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Physica B 357 (2005) 365–369

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Upconversion luminescence in Yb3+-doped yttriumaluminum garnets

Xiaodong Xua,b, Zhiwei Zhaoa,�, Pingxin Songa,b, Benxue Jianga,b,Guoqing Zhoua, Jun Xua, Peizhen Denga, Gilbert Bourdetc, Jean Christophe

Chanteloupc, Ji-Ping Zouc, Annabelle Fulopc

aCrystal Centre, Chinese Academy of Sciences, Shanghai Institute of Optics and Fine Mechanics, P.O. Box 800-211, Shanghai 201800,

People’s Republic of ChinabGraduate School of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China

cLaboratoire pour l’Utilisation des Lasers Intenses, UMR7605, CNRS-CEA-Universite Paris VI, Ecole Polytechnique,

91128 Palaiseau Cedex, France

Received 12 November 2004; received in revised form 29 November 2004; accepted 29 November 2004

Abstract

In this paper, we present results on upconversion luminescence performed on Yb3+-doped yttrium aluminum garnets

under 940 nm excitation. The upconversion luminescence was ascribed to Yb3+ cooperative luminescence and the

presence of rare earth impurity ions. The cooperative luminescence spectra as a function of Yb concentration were

measured and the emission intensity variation with Yb concentration was discussed. Yb3+ energy migration quenched

the cooperative luminescence of Yb:YAG crystals with doping level over 15 at%.

r 2004 Elsevier B.V. All rights reserved.

PACS: 87.64.Ni; 42.70.Hj

Keywords: Cooperative luminescence; Energy transfer; Yb:YAG

e front matter r 2004 Elsevier B.V. All rights reserve

ysb.2004.11.088

ng author. Crystal Centre, Chinese Academy of

hai Institute of Optics and Fine Mechanics,

211, Shanghai 201800, People’s Republic of

21 69918482; fax:+86 21 69918607.

sses: xdxu79@yahoo.com.cn (X. Xu),

.ac.cn (Z. Zhao).

1. Introduction

Yttrium aluminum garnet is the most widelyused laser material due to its unusual combinationof favorable chemical, optical, thermal, andmechanical properties [1,2]. Yb:YAG is a goodcrystal used as high power diode-pumped lasergain media due to small quantum defect (8.6%)between the pump and the laser photons resulting

d.

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X. Xu et al. / Physica B 357 (2005) 365–369366

in low thermal loading (fractional heating of lessthan 11%), long radiative lifetime of the upperlaser level (1.3 ms), large absorption width (morethan 15 nm at 940 nm), broad emission width,no excited state absorption and upconversionloss [3–6].

Cooperative luminescence is a special type ofupconversion in which two interacting ions in theexcited state return to the ground state simulta-neously, emitting one photon of the sum of theenergies of the single ion transitions. It wasobserved by Nakazawa and Shionoya in 1970 forYb3+ in YbPO4 [7]. Malinowski et al., reportedblue cooperative emission centered at 484 nm inYb:YAG planar epitaxial waveguides and foundthat some of the visible emission peaks wereexactly at half wavelength of the IR peaks [8].

In this paper, we have measured the cooperativeluminescence spectra as a function of Yb concen-tration. The influence of Yb concentration on thecooperative emission is discussed.

850 900 950 1000 10500

20

40

60

80

100

Abs

orpt

ion

coef

ficie

nt (

cm-1

)

Wavelength/nm

5 at.% Yb 10 at.% Yb 15 at.% Yb 20 at.% Yb 25 at.% Yb 50 at.% Yb YbAG

Fig. 1. Absorption spectra of Yb:YAG with six Yb concentra-

tion and YbAG crystals at room temperature.

2. Experiments

Yb:YAG and YbAG crystals were grown by theCzochralski method [9–11]. The 99.999%-pureraw materials were appropriately predried andweighed according to a specific molar ratio. Afterthe compounds were ground and mixed, they werepressed into pieces and put into an aluminumcrucible. The pieces were heated to 1350 1C for24 h. The charge was then loaded into an iridiumcrucible for crystal growth. During the growth, thepulling rate was 1 mm/h, the rotation rate was10–20 rpm, and the growth atmosphere was nitro-gen. The initial growth boundary in solid melt wasconvex toward the melt, so dislocations andimpurities were reduced or eliminated from thecrystal. After that, the growth boundary becameflat. To prevent the crystal from cracking, wecooled it slowly to room temperature after growth.All crystals had a blue–green coloration, whichcould be removed by annealing of the samples inair at 1600 1C for 36 h.

Samples for spectroscopic measurements werecut out of boules and the surfaces perpendicular tothe /1 1 1S growth axis were polished. The

thickness of the samples was 0.5 mm. Roomtemperature absorption spectra were measuredwith a Jasco V-570 UV–Vis–NIR spectrophot-ometer. The fluorescence spectra were acquired bya Triax 550 spectrophotometer with an InGaAslaser diode as the pump source (excited at 940 nm).The pump power was 700 mW.

3. Results and discussion

Fig. 1 presents the absorption spectra ofYb:YAG and YbAG crystals, attributed to the2F7/2-

2F5/2 Yb3+ transition. The maximumabsorption in the 850–1100 nm wavelength rangeis at 941 nm for 5 at% Yb:YAG, the main bandposition moved to the shorter wavelength direc-tion with increasing Yb3+ concentration and theband is centered at 938 nm for YbAG. Withincreasing Yb3+ concentration, the maximumabsorption coefficient increases from 5.67 cm�1

for a 5 at% Yb3+ concentration to 90.8 cm�1 forYbAG. There is self-absorption at the lasingwavelength of 1.03 mm in the crystals at roomtemperature.

Fig. 2 shows the emission spectrum of 20 at%Yb:YAG crystal at room temperature. The spec-trum is dominated by an emission band at 1030 nmwith smaller peaks on the high- and low-wave-length side. The emission bands can be assigned to

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950 1000 1050 11000

3

6

9

Flu

ores

cenc

e In

tens

ity (

a.u.

)

Wavelength (nm)

20%

Fig. 2. Emission spectrum of 20 at% Yb:YAG at room

temperature.

500 550 600 650 7000.0

0.2

0.4

0.6

0.8

1.0

Em

issi

on In

tens

ity (

a.u.

)

Wavelength (nm)

20%

Fig. 3. Upconversion luminescence spectrum of 20 at%

Yb:YAG at room temperature.

X. Xu et al. / Physica B 357 (2005) 365–369 367

transitions between the 2F5/2 excited state anddifferent crystal components of the 2F7/2 groundstate for Yb3+ in YAG.

The visible upconversion fluorescence spec-trum of 20 at% Yb:YAG crystal at room tem-perature is shown in Fig. 3. The energy levelscheme of Yb3+ ion is simple with only the2F7/2 ground state and the 2F5/2 excited state,separated by some 10 000 cm�1, and no energylevels in the visible. The 4f electrons of Yb3+ have

a relatively strong interaction with their surround-ings and the interaction between the 4f13 con-figurations of neighboring Yb3+ is strong incomparision with other rare earth ions, thestrong interaction gives rise to a high cooperativeluminescence transition probability. The blue(485 nm) emission could result from the coopera-tive process corresponding to the simultaneousradiative relaxation of a pair of excited Yb3+ ionsaccompanied by the emission of a visible photon inthe following manner [7]: Yb* (2F5/2)+Yb*

(2F5/2)-2 Yb (2F7/2)+hn, the photon energyof the cooperative luminescence is nearly exactlytwice the energy of the normal (single-ion)luminescence.

The concentration of rare-earth ions impuritiestested by ICP-AES method in our Yb:YAGcrystals are less than 10 ppm, starting withmaterials with a purity of 99.999% as used in thisstudy. Because rare earth elements are indeedchemically related, it is difficult to separate themfrom each other. Thus, impurities are inevitable.Looking at the Dieke diagram, one can see thatmany resonant energy transfers are possiblebetween trivalent rare earth ions. In particular, inthe 10 000 cm–1 energy range matching with theexcited state of Yb3+ ions, resonant energytransfer is allowed with the 4I11/2 excited level ofEr3+ ions and non-resonant energy transfer is alsoknown with Tm3+ or Ho3+ ions. This process wasobserved as concentration quenching in manyYb3+-doped oxides [12–14].

It is believed that the emission bands centered at538 and 669 nm shown in Fig. 3 should be ascribedto the presence of the Tm3+, Er3+, and Ho3+, etc.impurities in Yb:YAG crystal. The luminescenceat 538 nm may be considered as the upconversionenergy transfer from the 2F7/2-

2F5/2 Yb3+

absorption transition to the Er3+ 4I11/2-4F7/2

absorption transition followed by the 4S3/2-4I15/2

emission transition [14–16] or to the Ho3+

5I6-5S2 absorption transition followed by the

5S2-5I8 emission transition [17–19]. The transi-

tion 5F5-5I8 of Ho3+ ions, 4F9/2-

4I15/2 of Er3+

ions or 1G4-3H4 of Tm ions [16,20] may be

responsible for the up-conversion luminescencecentered at around 669 nm in Fig. 2. Theupconversion luminescence of these ions is excited

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X. Xu et al. / Physica B 357 (2005) 365–369368

by infrared light through two- or three-step energytransfer from Yb3+ ions in general.

It should be pointed out that the upconversionluminescence in Yb:YAG crystal is detrimental tothe IR laser operation due to the loss of excitedenergy; however, the Yb:YAG crystal with thesmall thickness will still be a promising media inthe microchip laser configuration. On the otherhand, utilizing the above cooperative sensitizationmechanism, Yb:YAG crystal also can be served asthe excellent host for Ho3+, Tm3+ or Er3+ ionsco-doping for visible upconversion laser operation.

The cooperative luminescence of Yb:YAG andYbAG crystals is presented in Fig. 4. Theluminescence intensity increases with increasingYb3+ doping level up to 15 at%, When Yb3+

concentration is over 15 at%, the luminescenceintensity decreases. At low doping levels below15 at%, the enhancement of Yb3+ ion concentra-tion embedded in the crystal increases the prob-ability of the cooperative luminescence. However,the luminescence intensity drops dramatically forthe samples with doping level over 15 at%. Thisdecrease can be explained by the dominance ofquenching centers in the host materials. Yb3+

energy migration becomes more probable as theion–ion separation decreases with higher dopantlevels, and reduces the cooperative luminescenceintensity as the excitation is quenched uponreaching a defect site.

470 480 490 500 510-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Yb concentration (at.%)

Em

issi

on In

tens

ity (

a.u.

)

3

4

52

67

1

Em

issi

on In

tens

ity (

a.u.

)

Wavelength (nm)

1 5%2 10%3 15%4 20%5 25%6 50%7 100%

0 20 40 60 80 1000.0

0.4

0.8

1.2

Fig. 4. Cooperative luminescence spectra of Yb:YAG with six

Yb concentration and YbAG crystals at room temperature.

4. Conclusion

The upconversion luminescence of Yb3+-dopedyttrium aluminum garnets at room temperaturewas measured. The blue (485 nm) emission wascooperative luminescence and the emission bandscentered at 538 and 669 nm were ascribed to thepresence of the Tm3+, Er3+, and Ho3+, etc.impurities. It was observed experimentally thatthere is an optimum Yb concentration: in this casewhich is 15 at% of Yb. For higher concentrations,Yb3+ energy migration quenched the cooperativeluminescence.

Acknowledgements

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

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