Jude Offelt Theory

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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, Chinab 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 2007Available 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 absorp-

tion and fluorescence spectra at room temperature were investigated. The largest absorption cross-sections, corresponding to5I8?

5G6 +5F1 absorption bands, are 31.0 10

20 cm2 and 23.8 1020 cm2 for p- and r-polarization, respectively. The strongestemission corresponding to the 5S2 +

5F4?5I8 transition can be induced by populating any higher energy level of Ho

3+ ion. The fluo-rescence 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 transitions5S2 +

5F4?5I8,

5S2 +5F4?

5I7,5I4?

5I8 and5I6?

5I8, respectively. Due to the influence of fluorescence trapping, the measured fluo-rescence lifetimes of 550 and 1193 nm should be longer than their actual values. Based on the JuddOfelt (JO) theory and polarizedabsorption spectrum, the spontaneous transition probabilities, the fluorescent branching ratios and the radiative lifetimes werecalculated. 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 havebeen attracting much attention in the field of laser physics.Ho3+ ion, as one of lanthanide ions, has been widely inves-tigated for achieving laser actions in various wavelengths,such as infrared, visible and ultra-violet regions.

There has been increasing interests in molybdate crystalswith general formula MRe(MoO4)2 (M = alkali metal andRe = rare earth) in the past few years, not only because oftheir large lanthanide admittance, but especially owing totheir good properties, such as high integral absorptionand fluorescence cross-sections, broadened lines of opticalspectra of rare-earth ions and the possibilities to obtain

tunable laser oscillation within wide range, and so on [14]. As a member of MRe(MoO4)2 family, NaGd(MoO4)2is regarded as an attractive laser host material candidate,which belongs to the scheelite (CaWO4) structure withspace group centrosymmetric I41/a [5] (in some late papers[68], it is stated that the refined space group of this crystalis non-centrosymmetric I4). The cell parameters are as fol-

lows: a = b = 5.235 A

, c = 11.538 A

[5]. Recently, lots ofwork has been emphasized on NaGd(MoO4)2 crystal[1,2]. However, up to now, no attention has been paid forthe investigation of Ho3+ doped NaGd(MoO4)2 crystal.This paper reports the growth and spectral properties ofHo3+:NaGd(MoO4)2 crystal.

2. Crystal growth

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

0925-3467/$ - see front matter 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.optmat.2007.12.012

* Corresponding author. Tel.: +86 591 83710369; fax: +86 59183714946.

E-mail address: [email protected] (X. Long).

www.elsevier.com/locate/optmat

Available online at www.sciencedirect.com

Optical Materials 30 (2008) 18731877
mailto:[email protected]:[email protected]


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[13]. Ho2O3 (99.99%), Gd2O3 (99.99%), MoO3 (99.95%),and Na2CO3 (99.95%) were weighed according to the stoi-chiometric composition of NaHo0.01Gd0.99(MoO4)2 inaddition to 2 wt% excess MoO3 to compensate for its vol-atilization loss during the process of crystal growth. Thepolycrystalline materials of 1 at% Ho3+ doped NaGd-

(MoO4)2 crystal were synthesized by solid-state reaction.First, the weighed chemicals were thoroughly mixed, thenafter grinding and extruding to form tablets, the mixturewas loaded into an alumina crucible, which was thenplaced into a vertical furnace, holding at 650 C for 24 hto carry out the reaction, then repeating the above processbut held at 900 C also for 24 h to assure adequatereaction.

The synthesized polycrystalline materials of Ho3+:NaGd(MoO4)2 were melted in a U50 50 mm

2 platinumcrucible using a 2.5 kHz frequency induction furnace. Asmall [0 0 1] orientated NaGd(MoO4)2 single crystal barwas used as a seed and the growing temperature was deter-

mined accurately by repeated seeding trials. The crystalwas grown at a pulling rate of 0.51.5 mm/h and a rotatingrate of 1030 rpm in slightly oxidizing atmosphere. At theend of the slow cooling process, the crystal was pulled outof the melting surface and cooled down to room tempera-ture at a rate of 515 C/h. A Ho3+:NaGd(MoO4)2 crystalwith dimensions up to U12 42 mm2 was obtained, asshown in Fig. 1. The as-grown crystal was black in colordue to an oxygen-deficient atmosphere [9], which neededto be annealed in the air to reduce color centers. A yellow-ish and transparent crystal plate was obtained after oxida-tively annealing at 900 C for 72 h (as also shown in Fig. 1).

The concentration of Ho3+

in the Ho3+

:NaGd(MoO4)2crystal was determined to be 0.24 wt% (0.73 at%) by ioniccoupled plasma (ICP) spectrometry (Ultima 2). Thus, thesegregation coefficient (K) in Ho3+:NaGd(MoO4)2 crystalwas calculated to be 0.73 according to the followingequation: K= C 0/C0, where C

0 and C0 are the concentra-tions 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 thecrystal meteorol model YX-200 instrument produced byDandong Radiative Instrument Co. Ltd. A crystal platewith dimensions of 13 8 1 mm3 was cut from the crys-

tal along and perpendicular to the oriented c-axis, i.e. opti-cal axis direction, which was polished for spectralmeasurements. Polarized absorption spectrum in the rangeof 4002100 nm at room temperature was measured byPerkinElmer UVVISNIR spectrometer (Lambda-900).An Edinburgh Analytical Instruments FLS920 Spectrome-ter was employed to measure the fluorescence spectra andfluorescence lifetimes excited with 452 nm pumping atroom temperature. The measuring information of the+Edinburgh Analytical Instrument FLS920 is as following:if the measured fluorescence lifetime is less than 10 ls, thepulsed H2 lamp (nF lamp) is used, the pulse duration ofwhich is 2 ns. Otherwise, if the measured fluorescence life-

time is more than 15 ls, the pulsed Xe lamp (lF lamp) isused and the pulse duration of which is 2 ls. The responsespeed of FLS 920 is controlled by time correlated singlephoto counting (TCSPC).

Polarized absorption spectrum of Ho3+:NaGd(MoO4)2crystal at room temperature is shown in Fig. 2, wherep- and r-polarizations are defined in terms of the E-vectorbeing parallel and perpendicular to the c-axis, respectively.The absorption cross-section was calculated by the follow-ing formula:

rabsk a

Nc1

where rabsk is the absorption cross-section, a is theabsorption coefficient, Nc is the concentration of Ho

3+

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 to5G5(

3G5),5G6 +

5F1,5S2 +

5F4,5F5,

5I6 and5I7 for p and r, respectively. The

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

400 600 800 1000 1200 1400 1600 1800 2000 22000

5

10

15

20

25

30

35

40

45

50

400 450 500

10

20

30

40

50

Wavelength (nm)

5I75I

65I4

5F

5

3K

8

+5F

2

+5F

3

5S

2

+5F

4

5G

6

+5F

1

5G

5,3G

5

abs

(10-20cm

2)

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

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relevant absorption cross-sections of the strongest absorp-tion peaks corresponding to 5I8?

5G6 +5F1 transition are

31.0 1020 cm2 and 23.8 1020 cm2 for p and r,respectively.

The JO theory has been extensively used to analyze theoptical 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 obtainedby the least square fitting between experimental linestrength (Sexp) and calculated line strength (Scal). The cal-culated process is as follows:Z

rkdk 8p3e2k

3hc2J 1

n2 22

9nSJJ0 : 2

where, rk is the absorption cross-section that can be ob-tained from the absorption spectrum, e is the electroncharge, k is the wavelength of the transition, h is the Plankconstant, c is the light velocity, J is the total angular mo-ment of the ground state (J = 8 in Ho3+ ion), and n isthe refractive index of the material.

According to the JO theory, the absorption linestrength for an electrical dipole transition from an initialstate J to a final state J0 can be expressed in terms of theintensity parameters Xt (t = 2,4,6) by

SJJ0 Xt

XtjhS;LJjUtjS0;L0J0ij2: 3

Ut jhfNWJjjUtjjfNWJ0ij2: 4

Here SJJ0 is the absorption line strength and U(t) is the re-

duced matrix, which can be found from Ref. [12]. The rootmean square (rms) deviation between experimental and cal-

culated line strengths is given by

rmsDS

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiXNt1

Sexp Scal2=N 3

r: 5

where N is the number of absorption bands. The values ofrmsDS are 1.99 1020 cm2 and 1.30 1020 cm2 for pand r, respectively. The experimental and calculated linestrengths are listed in Table 1.

The intensity parameters were obtained by the leastsquare fitting method. Compared with other Ho3+ iondoped crystals (see in Table 2), Ho3+:NaGd(MoO4)2 crys-tal has larger intensity parameters. Generally, the X2 valueis very sensitive to structure and covalence [13]. The largerX2 value of Ho

3+:NaGd(MoO4)2 crystal as shown in Table2 indicates its stronger covalence characteristics. The othertwo intensity parameters X4 and X6 are useful for calculat-ing the spectroscopic quality factor by the formulaX X4=X6: The high X value (7.85 and 4.87 for p and r,respectively) of Ho3+:NaGd(MoO4)2 crystal assures thatthis crystal is a promising material for efficient laser action.

In the uniaxial crystal, the effective intensity parameterscan be determined by the formula Xefft 2X

rt X

pt=3.

The results were calculated to be X2 = 2.26 1020 cm2,

X4 = 6.21 1020 cm2 and X6 = 9.30 10

20 cm2. Thenthe spontaneous radiative line strengths can also be calcu-lated using Xefft and Eq. (3). Here, the reduced matrix oftensor operators U(t) can be found from Refs. [15,16],

and the radiative transition rates can be calculated by thefollowing equation:

AJJ0 64p4e2

3h2J 1k3nn2 2

2

9SJJ0 : 6

The results for different final states should be summed upto give the total radiative transition rates as follows:

ATJ XJ0

AJJ0 : 7

After that the radiative lifetime s1r P

J0AJJ0 can be ob-tained, and the fluorescent branching ratio can be ex-

pressed by

bJ0 AJJ0

ATJ: 8

The calculated radiative transition rates, the fluorescentbranching ratios, and the radiative lifetimes for differentlevels are listed in Table 3. Within the uncertainty of theradiative JO magnitudes (about 15%), the Ho3+ radia-tive results of NaGd(MoO4)2 crystal can be considered assimilar to the results of NaBi(MoO4)2 and LiBi(MoO4)2hosts [6].

Polarized fluorescence spectra at room temperature

excited with 452 nm radiation is shown in Fig. 3. There

Table 1

Experimental line strength (Sexp) and calculated line strength (Scal) ofHo3+:NaGd(MoO4)2 crystal

Transition Wavelength (nm) Sexp (1020 cm2) Scal (10

20 cm2)

p r p r p r

5G5(3G5)?

5I8 419 419 3.59 2.47 3.52 2.915G6,

5F1?5I8 452 452 40.20 38.70 40.20 38.60

5F2,3K8?

5I8 469 468 1.31 0.36 0.99 1.045F3?

5I8 488 487 5.07 0.51 0.29 0.395S2,

5F4?5I8 539 539 4.02 3.28 2.36 2.35

5F5?5I8 643 643 2.51 3.15 3.28 2.96

5I6?5I8 1190 1156 1.51 0.86 1.02 1.17

5I7?5I8 1951 1953 1.65 2.63 2.72 3.00

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

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

Material X2(1020 cm2)

X4(1020 cm2)

X6(1020 cm2)

Reference

NaBi(MoO4)2 9.5 2.6 0.4 [6]LiBi(MoO4)2 10.1 2.7 0.5 [6]NaY(MoO4)2 14.87 2.89 1.24 [14]

LaF3 1.16 1.38 0.88 [15]

NaGd(MoO4)2p-Polarized 22.72 6.59 0.84 This workr-Polarized 22.30 5.45 1.12 This work

Z. Wang et al. / Optical Materials 30 (2008) 18731877 1875


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are four emission bands around 489, 550, 659, and 754 nmin the visible region (as shown in Fig. 3a), corresponding totransitions: 5F3?

5I8,5S2 +

5F4?5I8,

5F5?5I8, and

5S2 +5F4?

5I7 together with5I4?

5I8, respectively. Thestrongest emission corresponding to 5S2 +

5F4?5I8 transi-

tion can be induced by any higher energy level of Ho3+ ion.In the near-infrared region, there are two emission bands inthe vicinity of 1011 and 1193 nm attributed to transitions5S2 +

5F4?5I6,

5F2?5I5 and the transition

5I6?5I8,

respectively (as shown in Fig. 3b). The excited transitionmechanism of Ho3+ ion can be described with the help ofthe energy level diagram as shown in Fig. 4 [14,15].

The fluorescence lifetime decay curves excited by 452 nmpumping at room temperature are shown in Fig. 5. Fig. 5ac are corresponding to the lifetimes of 550 nm, 754 nm, and1193 nm, respectively. By fitting the luminescence decaycurve 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 transitions5S2 +

5F4?5I8,

5S2 +5F4?

5I7,5I4?

5I8, and5I6?

5I8,

respectively. It can be seen that the fluorescence spectra

overlap with absorption one around 550 and 1193 nm fromFigs. 2 and 3. The overlaps exhibit that there are fluores-

cence trapping influencing the measured lifetimes. So the

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

Transition k (nm) AJJ0 (s1) bJJ0 sr (ms)

5I7?5I8 1948 147 1 6.8

5I6?5I7 2819 48 0.15 3.0

5I8 1193 280 0.85

5I5? 5I6 3894 23 0.09 3.95I7 1635 124 0.485I8 889 111 0.43

5I4?5I5 5053 12 0.09 7.7

5I6 2199 51 0.395I7 1235 57 0.445I8 753 11 0.08

5F5?5I4 4214 0 0 0.1

5I5 2298 86 0.015I6 1445 253 0.035I8 659 5837 0.74

5S2?5F5 3650 2 0.00 0.4

5I4 1956 70 0.035

I5 1410 52 0.025I6 1011 320 0.115I7 754 1054 0.375I8 550 1348 0.47

5F4?5F5 3305 89 0.01 0.1

5I4 1852 39 0.005I5 1355 333 0.035I6 1005 1128 0.105I7 754 1865 0.165I8 550 8328 0.71

5F3?5F4 4880 18 0.00 0.10

5S2 4602 1 0.005F5 1959 162 0.025I4 1377 311 0.03

5I5 1074 916 0.095I6 843 1041 0.105I7 651 5228 0.535I8 489 2269 0.23

500 550 600 650 700 750 800

0

20

40

60

80

100

120

140

5F

3

5I8

5S

2

,5F

4

5I75

I4

5I8

5F

5

5I8

5S

2,5F

4

5I8

Intensity(a.u.)

Wavelength (nm)

800 1000 1200 1400 1600

0

10

20

30

40

5I6

5I8

5S

2,5F

4

5I6

5F

2

5I5

Intensity

(a.u.

)

Wavelength (nm)

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

0

5

10

15

20

25 5G5,3G

55G

6,5F

13K

8,5F

25F3

5S

2,5F

4

5F5

5I4

5

I55I6

5I7

5I8

Energ

y(103c

m-1)

Fig. 4. Energy level diagram of Ho3+ ion and the luminescence process by452 nm radiation pumping.

1876 Z. Wang et al. / Optical Materials 30 (2008) 18731877


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measured fluorescence lifetimes of 550 and 1193 nm shouldbe longer than their actual values. But around 754 nm thereis no overlap between the fluorescence and absorption

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

4. Conclusion

The Ho3+:NaGd(MoO4)2 crystal with dimensions of

U12 42 mm2

was grown by CZ method and its polarizedabsorption spectrum at room temperature was investigated.Six typical absorption bands can be seen from the absorptionspectrum, among which the bands corresponding to the tran-sition 5I8?

5G6 +5F1 have the largest absorption cross-sec-

tions. Based on the JO theory and the absorption spectrum,the spontaneous transition probabilities, the fluorescentbranching ratios, and the radiative lifetimes were obtained.Polarized fluorescence spectra at room temperature in visibleand near-infrared regions were also investigated. The peakattributed to the 5S2 +

5F4?5I8 transition is the strongest

emission in six main emission peaks. By the single exponen-tial fitting, the fluorescence lifetimes were calculated to be

3.4 ls, 3.5 ls, and 46.4 ls, corresponding to transitions5S2 +

5F4?5I8,

5S2 +5F4?

5I7,5I4?

5I8, and5I6?

5I8,respectively. Due to the influence of fluorescence trapping,the measured fluorescence lifetimes of 550 and 1193 nmshould be longer than their actual values.

Acknowledgement

This work is supported by the Young Scientists Innova-tion Foundation of Fujian Province (2003J041 and2006F3139).

References

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[5] M.V. Mokhosoev, F.P. Alekseev, V.I. Lutsyk, Phase Diagram ofMolybdate and Tungstate Systems, Nauka, Siberian branch, Novo-sibirsk, 1978 (in Russian).

[6] A. Mendez-Blas, M. Rico, V. Volkov, C. Zaldo, C. Cascales, Phys.Rev. B 75 (2007) 174208.

[7] A. Mendez-Blas, M. Rico, V. Volkov, C. Cascales, C. Zaldo, C. Coya,A. Kling, L.C. Alves, J. Phys. Condens. Matter 16 (2004) 2139.

[8] V. Volkov, C. Cascales, A. Kling, C. Zaldo, Chem. Mater. 17 (2005)291.

[9] G.M. Kuzmicheva, D.A. Lis, K.A. Subbotin, V.B. Rybakov, E.V.Zharikov, J. Cryst. Growth 275 (2005) 1835.

[10] B.R. Judd, Phys. Rev. 127 (1962) 750.[11] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511.[12] W.T. Carnall, P.R. Fields, K. Rajnak, J. Chem. Phys. 49 (1968) 4424.[13] A. Florez, S.L. Oliveira, M. Florez, L.A. Gomez, L.A.O. Nunes, J.

Alloys Compd. 418 (2006) 238.[14] X.A. Lu, Z.Y. You, J.F. Li, Z.J. Zhu, G.H. Jia, B.C. Wu, C.Y. Tu, J.

Phys. D: Appl. Phys. 39 (2006) 3755.[15] M.J. Weber, B.H. Matsinger, J. Chem. Phys. 57 (1972) 562.[16] E. Rukmini, C.K. Jayasankar, Opt. Mater. 4 (1995) 529.

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

Time (s)

550 nm

Model: ExpDec1

Chi2 = 0.0002, R2 = 0.99642

I0

= 0 +0

A = 8.2771 +0.11024

= 3.43321 +0.01702

Intensity(a.u.)

Time (s)

Intensity(a.u.)

Time (s)

Intensity(a.u.)

754 nm

Model: ExpDec1

Chi2 = 0.0003, R2 = 0.99547

I0

= 0 +0

A = 7.9777 +0.12338

= 3.54711 +0.02098

100 120 140 160 180 2000

5

10

15

20

25

30

1193 nm

Model: ExpDec1

Chi2 = 0.0507, R2 = 0.99913

I0

= 0 +0

A = 301.99507 +07.48625

= 46.40689 +0.54767

8 10 12 14 16 18 20 22 24 26

8 10 12 14 16 18 20

Fig. 5. Fluorescence decay curves of Ho3+:NaGd(MoO4)2 crystal by452 nm pumping at room temperature: (a) for 550 nm; (b) for 754 nm; and(c) for 1193 nm.

Z. Wang et al. / Optical Materials 30 (2008) 18731877 1877

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Jude Offelt Theory