Luminescence of Pr3+-doped garnet single crystals

Luminescence of Pr3+-doped garnet single crystals

Optical Materials 30 (2007) 30–32 www.elsevier.com/locate/optmat Luminescence of Pr3+-doped garnet single crystals V. Babin a,* , A. Krasnikov a, Y...

217KB Sizes 0 Downloads 72 Views

Optical Materials 30 (2007) 30–32 www.elsevier.com/locate/optmat

Luminescence of Pr3+-doped garnet single crystals V. Babin

a,*

, A. Krasnikov a, Y. Maksimov a, K. Nejezchleb b, M. Nikl c,d, T. Savikhina a, S. Zazubovich a

a

d

Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia b CRYTUR Ltd., Palackeho 175, 51119 Turnov, Czech Republic c Institute of Physics AS CR, Cukrovarnicka 10, 16253 Prague, Czech Republic Dip. Scienza dei Materiali, Universita di Milano-Bicocca, via Cozzi 53, 20125 Milano, Italy Available online 21 December 2006

Abstract The steady-state and time-resolved emission and excitation spectra as well as defects creation spectra were studied for Pr3+-doped Y3Al5O12 (YAG) and Lu3Al5O12 (LuAG) single crystals at 11–300 K in the 2–12 eV energy range. For Pr3+-related UV emission bands located at 3.0–4.2 eV and excited in the band-to-band and exciton absorption regions, additional excitation bands are observed around 6.9–6.35 eV (LuAG) and 6.7–6.25 eV (YAG), which are absent in the undoped crystals and in the excitation spectrum of the intrinsic emission. The wide intrinsic emission observed at 11 K around 4.9 eV, arising from the antisite defects ðLu3þ Al Þ, is excited only in the exciton band and in the band-to-band absorption region. Ó 2006 Elsevier B.V. All rights reserved. PACS: 71.35.y; 78.47.+p; 78.55.Hx; 78.60.b Keywords: Pr3+-doped scintillators; Garnets; Time-resolved spectroscopy

1. Introduction Garnet single crystals YAG and LuAG doped with Ce3+ have found their applications as scintillating materials due to an intense and fast Ce3+-related emission, high density and high mechanical and chemical stability. An introduction of Pr3+ into host material results in the Pr3+-related 5d–4f emission, which is shifted to higher energy with respect to Ce3+ emission and characterized by even shorter decay time [1–3]. Owing to these characteristics, Pr3+doped garnet single crystals can be considered as promising scintillator materials for imaging screens with high spatial resolution. In the present paper, steady-state and time-resolved emission and excitation spectra as well as defects creation

*

Corresponding author. Tel.: +372 7428 946; fax: +372 7383 033. E-mail address: [email protected] (V. Babin).

0925-3467/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2006.10.022

spectra were studied for Pr3+-doped garnet single crystals at 11–300 K and in the energy range 2–12 eV. 2. Experimental Single crystals of Pr3+-doped YAG and LuAG with different Pr concentration (shown in melt) were grown using the Czochralski method by CRYTUR Ltd.1 Luminescence investigations were performed at two different setups: (i) the measurements at 80–300 K were carried out with the vacuum monochromator VMR-2, emission spectra were measured with monochromator MDR-4. For the excitation spectra, the emission wavelength was selected with different sets of optical filters. The spectra were corrected for the detector spectral response. (ii) Synchrotron radiation facility of HASYLAB at DESY (Hamburg, Germany) was used for the measurements at 11–300 K (SUPERLUMI station). 1

CRYTUR Ltd., Palackeho 175, 51119 Turnov, Czech Republic.

V. Babin et al. / Optical Materials 30 (2007) 30–32

3. Results and discussion In the emission spectra of YAG:Pr crystals, at 80 K the Pr3+-related emission bands at about 3.9 eV and 3.22 eV and the wide intrinsic 4.81 eV are observed (Fig. 1). At 295 K, the intrinsic emission band should be located at around 3.95 eV, as it is strongly overlapped with the Pr3+-related emission bands. The 3.9/3.22 eV emission intensity ratio depends on Pr3+ concentration (2.4 in YAG:0.65%Pr and 1.5 in YAG:0.16%Pr). These effects point to the overlap of the Pr3+-related emission bands with the emission of some other centers. The intensity of the intrinsic 4.81 eV emission is relatively small as compared with that in [2]. As the Pr3+ concentration increases five times, this emission intensity decreases about five times, and the 3.9/4.81 eV emission intensity ratio increases 10 times. This effect can be explained by the competition in the energy transfer processes between the antisite defects and Pr3+ ions as well as by the reabsorption of the 4.81 eV emission in the Pr3+-related absorption bands. As the temperature increases from 80 K to 295 K, the 3.9 eV emission intensity decreases 2.3 times. In LuAG:Pr crystals, at 80 K the Pr3+-related 4.0 eV and 3.4 eV emission bands and the intrinsic 4.9 eV emission band are observed (Fig. 1). At 295 K, the intrinsic emission band should be located at 3.65 eV, i.e., it is overlapped with the Pr3+-related emission bands like in the case of YAG:Pr. In the 3.54–3.02 eV energy range, the emission spectra of the two samples with different Pr concentration are strongly different. The 3.22 eV band may arise from the undoped LuAG [4]. At 80 K the main band is located at 4.0 eV (LuAG:5.18%Pr) and at 4.06 eV (LuAG:1.17%Pr).

From 85 K to 295 K, the 4.0 eV emission intensity decreases about 1.2 times, but the intensity of the 3.4 eV emission does not practically change. As the Pr3+ concentration increases 4.4 times, the intensity of the 4.0 eV emission increases only 1.70–1.75 times, but the 3.4 eV emission intensity, 3.0–3.5 times. The intensity of the 4.9 eV band decreases about 7 times. In LuAG:1.17%Pr, the 4.0/4.9 eV emission intensity ratio is 7, in LuAG:5.18%Pr this ratio is 85. In the excitation spectra in YAG:Pr crystals at 80 K, the exciton band maximum is at about 6.7–6.9 eV (Fig. 2), and it depends slightly on the emission energy and Pr concentration. Indeed, in the excitation spectrum of the 3.22 eV emission, the exciton band is located at 6.7 eV in YAG:0.16%Pr and at 6.85 eV, in YAG:0.65%Pr. The comparison of the excitation spectra of the 3.9 eV emission of Pr3+-doped crystals (Fig. 3) and the intrinsic emission of undoped YAG crystals indicates the presence near 6.7 eV

80 K 300 K 60 YAG:Pr 0.16% 40 Eem =3.97 eV

80

Intensity [a.u.]

Emission was selected by ARC monochromator and detected with PMT Hamamatsu R6358P. The excitation spectra were corrected for spectral distortions. The emission spectra were not corrected.

20 0 80

80 K 300 K 60 LuAG:Pr 1.17% 40 Eem =3.4 eV

20 0 11

10

0.6 Intensity [a.u.]

50 0 200 150 100

80 K 300 K LuAG:Pr 5.18% Eexc =7.7 eV

x10

Intensity [a.u.]

100

50 0 5.5

5.0

4.5 4.0 3.5 Photon energy [eV]

3.0

Fig. 1. Emission spectra for YAG:Pr and LuAG:Pr crystals measured under approximately the same conditions.

8 7 6 Photon energy [eV]

5

4

YAG YAG:Pr 0.65%

0.8

x10

150

9

Fig. 2. Excitation spectra of YAG:Pr and LuAG:Pr. For each Eem, the spectra are measured at the same conditions.

1.0

80 K 300 K YAG:Pr 0.65% Eexc =7.7 eV

31

a

80K

2

1

0.4 0.2

3 0.0 LuAG 1.0 LuAG:Pr 5.18% 0.8 1 80K 0.6 0.4 3 0.2 0.0 11.0 10.5 10.0 9.5 9.0 8.5 8.0 7.5 7.0 Photon energy [eV]

b 2

6.5

6.0

Fig. 3. Normalized excitation spectra of the undoped (curves 1) and Pr3+doped (curves 2) YAG: (a) and LuAG (b) crystals. Defects (afterglow) creation spectra (curves 3) after irradiation of a sample at 80 K for 2 min.

32

V. Babin et al. / Optical Materials 30 (2007) 30–32

Intensity [a.u.]

0.8 LuAG:1.17%Pr 11 K

0.6 0.4 0.2 0.0 9

8

7

6 5 Photon energy [eV]

4

3

Fig. 4. Time-resolved excitation (filled symbols, Eem = 4.0 eV) and emission (open symbols, Eexc = 7.12 eV) spectrum. Circles – integral intensities; triangles – ‘‘fast’’ time-window (2–16 ns); squares – ‘‘slow’’ time-window (30–99 ns).

the band of the exciton perturbed by Pr3+. Another perturbed exciton or charge–transfer band is located near 6.25 eV. Excitation bands of the Pr3+ 4f–5d1,2 transition are located near 5.2 eV and around 4.3 eV. Under the band-to-band, exciton and 4.30 eV excitations at 80 K, the 3.9 eV emission intensity increases with the increasing Pr3+ concentration 1.3–1.4 times, but the 3.22 eV emission intensity, 1.1–1.2 times. However, under 6.25 eV excitation, the intensity increases much more (2.2 and 1.6 times, respectively). In the excitation spectra minima, it increases several times, which may point to the presence of some other centers, whose emission spectra are overlapped with the emission spectra of Pr3+ ions. In LuAG:Pr crystals, the positions of the exciton band at 80 K are 7.1 eV in LuAG:1.17%Pr and 7.25 eV in LuAG:5.18%Pr. The comparison of the normalized excitation spectra of the 4.0 eV emission of Pr3+-doped crystals (Fig. 3) and the UV emission of undoped LuAG indicates the presence of the band near 6.9 eV arising probably from the exciton perturbed by Pr3+. Another excitation band, arising probably from the perturbed exciton or chargetransfer transition, is located at 6.35 eV. Excitation bands of the Pr3+ emission are located at 5.15 eV and 4.40 eV, corresponding to 4f–5 d1,2 transition. The former band can be overlapped with the wide band located at 5.3–5.5 eV and observed in the excitation spectra of all the emission bands of all the undoped and Ce3+doped LuAG samples studied. As the Pr3+ concentration increases, the intensity of the 4.0 eV emission at 80 K increases 1.6–1.8 times, but the intensity of the 3.22 eV emission remains practically unchanged. As the temperature increases from 80 K to 295 K, the intensity of the 4.0 eV emission decreases 1.2–1.4 times, but the intensity of the 3.4 eV emission does not practically change. Different concentration dependences of the 4.0 eV and 3.4 eV emissions intensity may point to their different origin or, more probable, to the overlap of the 3.4 eV emission band with some other emission bands (e.g., with the 3.22– 3.14 eV emission bands observed in undoped LuAG). The

presence of the 3.22 eV band in the emission spectrum of the LuAG:1.17%Pr confirms this suggestion. The study of the time-resolved spectra of the LuAG:Pr crystal indicates that the slow decay component (SC) strongly dominates in the 4.9 eV emission. In the excitation bands of the 3.98 eV Pr3+ emission (Fig. 4), located at Eexc < 7 eV, the SC is 2.5 times weaker than the fast component (FC), but under excitation in the exciton and in the band-to-band region, the SC dominates. At Eexc > 7.0 eV, the excitation spectrum of the SC of the 3.98 eV emission corresponds to the excitation spectrum of the SC of the intrinsic 4.9 eV emission. Probably, the mentioned SC arises from the overlap of the wide 4.9 eV intrinsic emission band with the 3.98 eV emission band, but mainly, from the reabsorption of the slow 4.9 eV emission in the intense absorption bands of Pr3+ centers located in the 4.9–5.9 eV energy range. 4. Conclusions The steady-state and time-resolved emission and excitation spectra as well as defects creation spectra were studied for Pr3+-doped garnet single crystals. For the Pr3+-related UV emission bands, excited in the band-to-band and exciton absorption region, an additional excitation bands are observed around 6.9–6.35 eV (LuAG) and 6.7–6.25 eV (YAG). These bands may arise from the exciton states perturbed by Pr3+ ions or from the Pr3+-related charge-transfer states. The wide intense band peaking at 5.5–5.3 eV is present in the excitation spectra of all the emission bands and its position depends on the emission studied. The wide intrinsic emission observed at 11 K around 4.9 eV, arising from the antisite defects ðLu3þ Al Þ, is excited only in the regular exciton band and in the band-to-band absorption region. The intensity ratio of fast and slow components in the UV emission bands depend on the impurity content. Acknowledgements This work was supported by the EC – Research Infrastructure Action under the FP6 ‘‘Structuring the European Research Area’’ Programme (‘‘Integrating Activity on Synchrotron and Free Electron Laser Science’’); Estonian Science Foundation Grants No. 6548, 6538, 0548J; NATO Reintegration Grant No. 981411 and Czech GA AV project S100100506. References [1] M.J. Weber, Sol. St. Commun. 12 (1973) 741. [2] M. Nikl, H. Ogino, A. Krasnikov, A. Beitlerova, A. Yoshikawa, T. Fukuda, Phys. Stat. Sol. (a) 202 (2005) R4. [3] W. Drozdowski, T. Lukasiewicz, A.J. Wojtowicz, D. Wisniewski, J. Kisielewski, J. Cryst. Growth 275 (2005) 709. [4] V. Babin, K. Blazek, A. Krasnikov, K. Nejezchleb, M. Nikl, T. Savikhina, S. Zazubovich, Phys. Stat. Sol. (c) 2 (2005) 97.