Growth and optical properties of Ho3+:NaGd(MoO4)2 crystal

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Optical Materials 30 (2008) 1873–1877 www.elsevier.com/locate/optmat

Growth and optical properties of Ho3+:NaGd(MoO4)2 crystal Zujian Wang a,b, Xiuzhi Li a, Guojian Wang a,b, Mingjun Song a,b, Qian Wei a,b, Guofu Wang a, Xifa Long a,* a

Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, China Received 13 August 2007; received in revised form 16 November 2007; accepted 12 December 2007 Available online 11 February 2008

Abstract The Ho3+:NaGd(MoO4)2 crystal with dimensions of U12  42 mm2 has been grown by Czochralski (CZ) method. Polarized absorption and fluorescence spectra at room temperature were investigated. The largest absorption cross-sections, corresponding to 5 I8 ? 5G6 + 5F1 absorption bands, are 31.0  1020 cm2 and 23.8  1020 cm2 for p- and r-polarization, respectively. The strongest emission corresponding to the 5S2 + 5F4 ? 5I8 transition can be induced by populating any higher energy level of Ho3+ ion. The fluorescence lifetimes at room temperature are 3.4 ls at 550 nm, 3.5 ls at 754 nm and 46.4 ls at 1193 nm, corresponding to transitions 5 S2 + 5F4 ? 5I8, 5S2 + 5F4 ? 5I7, 5I4 ? 5I8 and 5I6 ? 5I8, respectively. Due to the influence of fluorescence trapping, the measured fluorescence lifetimes of 550 and 1193 nm should be longer than their actual values. Based on the Judd–Ofelt (J–O) theory and polarized absorption spectrum, the spontaneous transition probabilities, the fluorescent branching ratios and the radiative lifetimes were calculated. Ó 2007 Elsevier B.V. All rights reserved. PACS: 42.70.Hj; 78.20.e Keywords: Czochralski method; Ho3+:NaGd(MoO4)2; Optical properties; Molybdate

1. Introduction In the recent years, rare-earth ions doped materials have been attracting much attention in the field of laser physics. Ho3+ ion, as one of lanthanide ions, has been widely investigated for achieving laser actions in various wavelengths, such as infrared, visible and ultra-violet regions. There has been increasing interests in molybdate crystals with general formula MRe(MoO4)2 (M = alkali metal and Re = rare earth) in the past few years, not only because of their large lanthanide admittance, but especially owing to their good properties, such as high integral absorption and fluorescence cross-sections, broadened lines of optical spectra of rare-earth ions and the possibilities to obtain

tunable laser oscillation within wide range, and so on [1– 4]. As a member of MRe(MoO4)2 family, NaGd(MoO4)2 is regarded as an attractive laser host material candidate, which belongs to the scheelite (CaWO4) structure with space group centrosymmetric I41/a [5] (in some late papers [6–8], it is stated that the refined space group of this crystal is non-centrosymmetric I 4). The cell parameters are as fol˚ , c = 11.538 A ˚ [5]. Recently, lots of lows: a = b = 5.235 A work has been emphasized on NaGd(MoO4)2 crystal [1,2]. However, up to now, no attention has been paid for the investigation of Ho3+ doped NaGd(MoO4)2 crystal. This paper reports the growth and spectral properties of Ho3+:NaGd(MoO4)2 crystal. 2. Crystal growth

*

Corresponding author. Tel.: +86 591 83710369; fax: +86 591 83714946. E-mail address: [email protected] (X. Long). 0925-3467/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2007.12.012

Due to its congruent melting [5], the Ho3+:NaGd(MoO4)2 crystal can be grown by Czochralski (CZ) method

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[1–3]. Ho2O3 (99.99%), Gd2O3 (99.99%), MoO3 (99.95%), and Na2CO3 (99.95%) were weighed according to the stoichiometric composition of NaHo0.01Gd0.99(MoO4)2 in addition to 2 wt% excess MoO3 to compensate for its volatilization loss during the process of crystal growth. The polycrystalline materials of 1 at% Ho3+ doped NaGd(MoO4)2 crystal were synthesized by solid-state reaction. First, the weighed chemicals were thoroughly mixed, then after grinding and extruding to form tablets, the mixture was loaded into an alumina crucible, which was then placed into a vertical furnace, holding at 650 °C for 24 h to carry out the reaction, then repeating the above process but held at 900 °C also for 24 h to assure adequate reaction. The synthesized polycrystalline materials of Ho3+: NaGd(MoO4)2 were melted in a U50  50 mm2 platinum crucible using a 2.5 kHz frequency induction furnace. A small [0 0 1] orientated NaGd(MoO4)2 single crystal bar was used as a seed and the growing temperature was determined accurately by repeated seeding trials. The crystal was grown at a pulling rate of 0.5–1.5 mm/h and a rotating rate of 10–30 rpm in slightly oxidizing atmosphere. At the end of the slow cooling process, the crystal was pulled out of the melting surface and cooled down to room temperature at a rate of 5–15 °C/h. A Ho3+:NaGd(MoO4)2 crystal with dimensions up to U12  42 mm2 was obtained, as shown in Fig. 1. The as-grown crystal was black in color due to an oxygen-deficient atmosphere [9], which needed to be annealed in the air to reduce color centers. A yellowish and transparent crystal plate was obtained after oxidatively annealing at 900 °C for 72 h (as also shown in Fig. 1). The concentration of Ho3+ in the Ho3+:NaGd(MoO4)2 crystal was determined to be 0.24 wt% (0.73 at%) by ionic coupled plasma (ICP) spectrometry (Ultima 2). Thus, the segregation coefficient (K) in Ho3+:NaGd(MoO4)2 crystal was calculated to be 0.73 according to the following equation: K = C0 /C0, where C0 and C0 are the concentrations of Ho3+ ion in the crystal and in the raw material, respectively.

3. Optical properties The c-axis of the as-grown crystal was oriented by the crystal meteorol model YX-200 instrument produced by Dandong Radiative Instrument Co. Ltd. A crystal plate with dimensions of 13  8  1 mm3 was cut from the crystal along and perpendicular to the oriented c-axis, i.e. optical axis direction, which was polished for spectral measurements. Polarized absorption spectrum in the range of 400–2100 nm at room temperature was measured by Perkin–Elmer UV–VIS–NIR spectrometer (Lambda-900). An Edinburgh Analytical Instruments FLS920 Spectrometer was employed to measure the fluorescence spectra and fluorescence lifetimes excited with 452 nm pumping at room temperature. The measuring information of the +Edinburgh Analytical Instrument FLS920 is as following: if the measured fluorescence lifetime is less than 10 ls, the pulsed H2 lamp (nF lamp) is used, the pulse duration of which is 2 ns. Otherwise, if the measured fluorescence lifetime is more than 15 ls, the pulsed Xe lamp (lF lamp) is used and the pulse duration of which is 2 ls. The response speed of FLS 920 is controlled by time correlated single photo counting (TCSPC). Polarized absorption spectrum of Ho3+:NaGd(MoO4)2 crystal at room temperature is shown in Fig. 2, where p- and r-polarizations are defined in terms of the E-vector being parallel and perpendicular to the c-axis, respectively. The absorption cross-section was calculated by the following formula: a rabs ðkÞ ¼ ð1Þ Nc where rabs ðkÞ is the absorption cross-section, a is the absorption coefficient, N c is the concentration of Ho3+ ion which is 4.71  1019 cm3 here. It can be seen that there are six typical absorption bands from 400 to 2100 nm, attributed to the transitions of Ho3+ ion from the ground state 5I8 to 5G5(3G5), 5G6 + 5F1, 5 S2 + 5F4, 5F5, 5I6 and 5I7 for p and r, respectively. The

5

K8 + F2 + F3

5

5

50 40 30 20 10

5

5

20

F5

25

3

5

S2 + F4

3

30

5

G5, G5

2

35

-20

400

450

500

I7

5

10

I6

5

I4

15

5

σabs (10 cm )

40

π σ

5

45

G6 + F1

50

5 0 400

600

800 1000 1200 1400 1600 1800 2000 2200

Wavelength (nm) Fig. 1. As-grown Ho3+:NaGd(MoO4)2 crystal and a polished plate.

Fig. 2. Polarized absorption spectrum of Ho3+:NaGd(MoO4)2 crystal.

Z. Wang et al. / Optical Materials 30 (2008) 1873–1877

relevant absorption cross-sections of the strongest absorption peaks corresponding to 5I8 ? 5G6 + 5F1 transition are 31.0  1020 cm2 and 23.8  1020 cm2 for p and r, respectively. The J–O theory has been extensively used to analyze the optical characteristics of trivalent rare-earth ions in many host materials [10,11]. Based on the absorption spectrum, the intensity parameters Xt (t = 2, 4, 6) can be obtained by the least square fitting between experimental line strength (Sexp) and calculated line strength (Scal). The calculated process is as follows: Z 2 8p3 e2 k ðn2 þ 2Þ rðkÞdk ¼ S JJ 0 : ð2Þ 3hcð2J þ 1Þ 9n where, rðkÞ is the absorption cross-section that can be obtained from the absorption spectrum, e is the electron charge, k is the wavelength of the transition, h is the Plank constant, c is the light velocity, J is the total angular moment of the ground state (J = 8 in Ho3+ ion), and n is the refractive index of the material. According to the J–O theory, the absorption line strength for an electrical dipole transition from an initial state J to a final state J0 can be expressed in terms of the intensity parameters Xt (t = 2, 4, 6) by X S JJ 0 ¼ Xt jhðS; LÞJ jU ðtÞ jðS 0 ; L0 ÞJ 0 ij2 : ð3Þ t

U

ðtÞ

2

¼ jhf N WJ jjU ðtÞ jjf N WJ 0 ij :

ð4Þ (t)

Here SJJ0 is the absorption line strength and U is the reduced matrix, which can be found from Ref. [12]. The root mean square (rms) deviation between experimental and calculated line strengths is given by rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi XN rmsDS ¼ ðS exp  S cal Þ2 =ðN  3Þ: ð5Þ t¼1 where N is the number of absorption bands. The values of rmsDS are 1.99  1020 cm2 and 1.30  1020 cm2 for p and r, respectively. The experimental and calculated line strengths are listed in Table 1.

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Table 2 Comparison of intensity parameters for various Ho3+ ion doped crystals Material

X2 (1020 cm2)

X4 (1020 cm2)

X6 (1020 cm2)

Reference

NaBi(MoO4)2 LiBi(MoO4)2 NaY(MoO4)2 LaF3

9.5 10.1 14.87 1.16

2.6 2.7 2.89 1.38

0.4 0.5 1.24 0.88

[6] [6] [14] [15]

NaGd(MoO4)2 p-Polarized 22.72 r-Polarized 22.30

6.59 5.45

0.84 1.12

This work This work

The intensity parameters were obtained by the least square fitting method. Compared with other Ho3+ ion doped crystals (see in Table 2), Ho3+:NaGd(MoO4)2 crystal has larger intensity parameters. Generally, the X2 value is very sensitive to structure and covalence [13]. The larger X2 value of Ho3+:NaGd(MoO4)2 crystal as shown in Table 2 indicates its stronger covalence characteristics. The other two intensity parameters X4 and X6 are useful for calculating the spectroscopic quality factor by the formula X ¼ X4 =X6 : The high X value (7.85 and 4.87 for p and r, respectively) of Ho3+:NaGd(MoO4)2 crystal assures that this crystal is a promising material for efficient laser action. In the uniaxial crystal, the effective intensity parameters r p can be determined by the formula Xeff t ¼ ð2Xt þ Xt Þ=3. 20 The results were calculated to be X2 = 2.26  10 cm2, 20 2 20 2 X4 = 6.21  10 cm and X6 = 9.30  10 cm . Then the spontaneous radiative line strengths can also be calculated using Xeff and Eq. (3). Here, the reduced matrix of t tensor operators U(t) can be found from Refs. [15,16], and the radiative transition rates can be calculated by the following equation: AJJ 0 ¼

64p4 e2 nðn2 þ 2Þ2 S JJ 0 : 9 3hð2J þ 1Þk3

ð6Þ

The results for different final states should be summed up to give the total radiative transition rates as follows: X AT ðJ Þ ¼ AJJ 0 : ð7Þ J0

Table 1 Experimental line strength (Sexp) and calculated line strength (Scal) of Ho3+:NaGd(MoO4)2 crystal Transition 5

G5(3G5) ? 5I8 G6, 5F1 ? 5I8 5 F2, 3K8 ? 5I8 5 F3 ? 5I8 5 S2, 5F4 ? 5I8 5 F5 ? 5I8 5 I6 ? 5I8 5 I7 ? 5I8 5

Wavelength (nm)

Sexp (1020 cm2)

Scal (1020 cm2)

p

r

p

r

p

r

419 452 469 488 539 643 1190 1951

419 452 468 487 539 643 1156 1953

3.59 40.20 1.31 5.07 4.02 2.51 1.51 1.65

2.47 38.70 0.36 0.51 3.28 3.15 0.86 2.63

3.52 40.20 0.99 0.29 2.36 3.28 1.02 2.72

2.91 38.60 1.04 0.39 2.35 2.96 1.17 3.00

rms error (p) = 0.06885 rms error (r) = 0.04354

P 0 After that the radiative lifetime s1 r ¼ J 0 AJJ can be obtained, and the fluorescent branching ratio can be expressed by bðJ 0 Þ ¼

AJJ 0 : AT ðJ Þ

ð8Þ

The calculated radiative transition rates, the fluorescent branching ratios, and the radiative lifetimes for different levels are listed in Table 3. Within the uncertainty of the radiative J–O magnitudes (about ±15%), the Ho3+ radiative results of NaGd(MoO4)2 crystal can be considered as similar to the results of NaBi(MoO4)2 and LiBi(MoO4)2 hosts [6]. Polarized fluorescence spectra at room temperature excited with 452 nm radiation is shown in Fig. 3. There

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Table 3 Luminescence parameters of the Ho3+:NaGd(MoO4)2 crystal AJJ0 (s )

140 5

bJJ0

sr (ms)

1

6.8

Transition

k (nm)

5

I7 ? 5I8

1948

I6 ? 5I7 I8

2819 1193

48 280

0.15 0.85

3.0

I5 ? 5I6 5 I7 5 I8

3894 1635 889

23 124 111

0.09 0.48 0.43

3.9

5

I4 ? I5 5 I6 5 I7 5 I8

5053 2199 1235 753

12 51 57 11

0.09 0.39 0.44 0.08

7.7

5

F5 ? I4 I5 5 I6 5 I8

4214 2298 1445 659

0 86 253 5837

0 0.01 0.03 0.74

0.1

5

S2 ? 5F5 5 I4 5 I5 5 I6 5 I7 5 I8

3650 1956 1410 1011 754 550

2 70 52 320 1054 1348

0.00 0.03 0.02 0.11 0.37 0.47

0.4

5

F4 ? 5F5 I4 5 I5 5 I6 5 I7 5 I8

3305 1852 1355 1005 754 550

89 39 333 1128 1865 8328

0.01 0.00 0.03 0.10 0.16 0.71

0.1

5

4880 4602 1959 1377 1074 843 651 489

18 1 162 311 916 1041 5228 2269

0.00 0.00 0.02 0.03 0.09 0.10 0.53 0.23

0.10

5

5

5

5

5

F3 ? 5F4 S2 5 F5 5 I4 5 I5 5 I6 5 I7 5 I8 5

5

π

I8

σ

100 80

5

5

5

5

S2 , F4 I4

60

5

I7

I8

5

5

F5

40

I8

20 5

5

0

F3 I 8 500

550

600

650

700

750

800

Wavelength (nm) 40 5

I6

5

I8

π

30

are four emission bands around 489, 550, 659, and 754 nm in the visible region (as shown in Fig. 3a), corresponding to transitions: 5F3 ? 5I8, 5S2 + 5F4 ? 5I8, 5F5 ? 5I8, and 5 S2 + 5F4 ? 5I7 together with 5I4 ? 5I8, respectively. The strongest emission corresponding to 5S2 + 5F4 ? 5I8 transition can be induced by any higher energy level of Ho3+ ion. In the near-infrared region, there are two emission bands in the vicinity of 1011 and 1193 nm attributed to transitions 5 S2 + 5F4 ? 5I6, 5F2 ? 5I5 and the transition 5I6 ? 5I8, respectively (as shown in Fig. 3b). The excited transition mechanism of Ho3+ ion can be described with the help of the energy level diagram as shown in Fig. 4 [14,15]. The fluorescence lifetime decay curves excited by 452 nm pumping at room temperature are shown in Fig. 5. Fig. 5a– c are corresponding to the lifetimes of 550 nm, 754 nm, and 1193 nm, respectively. By fitting the luminescence decay curve with a single exponential function: I(t) = I0 + Aexp(t/s), the fluorescence lifetimes were 3.4 ls, 3.5 ls, and 46.4 ls, corresponding to transitions 5 S2 + 5F4 ? 5I8, 5S2 + 5F4 ? 5I7, 5I4 ? 5I8, and 5I6 ? 5I8, respectively. It can be seen that the fluorescence spectra

σ

20 5

10

5

5

S2 , F4 F2

5

I6

5

I5

0 800

1000

1200

1400

1600

Wavelength (nm) Fig. 3. Polarized fluorescence spectra of Ho3+:NaGd(MoO4)2 crystal excited by 452 nm pumping at room temperature: (a) visible emission and (b) near-infrared emission.

25

5

3

G 5, G 5 5 3

20

Energy (103 cm-1)

5

Intensity (a.u.)

5

147

5

S 2 , F4

120

Intensity (a.u.)

1

5

F3 5

15

5

K8, F 2

5

G 6, F 1

5

S2, F 4

5

F5 I4 5 I5

5

10

5

I6

5

5

I7

0

5

I8

Fig. 4. Energy level diagram of Ho3+ ion and the luminescence process by 452 nm radiation pumping.

overlap with absorption one around 550 and 1193 nm from Figs. 2 and 3. The overlaps exhibit that there are fluorescence trapping influencing the measured lifetimes. So the

Z. Wang et al. / Optical Materials 30 (2008) 1873–1877

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spectra. So there is no influence of fluorescence trapping onto the measured fluorescence lifetime of 754 nm.

1.0 550 nm

Intensity (a.u.)

0.8

4. Conclusion

Model: ExpDec1 Chi2 = 0.0002, R2 = 0.99642 I0 = 0 +0 A = 8.2771 +0.11024 τ = 3.43321 +0.01702

0.6 0.4 0.2 0.0 8

10

12

14

16

18

20

22

24

26

Time (μs)

1.0 754 nm

Intensity (a.u.)

0.8

Model: ExpDec1 Chi2 = 0.0003, R2 = 0.99547 I0 = 0 +0 A = 7.9777 +0.12338 τ = 3.54711 +0.02098

0.6 0.4

The Ho3+:NaGd(MoO4)2 crystal with dimensions of U12  42 mm2 was grown by CZ method and its polarized absorption spectrum at room temperature was investigated. Six typical absorption bands can be seen from the absorption spectrum, among which the bands corresponding to the transition 5I8 ? 5G6 + 5F1 have the largest absorption cross-sections. Based on the J–O theory and the absorption spectrum, the spontaneous transition probabilities, the fluorescent branching ratios, and the radiative lifetimes were obtained. Polarized fluorescence spectra at room temperature in visible and near-infrared regions were also investigated. The peak attributed to the 5S2 + 5F4 ? 5I8 transition is the strongest emission in six main emission peaks. By the single exponential fitting, the fluorescence lifetimes were calculated to be 3.4 ls, 3.5 ls, and 46.4 ls, corresponding to transitions 5 S2 + 5F4 ? 5I8, 5S2 + 5F4 ? 5I7, 5I4 ? 5I8, and 5I6 ? 5I8, respectively. Due to the influence of fluorescence trapping, the measured fluorescence lifetimes of 550 and 1193 nm should be longer than their actual values.

0.2

Acknowledgement

0.0

This work is supported by the Young Scientists Innovation Foundation of Fujian Province (2003J041 and 2006F3139).

8

10

12

14

16

18

20

Time (μs)

References 30

1193 nm

Intensity (a.u.)

25

Model: ExpDec1 Chi2 = 0.0507, R2 = 0.99913 I0 = 0 +0 A = 301.99507 +07.48625 τ = 46.40689 +0.54767

20 15 10 5 0 100

120

140

160

180

200

Time (μs) Fig. 5. Fluorescence decay curves of Ho3+:NaGd(MoO4)2 crystal by 452 nm pumping at room temperature: (a) for 550 nm; (b) for 754 nm; and (c) for 1193 nm.

measured fluorescence lifetimes of 550 and 1193 nm should be longer than their actual values. But around 754 nm there is no overlap between the fluorescence and absorption

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