Spectral properties of Nd3+-doped Sr3Ga2Ge4O14 crystal

Optics Communications 277 (2007) 385–389 www.elsevier.com/locate/optcom

Spectral properties of Nd3+-doped Sr3Ga2Ge4O14 crystal Anhua Wu

b

a,b,*

, Shangke Pan a, Jiayue Xu a, Hui Shen a, Norihito Saito b, Takayo Ogawa b, Satoshi Wada b

a Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, People’s Republic of China Solid-State Optical Science Research Unit, The Institute of Physical and Chemical Research (RIKEN), 2-1 Hirowasa, Wako, Saitama 351-0198, Japan

Received 16 March 2007; received in revised form 10 May 2007; accepted 11 May 2007

Abstract Neodymium doped strontium gallogermanate crystals were grown successfully by the Bridgman technique. The linear thermal expansion coefficients for the c- and a-axes were measured as 5.8 · 106 C1 and 6.5 · 106 C1. Absorption spectra, and fluorescence spectra, as well as fluorescence decay curves of Nd3+-doped Sr3Ga2Ge4O14 crystal, have been recorded at room temperature and used to calculate the absorption and stimulated emission cross-sections. Based on the Judd–Ofelt theory, three intensity parameters were obtained. The luminescent quantum efficiency of the 4F3/2 level was determined to be approximately 73.8% for this material. Compared with other Nd3+-doped laser crystals, Nd3+-doped Sr3Ga2Ge4O14 crystal displays special laser properties due to its disorder structure.  2007 Elsevier B.V. All rights reserved. Keywords: Nd: Sr3Ga2Ge4O14; Optical spectrum; Judd–Ofelt theory; Thermal expansion

1. Introduction Rare-earth-doped disordered crystals are interesting both as active medias for lasers and as spectroscopic models between simple crystals and glasses because of their spectral characteristics. Among oxide laser crystals with a disordered structure, Ca-gallogermanate-type A3BC3D2O14 (A = Ca, Sr, Ba, and RE = rare earth; B = Ga, Ta, Nb; C = Ga, Ge, Ta, Nb; D = Si, Ge) compounds have generated a great deal of interest as potential stoichiometric laser materials [1]. Their structure has a remarkable feature which admits huge amounts of trivalent rare-earth ions. Single crystal of Sr3Ga2Ge4O14 (SGG) belongs to the Ca-gallogermanate (CGG) structure and hence crystallizes in the trigonal system, the acentric crystal class 32, the space group is P321. The crystal structure of Sr3Ga2Ge4O14 * Corresponding author. Address: Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, People’s Republic of China. E-mail address: [email protected] (A. Wu).

0030-4018/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.05.020

consists of tetrahedral layers oriented perpendicular to the c-axis and separated by inter-layers formed of distorted Thomson cubes occupied by large ions, and octahedra, which is shown as Fig. 1 [2]. The tetrahedral layers consist of two types of tetrahedral (D and C positions) which can be occupied by cations with different valencies (Ga3+ or Ge4+). The statistical occupation of similar crystallographic positions in the lattice by cations with different valencies results in the formation of a variety of crystal fields at A lattice position where an activator ion (such as Nd3+) can be entered. As a consequence, the Nd3+ ions experience a disordered crystal field, resulting in macroscopic line broadening of the absorption and luminescence spectra [3,4]. The broadened active absorption bands improve the pump efficiency, while the increased luminescence linewidth enables the generation of shorter laser pulses, similar as with laser Nd3+: glasses. In this paper, the spectroscopic properties of Nd3+doped SGG crystal are systematically investigated based on the Judd–Ofelt theory while the linear thermal expansion coefficients are described.

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A. Wu et al. / Optics Communications 277 (2007) 385–389

0.005

a c

ΔL/L

0.004 0.003 0.002 0.001 0.000 200

300

400

500

600

700

800

900

1000

Temperature (K) Fig. 2. Linear thermal expansions as a function of temperature along the c- and a-axes.

0

a11 B aij ¼ @ 0 0 Fig. 1. Illustration of Ca-gallogermanate compound viewed along (a) [0 1 0] (b) [0 0 1] directions.

2. Experimental Nd3+-doped SGG crystal was grown by the Bridgman technique [5]. Thermal expansion coefficients of Nd3+: SGG along the crystallographic axes were measured by a Diatometer (Model DIL 402 EP) in the temperature range of 25–700 C. The sample lengths in the two crystallographic a- and c-directions were 17.02 and 17.04 mm, respectively. The sample was kept in a fused silica sample holder and heated at a rate of 3 C/min in the standard air atmosphere. Room temperature absorption spectra of these crystals were recorded by a Varian’s Cary 500 UV–VIS-NIR Spectrophotometer. The emission spectrum of the crystal was recorded at room temperature by a Jobin Yvon Instruments Fluorolog-3 spectrophotometer (France). 3. Results and discussion 3.1. Thermal expansion As a significant part of the pump power is converted into heat inside the material during laser operation, it is important to know its linear thermal expansion coefficients to predict how the material behaves when the temperature increases [6]. The thermal expansion coefficients of the crystal are important factors for crystal growth too. SGG belongs to the trigonal crystal system with two independent principal thermal expansion components whose matrix for the thermal expansion tensor is

0 a22 0

1 0 C 0 A a33

ð1Þ

The thermal components a11 (a11 = a22) and a33 can be determined by measuring the thermal expansion along the a- and c-oriented samples. The figure of linear expansions versus temperature is shown in Fig. 2. From the experimental results, we obtain the thermal expansion coefficients along the a- and c-axes that are positive between 298 and 973 K. The linear thermal expansion coefficient is defined as a¼

1 DL L0 DT

ð2Þ

Here L0 is the initial length of the sample at room temperature and DL is the change in length when the temperature changes DT. We can calculate the thermal expansion coefficient from the slope of the linear fitting of the linear relationship between DL/DT and the temperature. So the linear thermal expansion coefficients for c- and a-axes are 5.8 · 106 and 6.5 · 106 C1, respectively. 0 1 6:5 0 0 B C 6:5 0 A  106  C1 ð3Þ aij ðSGGÞ ¼ @ 0 0 0 5:8 The thermal expansions in the a-direction and c-direction are quite similar even though the thermal expansion in the a-direction is somewhat larger than that in the c-direction, hence thermal stress concentration can be avoided due to anisotropic thermal expansion. This is beneficial for the crystal growth and laser applications operating at high temperature gradients. 3.2. Absorption spectrum Since the Nd3+: SGG crystals were grown with seeds, the sample was cut perpendicular to the growth direction and optically polished to obtain flat and parallel faces. The Nd3+ concentration varies in different parts of the

A. Wu et al. / Optics Communications 277 (2007) 385–389

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4

G5/2 + 2G7/2, 4I9/2 ! 4F7/2 + 4S3/2, 4I9/2 ! 4F5/2 + 2H9/2 and 4I9/2 ! 4F3/2 respectively [3]. The most interesting absorption band is at about 806 nm, which is intrinsic pumping wavelength of Nddoped laser materials. Although the 806 nm is not the highest absorption peak in the Nd3+-doped SGG crystal, the FWHM (full width at half maximum) of absorption coefficient is 19 nm. The absorption cross-section rabs can be determined by the following equation: rabs ¼

Fig. 3. Absorption spectrum of Nd3+-doped SGG crystal.

Table 1 Comparison of FWHM and cross-section between Nd3+: SGG with other laser crystals crystal

Nd3+ Wavelength FWHM CrossReferences concentration (nm) (nm) section (·1020 cm3) (·1020 cm2)

Nd3+: SGG Nd3+: YAG Nd3+: YVO4

2.1

806

1.5

808

0.8

1.5

808

2

19

2.91

This work

7.0

[7]

27

[7]

A L  log e  N e

ð4Þ

Here A is the absorbance, L is the thickness of the polished crystal, and Ne is the rare-earth ions concentration in the sample. Also, the dimension of the ion concentration is atoms/cm3. Table 1 gives the parameters of the FWHM and cross-section rabs of some other laser crystals for comparison. Compared with traditionally Nd3+-doped laser crystals, the Nd3+-doped SGG crystal displays higher FWHM and lower cross-section. These special characterizations due to disorder structure suggest that it is worthy to investigate its spectrum properties. The experimental data obtained from the absorption spectra were used to calculate the dipole line strengths. The measured absorption line strengths Smeas(J ! J 0 ) for transitions from the ground state 4I9/2 manifold to the excited J 0 -manifold can be obtained from the absorption spectra using the following expression [8]: Z



2

8p3 e2 k ðn2 þ 2Þ 1 S meas ðJ ! J 0 Þ rðkÞ dk ¼ ð2J þ 1Þ 3hc 9n

ð5Þ



as-grown crystals due to compositional segregation, with an average of 2.05 at% as obtained by ICP-AES. Fig. 3 shows the absorption spectrum of Nd3+: SGG in the wavelength range from 400 nm to 1000 nm. The scan speed was 60 nm/min and the spectral resolution was 1 nm. The dominant transitions were attributed to 4f3–4f3 transition of Nd3+ ion, and the initial state of all these transitions is the ground 4I9/2 state of Nd3+ ion. These sharp absorption peaks 525 nm, 584 nm, 750 nm, 806 nm, and 879 nm correspond to the transitions from the ground state 4I9/2 to the excited states: 4I9/2 ! 4G7/2 + 2G9/2 + 2K13/2, 4I9/2 ! Table 2 Integrated absorbance, electrical dipole line strengths of Nd3+: SGG crystal R Excited state rðkÞ dkð1020 Þ 4

F3/2 F5/2 + 2H9/2 4 F7/2 + 4S3/2 4 F9/2 4 G5/2 + 2G7/2 4 G7/2 + 2G9/2 + 2K13/2 2 G9/2 + 2D3/2 + 4G11/2 + 2K15/2 2 P1/2 + 2D5/2 4

8.0884 30.8311 27.3331 2.2253 35.2107 11.2323 2.3482 0.3715

where k is the mean wavelength of the transition, h is the Planck’s constant, e is the electron charge, n is the refractive index (n0 = 1.795 and ne = 1.813 at k = 632.8 nm is equal to 6.0 mm1 [3]), c is the vacuum speed of light, J is the total angular momentum of the ground state, and r(k) is the integrated absorbance for each absorption band, which can be determined by the equation of the cross-section. Table 2 presents the integrated absorbance, electrical dipole line strengths of Nd3+: SGG crystal. Based on the Judd–Ofelt theory, the measured line strengths were then used to obtain the J–O intensity parameters X2, X4 and X6 by fitting the set of equations

k (nm)

888 805 748 667.5 579 516.5 467 435

Line strength (1020 cm2) Smeas

Scalc

0.561 2.359 2.251 0.205 0.3746 1.340 0.310 0.053

0.760 2.337 2.301 0.163 0.3762 1.110 0.234 0.100

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A. Wu et al. / Optics Communications 277 (2007) 385–389

Table 3 The J–O intensity parameters of Nd3+: SGG with other Ca-gallogermanate-type crystals Crystal

X2 (1020 cm2)

Nd3+: SGG Nd3+: SGG Nd3+: Ca3Ga2Ge4O14 Nd3+: La3Ga5Si2O14

X4 (1020 cm2)

X6 (1020 cm2)

64p4 e2

AðJ ! J 0 Þ ¼

 3

3hð2J þ 1Þ k

References

2.10 2.32 1.88

2.52 1.63 3.65

3.31 4.74 5.65

This work [3] [3]

2.4

4.6

3.4

[10]

AT ðJ Þ ¼

t¼2;4;6

(t)

Here U (t = 2, 4, 6) are the matrix elements of the unit tensor calculated by Carnall et al. [9]. After a least-square fitting of Smeas to Scalc, the three J–O intensity parameters were obtained. The three J–O intensity parameters of Nd3+: SGG crystal and other Nd3+-doped crystals are given in Table 3. 3.3. Emission spectrum The emission spectrum of the crystal recorded at room temperature is shown in Fig. 4. There are three main emission peaks whose wavelengths are centered at 902 nm (4F3/2 ! 4I9/2), 1070 nm (4F3/2 ! 4I11/2) and 1351 nm (4F3/2 ! 4I13/2), respectively. From this figure, the intense emission peaks are centered at 1070 nm, corresponding to the transition 4F3/2 ! 4I11/2 of Nd3+ ions. Since the radiative lifetimes are A(J ! J 0 ), the mathematical equation for the fluorescent branching ratio is given by [8]:

X

2

Xt jhðS; LÞ

t¼2;4;6

 J kU ðtÞ kðS 0 ; L0 ÞJ 0 ij2

ð7Þ

0

AðJ ! J Þ

ð8Þ

J0

Then the radiative lifetimes sR= 1/AT(J), so the mathematical formula for the fluorescent branching ratio is given by [9]: bðJ 0 Þ ¼

from the corresponding transitions between J and J 0 manifolds in the following equation: X 2 S calc ðJ ! J 0 Þ ¼ Xt jhðS; LÞJ kU ðtÞ kðS 0 ; L0 ÞJ 0 ij ð6Þ

X

nðn2 þ 2Þ 9

AðJ ! J 0 Þ AT ðJ Þ

ð9Þ

The emission line strengths, the calculated radiative transition rates, and the branching ratios are presented in Table 4. The stimulated emission cross-section rP related to the radiative transition probability can be defined by rP ¼

AðJ ! J 0 Þk2P 4p2 n2 Dm

ð10Þ

Here Dm is the full frequency width at half maximum, kP is the vacuum wavelength of the emission peak. Therefore, the emission cross-section rP of emission spectra centered at 1070 nm were measured to be 2.04 · 1020 cm2. Table 5 presents the emission cross-section of Nd3+: SGG crystal with other laser crystals. Obviously, the emission cross-section of Nd3+: SGG crystal is smaller than that of other Nd3+-doped laser crystals, such as Nd3+: YAG and Nd3+: YVO4. However, the emission band-width of Nd3+: SGG crystal is 20 nm, which is 10 times of those of other Nd3+-doped laser crystals. The disorder structures originated from statistical occupation of similar crystallographic positions in the lattice by cations with different valencies result in macroscopic line broadening of the absorption and luminescence spectrum. Fig. 5 presents the luminescence decay curve excited by 806 nm at room temperature in correspondence with the transition 4F3/2 ! 4I11/2 of Nd3+ ions at 1070 nm. The fluorescence lifetime was calculated to be 273 ls by fitting the fluorescence lifetime curve using the exponential function y = y0 + Aex/t [11]. The radiative lifetime sR of the 4F3/2 level of Nd3+: SGG crystal is calculated to be 370 ls according the equation sR= 1/AT(J). The linear relationship in the figure appears as a single exponential behavior, and the fluorescence life time could be obtained from the slope of the fitting line. So the luminescent quantum efficiency of the 4F3/2 level can be obtained according to the equation gc = sf/sR. The fluorescence time decay is 273 ls Table 4 luorescence branch ratio b and transition probabilities of 4F3/2 ! 4IJ Start level

Terminal levels

Wavelength (nm)

Scalc (1020 cm2)

A (s1)

b

4

4

901 1070 1351

0.766 1.707 0.702

1038.0 1381.1 282.4

0.3842 0.5112 0.1045

F3/2

I9/2 I11/2 4 I13/2 4

Fig. 4. Emission spectrum of Nd3+-doped SGG crystal.

A. Wu et al. / Optics Communications 277 (2007) 385–389

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Table 5 Comparison of emission cross-section between Nd3+: SGG with other laser crystals Crystal 3+

Nd : SGG Nd3+: YAG Nd3+: YVO4

Nd3+ concentration (·1020 cm3)

Wavelength (nm)

rP (·1020 cm2)

Emission band-width (nm)

References

2.1 1.5 1.5

1070 1064 1064

2.04 33 100

20 0.45 0.8

This work [7] [7]

possess special laser properties compared with other Nd3+-doped laser crystals due to their disorder structure. We applied J–O theory to calculated J–O intensity parameters, radiative probabilities, radiative branching ratios as well as radiative time. The fluorescence lifetime corresponding to the emission line 4F3/2 ! 4I11/2 of Nd3+ ions at 1070 nm is 273 ls, and Nd3+: SGG crystal has high luminescent quantum efficiency 73.8%. Acknowledgement

Fig. 5. The fluorescence time decay of Nd3+: SGG crystal.

shown in Fig. 5 and the radiative lifetime sR is 370 ls, thus the radiative quantum efficiency gc is 73.8%. We can see that Nd3+: SGG crystal has a large luminescent quantum efficiency. 4. Conclusion Nd-doped Sr3Ga2Ge4O14 (SGG) single crystals with Cagallogermanate (CGG) structure were grown successfully by the Bridgman technique. Nd3+: SGG crystals possess similar thermal expansions in the a-direction and c-direction, which is beneficial for crystal growth and laser application. Absorption and emission spectra of Nd3+: SGG crystal were presented. We measured the FWHM and calculated the absorption cross-section centered at about 806 nm of Nd3+: SGG crystals. Higher FWHM and lower absorption cross-section suggest that Nd3+: SGG crystals

The authors would like to acknowledge the financial support provided by the international cooperative project between Chinese Academy of Sciences and the Institute of Physical and Chemical Research (Riken). Also, Anhua Wu and Shangke Pan acknowledge financial support from National Natural Science Foundations of China (Grant Nos. are 50672110 and 60508007). References [1] R. Balda, J. AzKargorta, I. Iparraguirre, J. Fernandez, M.A. Arriandiaga, Opt. Mater. 8 (1997) 99. [2] R.B. Heimann, M. Hengst, M. Rossberg, J. Bohm, Phys. Status Solidi (a) 195 (2003) 468. [3] A.A. Kaminsky, E.L. Belokoneva, B.V. Mill, Y.V. Pisarevskii, S.E. Sarkisov, I.M. Silvestrova, A.V. Butashin, G.G. Khodzhabagyan, Phys. Status Solidi (a) 86 (1984) 345. [4] H.J. Eichler, D. Ashkenasi, H. Jian, A.A. Kaminsky, Phys. Status Solidi (a) 146 (1994) 833. [5] A. Wu, J. Xu, J. Ding, X. Li, Cryst. Res. Technol. 42 (2007) 451. [6] J.J. Carvajal, R. Sole, J. Gavalsa, J. Massons, F. Diaz, M. Aguilo, Chem. Mater. 15 (2003) 2730. [7] G. Wang, J. Synth. Cryst. (in Chinese) 27 (1998) 390. [8] M.J. Webler, Phys. Rev. 157 (1967) 262. [9] W.T. Carnall, P.R. Fields, K. Rajnak, J. Chem. Phys. 49 (1968) 4424. [10] A.A. Kaminsky, I.M. Silvestrova, S.E. Sarkisov, G.A. Denisenko, Phys. Status Solidi (a) 80 (1983) 607. [11] B. Wei, Z. Lin, G. Wang, J. Cryst. Growth 295 (2006) 241.