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Journal of Crystal Growth 310 (2008) 156–159

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Growth and spectral properties of Er:Gd2SiO5 crystal

Xiaodong Xu�, Guangjun Zhao, Feng Wu, Wenwei Xu, Yanhua Zong, Xiaodan Wang,Zhiwei Zhao, Guoqing Zhou, Jun Xu

Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, P.O. Box 800-211, Shanghai 201800, People’s Republic of China

Received 15 August 2007; accepted 27 September 2007

Communicated by K. Nakajima

Available online 5 October 2007

Abstract

Er3+-doped Gd2SiO5 (Er:GSO) single crystal with dimensions of + 35� 40mm3 has been grown by the Czochralski method. The

absorption and fluorescence spectra of the Er:GSO crystal were measured at room temperature. The spectral parameters were calculated

based on Judd–Ofelt theory, and the intensity parameters O2, O4 and O6 are obtained to be 6.168� 10�20, 1.878� 10�20, and

1.255� 10�20 cm2, respectively. The emission cross-section has been calculated by Fuechtbauer–Ladenbury formula.

r 2007 Elsevier B.V. All rights reserved.

PACS: 42.70.Hj; 81.10.Fq; 87.64.Ni

Keywords: A1. Doping; A2. Growth from melt; B1. Gadolinium compounds; B3. Solid state lasers

1. Introduction

Near infrared emission near 1.55 mm from Erbium-dopedmaterials has attracted much interest in recent years due toits wide applications in laser chemistry, nonlinear laserspectroscopy and other applications, such as laser mea-surement distance, laser radar and eye-safe lasers formedicine [1–3]. Er3+ ion has two absorption bands around800 and 970 nm, which coincide with wavelengths ofradiation from AlGaAs and InGaAs diode lasers. Up tonow, the spectroscopic properties of Er3+ in many crystalmaterials have been widely investigated [4–7].

Gadolinium oxyorthosilicate crystal Gd2SiO5 (GSO) is awell-known silicate that has been studied as a host forEu3+, Pr3+, Ce3+ and Yb3+ ions, with applications asphosphors, excellent scintillators and laser gain media, etc.[8–13]. As a host material, GSO possesses many qualitiessuch as low symmetry and large phonon energy [11–13].The spectroscopic characteristics of Er:GSO polycrystal-line were reported [14]. But to the best of our knowledge,

e front matter r 2007 Elsevier B.V. All rights reserved.

rysgro.2007.09.039

ing author. Tel.: +8621 69918485; fax: +86 21 69918607.

ess: xdxu79@yahoo.com.cn (X. Xu).

the crystal growth and spectral properties of Er:GSO singlecrystal have never been reported.In this paper, the growth of the Er3+-doped Gd2SiO5

single crystal by Czochralski method was reported and thespectroscopic properties of the crystal were studied.

2. Experiment

The Er:GSO single crystal was grown by Czochraskimethod in inductively heated iridium crucibles. Thestarting materials are Er2O3 (99.999%), Gd2O3 (99.999%)and SiO2 (99.995%) powder. The stoichiometric of rawmaterials according to the formula Er0.006Gd1.994SiO5 wereweighed. The mixture was grinded, extruded to form pieceswith diameter close to the inner diameter of the crucible athigh pressure, then sintered in an aluminum crucible at1350 1C for 10 h. The charge was then loaded into theIridium crucible for crystal growth. The GSO seed axis wasoriented at 60 1C relatively to the [0 1 0] axis and parallel tothe cleavage plane (1 0 0) to prevent the crystal fromcracking. The pulling rate was 1–3mm/h and the rotationrate of the seed was 15–30 rpm. High-purity nitrogen gaswas used as a protective atmosphere. In order to prevent

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400 600 800 1000 1200 1400 16000.0

0.5

1.0

1.5

2.0

2.5

3.0

Absorp

tion c

oeffic

ient (c

m-1

)

X. Xu et al. / Journal of Crystal Growth 310 (2008) 156–159 157

the crystal from cracking, the crystal was cooled to roomtemperature slowly after growth.

Samples for spectroscopic measurements were cut fromthe as-grown bulk crystal with two surfaces parallel to the(1 0 0) face and mechanically polished to spectral quality.The thickness of the sample was 3mm. The unpolarizedabsorption spectrum of Er:GSO was measured with aUV–vis–NIR spectrophotometer (Model V-570, JASCO).The luminescence spectra of the samples were recorded atroom temperature by a spectrofluorometer (Fluorolog-3,Jobin Yvon, Edision, USA) equipped with a HamamatsuR928 photomultiplier tube. A 940 nm continuous wavediode laser was used as the excitation source. The decaytime was measured by a computer controlled transientdigitizer. All measurements were performed at roomtemperature.

Wavelength (nm)

Fig. 2. Absorption spectrum of Er:GSO single crystal.

500 550 600 650 700

0

4000

8000

12000

16000

Flu

ore

scence inte

nsity (

a.u

.)

Wavelength (nm)

120000

160000

sity (

a.u

.)

3. Results and discussion

Fig. 1 shows the as-grown Er:GSO crystal with dimen-sions of + 35� 40mm3. The crystal boule was pink colorand free from cracks, inclusions and scattering centers.

The absorption spectrum of Er:GSO crystal from 330 to1700 nm at room temperature is presented in Fig. 2. Thesharp absorption bands of Er3+ ion centered at 365, 379,408, 451, 490, 520, 545, 656, 799, 972 and 1526 nm,corresponding to the absorptions from the ground state4I15/2 to the excited states 4G9/2,

4G11/2,2H9/2,

4F3/2–4F5/2,

4F7/2,2H11/2,

4S3/2,4F9/2,

4I9/2,4I11/2 and

4I13/2, respectively.The 4I15/2-

4I9/2 and4I15/2-

4I11/2 transitions, with absorp-tion bands at 799 and 972 nm, respectively, are importantbecause they are located in the emission bands of AlGaAsand InGaAs diode lasers. Therefore, the Er:GSO crystalcan be pumped by commercial high power laser diodes.There is self-absorption at the lasing wavelength of 1.53 mmin the crystals at room temperature and the absorptioncross-section at 1.53 mm is 1.11� 10�20 cm2.

Fig. 3 shows the emission spectrum of Er:GSO crystalat room temperature. In the range of 500–700 nm,two emission bands were observed, corresponding to

Fig. 1. The photograph of as-grown Er:GSO crystal boule.

1450 1500 1550 1600 16500

40000

80000

Flu

ore

scence inte

n

Wavelength (nm)

Fig. 3. Emission spectra of Er:GSO single crystal.

4S3/2-4II5/2 and

4F9/2-4II5/2 transition of Er3+ ions. The

emission band at wavelength range 1460–1620 nm corre-sponds to the 4I13/2-

4I15/2 transition of Er3+ ions, and thestrongest emission peak is located at 1527 nm.

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Table 1

The experimental and calculated oscillator strengths of Er:GSO crystal

Excited states

(ground state 4II5/2)l̄ (nm) f ed

exp (� 10�6) f edcal (� 10�6)

4GI1/2 381 19.86 20.764F7/2 491 1.90 2.522H11/2 523 10.20 11.754F9/2 655 2.21 2.814I9/2 795 0.211 0.4944I11/2 970 0.451 0.8044I13/2 1506 2.03�0.58ed ¼ 1.451.65

X. Xu et al. / Journal of Crystal Growth 310 (2008) 156–159158

The Judd–Ofelt theory [15,16] was applied to study thespectral properties of Er:GSO crystal in this work, Theexperimental line strength for electric dipole (ed) transi-tions Sed(exp) can be obtained from the absorption spectrumand the following formula:

SedðexpÞ ¼3hcð2J þ 1Þ

8p3e2l̄9n

ðn2 þ 2Þ2

ZsabsðlÞdl,

where sabs(l) is the absorption cross-section of the sampleat the wavelength of l, l̄ is the mean wavelength of theabsorption band, J is the total angular momentum of theground state, e is the electron charge magnitude, h is thePlanck constant, c is the speed of light in vacuum and n isthe refractive index.

The values of Sed(meas) calculated from the formulaabove can substitute to the following formula to determinethe oscillator strength parameter O2, O4 and O6 by least-square fitting of the following equation:

Sed ¼X

t¼2;4;6

OtjhðS;LÞJjjUðtÞjjðS0;L0ÞJ 0ij

2,

where /(S,L)J||U(t)||(S0,L0)J0S are the doubly reducedmatrix elements corresponding the transition from J to J0

manifold of the Ut unitary tensor operator of rank t, witht=2, 4, and 6. These matrix elements were taken fromthose determined by Carnall et al. [17] from the Er3+ ion inLaF3, since they are almost independent of environment[6]. The Judd–Ofelt intensity parameters O2, O4 and O6 forEr3+ in GSO crystal were 6.168� 10�20, 1.878� 10�20, and1.255� 10�20 cm2.

One the other hand, the line strength for magneticdipole (md) transitions between J and J0 manifolds isgiven by

Smd ¼1

4m2c2jhðS;LÞJjjLþ 2SjjðS0;L0ÞJ 0ij2,

where m is the electron mass, L+2S is the md operator.The md transitions are parity allowed between states of f11

configuration and subject to selection rules DS ¼ DL ¼ 0,DJ ¼ 0, 71 in the Russel–Saunders limit, the hostindependent matrix elements of L+2S was calculated byCarnall et al. [17].

The oscillator strength for the transitions from the initialJ state to the final J0 state, including both ed and mdcontributions, is given by

f ¼8p2mc

3hð2J þ 1Þl̄ðwedSed þ wmdSmdÞ,

where wed and wmd are the local field corrections for therefractivity of the medium, and are usually determined by

wed ¼ðn2 þ 2Þ2

9nand wmd ¼ n.

The experimental oscillator strength can be calculated by

f exp ¼mc2

pe2l̄2

ZsabsðlÞdl.

The experimental and the calculated oscillator strengthsare listed in Table 1. In the most case, the md (in the orderof 10�8) is unimportant and can be negligible comparedwith the ed (in the order of 10�6) contribution, excepted forEr3+ and Ho3+ ions. In Er3+-doped laser materials, asignificant contribution of the md component is involvedfor the 4I15/2-

4I13/2 transition. The md contribution to the4I15/2-

4I13/2 oscillator strength was 0.58� 10�6, which wassubtracted from the measured oscillator strength(2.03� 10�6) to obtain the purely ed oscillator strength(1.45� 10�6).The total spontaneous emission probabilities between

the excited state J and the terminating state J0 are given bythe expression:

AðJ; J 0Þ ¼ AedðJ; J0Þ þ AmdðJ; J

0Þ ¼64p4e2n2

3hl̄3ð2J þ 1Þ

ðwedSed þ wmdSmdÞ,

where Aed and Amd are the spontaneous emissionprobabilities of ed and md transitions, respectively. Thematrix elements were taken from those determined byWeber [18] from the Er3+ ion in LaF3. The A rate of anylevel is predominantly due to the forced ed component.However, the Amd emission probabilities for transition of4I13/2-

4I15/2,4I11/2-

4I13/2 and 4I9/2-4I11/2 have an

important md contribution [6].The radiative lifetime for excited state J can be

calculated as from the following formula:

trad ¼1P

J 0AðJ; J 0Þ

,

where the sum runs over all final states J0.The branching ratio can be determined by

bJJ 0 ¼AðJ; J 0ÞP

J 0AðJ; J 0Þ

.

The branching ratios, calculated ed and md transitionprobabilities, and radiative lifetimes for the main multipletsare listed in Table 2.

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Table 2

Branching ratios, calculated ed and md transition probabilities, and

radiative lifetimes of Er:GSO crystal

Transition b Aed (s�1) Amd (s�1) trad (ms)

4I13/2-4I15/2 1.000 177.2 61.7 4.19

4I11/2-4I15/2 0.839 244.2 3.44

4I11/2-4I13/2 0.161 29.1 17.7

4I9/2-4I15/2 0.748 223.9 3.34

4I9/2-4I13/2 0.237 70.9

4I9/2-4I11/2 0.015 1.2 3.4

4F9/2-4I15/2 0.912 2371.7 0.38

4F9/2-4I13/2 0.048 124.2

4F9/2-4I112 0.038 99.7

4F9/2-4I19/2 0.002 5.1

4S3/2-4I15/2 0.672 1802.3 0.37

4S3/2-4I13/2 0.273 731.4

4S3/2-4I11/2 0.021 56.4

4S3/2-4I9/2 0.035 92.6

X. Xu et al. / Journal of Crystal Growth 310 (2008) 156–159 159

The stimulated emission cross-section of 4I13/2-4I15/2

transition can be calculated by the Fuechtbauer–Laden-bury formula [19]:

semðlÞ ¼l5IðlÞ

8pn2ctR

IðlÞldl,

where I(l) is the emission spectral intensity at differentwavelength of Er3+ ions, t is the radiative lifetime of theupper laser level. The maximum value of emission cross-section is 1.52� 10�20 cm2 at 1528 nm.

4. Conclusion

Er3+-doped GSO single crystal was grown successfullyby the Czochralski method. The absorption spectrum andthe fluorescence spectra of the Er:GSO crystal wereinvestigated at room temperature. Using Judd–Ofelttheory, and the intensity parameters O2, O4 and O6 areobtained to be 6.168� 10�20, 1.878� 10�20, and

1.255� 10�20 cm2, respectively. The emission cross-sectionwas calculated by the Fuechtbauer–Ladenbury method andthe value is 1.52� 10�20 cm2 at 1528 nm.

Acknowledgments

This work is supported by the National Natural ScienceFoundation of China (No. 60608017), Shanghai MunicipalNatural Science Foundation (No. 06ZR14094), andChinese Academy and Sciences.

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