Journal of Crystal Growth 257 (2003) 272–275

Crystal growth and spectral properties of Yb3Al5O12

Xiaodong Xu, Zhiwei Zhao*, Jun Xu, Peizhen Deng

Shanghai Institute of Optics and Fine Mechanics, Crystal Center, Chinese Academy of Sciences, P.O. Box 800-211,

Shanghai 201800, People’s Republic of China

Received 11 May 2003; accepted 29 May 2003

Communicated by M. Schieber

Abstract

The stoichiometric laser crystal Yb3Al5O12 has been grown by the Czochralski method. The structure of the YbAG

crystal has been determined by X-ray diffraction analysis. The absorption and emission spectra of YbAG crystal at

room temperature have also been studied. The absorption, emission cross-section and fluorescence lifetime have been

estimated as 0.64� 10�20, 1.96� 10�20 cm2 and 270ms, respectively. The spectroscopic parameters of Yb:YAG and

YbAG crystal have been compared, and the results indicate that YbAG is a promising stoichiometric laser crystal.

r 2003 Elsevier B.V. All rights reserved.

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

Keywords: A1. Crystal structure; A1. Fluorescence lifetime; A2. Czochralski method; B1. Stoichiometric laser crystal; B1. YbAG

crystal

1. Introduction

Recent advances in high-performance InGaAslaser diode with a wavelength between 0.9 and1.1 mm have stimulated interest in diode-pumpedYb3+ lasers [1]. The Yb3+ ion has a low quantumdefect, leading to a low thermal load as a result ofthe energy mismatch between pump and laserwavelength. As a host material, Y3Al5O12 (YAG)possesses many qualities desirable for high-aver-age-power laser applications such as high thermalconductivity, excellent physical and chemical

properties [2]. Diode-pumped Yb:YAG lasers havereached a high level of development, with anoutput of 2.65 kW [3].The trivalent ytterbium ion’s simple [Xe]4f13

electronic structure allows for no excited stateabsorption, upconversion or low-concentrationquenching even at high doping concentrations ofYb3+ ions. As a result, the Yb3+-doped lasercrystals are favorable for compact and miniaturelaser design. Yb3+-doped YAG is one of themost promising laser-active materials in the nearfuture [4].YbAG and YAG are isostructural with only

about a 1.5% difference in unit-cell size and a200�C separation in their melting points, andthey have a cubic symmetry with space groupIa3d [5].

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*Corresponding author. Tel.: +8602169918482; fax:

+8602159928755.

E-mail addresses: xdxu79@sina.com (X. Xu),

zzw8006@sina.com (Z. Zhao).

0022-0248/03/$ - see front matter r 2003 Elsevier B.V. All rights reserved.

doi:10.1016/S0022-0248(03)01418-0

In this paper, we report Czochralski growth ofYbAG single crystal. The structure and spectralproperties of the crystal have also been measuredand compared to those of Yb3+:YAG with lowYb3+ concentration.

2. Crystal growth

YbAG crystal was grown by the RF-heatingCzochralski method with an iridium crucible(75mm in diameter). The start materials usedwere Yb2O3 (99.999%) and Al2O3 (99.999%),which were mixed in the stoichiometric ratio.The mixture was grinded, extruded to form pieceswith diameter close to the inner diameter of thecrucible at high pressure, then sintered in analuminum crucible at 1350�C for 24 h to obtainYbAG single-phase power. The XRD data showedthat it had nearly a single phase of YbAG. Thecharge was then loaded into the iridium cruciblesfor crystal growth. The pulling and rotation rateswere 1mm/h and 10–20 rpm under the nitrogen orargon atmosphere, respectively.The initial growth boundary in solid-melt was

convex towards the melt so that the dislocationsand impurity were reduced or eliminated. Afterthat, the growth boundary became flat. In order toprevent the crystal from cracking, the crystal wascooled to room temperature slowly after growth.Finally, a YbAG crystal with a size up to+3� 7 cm3 was obtained. The crystal boule wasblue and changed from blue to colorless after

annealing. Fig. 1 shows a sliced polished piece ofYbAG crystal after annealing.The YbAG crystal has a regular shape and its

structure has been determined by X-ray diffractionanalysis [6]. The compound has a cubic symmetry.The cell parameters are: a ¼ 1:193799 nm, b ¼ 90;V ¼ 1:70135 nm3 and the density of the crystalDC ¼ 6:621 g/cm3.

3. Spectral properties

3.1. Experiment results

Samples for spectroscopic measurements werecut out of boules and the surfaces perpendicular tothe /1 1 1S growth axis were polished. Thethickness of the samples was below 0.4mm. AJASCO V-570 UV/VIS/NIR spectrophotometerwas employed for acquisition of the absorptionspectra at room temperature. The absorptionspectra of Yb2+ and color center in YbAG crystalare shown in Fig. 2. The absorption of Yb3+ inYbAG crystal occurs only in the wavelength range900–1050 nm, and the spectrum is illustrated inFig. 3. There are four main absorption bands inYb:YAG, which center at 916, 938, 968, and1029 nm, respectively. The absorption coefficientof 938 nm is 90.8 cm�1. When the thickness of thesample is larger than 0.4 nm, total absorption willoccur around 938 nm and the results of measure-ment are incorrect. The absorption line-width(FWHM) of 938 and 968 nm is about 23 and

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Fig. 1. A sliced polished section of YbAG crystal after

annealing.

4

3

2

1

0300 400 500 600 700 800

unannealing YbAGannealing YbAG

Wavelength/nm

Abs

orpt

ion

coef

ficie

nt/c

m-1

Fig. 2. Absorption spectra of Yb2+ and color center in YbAG.

X. Xu et al. / Journal of Crystal Growth 257 (2003) 272–275 273

7 nm. A wide FWHM means that the laser crystalcan accommodate some thermal shift of the pumplight wavelength, and the output power of the laserremains stable. Therefore, the FWHM of absorp-tion band at pump wavelength is one of theimportant parameters for laser crystal.The fluorescence spectrum of YbAG crystal at

room temperature is shown in Fig. 4. It wasacquired by a TRIAX 550 spectrophotometer withInGaAs LD as the pump source (excited at 940nm).The decay time of 270ms was measured by acomputer-controlled transient digitizer. There aretwo strong emission peaks located at wavelengths1025 and 1036nm. The 35nm FWHM for thefluorescence band around 1036nm reveals that theYbAG crystal may be used as tunable laser medium.

3.2. Analysis

There are two absorption bands located atwavelengths 375 and 625 nm, respectively, inYb:YAG crystals grown by CZ method [4]. Thecrystal was grown at inert atmosphere, whichbrought a lot of oxygen vacancies and formed Re–F color center. Yb2+ that caused the absorptionband was attributed to Re–F color center. Theroom temperature color center spectra of YbAGare shown in Fig. 2. The absorption bands centerat 365 and 589 nm and move a little compared toYb:YAG crystal. The reason is possibly that Yb2+

exists in a different crystal field environment. Afterannealing, the absorption peaks vanish away.From Fig. 3 we can see that the YbAG crystal

has a strong and broad absorption band around938 nm, which is matched with the emissionwavelength of InGaAs laser diodes. The absorp-tion cross-section of Yb3+ is calculated accordingto the expression

sabs ¼ a=C; ð1Þ

where a is the absorption coefficient of Yb3+,C ¼ 1:413� 1022 ion/cm3 is the concentration ofYb3+ ions. The absorption cross-section at 938and 968 nm has been estimated: sabs(938 nm) at0.64� 10�20 cm2 and sabs(968 nm) at 0.35�10�20 cm2. The emission cross-section sem is givenby the following reciprocity equation [7]:

semðlÞ ¼ sabsðlÞZl

Zuexp½ðEZL � hc=lÞ=kT �; ð2Þ

where sabs is the absorption cross-section atwavelength l: Zl; Zu; k and EZL represent thepartition functions for lower and upper levels,Boltzmann’s constant, and zero-line energy that isdefined to be the energy separation between thelowest component of upper and lower states,respectively. The values of Zl; Zu; and EZL havebeen provided in Ref. [7]. The sem(1036 nm) is1.96� 10�20 cm2 as calculated from Eq. (2).We also measured the absorption and fluores-

cence spectra of 5.4 at% Yb:YAG crystal as shownin Fig. 5. The spectroscopic parameters ofYb:YAG and YbAG crystal are compared inTable 1. Except for the concentration quenchingeffect causing the fluorescence lifetime of YbAG to

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850 900 950 1000 1050 11000

20

40

60

80

100

Abs

orpt

ion

coef

ficie

nt (

cm-1

)

Wavelength/nm

Fig. 3. Absorption spectrum of YbAG crystal at room

temperature.

950 1000 1050 1100

Flu

ores

cenc

e in

tens

ity/a

.u.

Wavelength/nm

Fig. 4. Fluorescence spectrum of YbAG crystal at room

temperature.

X. Xu et al. / Journal of Crystal Growth 257 (2003) 272–275274

be shorter than that of Yb:YAG, other spectro-scopic parameters are similar for both crystals.It is well known that the threshold of a

continuous wave laser is inversely proportionalto the merit factor [8]:

M ¼ sabssemtfC: ð3Þ

A crystal with a large M factor may be a goodcandidate for solid-state lasers. The Yb3+ con-centration in Yb3+(5.4 at%):YAG crystal is7.51� 1020 ion/cm3. From the values of sabs; semand tf listed in Table 1, the merit factor can becalculated using Eq. (3), which is also listed inTable 1. TheM factor of YbAG is about four times

larger than that of Yb:YAG. Thus, the results ofthe spectroscopic parameters indicate that YbAG isa promising stoichiometric laser crystal.

4. Conclusion

The stoichiometric laser crystal Yb3Al5O12 hasbeen grown by the Czochralski method. Thestructure of the YbAG crystal has been deter-mined by X-ray diffraction analysis. The absorp-tion and emission spectra of YbAG crystal atroom temperature have also been studied andcompared to a lower doping Yb:YAG crystal. Theresults indicate that YbAG is a promising stoi-chiometric laser crystal.

Acknowledgements

The authors are grateful to Guosong Huang andShunguang Li of Shanghai Institute of Optics andFine Mechanics, Chinese Academy of Sciences formeasuring the emission spectra and fluorescencelifetime. This work is supported by the HighTechnology and Development Project of thePeople’s Republic of China (Grant No.2002AA311030).

References

[1] P. Lacovara, H.K. Choi, C.A. Wang, et al., Opt. Lett. 16

(1991) 1089.

[2] Hongwei Qiu, Peizhi Yang, Jun Dong, et al., Mater. Lett. 55

(2002) 1

[3] H. Bruesselbach, Presented at the Optical Society of

America 2001 Annual Meeting, Long Beach, CA, October

14–18, 2001.

[4] Peizhi Yang, Peizhen Deng, Jun Xu, et al., J. Crystal

Growth 216 (2000) 348.

[5] A.R. Reinberg, L.A. Riseberg, R.M. Brown, et al., Appl.

Phys. Lett. 19 (1971) 11.

[6] Xiaodong Xu, Zhiwei Zhao, Jun Xu, et al., J. Crystal

Growth 255 (2003) 338.

[7] L.D. Deloach, S.A. Payne, L.L. Chase, et al., IEEE

J. Quantum Electron 29 (1983) 1179.

[8] Yongyuan Xu, Xinghong Gong, Yujin Chen, et al.,

J. Crystal Growth 252 (2003) 24.

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Table 1

Spectroscopic parameters of Yb:YAG and YbAG crystal

Properties Yb3+(5.4 at%):YAG YbAG

Absorption peak (nm) 915, 941, 969, 1029 916, 938,

968, 1029

Absorption band-width

(nm)

20 23

Absorption cross-section

(10�20 cm2)

0.76 0.64

Fluorescence peak (nm) 969, 1030, 1048 1025, 1036

Emission band-width

(nm)

10 35

Emission cross-section

(10�20 cm2)

1.89 1.96

Fluorescence lifetime (ms) 1.15 0.27

M (10�22 cms) 1.241 4.786

AbsorptionEmission

Abs

orpt

ion

cros

s s

ectio

n/cm

2

1.0

0.8

0.6

0.4

0.2

0.0850 900 950 1000 1050 1100

Wavelength/nm

Flu

ores

cenc

e in

tens

ity/a

.u.

Fig. 5. Absorption and emission spectra of 5.4 at% Yb:YAG.

X. Xu et al. / Journal of Crystal Growth 257 (2003) 272–275 275