Journal of Crystal Growth 257 (2003) 297–300

Comparison of Yb:YAG crystals grown by CZand TGT method

Xiaodong Xu, Zhiwei Zhao*, Guangjun Zhao, Ping Xin Song,Jun Xu, Peizhen Deng

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

Shanghai 201800, China

Received 16 May 2003; accepted 12 June 2003

Communicated by M. Schieber

Abstract

Five atomic percent Yb:YAG laser crystals have been grown by Czochralski (CZ) and temperature gradient

technique (TGT) method. The absorption spectra of as-grown and annealed Yb:YAG crystals have been investigated.

There is no Yb2+ and color center absorption in TGT-grown Yb:YAG crystals; the absorption peak of CZ-grown

Yb:YAG crystals after O2-annealing at 255 nm is attributed to Fe3+ ions. The longitudinal distribution of Yb3+ ions

has also been studied; TGT-grown Yb:YAG crystal has a larger distributing inhomogeneity of Yb3+ although it is

large sized.

r 2003 Elsevier B.V. All rights reserved.

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

Keywords: A1. Spectral properties; A2. Czochralski method; A2. Temperature gradient technique; B1. Yb:AG crystal

1. Introduction

Recent development of InGaAs laser diode hasstimulated interest in Yb3+-doped solid-statematerials to be used as gain for high-efficiency,high-power diode-pumped solid-state lasers [1].The Yb3+ ion has only two electronic states, the2F7/2 ground state and 2F5/2 excited state, whichare separated by approximately 10,000 cm�1. The

next higher level is in the 5d configuration, whichbegins near 100,000 cm�1. The crystal field splitsthe 4f manifolds by approximately 700 cm�1, withthe result that Yb3+ lasers are quasi-three-level atroom temperature [2].

Among the numerous Yb-doped crystals,Yb:YAG is the most interesting one as high-powerdiode-pumped lasers gain media due to smallquantum defect (8.6%) between the pump and thelaser photons resulting in low thermal loading(fractional heating of less than 11%), longradiative lifetime of the upper laser level (1.3ms),large absorption width (more than 15 nm at940 nm), broad emission width, no excited state

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

doi:10.1016/S0022-0248(03)01455-6

absorption and upconversion loss [3]. In addition,as a host material, Y3Al5O12 (YAG) possessesmany qualities desirable for high-average-powerlaser applications such as high thermal conductiv-ity, excellent physical and chemical properties [4].

In this paper we report comparison of Yb:YAGcrystals grown by CZ and TGT method in spectralproperties and the longitudinal distribution ofYb3+ ions.

2. Experimental procedure

2.1. Crystal growth

The starting raw materials used were of highpurity (5N). The /1 1 1S-oriented Yb:YAGboules were grown by the Czochralski (CZ)method [3] and temperature gradient technique(TGT) method [5]. The CZ-grown Yb:YAG had ablue-green coloration, which did not appear in theTGT-grown Yb:YAG. The coloration can beremoved by annealing the samples in oxygen at1400�C for 36 h.

2.2. Sample preparation

Samples for spectroscopic measurements werecut out of the boules, and surfaces perpendicularto the /1 1 1S growth axis were polished. AJASCO V-570 UV/VIS/NIR spectrophotometerwas employed for acquisition of the absorptionspectra. The sample that was cut from the crystalsadjacent the seed crystal position and grinded topowder in an agate mortar was analyzed withinductively coupled plasma atomic emission spec-trometer (ICP-AES) for Yb3+ concentration. Thecrystal structures were measured by JUINERH .AJJ Camera. The data were collected andanalyzed by computer.

3. Results and discussion

3.1. Spectral performance

A particular color center was formed duringCZ-grown of Yb:YAG [3,6]. Its absorption spectra

are shown in Fig. 1. The crystal was grown at inertatmosphere, which brought a lot of oxygenvacancies and formed R-–F color center. Yb2+

that caused the absorption band was attributed toRe-F color center. Yb2+ and Re-F color center inYb:YAG are detrimental to the intrinsic spectro-scopic performances of Yb:YAG. They degradedthe Yb3+ intrinsic absorption at 900–1050 nm andthe emission intensity at 1028–1060 nm andshortened the fluorescence lifetime of Yb3+ inYAG host. The color center’s two absorptionbands are located at wavelengths of 368 and623 nm, respectively. Each absorption peak in-creases in intensity by annealing the sample inhydrogen at 1400�C for 12 h, and the main bandpositions move from 368 to 381 nm and form 623to 642 nm.

The absorption spectra of TGT-grown Yb:YAGcrystals at the wavelength range 300–800 nm arealso shown in Fig. 1. Although grown at inertatmosphere by TGT technology, and even anneal-ing the sample at hydrogen atmosphere, Yb:YAGcrystals do not exhibit the same absorption bands.The results indicate that there is no Yb2+ andcolor center absorption in TGT-grown Yb:YAGcrystal. It is of great use for laser performance ofYb:YAG.

Compared with CZ method, during the TGT-growth process, the temperature and the thermalfield are very stable and convection that disturbsthe solid–liquid interface does not appear [7].Yb3+ ions can easily substitute the position of

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300 400 500 6000.0

0.5

1.0

1.5

2.0

2.5

Wavelength/nm

Abs

orpt

ion

coef

ficie

nt/c

m-1

700 800

as-grown CZ H2-annealing CZ as-grown TGT H2-annealing TGT

Fig. 1. Absorption spectra of color center in Yb:YAG.

X. Xu et al. / Journal of Crystal Growth 257 (2003) 297–300298

Y3+ sites. From the results of cell parametersshown in Table 1, we can see that the effect ofYb3+ on the crystal lattice deformation in TGT-grown Yb:YAG crystals is smaller than that inCZ-grown Yb:YAG crystals. The cell structure ofTGT-grown Yb:YAG crystals is more steady.

The samples of TGT-grown Yb:YAG crystalswere cut out of the middle part of the boules,which did not touch the atmosphere of theenvironment during the crystal growth, but inthe CZ-growth process, the crystals were exposedto the inert atmosphere. Moreover, in the CZ-growth, zirconia, which is used as attemperatoraround the iridium crucible, can easily enter intothe melt from the top of the crucible. When Zr4+

was introduced into the Yb:YAG crystals, it couldbe a charge compensation of Yb2+, which madethe Yb3+ to be reduced to Yb2+ more easily. Thecontent of Zr ions was measured by ICP-AES andlisted in Table 2. We can consider that Zr ions inCZ-grown Yb:YAG crystals possibly act as acharge compensation of Yb2+.

The UV absorption spectra after annealing inoxygen are shown in Fig. 2. There is a weak peakaround 255 nm in CZ-grown Yb:YAG crystals.Chen et al. [8] used optical absorption and electronparamagnetic resonance (EPR) technique to studyiron impurities in YAG crystals and consideredthat the absorption band at 255 nm was attributedto an Fe3+ charge-transfer band, which was madeup of contributions from substitutional Fe3+ ionsin octahedral and tetrahedral sites. Jiang et al. [9]considered that the weak absorption peak at255 nm in YAG crystals grown by CZ is attributedto Fe3+ ions. Table 2 also shows the content of Feions in the crystals; the Fe ions in the CZ-growthcrystals are more than that in TGT-grown crystals.We conclude that the weak peak around 255 nm inCZ-grown Yb:YAG crystals is attributed to Fe3+

ions. In the CZ-growth, we used iron apparatus to

scrap the residual raw materials in the crucibleafter growth, so Fe ions might have enriched thesurface of crucible. They possibly entered into thecrystals along with the crucible during the nextgrowth process. But in the TGT-growth, almost allraw materials had been crystallized. In addition,Fe in zirconia attemperator and raw materials maybe a contributor to such a large Fe contamination.The pump transition wavelength of Yb:YAG is941 or 968 nm, so the Yb3+ ions have littleinfection on the laser performance.

3.2. Distributing inhomogeneity of Yb3+

TGT is considered as a method for the growthof large and high-quality crystals. We have grown

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Table 1

Cell parameters of Yb:YAG crystals

Growth method Yb3+concentration (at%) a (nm) b V (nm3)

CZ 5.4 1.20042470.000063 90.0 1.72983

TGT 5.4 1.20070470.000052 90.0 1.73104

250 3500

20

40

60

80

Abs

orpt

ion

Coe

ffic

ient

/cm

-1

Wavelength/nm

140

120

100

200 300 400

O2-annealing CZ

O2-annealing TGT

Fig. 2. UV absorption spectra of Yb:YAG crystals.

Table 2

Impurity ions in Yb:YAG crystals

Yb:YAG Fe (wt%) Zr (wt%)

CZ 0.11 0.035

TGT 0.010 o0.001

X. Xu et al. / Journal of Crystal Growth 257 (2003) 297–300 299

a 3 inches crystal without cores and scatteringgrain using TGT. We obtained the distributioncoefficient of Yb3+ in nominal 5 at% Yb:YAGgrown by CZ [10] and TGT [5]. The distributioncoefficient of Yb3+ in Yb:YAG crystals grown byCZ (1.09) is approximately equal to that inYb:YAG crystals grown by TGT (1.08).

The general distribution equation of Yb3+ inYb:YAG boules is

Cs ¼ C0kmð1� gÞkm�1; ð1Þ

where g is the fraction of the melt crystallized, km

is the distribution coefficient of Yb in Yb:YAGcrystals, and C0 is the initial concentration of theadmixture. We calculate the Yb concentration Cs

with a given g:The distributing inhomogeneity ofYb3+ in the crystals is defined as the following:

S ¼ ðCb � Cf Þ=L; ð2Þ

where Cb and Cf are the Yb3+ concentration atthe beginning and finishing position of the isodia-metric part, and L is the length of isodiametricpart of the crystal (mm). The Yb3+ concentrationsin the crystals calculated from Eq. (1) are listed inTable 3. The distributing inhomogeneity of Yb3+

calculated from Eq. (2) is also listed in Table 3.Although TGT-grown Yb:YAG crystal is largesized, its distributing inhomogeneity of Yb3+ is 17times more than that of CZ-grown Yb:YAGcrystals. The discrepancy of concentration distri-bution is a main factor that restricts the laserperformance of Yb:YAG lasers.

4. Conclusion

The absorption spectra of as-grown and an-nealed Yb:YAG grown by Czochralski (CZ) and

temperature gradient technique (TGT) methodhave been investigated. There is no Yb2+ andcolor center absorption in TGT-grown Yb:YAGcrystals and it is of great use for laser performanceof Yb:YAG. The absorption peak of CZ-grownYb:YAG crystals after O2-annealing at 255 nm isattributed to Fe3+ ions. The longitudinal distribu-tion of Yb3+ ions has also been studied, TGT-grown Yb:YAG crystal has a larger distributinginhomogeneity of Yb3+ although it is large sized.

Acknowledgements

This work is supported by the High Technologyand Development Project of the People,s Republicof China (Grant No. 2002AA311030).

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Table 3

Homogeneity of Yb:YAG crystals

Yb:YAG L (mm) Diameter (mm) Yb concentration (at%) S (� 10�5 at%/mm)

Cb Cf

CZ 150 30 5.38 5.11 1.8

TGT 45 76 5.26 3.87 30.9

X. Xu et al. / Journal of Crystal Growth 257 (2003) 297–300300