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Optical Materials 29 (2007) 1553–1556

Growth and optical properties of Yb3+-doped Ba3Y2(BO3)4 crystal

Ping Ma, Zhoubin Lin, Guofu Wang *

Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China

Received 2 August 2005; accepted 14 July 2006Available online 25 September 2006

Abstract

Yb3+:Ba3Y2(BO3)4 crystals with dimension of B20 · 30 mm3 and few cracks have been grown by Czochralski method.Yb3+:Ba3Y2(BO3)4 crystal congruently melts at 1222.7 �C. The strongest absorption band of Yb3+:Ba3Y2(BO3)4 is at 977 nm with anabsorption coefficient of 5.8 cm�1 and FWHM of 8 nm. The absorption cross-section of this peak is calculated to be1.23 · 10�20 cm2. The dominant feature of fluorescence spectrum is a broadband emission, extending from 925 to 1100 nm. The emissioncross-section is 8.05 · 10�20 cm2. The radiative lifetime and fluorescence lifetime are 0.587 ms and 0.937 ms, respectively.� 2006 Elsevier B.V. All rights reserved.

PACS: 42.70.Hj; 78.20.�e

Keywords: A1. Optical microscopy; A2. Czochralski method; A2. Single crystal growth; B1. Borates; B2. Solid state materials

1. Introduction

It well known that, the solid-state lasers emitting at 1 lmhave various applications in a wide range of fields, suchas military, industry, medical treatment and scientificresearches. Recently, with the development of InGaAsdiode lasers operating at 0.9–1.1 lm as the pumpingsources for Yb3+ laser medium, much attention has beenpaid on solid-state laser of Yb3+ ions operating at 1 ls. Tri-valent ytterbium Yb3+ has advantages over Nd3+ ion.Firstly, it has only two manifolds i.e. 2F7/2 ground stateand 2F5/2 excited state, which prevents laser losses owingexcited-state absorption, up-conversion and concentrationquenching. Secondly, there are 3 or 4 times longer emissionlifetime than that of Nd3+ ion in similar hosts. Thirdly, theYb3+ ion exhibits a strong and broad absorption at960 nm, which is suitable for InGaAs diode laser pumping.The borate with the formula M3Re2(BO3)4 (Ca, Sr, Ba;Re = Y, Gd, La), a new borate family [1–3], have beeninvestigated as potential laser host materials [4,5].

0925-3467/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.optmat.2006.07.017

* Corresponding author. Tel./fax: +86 591 83714636.E-mail address: wgf@ms.fjirsm.ac.cn (G. Wang).

Ba3Y2(BO3)4 belongs to orthorhombic system with spacegroup Pnma having the cell parameters a = 7.680 A,b = 16.5152 A, c = 9.009 A, Z = 4 [6]. This paper reportsthe crystal growth and spectroscopic properties of Yb3+-doped Ba3Y2(BO3)4 crystal.

2. Crystal growth

The result of DSC analysis showed that Yb3+:Ba3-Y2(BO3)4 crystal melted congruently at 1222.7 �C, asshown in Fig. 1, which was measured using a NETSZCH449C Simultaneous Thermal Analyzer. Since Yb3+:Ba3Y2(BO3)4 crystal melts congruently, Yb3+:Ba3Y2(BO3)4

crystal can be grown by Czochralski method. High puritychemicals of BaCO3, Y2O3, H3BO3 and Yb2O3 were usedas raw materials, and weighted according to the formulaBa3Yb0.10-Y1.90(BO3)4. A 3at.% excess amount of H3BO3

was also added to compensate for the loss caused by evap-oration of H3BO3 during reactions. Yb3+:Ba3Y2(BO3)4 wassynthesized by the means of solid-state reaction accordingto the formula Ba3Yb0.10Y1.90(BO3)4. After grinding andextruding to form pieces, the samples were placed in the

400 600 800 1000 1200-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.2

0.0

1222.7

Hea

t Flo

w (m

vv/m

g)

Temperature(ºC)

-0.4

Fig. 1. The curve of DSC of Yb3+:Ba3Y2(BO3)4 crystal.

850 900 950 1000 1050 11000

1

2

3

4

5

6

Abs

orpt

ion

Coe

ffici

ent (

cm-1

)

Wavelength (nm)

Fig. 3. Absorption spectrum of Yb3+:Ba3Y2(BO3)4 crystal at roomtemperature.

50000

1554 P. Ma et al. / Optical Materials 29 (2007) 1553–1556

Pt crucible and held at 1000 �C for 12 h, repeating onceagain to assure adequate solid-state reaction.

Yb3+:Ba3Y2(BO3)4 crystal was grown in a 25 kHz fre-quency furnace heating in an iridium crucible. The crystalgrowth system consists of a medium-frequency heater, acontrol unit of rotation and lifting, a control unit of thetemperature and the growing furnace. The synthesizedraw materials were melted in B60 · 40 mm2 iridium cruci-ble. Yb3+:Ba3Y2(BO3)4 crystal was grown at a pulling rateof 1 mm/h and a rotating rate of 20–30 rpm in N2 atmo-sphere. Since the grown Yb3+:Ba3Y2(BO3)4 crystal tendedto crack in the annealing process, the slow cooling rate of30 �C/h was used to the annealing process. Cracking-freeYb3+:Ba3Y2(BO3)4 crystal with dimension B20 · 30 mm3

was obtained, as shown in Fig. 2.The concentration of Yb3+ in Yb3+:Ba3Y2(BO3)4 crystal

was measured to be 6.7 at.% ± 0.002 on average by elec-tron microprobe analysis, from the samples that were takenfrom the top, middle and bottom part of the crystal. Thesegregation coefficient of Yb3+ ion in this crystal is thenestimated to be 0.67 ± 0.01. Thus, the actual Yb3+ concen-tration in the Yb3+:Ba3Y2(BO3)4 crystal was calculated tobe 4.69 · 1020 cm�3.

Fig. 2. Yb3+:Ba3Y2(BO3)4 crystal.

3. Optical properties

A sample with dimension 7 · 5 · 0.8 mm3 was cut fromthe as-grown crystal and applied to spectral measurement.The absorption spectrum, as shown in Fig. 3, was mea-sured using Perkin–Elmer UV–VIS–NIR (Lamdba-35)spectrometer. The broad absorption is due to 4F7/2!4F5/2 transition of Yb3+ ion. Three main absorption peaksare centered at 926 nm, 960 nm and 977 nm, correspondingto Stark levels of 4F5/2. The strongest absorption band at977 nm has an absorption coefficient of 5.8 cm�1 andFWHM of 8 nm. Then the absorption cross-section at977 nm was calculated to be 1.23 · 10�20 cm2.

The fluorescence spectrum, as shown in Fig. 4, wasrecorded using Edinburgh Analytical Instruments Fluores-cence Spectroscopy FLS920 with InGaAs diode laser as alight source at room temperature. The dominant featureof fluorescence spectrum is a broadband extending from925 to 1100 nm. The fluorescence lifetime at 1020 nm wave-length was determined to be 0.937 ms, as shown in Fig. 5.

900 950 1000 1050 1100 1150 1200

0

10000

20000

30000

40000

Inte

nsity

(A.U

.)

Wavelength(nm)

Fig. 4. Emission spectrum of Yb3+:Ba3Y2(BO3)4 crystal at roomtemperature.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.55000

5500

6000

6500

7000

Fluo

resc

ence

Inte

nsity

(a.u

.)

Time (ms)

Fig. 5. The lifetime decay curve of Yb3+:Ba3Y2(BO3)4 crystal at roomtemperature.

Table 1Spectroscopic parameters of Yb3+:Ba3Y2(BO3)4 crystal and other crystals

Crystals Yb:YAB[8]

Yb:YCOB[9]

Yb:CYB[10]

YB:Ba3Y2(BO3)4

(this work)

Ion concentration(at.%)

20 20 10 6.7

kp (nm) 975 976 976 977dabs (·10�20 cm2) 3.4 0.94 1.62 1.23kext (nm) 1040 1030 1025 1020dem (·10�20 cm2) 8 5.5 3.5 8.05sem (ms) 0.68 2.65 1.8 0.937srad (ms) 0.6 2.91 0.7 0.587bmin (%) 4.3 5.8 11 4.3Isat (kW/cm2) 8.8 8.2 3.6 17.2Imin (kW/cm2) 0.38 0.48 0.4 0.74

P. Ma et al. / Optical Materials 29 (2007) 1553–1556 1555

By the reciprocity method (RM), the emission cross-sec-tion dem can be obtained using the following formula [7]:

demðkÞ ¼ dabsðkÞZ l

Zu

exp½ðEl � hc=kÞ=kT � ð1Þ

where dabs(k) is the absorption cross-section at the ex-tracted wavelength k; h is Planck constant; k is Boltz-mann’s constant; T is the absolute temperature; Zl andZu are partition functions of the lower and upper levels,the lowest excited energy El is 10,235 cm�1. The partitionfunction Zk can be calculated using the following formula:

Zk ¼X

k

dk expð�Ek=kT Þ ð2Þ

Then the partition functions Zl and Zu were calculated tobe 1.46 and 1.48, respectively, based on energy-level sepa-ration values.

The fundamental relationship between spontaneous andstimulated emission rate can be expressed by Fuchtbanuer–Ladenburg (F–L) formula [7]:

dem ¼k5

8pcn2

1

srad

IðkÞRkIðkÞdk

ð3Þ

where c is the speed of light; k is the extracted wavelength;n is the refractive index; srad is the radiative lifetime. Thus,using this method, the radiative lifetime can be obtainedusing this method.

The minimum pump intensity Imin represents the mini-mum pumping intensity to invert population in a quasi-three-level-system. It is expressed by following formula [7]:

Imin ¼ bminI sat ð4Þwhere bmin is defined as the minimum inversion fraction ofYb3+ ions that must be excited to balance the ions exactlywith the ground-state absorption at the extraction wave-length. bmin can be obtained from following formula [7]:

bmin ¼dabsðkextÞ

demðkextÞ þ dabsðkextÞð5Þ

Isat is the pump saturation intensity, which can be obtainedfrom the following formula[7]:

I sat ¼ hc=ðkpdabssemÞ ð6Þ

Thus, all spectroscopic parameters of Yb3+:Ba3Y2(BO3)4

crystal were obtained and listed in Table 1.

4. Results and discussion

Yb3+:Ba3Y2(BO3)4 crystal with dimension up toB20 · 30 mm3 and few cracks has been grown by Czo-chralski method. Yb3+:Ba3Y2(BO3)4 crystal melts congru-ently at 1222.7 �C. Yb3+:Ba3Y2(BO3)4 crystal has thestrongest absorption band at 977 nm with the absorptioncoefficient of 5.8 cm�1 and FWHM of 8 nm. The absorp-tion cross-section at 977 nm is 1.23 · 10�20 cm2. The dom-inant feature of fluorescence spectrum is a broadbandemission, extending from 925 to 1100 nm. In comparisonwith the other materials (see Table 1), Yb3+:Ba3Y2(BO3)4

crystal has large emission cross-section. The radiative life-time and fluorescence lifetime are 0.587 ms and 0.937 ms,respectively. The fluorescence lifetime is longer than radia-tive lifetime. This is probably due to that in Yb3+-dopedmaterials there exists a large spectral overlap between thefluorescence and ground-state absorption, when the spon-taneously emitted photons from the metastable levels aretrapped by reabsorption of the ions in the ground state.These nascent excited-state ions then relax by spontane-ously emitting more photons. Then the photons are reab-sorbed again, and the entire process is repeated. As aresult, the fluorescence lifetime is longer than radiativelifetime.

Acknowledgements

This project was supported by the National NaturalScience Foundation of China (No. 60378031), and KeyProject of Science and Technology of Fujian Province(2001F004), respectively.

1556 P. Ma et al. / Optical Materials 29 (2007) 1553–1556

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