm of the Sm3 þ doped LCB crystal is measured, and shown in Fig. 4. The crystal was excited at the 405 nm wavelength and emission at 613 nm was observed. The obtained curve was fitted by a single exponential function described by the relation I ¼I0 exp(  t/τ) (I0 is the initial intensity at t ¼0, τ is the lifetime) giving experimental lifetime τexp of 1.967 ms, which is a little shorter than 2.113 ms of Sm:GGG [32], 2.406 ms of Sm:(Mg, Ca, Zr)GGG [30], and longer than 0.84 ms of Sm:LiNbO3 [28]. The luminescence quantum efficiency was calculated by the equation η ¼τexp/τrad ¼62.27%, where τrad is radiative lifetime. The result of fluorescence spectra shows that the Sm:LCB crystal has the most excellent emission property at 599 nm. As presented in Table 3, the longest fluorescence lifetime is 1.967 ms with the highest quantum efficiency of 62.27% and large emission cross-section. Thus, we can deduce that Sm:LCB crystal satisfies the fundamental spectroscopic conditions for laser emission. Investigations concerning the laser output for Sm:LCB crystal are in progress. 4. Conclusions Large transparent Sm3 þ -doped LCB crystal has been grown by the top-seeded solution growth method using Li2O–CaO–B2O3 as flux. The spectroscopic properties of Sm:LCB crystal have been investigated at room temperature. The Judd–Ofelt theory has been applied to evaluate the optical transition probabilities of Sm3 þ ions in Sm:LCB. Based on the Judd-Ofelt theory, the intensity parameters Ωt obtained by the least-square fitting method is Ω2

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