Composition-dependent spectroscopic properties of Nd3+-doped tellurite–germanate glasses

Composition-dependent spectroscopic properties of Nd3+-doped tellurite–germanate glasses

ARTICLE IN PRESS Physica B 405 (2010) 1958–1963 Contents lists available at ScienceDirect Physica B journal homepage: www.elsevier.com/locate/physb ...

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ARTICLE IN PRESS Physica B 405 (2010) 1958–1963

Contents lists available at ScienceDirect

Physica B journal homepage: www.elsevier.com/locate/physb

Composition-dependent spectroscopic properties of Nd3 + -doped tellurite–germanate glasses Jing Wan, Lihong Cheng , Jiashi Sun, Haiyang Zhong, Xiangping Li, Weili Lu, Yue Tian, Bo Wang, Baojiu Chen  Department of Physics, Dalian Maritime University, Dalian, Liaoning 116026, PR China

a r t i c l e in f o

a b s t r a c t

Article history: Received 24 October 2009 Received in revised form 4 January 2010 Accepted 6 January 2010

Nd3 + -doped tellurite–germanate glasses with various compositions were prepared by a conventional melt-quenching method. On the basis of Judd–Ofelt theory, the optical transition intensity parameters Ol (l = 2, 4, 6) of Nd3 + in all the samples were calculated from the absorption spectra. By using these three intensity parameters, the radiative transition properties, such as the transition rates, branch ratios for 4F3/2-4I11/2 and 4F3/2-4I9/2 transitions, and radiative lifetimes of 4F3/2 level of Nd3 + in all the glasses were carefully calculated. The compositional dependence of these spectroscopic parameters was analyzed. According to the McCumber theory, the emission cross sections for 4F3/2-4I9/2 of Nd3 + in various glasses were obtained, furthermore the optical gain properties for 4F3/2-4I9/2 transition in all the samples were also studied. & 2010 Elsevier B.V. All rights reserved.

Keywords: Judd–Ofelt theory Nd3 + Spectroscopic properties Intensity parameters Cross section

1. Introduction As well known, Nd3 + is one of the most widely investigated rare earth (RE) ions. Nd3 + can be incorporated into a variety of hosts, such as glasses and crystals, and gives the possible UV, blue, green, orange, red and infrared (IR) emissions. Therefore, Nd3 + can be a potential candidate as the luminescent center, especially, because of its two most important IR transitions, viz. 4F3/2-4I11/2 and 4F3/2-4I9/2 [1–4]. Among all the possible hosts for Nd3 + ions, glasses have attracted much attention mainly due to their advantages, for example, excellent mechanical and thermal stability, large solubility of RE ions, good capacity of glass configuration and low cost and so on [5]. It was reported that RE activated tellurite glasses are in possession of many good characteristics: high refractive index (  2), good glass stability, corrosion resistance, low vibration energy among oxide glasses, and wide transmission region [6]. In addition, germanate-based glasses combine high mechanical strength, high chemical durability, thermal stability, and good transmission in the infrared region [7]. Yanmin Yang et al. reported the influence of host composition on the thermal stability, optical transition properties of Er3 + , quantum

 Corresponding author.  Parallel corresponding author. Tel./fax: + 86 411 84728909.

E-mail addresses: [email protected] (L. Cheng), [email protected] (B. Chen). 0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.01.051

efficiencies, optical absorption edge in the tellurite–germanate glasses. It was found that introducing GeO2 into the tellurite glass improved the thermal stability and resulted in the blue shift of the optical absorption edge of the host matrix, meanwhile, the emission intensity of 4I13/2-4I15/2 increased but the upconversion luminescent intensity decreased [8]. In this present work, we prepared a set of Nd3 + -doped tellurite–germanate glasses in order to study the influence of glass composition on optical transition properties, especially for the transitions of 4F3/2-4I11/2 and 4F3/2-4I9/2. Our purpose is to supply some data for the future work, and to explore the potential applications of Nd3 + -activated glasses in the laser field.

2. Experimental procedures Nd3 + -doped tellurite–germanate glasses with the following chemical molar compositions were prepared by a conventional melt-quenching method. xGeO2–(80 x) TeO2–10ZnO–10BaO (here x =0, 20, 40, 60, 80; extra 0.5 mol% Nd2O3 were added for each sample). According to the designed compositions above, 5 g of each batch of the starting materials GeO2, TeO2, ZnO, BaO and Nd2O3 with analytical grade were weighed and well mixed in an agate mortar. Then the mixture was put in an alumina crucible and melted in a muffle furnace at 800 1C for about 10 min to obtain homogeneously mixed melt. After that, the melt was poured into a brass mold and then annealed at 400 1C for 4 h in order to

ARTICLE IN PRESS J. Wan et al. / Physica B 405 (2010) 1958–1963

remove mechanical stress. Finally, the as-prepared samples were cut and polished carefully to meet the requirement of optical measurement. According to the content of TeO2 and GeO2, the samples were labeled as T8G0, T6G2, T4G4, T2G6, T0G8. For instance, T4G4 denotes the sample with designed composition of 40GeO2–40TeO2–10ZnO–10BaO with 0.5 mol% of Nd2O3. The glass density was measured according to the Archimedes’ principle, using pure water as immersion liquid. The refractive indices were achieved by using a VASE (variable angle incidence) M-2000 Spectroscopic Ellipsometer (J.A. Woollam Co, Inc. USA). The absorption spectra were taken on a Shimadzu PC3101 Fluorescence spectrophotometer.

3. Calculations

ln½I0 ðvÞ=IðvÞ Nt

ð1Þ

where I0(n) and I(n) are the intensities of incident and outgoing light, N is the number of rare earth ion per unit volume, t is the thickness of the sample. According to Eq. (1), the experimental oscillator strength for a certain transition can be given as follows: Z mc sðvÞ dv ð2Þ fex ¼ 2 pe where m, e are the mass and charge of electron, respectively, c represents the light velocity in the vacuum. Based on the Judd–Ofelt theory [9,10], the theoretical oscillator strength of an electric dipole transition can be obtained from the following equation. ed fth ¼

8p2 mcv ðnðvÞ2 þ 2Þ2 X Ol ð/CJjjU l jjC0 J 0 SÞ2 9n 3hð2J þ 1Þ l ¼ 2;4;6

ð3Þ

where h is Planck’s constant, n(n) is the refractive index at wavenumber n, J is the angular moment of the initial level, Ol (l = 2, 4, 6) are Judd–Ofelt intensity parameters. /CJjjU l jjC0 J0 S is the reduced matrix element of corresponding electric dipole transition. In order to evaluate the uncertainty of the calculation, the root-mean-square (r.m.s) was also calculated by using the following equation: vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u N i uX ðfex f i Þ2 th ð4Þ d¼t N3 i where N is the number of absorption transition used in the Judd– i i Ofelt calculation; fex and fth are the experimental and theoretical oscillator strengths for ith transition. By using the obtained intensity parameters, the transition rates, branch ratios and radiative lifetimes for some important transitions intra 4f-configuration of Nd3 + can be obtained. The radiative transition rate of an electric dipole transition can be described as follows. AED ¼

64p4 e2 v3 nðn2 þ 2Þ2 X Ol ð/CJjjU l jjC0 J0 SÞ2 27hð2J þ 1Þ l ¼ 2;4;6

ð5Þ

According to the calculated results from Eq. (5), the branch ratio from the upper level (J) to the lower level (J0 ) can be derived by A 0 ðJ-J 0 Þ 0 J 0 AJ 0 ðJ-J Þ

bJJ0 ¼ P J

Accordingly, the radiative lifetimes of the upper level also can be expressed using the below equation.

tJ ¼ P

1

J 0 AJ 0 ðJ-J



ð7Þ

In terms of McCumber theory [11], we can estimate the emission cross section by using the absorption cross section data. The relationship between emission and absorption cross sections is

se ðvÞ ¼ sa ðvÞ

Zl exp½ðehvÞ=kT Zu

ð8Þ

where Zl and Zu are the degeneracies for the lower and upper levels, respectively. e is the effective energy difference between the lower and upper levels. k is Boltzmann constant.

4. Results and discussions

The absorption cross section s(n) at wavenumber n is determined from the following equation:

sðvÞ ¼

1959

ð6Þ

4.1. Judd–Ofelt analysis The optical absorption spectra of Nd3 + -doped tellurite– germanate glasses with different compositions were recorded at room temperature in the wavelength range 400–1000 nm, and are shown in Fig. 1. All the Nd3 + -doped samples display 12 absorption bands centered at 431, 463, 475, 513, 527, 585, 683, 743, 750, 797, 806 and 879 nm corresponding to the different transitions from the ground state 4I9/2 to the excited states 4P1/2, 2 K15/2(4G11/2), 2D3/2, 4G9/2(2K13/2), 4G7/2, 4G5/2(2G7/2), 4F9/2, 4S3/2, 4 F7/2, 2H9/2, 4F5/2 and 4F3/2, respectively. The observed absorption transitions and two radiative transitions interested are marked in Fig. 2. The intensity of each absorption band can be measured by its oscillator strength, which is proportional to the area under the absorption curve. The experimental oscillator strengths can be calculated by using Eq. (2). The Judd–Ofelt intensity parameters were derived by using least-square method, and the r.m.s were also calculated from Eq. (4). These parameters for all the samples are given in Table 1. The calculation errors are smaller than the values of most oscillator strengths. This means that the calculation results are reliable. It is generally accepted that the Judd–Ofelt intensity parameters Ol (l =2, 4, 6) are dependent on the host [1]. Among these three intensity parameters, O2 is most sensitive to the local structure changes of host. From Table 1 it can be seen that O2 changes with the variation of TeO2 content in the glasses. This fact implies that the Nd3 + ions occupy different sites in different glasses. It is well known that higher the O2 value, the more nonsymmetrical the ion site is. The sample with 20 mol% TeO2 displays maximum O2 value which means the Nd3 + ions occupy the sites which are the most nonsymmetrical. Generally speaking, O6 is related to the rigidity of glass host, O4 is determined by O2 and O6 [9,12–14]. The ratio of O4 to O6 defined as spectroscopic quality factor is a measure of lasing capacity for a rare earth doped material. For the studied samples, O4 and O6 change not obviously, thus implying that the host rigidity changes less than its local structure. With the increase of TeO2 content, the spectroscopic quality factors basically increase. This fact indicates that the samples with high content of TeO2 are more suitable for laser operation. In comparison, the intensity parameters for Nd3 + in some other host matrixes reported in the literatures [1,4,15,16] are also listed in Table 1. It is found that Ol (l = 2, 4, 6) are discriminating in different host matrixes. The T2G6 and K–Mg–Al phosphate glasses exhibit similar maximum O2, the alkali tellurofluorophosphate and oxyfluoride (silicate) glasses show the minimum O2. Except for the barium fluorophosphate glass, the O4 and O6 are similar with each other. This fact tells us that O2 is more sensitive to the crystal field than O4 and O6.

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2

4S 4 2K 15/2 G11/2

2

D3/2

4 4G 7/2

H9/2

4F 7/2

P1/2

4

4F 5/2 4F 3/2

3/2

F9/2

TG08 TG26 TG44 TG62 TG80 500

400

600

700 800 Wavelength (nm)

900

Te O

2

co nte nt inc rea se

2 4G 9/2 K13/2

s

Absorbance ( cm-1)

4G 2 5/2 G7/2

1000

Fig. 1. Absorption spectra of Nd3 + -doped tellurite–germanate glasses with different compositions.

30.0k Energy levels and Transitions 4 P3/2 D5/2

2

25.0k

4 4G P1/2 11/2 2K 2 D 15/2 3/2 2 K13/2 4 G9/2 4G 7/2 2G 7/2 4 G5/2 2 H11/2 4F 9/2 4S 3/2 4F 7/2 2 H9/2 4F 5/2 4 F3/2

Wavenumber (cm-1)

20.0k

15.0k

10.0k

4I 15/2

5.0k

4I 13/2 4

I11/2

4

0.0

I9/2

Nd3+

Fig. 2. Energy level diagram and transitions for Nd3 + in the tellurite–germanate glasses.

4.2. Optical transition properties Using the obtained Judd–Ofelt intensity parameters, the transition rates, branch ratios for 4F3/2-4I11/2 and 4F3/2-4I9/2 transitions, and the radiative lifetimes of 4F3/2 level for all the samples were calculated from Eqs. (5)–(7). Fig. 3 displays the dependence of the radiative transition rates of 4F3/2-4I11/2 and 4 F3/2-4I9/2 transitions on TeO2 content. It can be seen that with an increase of TeO2 content, the transition rates of 4F3/2-4I11/2

and 4F3/2-4I9/2 gradually increase. This fact tells us that the radiative transition rate can be controlled by adjusting TeO2 content in the tellurite–germanate glasses. The relationship between the branch ratios of both 4F3/2-4I11/2 and 4F3/2-4I9/2 transitions and the TeO2 content is shown in Fig. 4. It is evident that the branch ratios change slightly with the increase of TeO2 content. As widely known, the 4f electrons of trivalent rare earth ions are shielded by the outside 5s and 5p electrons that make the transitions intra 4f-configuration insensitive to the host

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1961

Table 1 Judd–Ofelt intensity parameters Ol (l = 2, 4, 6) (10  20 cm2), optical quality factors, emission cross sections and the r.m.s. for the studied glasses and some other samples doped with Nd3 + . Glass

O2

O4

O6

w = O4/O6

se (10  20 cm2)(4F3/24I9/2)

r.m.s (10  8)

Ref.

(10  20 cm2) T0G8 T2G6 T4G4 T6G2 T8G0 50(NaPo3)6–10TeO2–20AlF3–19LiF–1Nd2O3 (alkali tellurofluorophosphate) 45SiO2–25Al2O3–5CaO–10NaF–15CaF2–1NdF3 (oxyfluoride) 58.5P2O5 + 17.0K2O +14.5MgO + 9.0Al2O3 + 1.0Nd2O3 (K–Mg–Al phosphate) 78P2O5–10Na2SO4–10BaF2–2NdF3 (barium fluorophosphate containing sulfate)

5.17 6.18 5.42 5.42 4.39 3.54

2.48 3.99 3.52 4.09 3.97 3.76

3.25 4.10 4.31 4.10 3.53 4.46

0.76 0.97 0.82 0.998 1.12 0.84

0.82 1.34 1.52 1.65 1.41 0.86

16.18 17.60 27.61 15.16 25.35 16.90

This This This This This [1]

3.56

3.90

3.95

0.99

0.69

38.00

[4]

6.22

5.95

6.83

0.87

1.58



[15]

9.8

1.61

2.5



[16]

5.7

15.8

0.45

2400

4F 3/2

-> 4I11/2

4F 3/2

-> 4I9/2

0.40 4F 3/2

level

0.35

2000

Lifetime (ms)

Radiative Transition Rate (s-1)

2800

work work work work work

1600

0.30

0.25 1200 0.20 800 0

20 40 60 TeO2 content (mol %)

80

0

Fig. 3. Dependence of radiative transition rates of 4F3/2-4I11/2 and 4F3/2-4I9/2 on TeO2 content.

Branch ratio (%)

4 F3/2 -> I11/2

4F 3/2

60

20 40 60 GeO2 content (mol %)

80

Fig. 5. Relationship between radiative lifetime of 4F3/2 level of Nd3 + and GeO2 content.

matrix. Therefore, the wavelength variation of an f–f transition in the different host is not obvious, and usually f–f transitions of trivalent rare earth ions exhibit atom-like spectral profile. As a matter of fact, except for the hypersensitive transition, the branch ratio for a transition usually changes slightly as observed in this experiment. This fact enlightens us that the branch ratio cannot be changed by adjusting the glass composition. Fig. 5 shows the dependence of the radiative lifetimes of 4F3/2 energy level of Nd3 + on the glass composition. It can be seen that the pure germanate glass (T0G8) displays a maximum radiative lifetime, nevertheless, the pure tellurite (T8G0) glass displays a minimum radiative lifetime for the 4F3/2 level. For all the glasses, the radiative lifetime increases with the increase of GeO2 content. From this result, it can be deduced that the radiative lifetime of Nd3 + in tellurite (or germanate) glasses can be controlled by introducing proper amount of germanium oxide (or tellurium oxide).

70 4

0.15

-> 4I9/2

50

40

30

20 0

20

40 60 TeO2 content (mol %)

Fig. 4. Dependence of branch ratios of content.

4

F3/2-4I11/2 and

80

4

F3/2-4I9/2 on TeO2

4.3. Emission cross section and optical gain coefficient The absorption cross sections for the transitions from ground state to the excited states can be directly derived from Eq. (1).

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J. Wan et al. / Physica B 405 (2010) 1958–1963

0.9 T0G8

T6G2 T2G6

0.6

T0G8 0.8

P=1

T4G4

1.2

g (ν)/N

Emission Cross Section (10-20 cm2 )

1.6

T8G0

0.3

0.0 0.4

-0.3

0.0 10600 10800

10800 11000

11200

11400

P=0 11000

11200

11400

11600

11800

Wavenumber (cm-1)

11600 11800

Wavenumber (cm-1) Fig. 6. Emission cross sections of 4F3/2-4I9/2 of Nd3 + in various glasses.

1.5 P=1

T8G0 1.0 g (ν)/N

If the absorption cross section for a transition A-B is known, the emission cross section for the transition B-A could be then confirmed from McCumber theory, as described in Ref. [17, Eq. 10]. Fig. 6 shows the emission cross sections of 4F3/2-4I9/2 transition of Nd3 + in different samples. From Fig. 6, it can be found that the pure germanate glass (T0G8) exhibits minimum emission cross section, the pure tellurite glass (T8G0) exhibits maximum emission line width. Though the low content of TeO2, for example 20 mol%, was introduced into the germanate glass, the sample (namely T2G6) gained a large emission cross section which is almost close to the pure tellurite glass. From this result we can conclude that introducing TeO2 into germanate glass can increase the emission cross section of 4F3/2-4I9/2 transition. The differential emission cross sections for the studied glasses and some other glasses are listed in the sixth column of Table 1. It is found that the barium fluorophosphate glass containing sulfate has the maximum value, but the oxyfluoride glass has the minimum value. In our work, the emission cross sections of pure germanate and tellurite glasses are smaller than that of T6G2. The optical gain coefficient g at wavelength l can be determined by means of the formula g(l)= N2se(l) N1sa(l), where sa(l) and se(l) are the absorption and emission cross sections at wavelength l, and N1 and N2 represent the populations of the upper and lower levels, respectively [17]. If N= N1 +N2, then gðlÞ=N ¼ P se ðlÞð1PÞsa ðlÞ, where P ¼ N2 =N, which is dependent on the pump energy-density. By using the data of emission (4F3/2-4I9/2) and absorption (4I9/2-4F3/2) cross sections, the optical gain coefficients gðlÞ=N for all the samples were calculated. In the calculations, P takes the values starting from 0 to 1 with an increment of 0.1. As representatives, Figs. 7(a) and (b) show the optical gain coefficient as a function of wavenumber for the pure tellurite (T8G0) and pure germinate (T0G8) samples. It can be seen that the positive optical gain firstly appears at larger wavenumber when P is around 0.2. This fact tells that the studied samples reveal low threshold for 4F3/2-4I9/2 laser operation.

0.5

0.0

-0.5 P=0 10800

11000

11200

11400

11600

11800

Wavenumber (cm-1) Fig. 7. Optical gain coefficient spectra for 4F3/2-4I9/2 transition; here P is the ratio of 4F3/2 population to the sum population of 4F3/2 and 4I9/2 levels, and P increases with step of 0.1: (a) T0G8, (b) T8G0.

of Judd–Ofelt theory. It was found that the optical transition intensity parameter O2 changes greatly with glass composition, but O4 and O6 change slightly. There is almost no change in the branch ratios for 4F3/2-4I9/2 and 4F3/2-4I11/2 transitions, nevertheless, the radiative transition rates for these two transitions increase with the increase of TeO2 content. The radiative lifetime for 4F3/2 level increases with the increase of GeO2 content. Moreover, the emission cross section for 4F3/2-4I11/2 transition of Nd3 + changes also with glass composition. Amongst all the samples, the pure germanate glass presents a minimum emission cross section, the pure tellurite glass presents a maximum emission line width. The introduction of TeO2 is beneficial to the increase of emission cross section. The study on the optical gain properties proves that these Nd3 + doped glasses can provide a low threshold for laser operation of 4F3/2-4I11/2 transition.

5. Conclusions Acknowledgments The Nd3 + doped tellurite–germanate glasses with various compositions were prepared via a melt-quenching technique. Their optical transition properties were studied in the framework

This work was partially supported by NSFC (National Natural Science Foundation of China, Grant no. 50972021 and 50802010),

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Joint Program of NSFC-GACAC (General Administration of Civil Aviation of China, Grant no. 60776814), and Natural Science Foundation of Liaoning Province (Grant no. 20082139 and 20092147). References [1] M. Jayasimhadri, L.R. Moorthy, R.V.S.S.N. Ravikumar, Opt. Mater. 29 (2007) 1321. [2] K.U. Kumar, V.A. Prathyusha, P. Babu, C.K. Jayasankar, A.S. Joshi, A. Speghini, M. Bettinelli, Spectrochim. Acta Part A 67 (2007) 702. [3] G.N. Hemantha Kumar, J.L. Rao, K. Ravindra Prasad, Y.C. Ratnakaram, J. Alloys Compounds (2009). [4] D. Chen, Y. Wang, Y. Yu, E. Ma, F. Liu, J. Phys. Chem. Solids 68 (2007) 193. [5] E.O. Serqueira, N.O. Dantas, A.F.G. Monte, M.J.V. Bell, J. Non-Cryst. Solids 352 (2006) 3628.

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