Spectroscopic properties of Nd3+ ions in La2(WO4)3 crystal

Chemical Physics Letters 381 (2003) 598–604 www.elsevier.com/locate/cplett

Spectroscopic properties of Nd3þ ions in La2(WO4)3 crystal Yujin Chen, Xiuqin Lin, Zundu Luo, Yidong Huang

*

Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, 155 Yangqiao West Road, Fuzhou, Fujian 350002, China Received 12 September 2003; in final form 12 September 2003 Published online: 4 November 2003

Abstract The absorption spectra, fluorescence spectrum and fluorescence decay curve of Nd3þ ions in anisotropic monoclinic crystal La2 (WO4 )3 were measured at room temperature. Taking the optical anisotropism into account, the spectroscopic parameters, such as intensity parameters Xt ðt ¼ 2; 4; 6Þ, spontaneous emission probability, fluorescence branching ratio, radiative lifetime and fluorescence quantum efficiency were obtained by the Judd–Ofelt theory. The stimulated emission cross-section at 1058 nm wavelength was calculated to be 1.12
1019 cm2 . The good spectroscopic properties show that the Nd3þ :La2 (WO4 )3 crystal is a good candidate of solid-state laser and self-stimulated Raman laser media. Ó 2003 Elsevier B.V. All rights reserved.

1. Introduction In recent years, solid-state self-stimulated Raman lasers based on stimulated Raman scattering (SRS) become a practical and efficient class of laser devices, which can convert the wavelength of laser emission and received an intense attention as a promising application of medicine and laser remote sensing at a specific wavelength [1–3]. Among numerous crystals, tungstate crystals, such as PbWO4 , CaWO4 and KGd(WO4 )2 , have proved to be excellent stimulated Raman media and selfstimulated Raman laser media when doped with

*

Corresponding author. Fax: +86-591-3714946. E-mail address: [email protected] (Y. Huang).

rare-earth ions [3–5]. For example, the total stokes conversion efficiency beyond 50% is achieved for PbWO4 crystal [3]. Lanthanum tungstate crystal La2 (WO4 )3 belongs to the monoclinic structure with space group C2 =c and is a biaxial crystal. Some of the crystallographic and physical properties about the crystal can be found in [6]. Stimulated Raman conversion with an internal efficiency of 37% in a fiber-like lanthanum tungstate crystal and maximum laser output slope efficiency of 35% for a 3.2 at.% Nddoped lanthanum tungstate crystal have been reported [7,8]. However, because of the difficulty in distinguishing the two optical axes, to our knowledge, the detailed spectroscopic analysis of Nd3þ :La2 (WO4 )3 crystal, which is important to the evaluation of laser performance, has not been reported until now.

0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.10.046

Y. Chen et al. / Chemical Physics Letters 381 (2003) 598–604

In this Letter, in order to avoid the trouble of cutting the sample in special orientation as well as the errors brought by polarized absorption measurement for the case of anisotropic crystal, the three perpendicular unpolarized absorption measurement method (TPM method) [9] has been adopted for the calculation of effective intensity parameters Xt ðt ¼ 2; 4; 6Þ. By the TPM method, only a cubic sample cut in arbitrary orientation and the data of unpolarized absorption spectra measured in three perpendicular directions were needed. The detailed analysis showed that this method could obtain the data of required line strengths with enough accuracy [9]. From the intensity parameters, the fluorescence branching ratio and radiative lifetime are calculated. The fluorescence lifetime, fluorescence quantum efficiency and stimulated emission crosssection are also analyzed.

2. Experimental 2.1. Measurement of spectra and fluorescence lifetime A single crystal of Nd3þ :La2 (WO4 )3 was grown by the Czochraski method. The crystal sample was cut to a cube with dimensions of 2
2
2 mm3 in arbitrary orientation. All the surfaces of the cube were polished. The density of the crystal is 6.60 g/ cm3 [6] and the concentration of Nd3þ ions in the sample is 2.33
1020 cm3 (3 at.%). Absorption spectra, fluorescence spectrum and fluorescence decay curve were measured at room temperature. The absorption spectra were recorded on a spectrophotometer (Lambda 35, Perkin–Elmer) with a spectral range from 200 to 1100 nm. By using a CW Ti:sapphire tunable laser (Model 3900s, Spectra-Physics) as the pump source, the fluorescence spectrum was recorded

599

when the exciting wavelength was 885 nm. The fluorescent signals past a monochromator (Triax550, Jobin-Yvon) were detected by using a cooled Ge detector (DSS-G025T, Jobin-Yvon) associated with a DSP lock-in amplifier (SR830, Stanford). The spectral resolutions of both the absorption and fluorescence spectra are 1 nm. The fluorescence decay curve at 1058 nm, corresponding to the emission peak from the 4 F3=2 ! 4 I11=2 transition, was recorded by a spectrophotometer (FL920, Edinburgh) while a microsecond flash lamp (lF900, Edinburgh) was used as the pump source and the pump wavelength was 803 nm. The signals were detected with an NIR PMT (R5509, Hamamatsu). By fitting the curve, the fluorescence lifetime of the 4 F3=2 manifold could be estimated. 2.2. Measurement of refractive index A piece of un-doped lanthanum tungstate crystal plate was cut and one of the faces was polished. According to [10], the refractive index n of the crystal can be obtained from measuring the reflectivity R of the crystal by means of
2 n1 R¼ : ð1Þ nþ1 The values of reflectivity and corresponding refractive index of La2 (WO4 )3 crystal are listed in Table 1. By fitting these values, a rough Sellmeier equation of the refractive index is obtained n2 ¼ 3:682 þ

0:202 : k2 þ 0:021

ð2Þ

3. Results and discussion Since Judd and Ofelt published their papers in 1962 [11,12], the Judd–Ofelt (J–O) theory has become the most popular method in the analysis of

Table 1 The values of reflectivity and corresponding refractive index of La2 (WO4 )3 crystal k (lm)

0.4

0.5

0.6

0.7

0.8

0.9

1.06

R n

0.139 2.19

0.127 2.11

0.118 2.05

0.113 2.01

0.111 2.00

0.110 1.99

0.105 1.96

600

Y. Chen et al. / Chemical Physics Letters 381 (2003) 598–604

spectroscopic properties of rare earth ions in crystals and glasses, such as the calculations of intensity parameters Xt ðt ¼ 2; 4; 6Þ, spontaneous emission probability, fluorescence branching ratio, radiative lifetime and fluorescence quantum efficiency. Detailed analysis procedure of spectroscopic parameters of rare earth ions in various matrixes by the Judd–Ofelt theory can be seen in [13,14]. The absorption spectra of sample in three mutually perpendicular directions measured in room temperature are shown in Figs. 1–3. The absorption peaks are corresponding to the transitions from the ground state 4 I9=2 to the excited states. It is worth noting that the absorption band with peak at 583 nm wavelength has a very large absorption coefficient and wide FWHM of about 10 nm. In these absorption spectra, the most interesting is the broad absorption band with an FWHM of 7 nm and large absorption coefficient at 803 nm wavelength. Compared to the 1.04 nm FWHM of 0.9 at.% Nd3þ :YAG crystal [15], the Nd3þ :La2 (WO4 )3 crystal can be pumped more effectively by diode laser and not restricted to the temperature stability of the output wavelength of diode laser. The line strengths of the electric–dipole transitions from the ground 4 I9=2 (J ¼ 9=2) manifold to the excited J 0 -manifold can be obtained from the

Fig. 2. Room temperature absorption spectrum of Nd3þ :La2 (WO4 )3 crystal in the second one of the three perpendicular directions, when L ¼ 2:0 mm.

Fig. 3. Room temperature absorption spectrum of Nd3þ :La2 (WO4 )3 crystal in the third one of the three perpendicular directions, when L ¼ 2:0 mm.

room temperature absorption spectra by the following relation [16]  3hcð2J þ 1Þ  9n ; Smea J ! J 0 ¼ C 3 2  2 8p e kp ðn þ 2Þ2 N0

Fig. 1. Room temperature absorption spectrum of Nd3þ :La2 (WO4 )3 crystal in the first one of the three perpendicular directions, when L ¼ 2:0 mm.

ð3Þ

where N0 is the Nd3þ concentration and expressed in ion/cm3 , n is the refractive index of the sample, kp is the mean wavelength of the absorption band  is the average integrated absorbance. In the and C condition that the dimensions of crystal are small or crystal is cubic shape (our sample satisfies this  of each absorption band can be condition), C

Y. Chen et al. / Chemical Physics Letters 381 (2003) 598–604

expressed as the average value of integrated absorbance of the three mutually perpendicular directions (marked as i ¼ 1; 2; 3) ¼1 C 3

3 X

601

shown in Table 3. By a least-root-means-square fitting between Eqs. (3) and (5), the three intensity parameters X2;4;6 could be obtained: X2 ¼ 13:76
1020 cm2 ;

Ci

i¼1

X4 ¼ 5:19
1020 cm2 ;

ð4Þ

Z 3 1X ln 10 ¼ ODi ðkÞ dk; 3 i¼1 Li J !J 0

X6 ¼ 6:76
1020 cm2 : The value of root-mean-squares (RMS) deviation is 3.73
1021 cm2 , which indicates that fitting result is in good agreement with the experiments. The calculated line strengths from the intensity parameters by Eq. (5) are also listed in Table 3. Once the intensity parameters were obtained, the spontaneous emission probabilities Aed JJ 0 , corresponding to transitions from the 4 F3=2 -manifold (J ¼ 3=2) to the lower J 0 -manifolds 4 IJ 0 can be calculated by means of the following relation [16]   0 Aed JJ 0 J ! J

where ODi ðkÞ is the measured optical density as a function of wavelength k and Li is the thickness of samples in direction i. The mean wavelengths of the absorption bands and the values of the integrated absorbance are listed in Table 2. From the table, we can see that there is considerable difference between the values of integrated absorbance recorded in different directions. So for an anisotropic crystal, when Eq. (3) is adopted in the analysis of spectroscopic parameters, only considering the integrated absorbance of one direction is insufficient. According to the J–O theory, the line strengths of the electric–dipole transitions can also be expressed as

2

¼

64p4 e2 nð n2 þ 2Þ 3 9 3hð2J þ 1Þke X    2
Xt 4 F3=2 U ðtÞ 4 IJ 0 ;

ð6Þ

t¼2;4;6

     X  n  2 Scal J ! J 0 ¼ Xt 4f ½aSLJ U ðtÞ 4f n a0 S 0 L0 J 0 ;

where ke is the mean wavelength of emission bands and the values of U ðtÞ ðt ¼ 2; 4; 6Þ have been proposed by Kaminskii et al. [18]. Then, the fluorescence branching ratio is given as

t¼2;4;6

ð5Þ

where U ðtÞ ðt ¼ 2; 4; 6Þ are the matrix elements of unit tensor operators and had been calculated by Carnall et al. [17]. The measured line strengths are

Aed0 b ¼ P JJ ed : J 0 AJJ 0

ð7Þ

Table 2 Integrated absorbance of the three mutually perpendicular directions and average integrated absorbance of Nd3þ :La2 (WO4 )3 crystal at room temperature (unpolarized spectra) J -manifold

4

F3=2 F5=2 + 2 H9=2 4 F7=2 + 4 S3=2 4 F9=2 2 H11=2 4 G5=2 + 2 G7=2 2 K13=2 + 4 G7=2 + 4 G9=2 2 K15=2 + 2 G9=2 + 2 D3=2 + 4 G7=2 2 P1=2 + 2 P5=2 4 D3=2 + 4 D5=2 + 4 D1=2 + 2 I11=2 + 2 I15=2 2 I13=2 + 4 D7=2 + 2 L17=2 4

kp (nm)

876 805 749 685 631 585 526 472 431 355 332

 (nm/cm) C

Ci (nm/cm) i ¼1

i ¼2

i ¼3

54.12 146.12 124.12 8.05 1.98 536.68 92.44 14.03 2.85 50.26 4.54

88.37 185.21 164.28 9.60 2.21 422.51 91.23 16.89 3.80 60.72 5.25

95.92 199.48 170.62 10.96 2.26 434.78 93.49 17.40 3.61 56.42 4.92

79.47 176.94 153.01 9.53 2.15 464.66 92.39 16.11 3.42 55.80 4.94

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Y. Chen et al. / Chemical Physics Letters 381 (2003) 598–604

Table 3 The measured and fitted line strengths of Nd3þ :La2 (WO4 )3 crystal at room temperature SðJ ! J Þ (1020 cm2 )

J -manifold

4

F3=2 4 F5=2 + 2 H9=2 4 F7=2 + 4 S3=2 4 F9=2 2 H11=2 4 G5=2 + 2 G7=2 2 K13=2 + 4 G7=2 + 4 G9=2 2 K15=2 + 2 G9=2 + 2 D3=2 + 4 G7=2 2 P1=2 + 2 P5=2 4 D3=2 + 4 D5=2 + 4 D1=2 + 2 I11=2 + 2 I15=2 2 I13=2 + 4 D7=2 + 2 L17=2

Smea

Scal

2.06 4.91 4.53 0.31 0.07 16.88 3.64 0.69 0.16 2.85 0.26

1.56 4.88 4.70 0.34 0.09 16.93 2.90 0.49 0.21 3.32 0.11

The calculated values of Aed JJ 0 and b are listed in Table 4. The radiative lifetime sr of the 4 F3=2 manifold is the reciprocal of the summation of the total radiative transition rates and for Nd3þ :La2 (WO4 )3 crystal, the value is 129 ls. Fluorescence decay curve of Nd3þ :La2 (WO4 )3 crystal is shown in Fig. 4 in semilog scale. The linear relationship in the figure displays single exponential behavior of the fluorescence decay and the fluorescence lifetime could be obtained from the slope of the fitting line k, i.e., sf ¼ ð1=2:303kÞ. By line fitting, the slope of the fitting line is )0.0039 and the fluorescence lifetime of Nd3þ :La2 (WO4 )3 crystal is about 112 ls. Then, the fluorescence quantum efficiency g ¼ sf =sr is calculated to be 87%. Compared to other Nd3þ -doped crystal, this value is higher. The explanation is given in the following. For crystals, the rate of non-radiative transition is mainly caused by the multi-phonon relaxation and the effect of concentration quenching. In the Nd3þ :La2 (WO4 )3 crystal, the (WO4 )2 group is the main phonon group and the phonon energy is

Fig. 4. Room temperature fluorescence decay curve of Nd3þ :La2 (WO4 )3 crystal. The fitting result of single exponential decay is 112 ls.

below 900 cm1 [19]. So the multi-phonon relaxation rate is relatively small. On the other hand, the quenching parameter in Nd3þ :La2 (WO4 )3 crystal is large and twice that of Nd3þ :YAG, which is the result of the ionic bonding among La3þ ions and (WO4 )2 ions prevents Nd3þ ions from interacting with each other [8]. It shows that the effect of concentration quenching in Nd3þ :La2 (WO4 )3 crystal is not distinct. As a result of the two reasons mentioned above, the rate of non-radiative transition in the Nd3þ : La2 (WO4 )3 crystal is small and the radiative quantum efficiency is high.

Table 4 Spontaneous emission probabilities and fluorescence branching ratios for the 4 F3=2 ! 4 IJ 0 transitions of Nd3þ :La2 (WO4 )3 crystal Transition

AðJ ! J Þ (s1 )

b (%)

4

2969 3954 797 36

38.28 50.98 10.28 0.46

F3=2 ! 4 I9=2 4 F3=2 ! 4 I11=2 4 F3=2 ! 4 I13=2 4 F3=2 ! 4 I15=2

Fig. 5. Room temperature Nd3þ :La2 (WO4 )3 crystal.

fluorescence

spectrum

of

Y. Chen et al. / Chemical Physics Letters 381 (2003) 598–604

603

Table 5 Comparison of the spectroscopic parameters of Nd3þ :La2 (WO4 )3 crystal and other Nd3þ -doped tungstate crystals X2

X4

X6

sf ðlsÞ

sr ðlsÞ

g (%)

re

Ref.

Nd :La2 (WO4 )3 Nd3þ :KGd(WO4 )2 Nd3þ :PbWO4

13.76 12.67 7.13

5.19 10.15 3.35

6.76 7.48 2.69

112 110 175

129 119 188

87 92.4 93.1

This work [9,22] [3]

Nd3þ :PbWO4 Nd3þ :KY(WO4 )2 Nd3þ :NaBi(WO4 )2

7.53 8.80 30.9

3.15 3.11 12.0

3.06 3.16 9.3

170 154 122

198 196 143

85.8 78.6 85

1.12 3.4 0.45 (?) 0.57 (jjc) 0.28 0.54 1.6

Crystal 3þ

[4] [20] [21]

(X2;4;6 are in unit of 1020 cm2 and re is in unit of 1019 cm2 ).

The room temperature fluorescence spectrum of the sample was recorded in a spectral range from 1020 to 1120 nm, corresponding to the transition of 4 F3=2 ! 4 I11=2 and is shown in Fig. 5. The emission peak is located at 1058 nm wavelength and the FWHM of emission band is about 11 nm. The stimulated emission cross-section could be estimated from the fluorescence spectrum by the F–L equation [13]: re ¼

k5e bJ !J 0 I ðke Þ R ; 8pcn2 sr kI ðkÞ dk

ð8Þ

where ke is the fluorescence peak wavelengths and IðkÞ is the fluorescence intensity at wavelength k. From the parameters obtained above, we could estimated the stimulated emission cross-section of Nd3þ :La2 (WO4 )3 crystal, re ¼ 1:12
1019 cm2 . In Table 5, the main spectroscopic parameters of some Nd3þ -doped tungstate crystals are listed. This table reveals that the spectroscopic parameters of Nd3þ :La2 (WO4 )3 crystal are comparable to those of other Nd3þ -doped tungstate crystals. It suggests that Nd3þ :La2 (WO4 )3 crystal is a good candidate of media for solid-state laser and selfstimulated Raman laser.

4. Conclusion Absorption spectra, fluorescence spectrum and fluorescence decay curve of Nd3þ :La2 (WO4 )3 crystal were measured. Based on the TPM method, the spectroscopic parameters have been calculated by the J–O theory. The intensity parameters Xt were obtained X2 ¼ 13:76
1020 cm2 , X4 ¼ 5:19
1020 cm2 , X6 ¼ 6:76
1020 cm2 . The radiative and

fluorescence lifetimes are 129 and 112 ls, respectively. So the fluorescence quantum efficiency g ¼ 87% was estimated. By the F–L equation, the stimulated emission cross-section was calculated to be 1.12
1019 cm2 . Comparing the spectroscopic parameters of Nd3þ :La2 (WO4 )3 crystal with those of other Nd3þ doped tungstate crystals, we may expect the Nd3þ :La2 (WO4 )3 crystal to be a good candidate in solid-state laser and self-stimulated Raman laser applications. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 60088004), the Fujian Provincial Science and Technology Foundation of China (No. 2002H004), and Chinese Academy of Sciences.

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