Doped Sc2o3 Crystals Grown by the Micro-Pulling-Down Method

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Optical Materials xxx (2006) xxx–xxx www.elsevier.com/locate/optmat

3+ Growth and spectroscopic properties of Yb -doped Sc2O3 crystals grown by the micro-pulling-down method

Rayko Simura a,*, Anis Jouini a, Mun Ji-Hun a, Alain Brenier b, Akira Yoshikawa a, Georges Boulon b, Tsuguo Fukuda a

a Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1 Aoba-ku, Sendai 980-8577, Japan b Physical Chemistry of Luminescent Materials, Claude Bernard/Lyon 1 University, UMR 5620 CNRS, Villeurbanne 69622, France

Abstract

3+ Room temperature absorption and fluorescent emissions were investigated for Yb -doped Sc2O3, which were grown by the micro- 3+ 2 3+ pulling-down (l-PD) method. The decay time for infrared Yb ( F5/2) fluorescence was 561 ls in the case of 0.5% Yb -doped Sc2O3.It is observed that this value increases slightly when the Yb3+ concentration becomes 10 times higher (593 ls for 5%). We understand that it is due to self-trapping effect. Ó 2006 Elsevier B.V. All rights reserved.

PACS: 81.10; 32.70.C; 42.55.X

3+ Keywords: Rare earth sesquioxides; Yb :Sc2O3; Luminescence; Life time

1. Introduction Czochralski technique (not well established for sesquioxides growth [1]), the Heat Exchanger technique, the floating Among host laser crystals, some of the rare earth sesqui- zone method and the laser heated pedestal growth. How- oxides (Sc2O3,Y2O3, and Lu2O3) with high melting tem- ever, these methods are limited by the size and the low opti- perature (2430 °C) are known for their superior thermo- cal quality of the obtained single crystals. In order to grow mechanical properties [1], such as higher heat conductivity large single crystal with high quality, crystal growth tech- (16.5 W/mK, 13.6 W/mK, 12.5 W/mK, respectively) than nique by pulling directly from melt with seed and crucible YAG (11 W/mK) [2,3], optical properties such as low pho- should be developed for this high melting temperature non energy and wide transparency from the visible to infra- material. Our group have already show the growth result red region [1]. Those sesquioxides can be easily doped with of Y2O3 using the l-PD method [5]. The method was all rare earth ions, furthermore, some of the transition applied for Sc2O3 in this paper. It was reported that the metal ions can also be accepted. This unique characteristic melting temperature of rare earth sesquioxides are makes it possible to create a large number of lasing wave- 2430 °C, 2430 °C, 2450 °C, for Y2O3,Sc2O3 and lengths [1]. Due to these advantages, several studies of Lu2O3, respectively [1]. However, during our experiment, emission and lasing experiments have been done with rare the melting temperature of Sc2O3 and Lu2O3 seems much earth sesquioxide ceramics and single crystals. Single crys- higher than Y2O3, which makes its growth very difficult. tals were grown by Verneuil method, the flux method, the Efforts have been made to overcome this problem and the results will be reported here. For laser application, Sc2O3 is one of the promising 3+ * Corresponding author. Tel.: +81 22 217 5167; fax: +81 22 217 5102. materials for the Yb -doped laser because of its high oscil- 3+ E-mail address: [email protected] (R. Simura). lator output yield [4]. As a candidate for Nd substitution,

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Yb3+ has many advantages: (1) simple energy diagram with equipped with a 1-lm-blazed grating (reciprocal disper- 2 2 ground state F7/2 and excited state F5/2, which means no sion: 2.4 nm/mm). The detection of the luminescence signal excitation state absorption and no up-conversion; (2) was carried out using a Peltier effect cooled R1767 Ham- higher concentration doping is possible because of weak amatsu photomultiplier tube, which has no response for concentration quenching; (3) infrared pumping based on wavelengths longer than 1150 nm. The resulting signal InGaAs laser diode is possible. Using this small pumping was then processed using a SR 250 gated integrator and device is the advantage for making the laser equipment boxcar averager from Stanford Research Systems provid- simple and small. In this paper, successful crystal growth ing an integrated signal to a DAC card coupled to a com- 3+ result and optical properties of Yb :Sc2O3 will be given. puter for the fluorescence spectra measurements. The fluorescence life times were recorded using a Lecroy 9410 2. Experimental digital oscilloscope coupled to the same computer.

2.1. Micro-pulling-down method 3. Results and discussion

Sc2O3 single crystal was prepared with the micro-pull- 3.1. Crystal growth ing-down method [5]. For the high melting temperature 3+ rare earth sesquioxides, zirconia ceramics were used for Obtained undoped, 0.5%, 5% Yb -doped Sc2O3 single insulators instead of alumina ceramics and Rhenium crystals are 4.3 mm in diameter and 10 mm in length with instead of Iridium for crucible, with gas flow of Ar and transparent pale yellow, red, and brownish color, whose 3+ 3% H2 to suppress Rhenium oxidization [1,5]. Starting colors do not depend on Yb concentration. Crystals materials were prepared from the stoichiometric mixture became colorless after annealing with 1400 °C 24 h. Nei- of 5 N pure Sc2O3 and Yb2O3 powders. They were thor- ther visible inclusions nor cracks were observed for all oughly mixed and put into the crucible, and which were transparent crystals. heated up to the melting temperature (2430 °C). W–Re wire was used for seeding during initial crystal growth. 3.2. X-ray characterization Further crystal growth attempts were carried out using Sc2O3 crystal seed obtained in the initial experiments. Powder X-ray diffraction analysis shows that undoped Growth rate was 0.02–0.1 mm/min. and Yb3+-doped scandia crystals were single phase. Scan- dia has cubic crystal system with the space group of Ia3. 2.2. Chemical and structural analysis The calculated lattice parameter increases with Yb3+ concentration from 9.85 to 9.9 A˚ for 0–5% Yb3+. Phase characterization and lattice parameter calculation were carried out from the powder X-ray diffraction data 3.3. Chemical characterization obtained from RIGAKU RINT2000. The diffraction data was collected in the 2h range from 10° to 80°, which was Line profiles of Yb3+ concentration along the growth performed in air and under room temperature (RT). The axis of the doped crystals were obtained by the electron X-ray source was Cu Ka (40 kV, 40 mA). The chemical probe micro-analysis (EPMA). Yb3+ concentration is compositions of grown crystals were determined using almost constant, except for the beginning of crystallization. EPMA JEOL JXA-8600L. In the beginning part, the Yb3+ concentration is lower than the unity and increases immediately, and then it keeps con- 2.3. Optical analysis stant value along growth axis. Optical properties are exam- ined in the region of constant concentration. Optical absorption spectra were recorded at RT with Lambda 900 UV–VIS–NIR Perkin–Elmer dual beam spec- 3.4. Optical characterization trometer. Fluorescence spectra under selective pulsed laser 2 3+ 12 excitation in the F5/2 multiplets of the Yb (4f ) ground Crystals were sliced into disks, with 4 mm in diameter state configuration were recorded using an optical para- and thickness of about 2 mm, were well polished and pre- metric oscillator pumped by a Nd:YAG laser. The energy pared for the optical measurements. 3+ of the beam impinging on the sample was always main- The infrared absorption spectra of Yb -doped Sc2O3 tained around 1 mJ/pulse throughout the measurements. show broad bands around 890 nm, 940 nm and 976 nm cor- 2 2 The excitation laser beam was focused onto the samples responding to F7/2 ! F5/2 transitions (Fig. 1). The last under glancing incidence in all cases, in order to prevent one corresponds to the 0-phonon (transition between the 2 the predominant part of the reflected laser light to be col- lowest multiplet components of the F5/2 ground state and 2 lected together with the fluorescent emission. The image the F7/2 excited state) is well suited for InGaAs laser-diode of the laser spot on the sample was then formed onto the pumping. The RT emission spectrum is well structured with entrance slit of a Jobin-Yvon monochromator with a a strong infrared emission around 1.04 lm. This emission is two-lens optical device. This monochromator was nearly completely due to Yb ions located in C2 sites. Low

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R. Simura et al. / Optical Materials xxx (2006) xxx–xxx 3

1-5 0.5% Yb-Sc O 1 0.6 2 3 I(t)=I +Aexp(-t/τ) 5.0% Yb-Sc O 0 2 3

0.5 x = 5 mol% → τ = 593 μs 1-6 x = 0.5 mol% → τ = 561 μs 0.4

1-7 0.1 0.3 Absorbance (a. u.)

0.2 2-5 3-5 Normalized intensity (a. u.)

0.1 850 900 950 1000 1050 1100 0.01 Wavelength (nm)

700 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.5% Yb3+-Sc O 5-3 Time (ms) 2 3 600 3+ 5.0% Yb -Sc O 2 3+ 2 3 Fig. 3. F5/2 (Yb ) life time profiles as a function of the ytterbium 500 concentration in Sc2O3 single crystal. kexc = 976 nm, kem = 1040 nm, T = 300 K. 400 Using absorption and emission spectra, an attempt of 300 Yb3+ energy levels assignment is proposed and the calcu- 5-1 5-2 Intensity (a. u.) (a. Intensity 200 lated energy levels are shown in Fig. 2. The Stark splitting is quite large in Yb-doped Sc2O3 compared to other sesqui- 100 oxide, due to the high crystal field. This leads to a high 1 0 ground state splitting (1110 cm ), which can reduce the reabsorption mechanism in such quasi-four-level laser 920 940 960 980 1000 1020 1040 1060 1080 system. Wavelength (nm) 3+ 2 The infrared Yb ( F5/2) fluorescence decays exponen- Fig. 1. Room temperature absorption (upper) and emission (lower) tially with a lifetime equal to 561 ls in the case of 0.5 mol% 3+ spectra of (Sc0.95Yb0.05)2O3 and (Sc0.995Yb0.005)2O3 single crystal under Yb -doped Sc2O3. This value is slightly affected to be infrared pulsed laser excitation at 976 nm. The indicated peak assignments 593 ls when the Yb concentration is 10 times higher, which are shown in Fig. 2. is due to self-trapping effect (Fig. 3).

4. Conclusions

cm-1 7 We established the growth technology of undoped and 3+ 10500 6 2 Yb -doped Sc2O3 crystals were grown by the l-PD 5 F 10000 5/2 method and spectroscopic investigation were carried out. X-ray characterization shows that lattice parameter 9500 increase with Yb3+ concentration. From RT absorption 3+ 9000 and emission, energy levels diagram for Yb in Sc2O3 is obtained and the Stark splitting is quite large in Yb-doped Sc2O3 compared to other sesquioxide, due to the high crys- 1500 3+ 2 tal ﬁeld. The lifetime of the Yb ( F5/2) is around 561 ls 4 3+ 1000 for 0.5% Yb -doped Sc O , which increases to be 593 ls 2F 2 3 7/2 3 for 5% due to self-trapping eﬀect. 500 2

0 1 Acknowledgements Absorption Emission Yb3+ The authors thank to Dr. Vladimir V. Kochurikhin for

3+ fruitful discussion about crystal growth. Fig. 2. Energy levels diagram of Yb in Sc2O3 single crystal. References temperature measurements and site-selective excitation are needed to detect emissions from C3i sites, as it was shown [1] L. Fornasiero, E. Mix, V. Peters, K. Petermann, G. Huber, Crystal in the case of Yb-doped Lu2O3 [2]. Research and Technology 34 (1999) 255.

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[2] K. Petermann, G. Huber, L. Fornasiero, S. Kuch, E. Mix, V. Peters, [4] A. Brenier, G. Boulon, Europhysics Letters 55 (2001) 647. S.A. Basun, Journal of Luminescence 87–89 (2000) 973. [5] J-H. Mun, A. Novoselov, A. Yoshikawa, G. Boulon, T. Fukuda, [3] V. Peters, E. Mix, L. Fornasiero, K. Petermann, G. Huber, S.A. Materials Research Bulletin 40 (2005) 1235. Basun, Laser Physics 10 (2000) 417.

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